Talk:Time dilation/Archive 2009

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Question

When the light is sent to clock A it will not only be seen until I reach C. I think the light or clock will be seen at every moment after it hits A (clock) such that it will be first seen when I reach the point B (as if I was at rest) and onwards in an infinite series of points pivoting from A i.e. from AB and onwards until to AC ad infinitum and each point will show a different time on the clock A... --WaleedAddas (talk) 07:27, 15 April 2009 (UTC)--WaleedAddas (talk) 07:27, 15 April 2009 (UTC)

Rechecking of age due to time dilation

“If the twin aboard the spaceship went to the nearest star, which is 4.45 light years away at 86 percent of the speed of light, when he returned, he would have aged 5 years. But the earthbound twin would have aged more than 10 years!” said Kak. (http://www.physorg.com/news90697187.html)

Let the twin ant had also rocket away along with person twin, when returned have aged 5 years but the earthbound twin ant would have aged more than 10 years!

Let the inside width of the spaceship is x unit long and at rest an ant covers the back and forth distance x in (say) five years with constant speed for both moving observer and stationary observer (back elevation or front elevation of spaceship).

As soon as the spaceship starts its flight an ant also starts its journey along the straight line x inside the ship. As an ant can’t decrease or increase its constant speed/ pace inside ship and there is also no length contraction widthwise of the ship, so the only difference is the moving clock slow down as compared to the stationary clock, therefore find the following after the whole space flight.

How many number of trips (width of inside ship or back and forth distance x) covered by an ant inside the ship w.r.t to both clocks?

What will be the (difference in) age of an ant w.r.t to both the inside no. of trips and rocket away at 86 % of speed of light? 96.52.178.55 (talk) 04:49, 3 June 2009 (UTC)khattak

Continued

Now if aforementioned ant (rocketing) that is back and forth with constant speed, inside widthwise of ship, increases its speed to say 0.8c and as the rocketing person doesn’t feel its traveling speed (0.86c) inside ship therefore becomes the stationary observer for this ant therewith

Now an ant (rocketing) has two speeds wrt stationary person observer on ground

One is the ship’s speed 0.86c inside which an ant (rocketing) is traveling in its forward direction (say for simplicity along Y-Axis) and the other is back and forth across the ship which is 0.8c (say for simplicity along X-Axis)

While w.r.t rocketing person (that became stationary observer for rocketing ant’s crosswise motion) inside ship (which is also moving with 0.86c in its forward direction), it has only one back and forth speed (0.8c) across the ship

Question:

What will be the clock reading of this ant (rocketing) that is back and forth with 0.8c widthwise inside ship (which is also moving with 0.86c in its forward direction) w.r.t the stationary observer on ground and rocketing person (that became stationary for rocketing ant’s crosswise motion) inside a ship (which is also moving with 0.86c in forward direction)?

I don’t know how to draw a diagram here but please draw a diagram before dealing the problem. Thanks 204.191.225.73 (talk) 05:19, 27 September 2009 (UTC) Khattak

An intersting question

Let there are two planets A and B some light years apart in space. The light pulses cross[*} each other at the middle, if fired simultaneously from each planet towards each other.

[*]For crossing, imagine a two-way traffic on a divided single road OR each track of a railway line represents a pulse but their direction is opposite.

Now there is a transparent space ship of certain length which started its journey with (say) 0.95 C from A to B. After a very short interval, light pulse are fired from both A and B towards each other.

As the pulse’s velocity “c” (fired from A) which is just behind the ship towards B is greater than the ship’s 0.95C therefore after sometime this light pulse will

1- Enters the ship through its back

2- Travels inside the ship; longer( c+v) for outside observer while c for inside)

3- Leaves the ship through its cockpit towards B

While the pulse which is fired from B, from opposite direction will

1- Enters the ship through its cockpit

2- Travels for some time inside the ship; longer (c) for inside observer while c-v for outside)

3- Leaves the ship through its tail towards A

Both pulses cross each other in the ship such that one on the left side whiles the other on the right side of longitudinal axis of ship. A stationary observer on asteroid is also watching the whole scene; a ship and both pulses.

Since velocity of ship is just below the velocity of light therefore the travel time of light inside the ship for outside observer will be very very longer and for inside observer will be the only velocity of light as per Einstein’s postulate.

Now please answer the following questions while keeping in mind c+v and c-v equation.

1- What will be inside travel time or passing and crossing sight distance of both pulses for the both the moving and stationary observer or simply write the c+v and c-v equation for above scenario?

