Talk:Speed of light/Archive 3

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Measurement of the Speed of Light - Ole Rømer

Rømer observed that Io revolved around Jupiter once every 42.5 hours when Earth was closest to Jupiter. He also observed that, as Earth and Jupiter were moving apart, Io's revolution seemed to take longer.

This statement is ambiguous because it implies that the movement of the Earth away from Jupiter is what makes Io's observed orbital period longer. The Speed of Light in a vacuum is constant and not affected by the movement of the observer or the subject.

Rømer made his observations by taking a previously measured time for Io's orbit about Jupiter, and making a schedule so he could observe the eclipses. He noted that Io was eclipsed earlier than the schedule predicted when the Earth was close to Jupiter, but was late when it was far away. Therefore, whether the Earth was moving toward or away from Jupiter was largely irrelevant, it was their distance apart and its affect on his schedule that he observed and measured.

Good point, we should clear up that ambiguity. Please feel free to adjust the text accordingly if you have not already done so. Regards & happy editing, Wile E. Heresiarch 14:11, 12 Apr 2005 (UTC)

Not entirely. In the paper (which was translated into english from the orginal french), what matters is whether we are on the approaching or the receding segment of our orbit. It says explicitly that it is the motion, relative to Jupiter that matters, and this agrees with standard physics today (Doppler effect in the non-relativistic limit for ligth). Ulcph 17:59, 29 August 2006 (UTC)

Roemer time-lag not similar to Doppler effect

From the article (about Roemer's detection of a variable time-lag in the occultations of Io): "These observations are akin to what today is known as the Doppler effect."

This statement is not correct. It is not the relative velocity of the two systems (Earth and Jupiter/Io) that is causing the time offset, but instead simply the distance between them, combined with the finite speed of light.

Reference to a Doppler effect is undesirable in this astronomical situation because of the potential for confusion with red shift (which is precisely a Doppler effect) or with the aberration effect that Bradley used for this purpose (which could reasonably be said to be akin to the Doppler effect, since both are velocity-based).

The effect is more like echolocation, or perhaps a closer analogy would be the determination of the speed of sound by measuring the differences in thunder delays between lightning flashes to points at known distances. But I do not see an analogy close enough that I think it enhances the article. Hunter 23:10, 17 Apr 2005 (UTC)

The statement "These observations are akin to what today is known as the Doppler effect." is correct. The statement "red shift (which is precisely a Doppler effect)" is wrong. So is much else in the above comment. If it is any consolation, you will find obviously non-sense statements even in well-known textbooks, to the effect that Rømer measured occultations at the closest and farthest points in Earth's orbit. And we wonder how he may have observed Io in broad daylight, with the Sun in between! With all due respect, this misconception seems to have been propagated by Newton himself, in Opticks. Not that we may assume that it was his intention. If in doubt, look up Rømer's original article and check his graphic! - Prof. Guest, Aug. 19, 2006

Now hear this! It is generally recognized that what Rømer observed is the optical (non-relativistic limit) Doppler effect. Indeed, what else could it be? It is a helpful insight for the readers. It would be desirable if people would refrain from editing this out, apparently based on incorrect notions, like the ones expressed in the discussion above. There is, for some reason, an incredible amount of misunderstandings about this particular piece of physics. See the compilation in A. Wroblewski, "de Mora Luminis: A spectacle in two acts with a prologue and an epilogue", Am. J. Phys. 53, 620 (1985), and then check your favorite textbook. Just so that you can appreciate how bad it actually is, here are, taken from this paper, the dates ascribed in textbooks to the discovery: 1656 (1 instant), 1666 (3), 1673 (1), 1675 (14), 1676 (24 correct), 1678 (1), 1876 (1). Related with all due respect. Ulcph 21:28, 31 August 2006 (UTC)

Vehicle

If you were traveling in a vehicle going at the speed of light and you turned your headlights on, would it do anything? Yofoxyman 00:11, 25 April 2006 (UTC)

First of all, it's impossible for a massive object to travel at the speed of light. Second, even if you could travel at the speed of light, in your own reference frame, the very same instant you achieve that speed you would simultaneously slam into your destination, no matter how far away it is, so you couldn't actually spend any time cruising at the speed of light and doing experiments.

On the other hand, if you're traveling in a vehicle going at just under the speed of light and you turn your headlights on, they will emit light which, in your own reference frame, travels away from you at the speed of light. A stationary observer will observe the light separating away from you at a much slower speed; the apparent discrepancy between these viewpoints is explained partially by time dilation. Melchoir 00:30, 25 April 2006 (UTC)

I would have to argue against your first point. It implies that the universe is finite and that one could theoretically reach any point instantaneously by traveling at the speed of light. I was thinking that since you are traveling at the speed of light and as are your headlights, the distance that they seem to travel in front of your vehicle would remain constant. Also, at that incredibly high speed, you would most likely be "over-driving your headlights." So because of that I think that if one were traveling at light speed, headlights wouldn't be much use. Yofoxyman 02:46, 25 April 2006 (UTC)

Yes, if you travel at the speed of light, you reach your destination instantaneously. Melchoir 03:09, 25 April 2006 (UTC)

Don't you reach your destination at the speed of light, 300 million meters per second? If you travel to the nearest star instantaneously, surely you would be a few years ahead of the light that follows the same path, and travels, as I understand it, at the same speed? Richard001 06:49, 2 August 2006 (UTC)

I meant "instantaneously in your own frame of reference". Melchoir 07:02, 2 August 2006 (UTC)

Melchoir means that if you had a clock on board it wouldn't have gotten in a single tick before you arrived at your destination. P0M 02:41, 8 December 2006 (UTC)

I may not understand this theory of relativity but how is light speed infinite and then stay again a constant what ever speed that is?

In YOUR time frame, as the traveler, time dilation distorts your timeframe so much that you would in fact reach your destination "instantaneously"... at least to YOU. Your body would not age, but when you step out of your ship, you'd find the universe has aged much more since then. When you move at speeds approaching c (from 0) the ratio of your experienced time to "real time" approaches 0 (from 1).

In Milky Way it states The main disk of the Milky Way Galaxy is about 80,000 to 100,000 light-years in diameter, surely you must at some point in the universe be able to do the headlight experiment, if you can travel for years at that speed!! Paddy :-) 18:41, 2 December 2006 (UTC)

If you put a clock into a box and disconnect the battery as you hand it to UPS it doesn't matter how far away you send it. It will still show the same date and time when it gets to the other end. P0M 19:18, 2 December 2006 (UTC)

So is the speed of light not a measurement of speed? Are you saying the speed of light is instantaneous? What is 1,079,252,848.8 km/h then? surely it would take you two hours sitting in a car if the distance from A to B is 120km and you where driving at 60km/h. Surely if C to B is two years light and you are traveling at the speed of light it would take you two years to get there and I think two years is plenty of time to notice it.

Not being funny, just asking because it really confuses me. Paddy :-) 21:27, 7 December 2006 (UTC)

It's a confusing matter, no doubt about it. The speed of something is the number you get when you divide the distance it travels by the amount of time it takes to get there. As I learned in grade school, d = r * t (distance equals rate times time). As long as you confine things to the kinds of experiences I was familiar with in grade school, nothing is going to look strange.

Now let's set up a simple experiment. We have a hollow tube or pipe. You can think about a ten foot long piece of the plastic pipe about 3" in diameter On the outside of the tube somewhere we put a few flashlight batteries in a box and we run two pairs of wire out of the box. One goes to a little light that we hang out in front of one end of the tube (or we could glue it to the inside of the tube just far enough inside that it wouldn't get bashed by things. On the other side we put a photoelectric screen that is shielded so it doesn't see light from the LED we just fixed to the other side, but it will receive light from the far end of the pipe. Then we go to the other end of the pipe and duplicate the same set-up. When we are done we have a matched pair of emitters and detectors. Their distance apart is the same. So in both cases D = 10'. Now we have to add a really expensive piece of equipment. It turns on one light and then measures the length of time before the blip of light it just sent on its way reaches the other end of the tube. (If we had a longer tube we could buy a cheaper measuring device I guess.) We have one timer for each pair. We know that the speed of light is supposed to be 186,000 miles per second. So we can fill in our formula: 10 feet = 186,000 miles/second * t seconds. So we turn on our experimental apparatus and the two timers show just exactly the time t that was predicted by the formula. Big deal. Physicists have known the speed of light for decades and have confirmed it time after time, so we shouldn't be surprised if we get the same answer. So why did we spend a cool million dollars buying that clock that measured the amount of time it took for light to go ten feet. It's such a tiny number it doesn't matter anyway, does it?