2- Since there is length contraction and difference in clocks therefore how would both the observers’ notices the pulses which leave the ship?

I hope I have explained things clearly enough that you can understand what I mean. 96.52.178.55 (talk) 04:33, 18 July 2009 (UTC) Quoth Khattak #1

This is a very interesting question. The short answer is that observers on the planets and observers on the ship will disagree about what order events occur in, and how long they take. Their observations are related by the Lorentz transformation discussed in the article, and so are self-consistent; they just represent different ways of drawing "space" and "time" axes on a spacetime diagram. I'll draw up a diagram of this particular situation, and post it in a few minutes. Stay tuned! --Christopher Thomas (talk) 06:43, 18 July 2009 (UTC)

And here's the diagram. From the planets' reference frame, the light rays are emitted at the same time but are resident in the ship for different lengths of time. From the ship's reference frame, the light rays are emitted at different times but are resident in the ship for the same length of time.

For an animation of the space and time axes changing as velocity changes, you can also check a previous diagram I made during a discussion about time travel. I hope this explanation is useful to you! --Christopher Thomas (talk) 07:35, 18 July 2009 (UTC)

Your answer is fine with relativity but my intent was to bring one’s attention towards the following common sense

Let say for example, when both pulse and ship are moving in same direction

For inside observer, the pulse after entering the ship through its tail, will pass through the cockpit within one second or two, while

For outside observer, the pulse entering the ship through its tail will travel inside ship (which is also moving say with 0.99c) for hours and hours…

Thus there is an eye popping difference in duration of both events (one second and hours and hours). thanks 96.52.178.55 (talk) 19:52, 30 July 2009 (UTC) Khattak

Quick clarification question

I must be misinterpreting the meaning of the variables in the basic formula (in the overview section), and I was hoping someone could clear up my confusion.

If you are on a spaceship traveling 4/5 light speed for 10 years (in your frame), you will age 10 years. During this time, your friend on Earth should have aged 50/3 years [(1-(4/5)^2)^.5= 3/5]. Isn't 50/3 years the "proper time" since it is the time recorded by the stationary observer? Basically, my question is: shouldn't the proper time always be greater than the moving observer's time? During any given interval between two events taking place on Earth, the clock on the moving spaceship will tick fewer times than the stationary clock on Earth; so doesn't that mean the proper time is greater?

I must have it backwards, since otherwise the formula in the overview section would be incorrect, right? —Preceding unsigned comment added by Jakemhall (talk • contribs) 10:48, 30 July 2009 (UTC)

By definition proper time between two events is measured on a clock present at both events, whereas coordinate time between two events is measured with some clock not present at both events. If my ship is traveling from Earth to Star, the time between the events of departure and arrival as measured on my clock is the proper time between the events, whereas the time between the events as measured on the Earth clock is the Earth's coordinate time between the events. DVdm (talk) 12:39, 30 July 2009 (UTC)

Comments by 205.153.156.222 (talk · contribs · WHOIS)

Those who argue that time 'dilation' is a fact usually interchange velocity with speed for light which is an error. For example, in this article the time 2D/c and 2L/c seem different, but the times are actually the same event, and both observers time the event the same IF they time it correctly. Notice that the vector component speed with which the light travels upward (North)while angle traveling toward the mirror to reach it in its shifted location is v(North)= c(L/D,)where L/D is the sine of the angle the ray travels from the horizontal as seen in the second figure. When this component for light is placed into the equation for time (stationary), it becomes t = 2L/[c(L/D)] = 2D/c and is the same as the other expression (moving). Time does not actually 'dilate' simply because an observer and his clock are in motion. It is just that if one uses light as a time-keeping device, you must use the proper component of velocity for the travel direction of the pulse involved. The magnitude of the velocity of light can vary from +c(East) to -c(West). Time dilation was invented in 1905 when Einstein mistakenly thought that (since light rays travel in straight lines) c and v could be always interchanged. But speed and velocity I assure you are different when an angle-traveling ray is used to compute time in one instance, and then that ray is used again in a different diagram without considering that its velocity in the direction of interest is now a component that is less than c. Velocity is only c in the propagation direction itself, which occurs here at an angle above the horizon toward the shifted mirror. The sationary observer does not compute time correctly because he ignores, or is unaware of, the lateral shift of the light and mirror ongoing at the same time. Time dilation is only an apparent phenomenon that occurs if calculations do not consider the component velocity for a directed pulse of light. Anyone can exceed the velocity of light. Simply point it North and you run West to reach the target. You beat the velocity of light toward the target. Or reflect a beam on a mirror; when the light returns to its starting position the velocity at that position was zero. You continue running,and don't turn around, you exceed the velocity of light.

Richard Sauerheber, Ph.D. —Preceding unsigned comment added by 205.153.156.222 (talk)

"When this component for light is placed into the equation for time (stationary)..." => This component ( namely c(L/D) ) has actually no business being placed into the equation for the stationary time, and is irrelevant for the stationary observer. The entire idea behind all this is that both observers attribute the same speed c to a light signal along its path. For the stationary observer the path is vertical. For the moving observer the path is tilted.

Perhaps you overlooked the first line of the section. it says: "Time dilation can be inferred from the observed fact of the constancy of the speed of light in all reference frames. It looks like you missed the essence of the derivation.

Hope this helps. - DVdm (talk) 08:03, 18 September 2009 (UTC)

Of course it is true that the intrinsic speed of light is constant in all reference frames. This was established first by James Maxwell, knowing that light is massless EM radiation that must travel at this constant speed. That is not the issue. The issue is the fact that the relative velocity of light and its components of velocity are not synonymous with intrinsic speed in the propagation direction. The moving observer is unaware the light ray strikes the mirror from an angled path that only the stationary observer can see. It is a mismatch of vectors to use speed c for a Northward component for the beam that actually travels NorthEastward. If one insists on using c for both times, then, to avoid vector algebra mishap, the time computed for the moving observer is 2[Dsin(q)]/c where sin(q)= L/D which again rearranges to the same expression as for the stationary observer. The moving observer must know his speed relative to the stationary observer or else he simply will calculate the wrong time for the event if he insists on using intrinsic speed c.

Again, this is very old news because the diagram shown in the article has been published in many Physics textbooks for decades. Rest assured, there is a huge difference between speed and velocity for light, even though authors have not thought this through while pronouncing the significance of the diagram. All beams from a lit candle for example travel at intrinsic speed c. None however have the same velocity. Some point East at +c, some point West at -c. To compute the time for one to hit a mirror that is in motion, it is necessary to use the proper component of distance for a given beam, or to use the component velocity involved for the distance one wishes to specifically travel. Any other computation is a vector algebra mismatch and yields the result that time is somehow different for the same single event because of one's perspective view. Please understand that another stationary observer positioned far East of the ship, who also can only see the vertical distance L, would also compute the wrong time of 2L/c even though he is stationary, not a moving observer. The time 2L/c only describes the time required for a light ray to the mirror and back if the ship itself were not moving at all. On this, all observers agree. It is most unfortunate that this mismatch has had such a formidable place in so many texts since the 1950's or so. Again, it is not that the moving observer on the ship (and the statonary observer far East of it) are watching a vertical path. What they see is the illusion of perfect vertical. The ground observer is in a better position to compute time because for him it is obvious the event involved a beam that angle-traveled to catch the mirror in its shifted location upon arrival. An outer space observer watching a pithcer throw a ball to a catcher knows full well the whole earth orbits in space and he sees the ball, pitcher and catcher all shift as well. The fan in the stands sees only the distance the ball travels relative to the ground because he also shits with the earth. This is what relativity is all about as originally introduced by Einstein. Perception (vertical length L) is not necessarily reality (actual light ray travel path D).

Richard Sauerheber, Ph.D. —Preceding unsigned comment added by 205.153.156.222 (talk • contribs)

Remarks:

• "The issue is the fact that the relative velocity of light and its components of velocity are not synonymous with intrinsic speed in the propagation direction." ==> Like I said, but this time using capitals for more emphasis, the entire idea behind all this is that both observers attribute the same SPEED c to a light signal along its path. For the stationary observer the path is vertical. For the moving observer the path is tilted. You really overlooked the first line of the section. It says: "Time dilation can be inferred from the observed fact of the constancy of the SPEED of light in all reference frames. It looks like indeed you missed the essence of the derivation.
• Now please have a look at Wikipedia's talk page guidelines. A talk page is meant to discuss content and format of the article. It is not a place for posting "originally researched" esseys and musings about the subject. As you can read in wp:ESSAYS#essay "Personal essays belong in the User namespace."
• An article's talk page is also not supposed to be a place where we educate people in the subject of the article. So, perhaps, instead of replying to your essey, I should have deleted it altogether. Someone still might decide to do this, so unless you have a specific sourced proposal to modify the article, perhaps it's better not to insist.