But wait. What happens if we are moving? Suppose that we did the same experiment by arranging the tube vertically and dropping a BB pellet from one end of the tube and timing when it hit the other end of the tube. Would it matter whether the tube was moving? Of course it would. If we dropped the BB and simultaneously accelerated the tube very rapidly, the bottom of the tube would meet the BB much sooner. If we held the BB and the tube vertically at the top of a tall building and let go of the two of them at the same time, then they would fall together (if there were no air resistance they would stay together perfectly). So we assume that if we emit a proton at the top and accelerate the tube toward where we emitted the proton, then the proton would hit the other end sooner. And we assume that a proton emitted from the other end and "trying to catch up with" the moving target at the other end of the tube ought to take longer to get there since the target is running away from the photon. And this is where things get strange. No matter how fast we move this tube, we always get the same answer. Even if everything is in vacuum, it is as though our tube carries its own light propagation medium along with it, and light moves through that medium at the same speed regardless of anything else "because" the medium moves along with the tube. Which is crazy because there is no "medium" that sticks with the tube -- especially in the vacuum of outer space.

Fortunately, we do not have to fabricate a space ship to perform this experiment. We already have Spaceship Earth, and it is already moving plenty fast to notice a difference. If we stand on the north pole we are moving toward some stars in one direction and away from some stars in another direction. So we carry our expensive piece of sewer pipe up to the north pole (or down to the south pole if that is closer) and measure away. Even if we stand on our heads the speedometer always reads 186,000 miles/second.

If a meteor came cruising in at just a few feet per minute faster than we were moving away from its point of origin, then it would land lightly in the snow. If it were coming from the opposite direction it would hit at tremendous velocity and probably fry us if it hadn't already burned up in the atmosphere. Speeds of ordinary things are additive. But the speed of light is not, and that is totally weird. Note that we can't even explain things away by claiming that the tube has gotten longer or shorter to compensate for our motion and make our calculations come out right--because any change in its length that made the speed of light correct in one direction would magnify the defect in the opposite direction.

So it's not that light does not have a speed, it's just that its speed is sort of a private matter between it and us. Light has a deal with me that it will always go from the tips of my fingers to my eyes in exactly the same amount of time no matter whether I'm moving toward Sirius at a fantastic speed, moving away from it at a fantastic speed, or what.

Another way of conceptualizing this paradox is to say that I never move with respect to whatever medium it is that light waves vibrate in. I move with respect to other things. Those other things never move with respect to whatever medium it is that light waves vibrate in (at least as far as they are telling me, and who am I to dispute them?), they just move with respect to me. And that idea is totally nutty. It says that I am in my boat on the ocean and I am not moving, and you are on your boat on the ocean, and you are not moving, yet we are moving with respect to each other. The "only" way that could happen would be if we shrank the ocean between us when we thought we were moving together or we expanded the ocean between us when we thought were were moving apart. Crazy, crazy, crazy.

I always get a bit sick when I try to think about these things, and I have to go back to the mundane business of grading final exams now. Let's see whether (a) you can get your head around what I've said so far, and (b) whether anybody else points out any flaws in it. Also, I think there are some time dilation simulations online, ones that should be in the notes to this article. You could play with those until you get used to the idea of your time slowing to a crawl (from my point of view, of course) as you get up close to the speed of light. P0M 02:41, 8 December 2006 (UTC)

question

suppose one could travel at the speed of light, and the place you wanted to go was 40 lightyears away, would it take 40 years to get there? --Revolución hablar ver 05:59, 29 April 2006 (UTC)

To a "stationary" observer, it would appear so. But in your own frame of reference, it would take no time at all. Melchoir 06:10, 29 April 2006 (UTC)

eh? yeah, sorry, continuing with that, would you, i dunno, say age 40 years if you travelled 40 light years at the speed of light, even if it felt like no time at all?

Hun? so light by itself has infinte speed?

You can think of it that way. As you accelerate towards the speed of light time slows down relative to a stationary observer, as you reach the speed of light, time stops! So you can travel an infinite distance without observing any passage of time. However it is not this simple as distance also contracts. So the observer at the speed of light sees no distance, taking obviuosly no time, and not requiring an infinite speed. In the moving frame it takes no time as they travel no distance. The idea of infinite speed comes from a stationary observer, using time from the moving frame but distance in the stationary frame, as long as you work in the same frame, everything remains consistent. I think its best not to worry about these things too much its just maths. Obviously it is impossible for an observer to actually travel at the speed of light, so this is all speculation. The slowing down of time has been observed though, for example fast moving particles take longer to decay than slower ones, i.e. time for them is slower.Jameskeates 10:42, 17 August 2006 (UTC)

Light has infinite speed in it's own perspective. And time is 'stoped' at the speed of light. So, if for instance a photon leaves the Sun towards the Earth, get here and is reflected back, it would arrive at the Sun in time to see itself leave? Doesnt this mean that for light the Sun and the Earth are actually in the same place? Isnt the universe all in the same 'place' then? And can time stop and still exist at all? Doesnt all this mean that light is actually outside time and space? Should we think of all this as some kind of Einstein's metaphysical implication of omnipresent light? Is one photon all the photons there are? Does all this make any sence? (Uanbiing 23:54, 26 January 2007 (UTC))

You raise an interesting question. It is a contrary-to-fact conditional, i.e., it asks, "If travel at the speed of light were not impossible for something that could have awareness, then what would the awareness of such a rapidly traveling entity be like.

We move through space-time. The faster we move through space, the slower we move through time, and it is all part of the same system.

We can only approach an understanding of what it would be like to travel at the speed of light by approaching the question as a limit case and asking what happens as a sentient observer makes faster and faster trips between two sample locations.

Suppose a Klingon battle cruiser is making a series of patrols in the neighborhood of Earth. The first time is flies over the north pole of Sol and the south pole of Wolf 359, and it is moving at .1 c. The arachnoids of Wolf 359-4 see the life signs of the Klingon crew and think their heart rates are rather slow. The Klingons perceive the life of that fourth planet as moving slowly too. The next patrol is made with an improved cruiser that travels at .5 c. Earthlings see the Klingons as slow-moving individuals, and vice-versa. And the trip between the two stars takes much less time according to the measurements of Earthlings. Similarly, the clocks on the ship, and the life processes of the Klingons, are slowed from the perspective of Earthlings. The Klingons, on the other hand, find their body processes the same as measured by their clocks. From their perspective their heart rates, etc., stay the same as before. The third mission is performed by a cruiser that goes at .9 c. Earthlings and Arachnoids agree that the cruiser is taking about 3.5 years to travel from system to system. The Klingons' clocks and biological processes still look normal to them, and their clocks hardly move during the trip. So stars going by look like telephone poles seen from a speeding car.

The faster the cruiser goes, the more the stars being bypassed are merely a blur in their foward looking video screens. From the standpoint of sentient observers on planets they pass, their motions are more and more glacial until it seems that they are frozen.

Finally, they are going so fast that each star looks to them like one frame in a movie taken of a single streetlight. The "single star" they see on their screen changes color and size in a random way until finally they are going so fast that even the changes blur into a steady perception due to the persistence of their vision.

So it looks as though if they could pay the fuel bill and actually accelerate to c then their body processes would stop, Not only that, all of the ship-board instruments would freeze. It would be "the same time" for each star visited by their in-stasis bodies.

The fact that the clocks of the Klingons have been stopped does not affect other processes, it just affects how the Klingons view/understand other processes. If some trickster invented a temporal stasis machine and tricked me into his laboratory in San Francisco at Noon on 1 January 2009, froze me, and shipped my frozen body (and frozen wrist watch) to New York City, woke me up, and showed me to the door, then I think I would be quite surprised. Suddenly it is the middle of the night, my watch is wrong in date and time according to local clocks, and I am on the other side of the continent when I open that door and go "back" outside. I call my home in S.F., and my family members want to know where I have been the past two days. (I was shipped UPS.) If they kept me on ice for a century, then according to my experience only a moment (representing the time it took to put me under) had passed, but the universe had gotten older under my feet. But who is to say that my clock is wrong and the clocks of everybody else are right? (People who have remained in a coma for years must have a terrible time accomodating to the changes that have occurred in the decades they have been unconscious.)

If the evil genius plays the same trick on several individuals, stopping all their clocks for the same length of time (according to his clock), that fact does not make those several individuals the same individual.