Please click the provided links and read carefully - Thanks. DVdm (talk) 18:26, 18 September 2009 (UTC)

Note to anonymous 205.153.156.222: I have reverted the modifications your made to your comments after I had replied to them. Please have a look at your talk page. Thank you. DVdm (talk) 22:31, 18 September 2009 (UTC)

Again, I fully agree that the constancy of the speed of light in all frames of motion is true. Whether an observer runs toward or away from a light beam, the intrinsic speed of that beam is fixed at c. An observer's actions have nothing to do with the intrinsic travel speed of the light. Now that this is established, please let's move on to the error in the Wikipedia article. The light beam itself traveled 2L in time 2L/c, which is correct. What is not correct is to violate that principle of the constancy of the speed of light for all reference frames by claiming in the second figure (of the same light event)that a moving observer would compute time differently than 2L/c. If that observer does not correct for the extra distance he accumulated, that the light beam did not, then his computed time is false. 2D/c has nothing to do with what the light ray itself actually traveled. The difference in distance was due to relativity, where the observer ran away from the apparatus. He uses an incorrect distance and implies that the light itself actually traveled such a path when it did not. If a car traveled at fixed speed 50 mph North and I ran away from it, the time for it to travel one mile is not changed, even though I perceive the relative distance between me and the car is greater than what the car actually traveled.

Understand that Einstein in 1905 first coined 'time dilation' and correctly described that the relative velocity between a moving rod and a light beam is not simply the intrinsic speed of light c. It is c+v for the rod moving toward the beam, or c-v for a receding rod (Einstein, Electrodynamics of Bodies in Motion, 1905). Confirming this, Arthur Otis published his textbook (Light Velocity and Relativity, Burckel, N.Y., 1963)proving that detectors moving away from sunlight detect decreased light frequency, not because the intrinsic wavelength of the light could be altered by a detector of course, but because the relative velocity between the observer and the beam front was c-v, as Einstein had written. Finally, much additional data have been published online that clarifies this quite well (Sauerheber, R., Clarifying the Nature of Light, www.lulu.com). Time dilation refers to a miscalculation with a vector mismatch, in all cases such as presented in the Wikipedia article, and is not an actual real 'slowing' of absolute time. R.Sauerheber, Ph.D.

Rsauerheber (talk) 01:10, 19 September 2009 (UTC)

Remarks:

• "... 2D/c has nothing to do with what the light ray itself actually traveled." => For the moving observer the light signal travels a distance 2D at speed c, so for the moving observer it takes a time 2D/c. That is all there is to say about this. Remainder of essay unread.
• I have added 4 references supporting the section's derivation. They show that your personal analysis is wrong.

Now please take your faulty original research elsewhere. You are disrupting this talk page. Thank you. DVdm (talk) 08:01, 19 September 2009 (UTC)

I'm sorry that you did not read the entire paragraph, but it is not only I who disagrees with the second graph. Albert Einstein, again in the above reference, clearly wrote that observers receding from a light beam front accumulate distance from it at a relative velocity of c - v, while of course light's intrinsic speed is fixed at c. For various reasons over many decades, many authors seem unaware of this truth and continue to believe that even relative velocity is somehow fixed when an observer is in motion. In my text and in that of Otis above, there is no doubt that two light beams traveling in opposite directions accumulate a total distance at a rate of twice c. Because no change will be forthcoming in the article distinguishing intrinsic speed, intrinsic velocity, relative speed and relative velocity, all four of which have completely different meanings, then I cannot recommend this Wikipedia article to any of my physics students, friends or family. One day everyone will know the truth about 'time dilation' because, sculpted on the Einstein memorial at Washington, D.C. are his own words "if one studies a topic and learns the truth about it, it is his duty to explain it to others". Good luck to you.

Rsauerheber (talk) 16:51, 19 September 2009 (UTC)

Remarks:

• "...did not read the entire paragraph..." => There is no point in reading what is built opon a glaring error.
• "...two light beams traveling in opposite directions accumulate a total distance at a rate of twice c..." => Yes, as measured by a third party observer. That is what we call closing speed aka relative speed as seen by a third party observer. You simply fail to understand the setup of the section at hand. I know there are ways to derive the time dilation equation using vertical vector components or closing speeds, but this section does it another way. You can check the provided sources by following the links and reading the relevant pages on-line, trying to understand the set-up. Failing that, this is not the place for us to help you understand. So unless you come up with at least 4 reliable sources supporting your view that this method is faulty, you are disrupting this talk page. I'm sorry, but that is how Wikipedia works.