If the evil genius stops my clock for ten days but doesn't move me, then I will blink and the bright light from the window may suddenly blank out. But I won't see myself coming in the door. I will just see the environment as changing. People that were seated across the table from me may be gone, the flowers in the vase may suddenly be shriveled and dead, etc.

My understanding of travel is that I walk, pilot a plane, or whatever, that the numbers on my wrist watch change, and that my physical location changes. Speed is understood as "how long does it take to go a certain distance." If, according to my clock, I am at 12:00 in SF and then next time I glance at my watch it is 12:00:30 and I am in NYC, then I will infer that I have traveled some 3000 miles in less than a second. (I am aware that I may have been in NYC a while before I looked at my watch again, so 30 seconds is just the maximum time that it might have taken.) If I have some kind of recording GPS with a very accurate clock I may decide that it took no appreciable time to travel that distance and I will suspect that my speed might have been infinite.

What if I argue the other way around D=RT, T = 0, therefore D = 0 no matter what the Rate was. But that's assuming that R was some finite value. If I am limited to c, then D = c * 0, and indeed D = 0. That logical and mathematically correct calculation does not mean that D = 0 for somebody whose clock is not frozen. It just means that "in an instant" I am transported from SF to NYC. My experience, naively, is going to be that I have to be in the same place. It's a natural mistake since I have been totally out of operation during the UPS ride across the continent. If this kind of thing happened as a matter of course, then "everyplace is the same place" would make a kind of sense. (I have a science fiction story somewhere about people who have little "instant transport" cabinets in their homes. They just think "Paris" as they enter the rotating door and when they rotate back out they are thousands of miles away.) As long as you have the time freezer available, distance is irrelevant -- not because it is not there but because it is not experienced.

Technically speaking, time does not exactly stop. Time is hard to think about. If you give an operational definition of time, then it gets lots easier to think about. People have measured time in lots of ways, some more reliable (e.g., for developing film in the darkroom) than others. A water clock might not be reliable for timing some process because the several "identical" water clocks will calculate different amounts of time when run together, and, worse, the same water clock may win or lose in comparison to another water clock on another run of the same experiment. So what we want, when we measure time, is something that happens in a very regular way and that happens very fast. I might have an atomic wrist watch someday. It would have a vibrating system composed of some atomic process. It would happen very rapidly, and it would be extremely regular (since there is no friction involved, etc.). The mechanism is conceptually very simple. I have attached a counter that turns over once each time the atomic process vibrates one time. So the time it takes to cook an egg is the number of "ticks" made by this tiny vibrating system during the time the egg is in the boiling water. I could have two atomic watches, one on each wrist. If I travel from Sol to Alpha Centauri at a large fraction of c, then my heart rate and other bodily processes, as measured by my wristwatches, will stay 70 or whatever is my normal rate, and my watches will stay synchronized. Maybe my on-board clock sends out a signal beep once every day. It takes a while for the signal to reach earth, but eventually earth gets its own record of my clock ticking away. What earth sees is that it takes more than a day between beeps.

Time is my time. Time is your time. But my time is not your time. Even so, there is a definite relationship between the two. With just high school algebra one can work out the relationship. So it is possible for the earth observer to learn the speed of the space ship relative to earth and calculate how long it will take between signals from the "24 hour clock" onboard the speeding ship.

Imagine that the clock onboard the ship consists of a light emitter on the long axis of the ship, a mirror on a long boom perpendicular to the long axis of the ship, and a detector on the light's return path that is attached to a counter. A blip of light is sent to the mirror and back, it hits the detector, gets counted, and simultaneously fires off another blip. From the perspective of the people on the ship, the distance light travels is two times the length of the boom. From the perspective of people on Earth, the distance light travels depends on how far light has to go to catch up with the mirror on the boom and then catch up with the detector -- catch up because it is shooting at moving targets. If the ship is travelling at the speed of light, then light will never catch up with the mirror because the mirror is moving away at just that speed. It's like one twin trying to catch the other when the other has a slight head start. So light never gets back to the detector, and the detector never counts even a single blip. So the time of the space traveler stops. No process, no interaction, is mediated at faster than the speed of light, so no process can occur when the "target" of the process is moving faster than the transmission speed of the process. For example, a jet fighter could not catch up with the machine gun bullets it shoots unless the jet's speed is greater than the bullets' speed. So a jet fighter cannot shoot itself unless it can fly fast enough to get in front of its own bullets. (Actually, jet fighters in a power dive have shot themselves, but that is because they indeed can fly faster in that special case.)

The fact that my clock can't click over does not influence whether your clock can click over. But the relative nature of all motion can, I think, create some paradoxes. Could we mean anything at all by the word "speed" unless we had some relatively stable system to define "location"? If there was an almost empty universe with only two space ships in it, they might be in a near collsion course. Is one moving and one still? Are both moving? The only thing they could calculate would be a single speed -- their closing speed. What would happen if we examined different cases, cases where their closing speed came closer and closer to c? I see myself as "still" and see your spaceship approaching mine at .9 c. You say, "No, I am still and you are approaching me." We each see the other's clock as going very slowly. It looks as though at a closing speed of c I should see your clock as stopped, and you should see my clock as stopped. But we should each see our own clocks as behaving just the same as they always have. After all, the only thing that has changed is that you have gotten close enough to me to make us aware of each other.

If we could have a closing speed higher than c, then we would not see each other until after we had passed in the night. Then the light from your spaceship's headlights would arrive from the nearest point, then from slightly farther away, and so on. In other words, you would appear to be moving away from me at something greater than the speed of light. If you had been broadcasting a radio message to me, I would receive it in reverse. I guess I could tape it and play the tape in reverse. The tape could give details of the clock being used, so the crew on my ship could figure out how to interpret your time signals.

More after I do some math. P0M 14:41, 27 January 2007 (UTC)

Math still in progress. Is it clear that if something like a bullet is faster than the sound that firing the gun makes, then the bullet will strike before the firing of the gun is heard? And in general, if something like a plane is going faster than the speed of sound you will see it before you will hear it, and you may hear the sound that it generates from nearby before the sound it generated on the way in can reach you? P0M 21:29, 27 January 2007 (UTC)

Even though in an imaginary universe where ships could go faster than light you would expect to see them suddenly appear (probably in a terrible burst of X-rays) and appear to go off in 2 opposite directions, in our universe that kind of thing can't happen.

In a universe with only two spaceships, what would be the relationship between the clocks onboard the two ships?

Δt=2m/c (1/(1-(V²/c²))-²) (The other ship's time correction as computed by my ship)

Each spaceship sees its counterpart in exactly the same way, so if the Velocity between the two were greater than c then the equation would become invalid.

As V approaches c, Δt approaches infinity, i.e., it would take forever for the other person's clock to tick. Since there is no preference permitted, both spaceships being equally valid observers, If your clock were to stop then my clock would also stop. Strangely fair, no?

With only one spaceship in an otherwise empty universe there is no speed. With two spaceships, there is speed, and there is a speed limit.

If a spaceship could be going faster than c, then what would happen to its clock?

Let's look at what would happen if the spaceship were going at just under c. Light has to travel farther in a light clock than the spaceship travels in order for the clock to tick once. If light is just a little faster than the ship it will eventually catch up with the detector and cause the clock to tick.

What would happen if the spaceship were going at c? The light could not catch up with the detector because it has to travel the little extra distance imposed by the length of the mast.

What would happen if the spaceship were going at something greater than c? It would in that case, too, never catch up with the detector.

Is the only thing that gives meaning to the idea of the speed of the spaceship moving away from the point that light was emitted the presence of the other spaceship? Without another spaceship there is no meaning for the idea of the "speed of the spaceship." One could determine the speed of light within the spaceship, and that would be all. With the other spaceship in the picture, speed has meaning, but it also has a limit.