Good luck to you - and even more so, to your physics students. DVdm (talk) 19:07, 19 September 2009 (UTC)

But D and L are also vector quantities. A tree to a stationary observer appears to have a height with a different magnitude than for an observer who runs toward it, but yet the height is the same for both observers anyway. The same is true here for the path of a light beam. If the actual path is D, then another observer mistakes it for L. [If the actual path is L, another observer mistakes it for D.] And yes my students are fine; they learn the ways of the textbook and then the truth also. Thanks. --205.153.156.222 (talk) 16:39, 21 September 2009 (UTC)

Should the falling of an object both in gravity and space be treated differently

The Lorentz time dilation equation is derived using the concept of a light clock in which a pulse of light being continually reflected backwards and forwards between two mirrors for moving observer and zigzag for stationary observer.

1- The above concept was taken from a moving train (falling of object from ceiling of train under the influence of earth's gravity) but since in space there is no GRAVITY therefore an emerging pulse should follow the straight downward path from where it is emerged, and shouldn’t move along the direction of ship for both stationary and moving observers. (In other words the free fall of pulse shouldn’t be influenced by the ship’s velocity)

2- As light carry momentum and can push/ veer objects easily in SPACE therefore, should a ship swerve with a new resultant velocity when a pulse touches the second mirror? Further, any flat object or flanks of ship could reflect a pulse in thought experiment, so is there any special reason in using a mirror?

The same is applied to the Einstein’s elevator

Your comment please! Thanks

68.147.38.24 (talk) 01:57, 4 October 2009 (UTC) Khattak

Remarks:

• "...reflected backwards and forwards between two mirrors for moving observer and zigzag for stationary observer." => Actually, in the article it is, from the standpoint of the clock, upwards and downwards for stationary observer, and zigzag for moving observer.
• "The above concept was taken from a moving train..." => Where exactly in the article is this stated?
• "...should a ship swerve with a new resultant velocity when a pulse touches..." => Yes, slightly, with each bounce up, down, up, down, etc. Irrelevant in the context of the article.
• "...special reason in using a mirror?" => "Flat reflective object" is 3 words, 20 letters and 2 spaces" whereas "mirror" is 1 word with 6 letters. :-)

DVdm (talk) 09:46, 4 October 2009 (UTC)

Sorry about the aforementioned mistakes

The description of pulse bounces between top and bottom mirror, shown in article is very fine with question.

Moving train is not stated in the article but my intent was basic relativity 68.147.38.24 (talk) 16:34, 4 October 2009 (UTC) Khattak #1

recent edit added errors, pls discuss before restoring material

A recent edit has been reverted because it added errors and seems to have been made without reading the article: the new material is reproduced here, some reasons are given below why the edit is erroneous and misleading, pls discuss before replacing the material.

Time dilation is one of the phenomena explained by the two subvarieties of the theory of relativity in physics. In special relativity it happens when two observers in inertial, free-falling motion relative to each other measure each others' passage of time. Even if they have identical clocks running at the precise same pace as they see it, from the point of view of each observer the other observer's clock will still run at a slower pace than one's own. This symmetry is commonly thought to be counter-intuitive. This unobviousness perhaps was the primary reason why relativity was such a contested hypothesis at first, and why it also remains an item of popular curiosity even today.

To notice the effect, either the velocity difference has to be large enough to approach a significant proportion of the speed of light, or the period over which the measured times are accumulated must be sufficiently large. Otherwise we are working near the classical limit of the theory, which reduces to the much earlier and simpler laws of nature governed by Galilean invariance. That already contained the core of relativity, but it did not anticipate that time as well could be treated as a dimension, subject to symmetrical transformation between observers. It thus took Lorentz's, Einstein's, and Riemann's, among others', insights to relativize time as well.

In general relativity the effects of gravitation on both time and space are added to the same basic theory. Free inertial fall no longer means simply moving in a straight line in a flat Minkowski space-time. It now means locally moving along the shortest path known to one, in the way the instantaneous environment presents the shortest path. The local, momentary description of time dilation as experienced by two close-by observers relative to each other does stay exactly the same. But on a longer scale, the very definition of free fall is now also governed by the continual interaction between space-time distorted by the presence of mass (which alters both distance and elapsed time), and the inertial fall of mass in new directions, along the continually varying sense of "going down the gravitational well in the straightest direction".

These dynamics lead to extremely nonlinear behavior, global instability, and singularities, such as the black hole. This behavior can lead to extreme and unexpected forms of time dilation, such as the ones encountered by two observers closely orbiting an event horizon on mutually slanted orbits, or the oddities of the ergosphere of a Kerr-Newman type black hole.