The limitation on speed seems in some sense solipcistic since it has to be expressed in terms of what one ship would calculate for the operation of the clock on the other ship. Is there something about the universe that imposes this speed limit and that is only made comprehensible to us when we bring in the idea of another physical system to make speed an objective phenomenon? If there were a single spaceship in a universe, would it have a speed relative to something that is unobservable? But then when a second spaceship was introduced into the universe one of them might be "really" still and the other "really" moving, and that idea is inconsistent with relativity. P0M 03:06, 28 January 2007 (UTC)

Stupid question... (I'm not a math major.) Does relativity assume that the clocks themselves measure time correctly? I mean, clocks as we know them were created to sit stationary on a shelf, wall, or the ground, always in the exact same pull of gravity. What if the very laws of physics themselves change as we accellorate or move away from Earth's gravitational pull, in a way that causes the clock to act slightly different? Basically, no more or less time has passed for anyone or anything, but whatever changes in gravity or motion affect the clock on a subatomic level, causing it to "move differently". I guess my personal problem with relativity is that time is a human invention. I think it's a bit off for the same reason I don't believe in time travel. (Which I guess Einstein's Relativity assumes is possible.) I dunno, maybe what I'm saying blows the whole thing apart, or maybe it's the icing on the cake. And that's what I have trouble understanding.76.18.95.22 20:27, 22 March 2007 (UTC)

The short answer is that time is defined as the thing that clocks measure, so a well-built clock cannot fail to measure time. For a longer answer, try the reference desk; this is just the talk page for the article on the speed of light. Melchoir 20:41, 22 March 2007 (UTC)

Question.

If one particle is travelling at the speed of light in one direction and another particle is travelling at the speed of light in the opposite direction and they collide then is the combined speed at point of impact 2x speed of light ?

Um. No. If you and your friend is running against eachother you'll sooner or later hit. The force will be absorbed by the other and you'll gain the exact same force per kg of mass in the other direction. Please correct me if I'm wrong (14years, no uber physican). Cybesystem 00:11, 2 August 2006 (UTC)

nha if a car hits a brick wall at 70 mph is not the same if 2 car are going at eachother at 70 mph tthier force adds up to 140 mph of one car if both cars have similar mass or the mass is doubled with the ligth the momentum is 2X i belive unless there is some werid relativity law i dont understand

No. The speed of impact in the frame of reference of the particles will be less than or equal to the speed of light. i.e. the colliding particles will not experience the collision at twice the speed of light, due to the effects of relativity.Jameskeates 10:39, 17 August 2006 (UTC)

Unless I'm mistaken, the end result would be the two particles moving away from each other at lightspeed. (unsigned)

Our everyday experience convinces us in many ways that velocities are additive. The same rules apply when two soccer players are running on a collision course. We cannot notice the relativistic effects because there is a divisor involved (1-(v²/c²)). The speed of light squared is a huge number, and the speed of a man running (much less even than v = 1 mile/second) squared is a tiny number -- so small that for practical purposes it amounts to 0. Something very close to 0 divided by a really huge number yields something even closer to zero. 1 minus that number is so close to 1 that for any practical purpose one would just use 1.

If you plug in v = 0 then you get 1. However, if you plug in the value of c for v, you get (1-(1/1) = (1 - 1) = 0. When you divide something by 0 you get infinity, a practical impossibility. So the nearer you get to v = 0 the less you can see and the nearer you get to v = c the greater the correction that is needed to our ordinary idea of additive velocities. Since the mass of an object increases as it accelerates, it gets more and more difficult to boost speed another notch, too. P0M 15:54, 3 October 2006 (UTC)

Citation for Subluminal studies?

I think with as many references to studies as there are in the subluminal section, there should be some citation... --HantaVirus 14:05, 21 July 2006 (UTC)

Since this is a two-year-old Featured Article, it doesn't have the inline citations one would expect. I can look for a couple for that section... Melchoir 23:58, 21 July 2006 (UTC)

Question

Hi. I am having a hard time understanding how the speed of light is measured (and not "defined"). I hope someone will clarify this process for me. I've seen the diagrams that show lightbeams reflecting off of mirrors, but I can't make sense of what these images mean?

For instance, in the real world, there is what I call a "source" i.e a source of heat, light or sound. There is also a "receiver"; like your ears, eyes, or skin. From the diagrams given, I see a "source" sending out a beam of light, that is then redirected using mirrors, but what is the instrument that "receives" this signal? and how does it work? 2c me 04:23, 29 July 2006 (UTC)2c me

Don't light have a charge? If so, can you measure when the charge is changed in the reciver? I'm also confuzed Cybesystem 00:08, 2 August 2006 (UTC)

2c me: could you specify which diagrams or setups you find confusing, and whether any of them in Wikipedia need improvement?

Cybesystem: light doesn't carry charge, although it can certainly move around the charges already present in whatever it strikes. (If light did carry charge, then it would be able to support longitudinal polarization modes... but it doesn't.) Melchoir 00:18, 2 August 2006 (UTC)

In the 1850s, I assume that a human eye was the detection equipment used with the Fizeau-Foucault apparatus. In the 1890s, I also assume that the human eye would have been used with early versions of the Michelson-Morley experiment. These days, a light-sensitive semiconductor device would be used. Do those articles explain the diagrams and the experiments in sufficient detail for you to understand what is going on? -- ALoan (Talk) 10:01, 2 August 2006 (UTC)

Going at the speed of light and turn on the lights

Dumb question, but I do really want to know (can also be included in the A.)

Scenario:

You are traveling at the speed of light (though it might be impossible, but imagine), in your "spacecar". You then turn the front lights on. What happends?

Ok, I'm no physican but heres ny therory, please correct me:

Since Mass is another form of energy, energy is another form of mass. Therefore light have mass. We also know that the speed of light is constant, does not change if the car moves. And we are moving at the speed of light. Since the light can't go faster than we are, the light will be "trapped" inside the lightball. The light will continue to come, and get "stuck". As the amount of light increases, so does the mass. When the lightball is as full as it can be the light will create a preassure inside the lamp. When the preassure exeeds the force the lightball can take it will explode.

So, what do you think of a 14yr olds therories? Cybesystem 00:22, 2 August 2006 (UTC)

This doesn't seem like such a dumb question; it's actually very popular. My own answer is above at #Vehicle. For a longer answer, try this link. Melchoir 00:40, 2 August 2006 (UTC)

according to the thoery it is impossible to be going at speed of light casue it says if you apporach the speed of light ur mass increases infintyly and the engery requied would also be infite w/e lets says ur going .99 of the speed of light according to the theory the speed of light is the smae to all obververs so to ur it is going at the speed of light but lets says there is some guy doing how fast light is going experiment lets says as soon as you turn on the head lights the observer only see the light going the 1% outa the 99% ur going you you in the car see light go 100% its werid but idk even if the driver was apporaching a wall that was senstive to light and saw the light hit it from far away while the second obsever was near the wall apparantly she wouldn't see it unless he was getting extremly close to the wall as the light was going the 1% of the speed that she was observing. if what i said is wrong plz tell me what is right according to the thoery?

If you go the speed of light, you cannot turn on your lamp because no time is passing for you — you are frozen at that instant. At any speed, even if only a snail's pace slower than light, you will see the illumination of your light travelling at the speed of light. Interior cabin lights would act perfectly normal to you. An outside observer would see the light from the headlights go at the speed of light, which is only fractionally faster than the car. She would also see it blue (if approaching) or red (after passing her). —Długosz

so the light would hit the wall at the same time for both observers?

Relativity does not require events to happen simultaneously for both observers, it is possible for the light to be observed hitting at different times. There are even situations where the order of events can be altered for different observers raising interesting questions about cause and effect.Jameskeates 10:45, 17 August 2006 (UTC)

Question.

This may be another dumb question, but I haven't yet found a clear answer to this question elsewhere and I'm trying to understand relativity from the ground up rather than just accepting it on face value...

What is the observable evidence and/or assumptions that have persuaded scientists to believe that the speed of light is constant for the viewer regardless of the viewer's speed relative to the source?

The earth is moving-with-respect-to most of the stars that are visible to us. They are all going about their own business, and while we are all part of galactic rotation we should have considerable speed relative to most stars. For one thing, our motion around the sun gives us motion toward and away from stars that fall on the plane of our revolution depending on the time of the year, i.e., whether we happen to be going toward or away from them.

So if we put our light speed measuring device on the north pole (so we could ignore the daily rotation of the earth) we would be in a position analogous to a policeman riding a merry-go-round and carrying his radar speed measuring device. If he happens to be moving toward a person seated in a chair, then his device will show a positive velocity, i.e., they are moving closer together, and then when he gets as close as possible to that individual as the merry-go-round will take him, he will enter the second half of his revolution where he is moving away from the seated person, and his gauge will show that the person has a negative velocity, i.e., they are moving farther apart. Somebody who is walking toward the merry-go-round at a steady pace will have different speeds depending on where the policeman is in his revolution around the center of the merry-go-round.