---

Sample of reasons against the material in the edit:

• There are not two subvarieties of relativity theory: special relativity deals with the special restricted case where gravitational masses can be ignored, general relativity deals with the general case where that simplifying restriction is not available.
• It's an error to say that the other observer's clock will always run slower. The case of gravitational time dilation is assymmetrical -- as explained in the pre-edit text.
• A tag was recently attached, complaining of excessive technicality. That did not apply to the pre-edit lead paragraph which was short and in reasonably clear and plain English but the large edit has added much length and technicality, some of it also unsourced.
• 'Butchering the intro' is not a reason to make further edits, it is easy to return to the pre-edit position so that the proposed changes can be considered.

Terry0051 (talk) 21:28, 19 October 2009 (UTC)

Thank you for the revert; peer review is never bad, and when it rejects your edits, that's positive evidence that it has served its purpose. In the interest of developing the article, I'd like to respond, though. Let's see if we can synthesize something better as a result some discussion.

• First, the point about technicality caught me by surprise. In light of that, most of my viewpoint should perhaps have been included in the body, and not the introduction. Still, I *was* trying to make the idea of time dilation more intuitive to a lay person. Perhaps some of my wordings could still be utilized to that end?
• I do have the tendency towards prolixity, so perhaps caused some unnecessary/unwanted bloat. I also believe that is a matter of judgment, given the complexity and the exotic nature of the topic, which might call for more girth even at the beginning, for the sake of the casual reader.
• You are correct in that special relativity is subsumed by general relativity, however historically they have been two separate developments in physics, derived from several mathematical and empirical starting points, at a relatively wide interval to each other (see e.g. Annus mirabilis. Educationally they have usually been presented in separation from each other, one reason for which is that quantum theory has only been fully reconciled with SR, whereas quantum gravity, the unification with GR, let alone a truly unified theory of all of the basic forces, let's us wait for it for now. Then, intuitively speaking, and if I remember correctly, the only case in which the two theories coincide fully is at a single point and wrt massless inertial observers; physically that is then about coincident, massless bosons alone, which doesn't really cover most of the humanly relevant turf. Finally, given the dynamic instability of GR, relativistic manifolds can contain points near singularities of various sorts which will make the region of effective convergence between SR and GR arbitrarily small, which I think from the human perspective justifies calling them two subvarieties of a theory. (Also notice that even I didn't call them two different theories, but just two varieties of the same theory. Not that it's that exact in the end, though.)
• I believe I only claimed what was indeed already said in the text, which is that two inertial observers will observe symmetric time dilation wrt each other. This in fact holds even under general relativity (something which should be amply clear even from Wheeler et al.'s classic Gravitation; unfortunately I don't own the tome, and thus cannot quote a proof). What messes up the analysis currently presented in the article wrt my viewpoint is that a person standing on the top of the Earth is in fact not an inertial observer; instead he is continuously accelerated upwards, to stay "still", by the incompressibility of the Earth's crust, which at the lower level of physical explanation is essentially about the incompressibility of the electron (a fermion) cloud of atoms, which then mostly happens because of the Pauli exclusion principle, and to a considerably lesser extent because of Coulombic repulsion between pressed-together nuclei. (I don't remember the mass of the spherical threshold body where these interactions start to overcome exclusion, at the center; perhaps the articles surrounding neutron star might convince you there.)
• I'm formally diagnosed as being emotionally unstable, and my use of language tends to reflect that in its colorfulness. Perhaps I should instead have said that, as per Wikipedia policy, I was just "being bold in my modifications". It perhaps wouldn't have rattled you unnecessarily.
• I don't have a physics library at my immediate disposal. But if you could flag those parts of my comments which need verification, I could perhaps present approximate reasoning and/or citations, or some other form of credible backing for my interpretation, from memory. That way the stuff could at least be considered for inclusion, even if it deserved citation-wanted-type tagging from the very start.
• All in all, I hope we're off into a fruitful discussion. I wish it'll lead to a better article in all.

Decoy (talk) 21:59, 20 October 2009 (UTC)

[From Terry0051] Thanks for your thoughtful comments. Can I suggest that one hopefully fruitful route might be, to identify/seek consensus on which core facts should be communicated in the intro or opening sections (checking that there's consensus on the facts themselves!) and then working out how to put them briefly & clearly in suitable encyclopedic style.

I have to apologise for having only a short time right now, I'll get back to it in a couple of days, but points I'd suggest are:

• (a) The whole thing about one and the same ideal standard-rate clock showing apparently different rates to local and distant observers, is likely to be found really counter-intuitive by most readers. I doubt if there's a way to get over that, apart from briefly and clearly indicating what happens, initially without encumbering it by technical theory.
• (b) I'd suggest that the first thing to communicate is a brief indication of the established phenomena. After doing that (in a technophobe-friendly way so far as possible), later sections in the article can start getting into how the theory explains/predicts/describes them.
• (c) I don't know if we differ on this one, but I'd suggest it's really important to include at the outset (at least generically, if not also specifically) the case where the distant standard clock looks faster (as seen from here), as well as the cases where it looks slower. That's because the 'faster' case (in other words, where 'we' are more time-dilated than 'they' are), includes the important real-world examples of atomic clocks on GPS satellites and on mountain-top observatories (as seen by 'us' from the sea-level POV).
• (d) Inertial reference frames and suchlike stuff are really technical -- the more so as they are scarcely realizable hereabouts (except within approximations that need their own discussion which tends to be technical too), and they look highly theoretical as well.
• (e) I agree it's useful to indicate early on the sort of quantitative scale on which the effects occur, to show how they have really only been seen as a result of modern high-precision measurements. But I suggest that vague quantity-words don't really communicate this: for example, what is a 'significant' proprtion of the speed of light for this purpose? I'd suggest making reference to real-world examples and saying by how many microseconds per chosen time unit the GPS satellite clocks have to be slowed down so that they do not appear too fast from the Earth's surface, and/or how many microseconds have to be allowed for in terrestrial clock-synchronization to account for the velocity effects, examples of that sort.

With best wishes in the meantime Terry0051 (talk) 13:36, 21 October 2009 (UTC)

(Returning to offer supplementary suggestions answering in part the question posed about 'what points are unsourced & would need [specific] RS support?') The relevant points for specific support seem to include the suggested facts about:

• two observers closely orbiting an event horizon on mutually slanted orbits,
• the oddities of the ergosphere of a Kerr-Newman type black hole (reference to the linked articles does not provide the expected support for these),
• the nature of the contrast mentioned in the para about Minkowski space, (when clarified, this would probably turn out to need RS support if not added by the clarification),
• the proposed statements about inertial observers seem to need clarification and specific support, e.g. for what happens at the Earth's poles as suggested.

With good wishes, Terry0051 (talk) 19:31, 28 October 2009 (UTC)

Recent erroneous edits 10 December 2009

Anonymous user 213.100.87.94 has repeatedly introduced edits stating that the earth's orbital motion and the galactic motion of the solar system should be taken into account for terrestrial time scales.

Standard time scales used on the surface of the Earth include Terrestrial Time, which is closely related (by an offset) to international atomic time and (by a scale factor) to Geocentric Terrestrial Time. But these are coordinate time scales, and the coordinate system to which they refer is a geocentric one, moving with the Earth. Accordingly, the Earth's orbital motion around the Sun and the galactic motion of the solar system are not relevant to these time scales. (There are indeed other coordinate time scales referred to the solar-system barycenter, including TCB and TDB, for which the earth's orbital motion and its (varying) position in the relevant gravitational potential wells are taken into account.)

As already stated in DVdm's edit summary, the effect of the Earth's rotational motion has indeed been taken into account for the synchronization of Earth-based atomic clocks. The concept 'stationary clock' was not present in the article before the edits by 213.100.87.94, and really does not play a part. Accordingly this edit (*) and others like it by 213.100.87.94 do appear to be in error: (*: "In reality however, there are no stationary clocks, given that the earth rotates on it's axis with a surface velocity of 465 m/s, and orbits the sun at a mean velocity of 14,893 m/s, within a solar system which itself orbits the outer edge of the Milky Way Galaxy at a velocity of 2,092,990 m/s, these velocities must also be included in the equation, but in fact they never are.") It seems appropriate to remove this statement.

Terry0051 (talk) 20:35, 10 December 2009 (UTC)

I have removed the statement. It is original research. I also have left a message on the user's talk page. DVdm (talk) 20:52, 10 December 2009 (UTC)