If the velocity of light were like other velocities, i.e., the velocities of things we experience in our travels, then the velocities of photons approaching from the seated person (or his flashlight) should pass through our measuring device faster when we are moving toward the seated person because the whole device is moving toward the photons as the photons are moving toward it. If the photon were a bird flying in the sky and the light-speed-meter were an aircraft with a tubular fuselage open at the front end, then the bird would hit the back end of the fuselage in more or less time depending on how fast the back end of the fuselage were moving toward it.

So what people expected to see when their light-speed-meter was directed at stars in different parts of the sky was that the speed was higher when moving toward some star and slower when moving away from it later in the year. And since the earth would have different velocities with respect to most stars, almost every measure of the speed of light should be different.

The expectations that people had formed by thinking about the velocities of moving objects like people on ferris wheels with respect to people sitting on the ground, etc., were not supported when the speed of light was measured. The speed of light was always measured as being very nearly the same. (See the article for information about experimental results that show some minor differences.) Many people tried to explain away this surprising result, but Einstein concluded that it would be better to draw conclusions on the premise that the speed of light is a constant. That turned out to be a very productive change in the formation of the models by which we try to understand the universe.P0M 08:55, 2 November 2006 (UTC)

I've read about the measured speed of light being the same from binary stars (going in opposite directions), but that seemingly could also be explained by the speed being independent of the source (rather than being independent of the relative speed of the viewer).

That's the right idea, you just haven't taken it quite far enough. The speed of light is now understood to be independent of both the light source and the light observer. P0M 08:55, 2 November 2006 (UTC)

It's easy to comprehend the speed of light being independent of it's source (similar to the speed of sound being independent of its source, though without a medium to propagate in). And, other than that idea (constant speed of light as viewed by the observer), relativity formulas would seem to describe what a distant viewer would observe rather than what is actually, physically occurring.

Our ordinary experience of the world leads us to conceive of sound moving between objects that have locations and moving in a medium that itself has location. (If you were measuring the speed of propagation of a water wave while traveling in the Gulf Stream you would have to decide whether you were measuring the speed of the wave relative to your raft or relative to islands and continents.) But the "medium" through which light travels escapes our ideas of location because it has no fixed points in it, no islands, no continents, no sea bed, no shores. It is difficult for humans to get their minds around that lack of location. For one thing, we think of locations as absolutes. Pike's Peak is where it is, and as far as we are concerned it is there for eternity. While we are in our everyday minds we do not consider that, relative to the sun for instance, that mountain is making rather intricate high-speed motions. In fact, things have location only with respect to other things.

Where we really get stuck is on the idea that we, and nothing in the universe, moves with respect to whatever "medium" it is that light propagates in. At first that idea seems acceptable, especially if we have a tendency to be ego-centric or solopcistic. Of course the underlying "firmament of reality" doesn't move with respect to me--because I am special. But since other things are whizzing all around me it must be that they are moving with respect to this so-called aether in which light vibrates. But, perversely, they all maintain that they are the unmoving ones. They tell me that they have to be the unmoving ones because, for them, the speed of light is always constant.P0M 08:55, 2 November 2006 (UTC)

That obdurate fact constitutes a great paradox, no? P0M 08:55, 2 November 2006 (UTC)

Einstein said in his book "relativity" concerning the idea that the speed of light is constant to the observer (regardless of velocity relative to the source): "Who would imagine that this simple law has plunged the conscientiously thoughtful physicist into the greatest intellectual difficulties?"

Right. You've got to re-think everything you thought you knew about motion and location--and whether or in what sense they are objective.P0M 08:55, 2 November 2006 (UTC)

So it seems like the perceived "strangeness" of relativity comes from the idea that the speed of light is constant to all observers, which leads to the ideas of actual time and space dilation (as opposed to perceived time and space dilation by an observer). That leads me to question what the evidence is for a constant observed speed of light, regardless of the speed of the observer relative to the source.

If the speed of light were not constant we would notice it about as easily as we recognize that the frequency of light does change depending on our motion relative to the light source.P0M 08:55, 2 November 2006 (UTC)

And assuming that evidence is rock solid, are there any other theories/interpretations that would make the consequences of relativity more intuitive than the standard interpretation?

When something is "intuitively obvious" it is generally because it fits in with our everyday experience of the world. To be able to think of these phenomena without getting tangled up in preconceptions of which we are either aware or which manage to sneak back in anyway, we probably would need to teach ourselves to think in something like Loglan. For a simple example, what does it mean for two things in an otherwise empty universe to move? If by great good luck they happen to be near enough to see each other, it will be clear to both of them that for some time they are getting closer to each other, then they are side by side, and then they are getting farther apart. But each of them may claim that it is "still" or, perhaps, each of them may claim that it is "in motion." And, like the person sitting in a railway car, it may at one time consider the other to be moving, and then experience a shift of consciousness and consider the other to be at rest while it is moving. We need a language in which the standard way of conceptualizing things is that object x and object y have a closing velocity, V_xy. P0M 08:55, 2 November 2006 (UTC)

Gravitational lensing

I just removed the following claim:

In an analogous way, the light speed is also affected by gravity. This gives rise to the phenomenon of gravitational lensing, in which large assemblies of matter can refract light from far away sources, so as to produce multiple images and similar optical distortions. The constant speed of light then belongs to those who may be in free fall, or for other reasons may disregard such effects of gravity on light.

This is not true in any sense that I can make out. There are certainly coordinate systems in which the speed of light appears to be different (just double t, say) but one of the fundamental principles of general relativity is that the speed of light is the same in any inertial frame. Gravitational lensing occurs because spacetime is curved, not because the speed of light changes.

Maybe you can't make it out, but to edit here you are obliged to know your physics. For electromagnetic waves, the gravitation (curvature if you like) has a representation as refractive index, so these are really equivalent viewpoints. These are not inertial frames, of course, this is general, not special relativity. You will find that in standard textbooks. Ulcph 20:13, 1 September 2006 (UTC)

"Refractive index" does not appear in the index of Misner, Thorne, and Wheeler's Gravitation, a (voluminous) standard text on the subject. They use units in which c=1 (measure everything in meters), a nonsensical choice if the speed of light changes. Constancy of the speed of light is such a fundamental principle of both of Einstein's theories of relativity that I can't find somewhere they explicitly state that it doesn't change in a gravitational field.

Merely because there is an analogy with a situation in which light changes speed does not mean that light actually changes speeds. And we are not obligated to "know physics" to delete unsupported claims; if you wish to make an assertion, it is your obligation to provide a citation.

By all means: §90 in L.D.Landau and E.M.Lifshitz: The Classical Theory of Fields, 3rd revised English edition, Pergamon Press (1971). It is standard physics, in all courses and textbooks (including the quoted one), that the speed of light is not constant in general relativity. And I sincerely hope that it says so in a large number of articles here too. If you want articles here to be respected, then you are indeed obliged to know your physics, before you inform the public on that topic, or remove statements by others. Ulcph 10:42, 26 September 2006 (UTC)

Please include detailed citations *in the article* when making a disputed claim. What exactly do Landau and Lifshitz say about the speed of light in a gravitational field? Since the speed of light is used to define the meter, it's not actually possible for it to change in a gravitational field; what actually changes is the length of the centimeter. Perhaps Landau and Lifshitz are using some odd coordinate system in which the speed of light seems different?

I am not going to debate elementary textbook physics with someone who does not even care to create an account and sign a name. That is trolling. You have got more than sufficient references and links to find out for yourself, if you are really interested. Besides, gravitation is not the topic here. You persist in confusing "speed of light" = real electromagnetic wave propagation along a null-geodesic with "speed of light" = the constant c (which of course is constant by definition, as explained in the present article, and which we set = 1 in certain unit systems). Ulcph 13:40, 29 September 2006 (UTC)

Yes, well, none of my textbooks agrees with you, which is the problem. I'll drop by the library when I get a chance and see what Landau and Lifshitz (the only reference or link which you have given to support your claim) actually say. Light in a vacuum always propagates along null geodesics, gravitational field or no gravitational field. This is a result of the Lorentz invariance of Maxwell's equations.

As for creating an account, Wikipedia allows anonymous editing for a reason, and in any case my lack of an account does not affect the truth of your claim.

Overview

The opening paragraph seems redundant:

"According to standard modern physical theory, all electromagnetic radiation, including visible light, propagates (or moves) at a constant speed (or velocity) in a vacuum. This physical constant is commonly known as the speed of light and denoted as c. This speed c is also the speed of the propagation of gravity in the theory of general relativity."

Most of this was said right before, or will be right after. I suggest to delete it, in the interest of readability (but haven't).

The author of the last sentence probably was referring to a linearized approximation, for the propagation of a "small" wave-like disturbance in otherwise gravitation-free space. Of course, in general relativity coordinates are freely chosen, and speeds can come out as nearly anything, merely as a consequence of this. It is not covariant. To obtain the constant, go to a locally inertial frame (free fall), but you only get it locally, and light will still bend globally. This would be rather much to saddle the reader with right up front. Besides, it is not directly testable (so far), and only established indirectly in the binary pulsars - but convincingly, of course. Besides, this is, after all, an article about light. Ulcph 23:41, 8 September 2006 (UTC)

Question.

Although there is much reasoning over this, lets say for now, that light does have a mass. Would it therefore be possible to move something using light, if the 3 factors which make up light were strong enough?

I'm not sure what you mean by "3 factors." But light has momentum, so you can use it to move things anyway, in principle. You'd need very high total energy, but very low individual photon energies to avoid blowing the object apart; but low energy means long wavelength and a tendency to pass through things, so this isn't practical in the slightest.

In the future, can you please ask science questions at Wikipedia:Reference desk/Science? Article talk pages are for discussing changes to the articles. -- SCZenz 15:55, 12 September 2006 (UTC)

You may want to consult the article Radiation Pressure. Ulcph 04:19, 13 September 2006 (UTC)

It's entirely practical. That's how solar sails work. 67.87.115.207 09:48, 30 December 2006 (UTC)

Photon in Peer Review

Hi all,

The Photon article is now in peer review, in preparation for its Featured Article candidacy. Could you please give us some tips on reaching FA? Your article is already FA (congrats!) and it's very close to ours in its subject. Thanks! Willow 11:40, 15 September 2006 (UTC)

Doppler and GPS?

The article says that Doppler effects have to be taken into account in understanding how global positioning satellites work, but it does not actually give a rationale for that statement. On the other hand it mentions the Doppler effect in the context of the accurate synchronization of the clocks on satellites and on the ground. Doppler effects change the apparent frequency of waves emissions. Such effects might have an impact on the tuning of radios for reception or transmission, but Doppler effects would not have any effect on the times reported over those frequencies. A relativistic effect would, on the other hand, have an inevitable effect. Time dilation is well known to occur when, e.g., two clocks are synchronized, one is sent into orbit and remains there for some time, and then the two clocks are reunited. Less time has passed for the clock that was in orbit.

So is there a misidentification of the time dilation phenomenon as being "Doppler effect"? Or are both factors involved somehow? P0M 06:11, 19 September 2006 (UTC)

Both are involved, the satellite you receive from is higher in the Earth gravitational field, hence time is faster there on its clock (the time dilatation part), and also, it (and you) are both moving, which involves a Doppler effect.

Many around here do not seem to realize that, a clock is simply a periodic phenomenon, and so a signal from it: tick-tock-tick-tock... is a wave with a frequency (more precisely, a Fourier spectrum of frequencies). It can be (relativistically) Doppler shifted, and that is what-it-is-called (!) in physics. In vacuum, all the frequency components get equally shifted, red on recession, blue on approach (no dispersion). This also applies to Rømer's observations.

One could argue (I don't) that, it is not historically correct to call the relativistic Doppler effect after Doppler, for he only got it (for sound, which is not relativistic) about 200 years after Rømer. So why not call it the "Rømer effect". But the world doesn't work that way, it's called Doppler - and there is nothing you can do about it. Besides, the idea that pulses come faster when you run into them is probably as old as man. Ulcph 19:40, 23 September 2006 (UTC)

When there is communication from satellite to satellite and from satellites to ground, they are surely sending "this bird is currently at x, y, z, t" signals, not listening to the ticking of each other's clocks. For one thing, to do so with Doppler effects involved the listening devices would have to have a clock speed faster by an order of magnitude or so than the speed of the clocks they are listening to, else they could not measure the doppler-induced discrepancies. Why would they use their most accurate clocks to supervise less accurate clocks that were doing the work on which calculations were being based? It would be more efficient and accurate to use the best clock to measure elapsed time and periodically send out a timing pulse for synchronization purposes. The person who turns on his global positioning device probably hasn't had it working in standby mode, counting ticks from satellites in view -- and what would happen to those counts when contact with the satellite was lost for whatever reason?

How many external devices listen to the ticking of the atomic clocks that form our standard measurements of time? P0M 21:08, 23 September 2006 (UTC)

I just made an initial search for software involved with the use of GPS information and found that what satellites send to ground GPS devices, at least, are a few kind of "struct" formulated messages that contain "latitude, longitude, speed, bearing, satellite-derived time, fix status, and magnetic variation." (http://www.codeguru.com/vb/mobile/pocketpc/article.php/c8079/) The time appears to be a six-digit base-10 number. I believe military applications have access to higher accuracy information, so there must be some way to shield that information from ordinary users. If that is the case then it seems even more unlikely to me that the actual "ticks" are being broadcast and received.

If there is a doppler effect involved the article ought to be able to explain clearly what it is and how it is used and/or compensated for. P0M 21:22, 23 September 2006 (UTC)

With a little more time to scrounge around and see what people are actually doing, it appears that some sophisticated error compensation is involved in actual applications. Doppler variations of any highly regularly emitted "metronome" signals will give information on the velocity seen between emitter and receptor. (Change in the angle between the path of the satellite and the path of the receiver would vary the Doppler shift.) Is this the Doppler effect that you are talking about? P0M 16:38, 24 September 2006 (UTC)

"Many around here do not seem to realize that, a clock is simply a periodic phenomenon". The reason most people do not realize it is because it is not true. A clock does not merely announce when a second has elapsed, it also attaches a label to those seconds. When when looks at a clock, one looks at THE CURRENT TIME DISPLAYED, not how many seconds tick by while one is looking at it. What in the world would be the purpose of looking at the latter?Flarity 23:54, 24 September 2006 (UTC)

Depends on what is is. A "clock" in a computer is an oscillating circuit that functions like a metronome. It "ticks" at an extremely regular rate, and then in some applications another circuit counts the number of ticks it detects.

I suspect that Ulcph's original point was that the output of such a clock is, or at least approximates, a square wave, and Doppler effects can occur just as well with square waves as with sine waves. One of the things that a GPS device trying to locate itself is to look not only at where a given satellite is supposed to be and compute how far one is away from it by asking how long the satellite clock's signal took to reach the ground station. (If my brother and I synchronized watches and the next day he calls me to say it is 10:10 p.m. when my watch tells me it is 10:20 I may conclude that he is no longer on this earth.) But if a clock is sending out a timing beep every second and I am receiving timing beeps every .9 seconds, I can conclude that the transmitting unit is moving toward me at a rate that I can calculate.

As far as I have been able to learn, however, Doppler effects are not directly relevant to a discussion of the speed of light. P0M 06:54, 26 September 2006 (UTC)

A quote may help: "Although clock velocities are small and gravitational fields are weak near the earth, they give rise to significant relativistic effects. These effects include first- and second-order Doppler frequency shifts of clocks due to their relative motion, gravitational frequency shifts, and the Sagnac effect due to earth's rotation. If such effects are not accounted for properly, unacceptably large errors in GPS navigation and time transfer will result." from N. Ashby: Relativity in the Global Positioning System, available at http://www.livingreviews.org/Articles/Volume6/2003-1ashby. This is what I stated in general terms suitable to this article. And it is indeed remarkable that your GPS device relies on both what Rømer discovered (finite light speed) and on the means he used (now known throughout physics and technology as the Doppler effect for light). Ulcph 10:08, 26 September 2006 (UTC)

Two comments on readability for the average well-informed reader. (1) The quoted text is slightly confused in grammatical structure because the referent for the word "they" is a little unclear. For instance, is it saying that the clocks themselves have very small gravitational fields, that the clocks are operating in the (relatively) small gravitational field of the earth? Nevertheless, the average reader can figure out what the author is trying to say, and it is not going to be beyond their ken (even though "Sagnac effect" would need a link." It is much better for this average well-informed reader to have something that is clear if a little difficult (perhaps in a footnote) rather than to have something that is so vague that it suggests multiple interpretations to those readers. (2) When you say, "This is what I stated," the reader is left guessing what the word "this" refers to, and left guessing as to which of your many statements is meant by "I stated in general terms suitable to this article." P0M 13:50, 26 September 2006 (UTC)

In the quote I supplied above (which we can't very well change) the first "they" of course refers to "clock velocities" and "gravitational fields", to be "accounted for properly". My contribution, based on this work, referred to above as "this is what I stated in general terms", was/is the sentence:

" It is remarkable that, to work properly this method [the GPS] requires to take into account (among many other effects) the relative motion of satellite and receiver, which was how (on an interplanetary scale) the finite speed of light was originally discovered (see the following)."