To User:DVdm, It is not original research. No original research has been included in the edit, also the edit is not erroneous, the fact that a geocentric model is used in time dilation calculations is exactly the problem, this is not a geocentric system, it is a galactic system. the basis for the data may be found in several wiki articles such as Galactic Year, Sidereal time, and Astronomical unit... the equatorial circumference of the earth = 40075160 m and 1 sidereal rotation = 86164.091 s therefore the equatorial surface velocity = 465.1028 m/s 1 Astronomical unit = 149597870700 m and tropical year J2000 = 31556925.9747 s therefore 2 pi · AU = 939951143167.591 m / 31556925.9747 s = 29785.8905 m/s 1 Galactic Year = 250 000 000 earth years = 7889231493675000 s the galactic radius (Gr) = 250 000 LY = 2365182618145200000000 m where 1 LY = 9460730472580800 m therefore the galactic circumference = 2 pi · Gr = 14860880675126467046254 m / 7889231493675000 s = 1883691.8 m/s This is not original research, it's simple arithmatic based on common astrophysical knowledge. You may ignore the fact if you wish, however whether we like it or accept it or not.. Stationary Clocks Do Not Exist. —Preceding unsigned comment added by 213.100.87.94 (talk) 22:18, 10 December 2009 (UTC)

Nobody has said that stationary clocks do exist, they were not mentioned before your edit, nobody suggested that this is relevant, and it appears irrelevant. You clearly appear to be saying or at least implying, that coordinate time scales relative to the geocenter, and observed/evaluated at the earth's surface, are somehow affected by the orbital and galactic motions of the earth and solar system. Have you a reliable source for this --(or for any other statement that makes a relevant connection between these motions, or their velocities, and geocentric time scales observed/evaluated from the earth's surface)? Terry0051 (talk) 22:37, 10 December 2009 (UTC)

(Edit conflict with Terry0051)

Stationary clocks do exist: you have one around your wrist. It is stationary for you. You might not be an inertial observer, but to you (and probably to me as well) your clock is perfectly stationary. Other than that, the clocks, as discussed in the text, are not inertial, as your numbers show. We all know that. We also know that all the above effects are either taken into account (Earth rotation), or cancelling out as they are identical for all clocks involved. So first of all, your added sentence was irrelevant in the context of the article.

(Please use proper indentation for comments and use the four tildes (~~~~) to sign them? Thanks and cheers. DVdm (talk) 22:44, 10 December 2009 (UTC)

The term "Stationary Clock" has not been mentioned in the article, it has only been alluded to. My intetion was to point out the fact that stationary clocks and stationary observers do not exist in reality. the earth is not a stationary object in a stationary fixed point in space-time, it is an object in constant motion, moving along with everything else in the solar system, orbiting the galaxy at a velocity of 2 million m/s. The illusion of stationary references, such as earthbound atomic clocks, or wristwatches is self deceptive. We are all moving at 0.6% the speed of light! 213.100.87.94 (talk) 23:14, 10 December 2009 (UTC)Mathzilla

(Final attempt)

With respect to the "as good as inertially moving" centre of the Earth, the equator clocks on the surface of the Earth are moving in a simple circle with a tangential speed of 465 m/s. That is taken into account in the calculations of Hafele/Keating type experiments mentioned in the article.

The effects of the other motions around the Sun and around the centre of the galaxy (and so on) are irrelevant for 2 reasons:

1. No observer is sitting at a place where they measure all of us to be moving at 0.6% of the speed of light. The article talks about clocks and measurements made by observers on Earth.
2. If some observer would be sitting there, they would measure all the clocks in the experiment to be moving at the same speed, and that would have no differential effect on the clocks.

DVdm (talk) 08:13, 11 December 2009 (UTC)

Time dilation diagram

Lorentz factor as a function of velocity in natural unit c, notice that for small velocities (less then 0.1 c), γ is approximately 1

I created the following diagram which show's time dilation Δt' (Y axis) from the frame of reference of a moving observer traveling at the speed V (X axis in natural unit c),i think this is helpful diagram because it clearly show that for small friction of the speed of light, time dilation is approximately one (almost flat line for x from 0 to 0.1). what do you think? should i add the diagram to the article? —Preceding unsigned comment added by Zayani (talk • contribs) 18:17, 19 December 2009 (UTC)

Looks good, but perhaps replace the name of the vertical axis Dt' with Dt'/Dt, or with gamma, or with gamma = Dt'/Dt. - DVdm (talk) 21:07, 19 December 2009 (UTC)

Done, changed the name of the vertical axis to γ=Δt'/Δt zayani (talk) 23:09, 19 December 2009 (UTC)

Added as shown to the right zayani (talk) 10:09, 20 December 2009 (UTC)

As velocity is really a vector quantity, I made a few minor changes to the caption. I turned it into

• Lorentz factor as a function of speed (in natural units where c=1). Notice that for small speeds (less than 0.1), γ is approximately 1

Can you perhaps also change the name of the horizontal axis? Cheers, DVdm (talk) 11:06, 20 December 2009 (UTC)