Rather than quoting from a technical paper, I thought it proper to state the main point "in general terms". You seem to agree. We have a Sagnac effect article. Apart from such general mention, interested readers would perhaps want to look at Global Positioning System. I do not consider myself sufficiently expert on this to answer some of the questions you posed. However, we can be fairly sure that the maintainers of the system at NO would be monitoring the raw waves in real time, and taking all these things into account. Ulcph 09:10, 27 September 2006 (UTC)

Speed of Light

Why is the speed of light 1,079,252,848.8 km/h and not something else?

One answer is that we don't know. Another answer is that the speed of light is the universe's fundamental "conversion factor" between space and time, and we define our unit systems around it. See meter#Timeline of definition. -- SCZenz 02:42, 20 September 2006 (UTC)

You can look at it this way: if the speed of light were twice as great, then everything, from atoms to cells on up to humans, would be twice as big -- so you wouldn't notice. Melchoir 02:51, 20 September 2006 (UTC)

Because the meter and the second were historically defined in terms of the size of the Earth and the length of a day, both totally arbitrary pieces of data from a physical perspective. In the most universally natural set of units, the speed of light is simply 1. 67.87.115.207 09:47, 30 December 2006 (UTC)

Yep, our basic units of measurement were all made up before anyone had the slightest notion about the definition of the speed of light, never mind it's a constant, so the distance/time value we use seems arbitrary. In truth, we've defined it using arbitrary (in effect random) units. Gwen Gale 04:29, 24 February 2007 (UTC)

SI

Why isn't the International System of Units (SI) upheld in this article?

Wikipedia is international.

A quick search of the article shows that, except for one historical reference, the speed of light is always given in SI first and imperial units are given in parentheses. Unless one has an ideological need to "kill off" the imperial units that is strong enough to mitigate against putting difficult matters into somewhat more familiar terms for people who do not use SI in daily life, there is no need to restrict an article to mention of one set of units. P0M 17:40, 7 October 2006 (UTC)

Acceleration of a photon

When a photon is created, is its initial velocity equal to c or does it start at zero and then accelerate to c?--Just James 23:44, 11 October 2006 (GMT+10:00)

If light were a particle, then it could be accelerated if we could get hold of it somehow. If light were a wave, then its speed would depend entirely on the vibratory characteristics of the medium the wave is traveling through. Both particle and wave models help people to think about light sometimes. In the case of the speed of light, the wave model is more consistent with observations.

Light moves through things like lenses slower than it moves through air or vacuum. But when light comes back out of the lens its speed goes back up to what it was before it hit the lens. If a bullet were shot through a bag of gelatin its speed would be reduced by the gelatin, but its speed would not go back up to the speed it had coming out of the barrel of the gun.

If light has 0 mass and f = ma, then a =f/m would suggest an infinite velocity from any applicable force. Clearly we do not find results of that kind, so that is another reason to believe that light does not get accelerated.

If there are any exceptions the abovementioned conditions/limitations then they might make interesting additions to the article. P0M 23:18, 11 October 2006 (UTC)

OK that makes sense. Acceleration can also occur by a change in direction. Light can be bent by a gravitational source, thereby changing its direction. In that particular scenario is it better to think of light as a particle rather than a wave?--Just James 16:07, 12 October 2006 (GMT+10:00)

Actually, to make something with mass change its direction you have to apply a force to it. So we're back to being unable to accelerate a massless entity. We cannot decelerate it either. But in its "guise" as a vibration we can decelerate the speed of wave propagation of "empty space." I think the article explains why light goes slower when it is traveling through glass than when it is traveling through empty space. Now think of a rank of soldiers, elbows hooked by elbows, moving across a trackless terrain. At some point they encounter a wedge shaped plot of very muddy land. The guys on the left end of their line get through it in two steps, but the closer to the right end we get the longer is the distance through sticky mud. So the guys who get through on the left side go faster, overall, than the guys to their right. As a result the whole line moves as though it were pivoting around the guy on the far right end. So the line bends in its direction of travel. The wave (or wavefront) would work the same way except that light "waves" are slowed down because their rate of wave propagation is changed in the "mud" and not because they are physically grabbed by attractive forces analogous to the friction provided by mud. P0M 07:41, 12 October 2006 (UTC)

To give two briefer comments on all this:

1. A photon always has speed c, and starts at this "speed." This is a fundamental principle of the theory of relativity (well, that massless particles behave this way is such a principle—and the fact that photons are massless is confirmed to very great accuracy).
2. In terms of photons, the slower speed of light in matter can be thought of as individual photons being absorbed and re-emitted so that the wave as a whole moves more slowly. -- SCZenz 07:46, 12 October 2006 (UTC)

I forgot to mention that what gravity is understood to do is not to bend light but to distort space. A beam of light always takes the shortest distance between points. As long as we are dealing with Euclidean space, light's path is always a straight line as it is conceptualized in Euclid. But if space is stretched out of shape near masses, then light in traveling a "shortest distance" takes a path that conflicts without ordinary idea of a straight line. If we observe a star on a regular basis and have a clear idea of its position in relation to nearby stars, e.g., it is one degree away from one other star and ten degrees away from a third star, we may find that the star "moves" when light from that star must pass nearby our sun (something we can only verify during a solar eclipse). So it is not that massless light is attracted by the mass attraction of the sun, but that space-time is more and more distorted the nearer we get to the sun. There is a clear discussion of the distortion of space in George Gamow's book One, Two, Three... Infinity. It is old now, but Gamow was writing for his young son and often does a good job of anticipating the needs of people who haven't spent half a dozen years in a physics lab. Your library ought to have a copy of it. P0M 08:14, 12 October 2006 (UTC)

Indian Estimate

"Thus it is remembered: [O Sun] you who traverse 2202 yojanas in half a nimesa."

Sayana's statement comes very close to the actual speed of light, and has been called the most astonishing "blind hit" in the history of science.[5]

4404 yojanas/nimesa roughly equals 18333 m/s. Which is hardly close to the speed of light. Admittedly, it's also "unimaginably fast" from a medieval perspective, but not an amazing guess, as is implied.

I know there are different interpretations for the Vedic measures. I used the ones that would yield the fastest result.

Would anyone check this?

croketephji 14:58, 23 October 2006 (UTC)

I too calculated this out. A yojanas is about equal to 14,630 meters (if we equate 1 yojanas to 9.09 miles). 2202 yojanas, then, is about 32,215,260 meters. Half a nimesa is about 0.267 seconds. Thus, it's about 120,656,404 meters per second, compared to about 300,000,000 in reality. Good try, but I would have been more impressed if I had gotten 300,000,000 or 400,000,000.

Taking the highest estimate of a yojanas (16 km), we get 35,232,000 meters, which equals 131,955,056 meters per second. I don't think it's good enough to have the honor of the 'most astonishing "blind hit" in the history of science'.

I found this discussion of that verse. It seems to me a lot like: "It's correct if you interpret the units of time and distance in the right way"... Chovain 05:24, 30 November 2006 (UTC)

It need not be a 'blind hit'. If some of the claims at Hindu Astronomy are valid, and if they had also had telescopes, the speed of light could have been worked out using something like Ole Rømer's method.

There is some evidence in Western history of telescopes known before the official discovery. Why not also in Hindu culture - where it would probably have been kept as a 'secret art' and eventually lost.

Even if the estimate is less than half of the actual speed of light, it is astonishing to get to within the right order of magnitude. Light could be a thousand times faster or slower and we'd see nothing obvious. --GwydionM 19:57, 8 January 2007 (UTC)

question: How long it takes sunlight (in minutes) to reach earth.

My 7 year old has science homework each week and some simple answers in wikipedia would be greatly appreciated by harrassed parents (such as myself) who have never studied science at all.

Her questions are:

 A) Light travels at ________ km/h.
 B) The sun is _________ kilometres from the earth.
 C) Therefore it takes sunlight ___________ minutes to reach earth.

It took me over an hour to find the answer to B, 150 million and I give up on the others unless someone kind can answer.

My first port of call for B was to look at the wikipedia article on Sun. In the information box on the side it answers both B and C. This took me about 10 secs, but I guess you have to know where to look.

The point of C though is to work it out: using speed = distance/time, you can rearrange to give time=distance/speed.

Since the answer to part A is in kilometres per hour, the time you calculate will be in hours. To convert to minutes, you need to know that there are 60 minutes per hour.

Thus you multiply what you get by 60, to give you the time in minutes. LeBofSportif 02:44, 31 October 2006 (UTC)

I think I was in first grade when I was 7. Maybe the teacher gave these figures out and the students were supposed to take notes?

Anyway, here is a series of calculations you can take your child through. The math is actually trivial if you deal with rounded numbers and aren't afraid to off by an amount of time that you probably couldn't measure anyway.

Earth’s orbit = about 150 million km or about 150,000 million meters
Speed of light = about 300 million meters per second
Distance = Rate * Time so
Time = Distance/Rate
Plug in the numbers
Time = 150,000 million meters/300 million meters per second
The million multipliers in numerator and denominator “cancel out”, so Time = 150,000 meters/300 meters per second and doing some more canceling we get:
Time = 1500/3 seconds = 500 seconds = 500/60 minutes = 50/6 minutes = 8 1/3 minute

(That's my way. It's easier to see the math. Here is how the teacher wanted it:)

Earth’s orbit = about 150 million km.
Speed of light

= about 300*60*60 million meters per hour

= about 3*6*6*10,000 million meters per hour

= about 108*10,000 million meters per hour

= about 1080 million km per hour

t=d/r= 150 million km/1080 million km per hour, and “canceling”
t = 150/1080 hours = 15/108 hours = (15*60)/108 minutes = 900/108 minutes

What anybody who has ever gotten a physics problem wrong by misplacing a decimal point will do at this point is to mentally calculate a simpler problem: t = about 900/100 minutes, i.e., about 9 minutes. That’s a little higher than 900/108, which is 8 1/3 minutes, and the figure most people name as the approximate time for light to reach us from the sun is 8 minutes. But all the even numbers we started from are “round-offs” anyway. P0M 03:28, 31 October 2006 (UTC)

Speed of light in the Qur'an

it's was mentioned in the Qur'an (Arabic script: يدبر الأمر من السماء إلى الارض ثم يعرج اليه في يوم كان مقداره الف سنة مما تعدون) (سورة السجدة الآية 5).

Translation: (Allah) Rules the cosmic affair from the heavens to the Earth. Then this affair travels to Him a distance in one day, at a measure of one thousand years of what you count. Quar'an 32:5

Check the following link: http://www.speed-light.info/angels_speed_of_light.htm

This link is so full of logical holes its hard to even know where to begin. Just the initial statement that what the Qur'an is saying relates in any way to the speed of light is quite a stretch. I still cannot understand where the 12000 they are using initially comes from. It is true that there are approximately 12 lunar months in a year (actually somewhere between 12 and 13), but why does that matter? Why should we use the distance the moon travels, and not the distance the Earth travels in 1000 years? Most of the article is just complete nonsense, but I'll just pick an argument at random that is vital to the author's reasoning and debunk it, thus invalidating his argument:

"When we vectorially remove the energy gained from this force we can calculate the total energy and hence the length of the lunar orbit outside gravitational fields." Not only is the "length" of the lunar orbit not defined with no gravitational field - it would not orbit at all in this case - but how can you "vectorially" remove energy when energy is a scalar quantity? Grokmoo 19:34, 1 November 2006 (UTC)

The calculation has mathematical errors on that page. Not that anyone cares, it's a crank site anyway.LeBofSportif 17:02, 2 November 2006 (UTC)

I completely agree,it is a non-sense; taking only in consideration the potencial interest this theme could aport to the article, this should be eliminated, as does not refer, unlike the Indian or Medieval theories, to any experiment or theory.

If the purpose was to put a reference to an attemp made by a arabian scientist (or islamic, sorry if it causes confusion, i am not sure of the correct word) to measure the speed of light, it already exists. A reference to Avicenna is done, one of the greatest scientist that ever existed.

With all the respect for the Islamic belief, what the autor did was just a game with numbers, extracted from the Qu'ran. This, as a scientific article, is not the place for such comments about religion, neither a place to praise nor eulogize Allah. And with no scientific interest, I proceed to erase it, seeing that the previous complaint has been ignored, it is completely obvious that has not scientific nor historical interest.

Sorry for my english, I am from Spain. Thank You!

Problem in the overview?

The third paragraph of the overview says:

If information could travel faster than c in one reference frame, causality would be violated: in some other reference frames, the information would be received before it had been sent, so the 'effect' could be observed before the 'cause' is. Due to special relativity's time dilation, the ratio between an external observer's perceived time and the time perceived by an observer moving closer and closer to the speed of light approaches zero. If something could move faster than light, this ratio would not be a real number. Such a violation of causality has never been observed.

This passage is confusing. It should be rewritten to reflect the fact that there are at least three reasons why faster than light travel is believed to be impossible. Reason one involves the space and time coordinates of two events as observed from two reference systems moving at substantial fractions of c with respect to each other, and the possibility of arranging a series of transmissions among observers in the two reference system so that information could be received before it was sent by the observer who makes the first transmission. The second reason involves the physical issue of how any interactions that depend on speed of light propagations (e.g., exchange of photons) could be perceived to operate by an observer in a reference system moving at a speed greater than c with respect to the reference system in which the (clock or other) physical operation is occurring. The third reason is that once we have set up the equation that represents the time of one system of reference and the time it observes in the second system of reference, we can calculate meaningful results as long as the speed between the two systems of reference is less than c. If the speed presumed between the two systems of reference is set to c then we get a divison by zero in our result and cannot continue the computation. If the speed is arbitrarily set to some value greater than c we get an equation that involves the square root of a negative number. P0M 19:43, 14 November 2006 (UTC)

Question on speed of light?

Ok,my question is on this page it was stated that one could theoretically reach any point instantaneously by travelling at the speed of light. if this is true could travel at the speed of light in a circle and see your self leave?SmallBrain 05:29, 1 December 2006 (UTC)

In real estate it's all location. In relativistic physics it is all point of view. If you could go at the speed of light in a circle (without being squeezed into anchovy paste by the centrifugal forces of course) you could perhaps get back to your starting point without your on-board clock getting in a single tick, but that's just because your clock is "frozen." But that would not tell us anything about what had happened in the part of the universe that was going at another clock speed. Years might have passed and you might have become a legend. That kind of analysis is the basis of the "twins paradox" according to which one twin goes to and from Alpha Centauri at some substantial fraction of c only to discover on his return that the other twin has aged much more than he has.

We can divide things by two. We can divide things by three. We can even picture for ourselves and write out and even do the real-world operations by which we would divide a candy bar into x number of pieces. Similarly we can multiply things by two, three, etc. But when we talk about dividing things by 0 or multiplying things by infinity, we are dealing with an abstraction, and we are giving ourselves a set of operations that in each case goes on withing ever ending, so it describes a job that never is actually accomplished. So never fear, it will never happen.

When we work with time dilation we are using an equation that involves 1 divided by the square root of (1 - V² / c²) so the closer V gets to C the closer the fraction gets to 1, and the value of 1 minus the fraction gets closer and closer to 0. When that factor is in the divisor that means you end up dividing something by 0, which is something that you really cannot do. But the closer you get to doing it, the greater the time dilation or stretching longer, and if you could get to zero that would mean that it would take forever to get anything done. But that's only an abstraction. It's kind of like the real Zeno's arrow I guess. The faster something is going away from us the harder we have to push it from behind to get it to go any faster, and eventually we would have to give it an infinite push to get it to go the last little bit to a speed of c, so we can't do it, which either spoils or saves the whole thing depending on your emotional point of view. (I hope I'm remembering all of this correctly.) P0M 03:18, 8 December 2006 (UTC)

Error Found

Ok. So the article says that the speed of light is exactly 299,792,458 meters per second, and 1,079,252,848.8 kilometers per hour. But I know for a fact that the speed of light is a few trillion miles per hour, and measured in kilometers, the number should be higher. So I multiplied the 299,792,458 m/s by 3600 for the number of seconds in an hour, and by 1000 because we're doing kilometers, not meters. I got this number: 1079252848800000 km/s. Apparently someone had forgotton to calculate a few more zeros into the number. Could an expert on this tell me if this is the correct number before I edit the page? Stevv 21:43, 19 December 2006 (UTC)

The article as it stands is correct. Remember that if I am going at 1000 m/s then that is the same as 1 km/s. i.e, you must divide by 1000 to convert to km/s. Then multiply by 3600, for seconds->hours. The correct answer is: 1,079,252,848.8 LeBofSportif 08:57, 21 December 2006 (UTC)