Wikipedia:Reference desk/Archives/Science/2022 August 16

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is a transcluded archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

August 16

Light travel delay and observation of astronomical objects

If I understand correctly, when the light from a distant space object reaches the observer the observed object moves elsewhere, effectively making the observed object a sort of a ghost-like hologram. Because of that does the time delay of travelling light require position and time adjustments of observed objects (such as stars, including our Sun)? Does it affect position-sensitive measurements, such as parallax? 212.180.235.46 (talk) 11:19, 16 August 2022 (UTC)

The stars that move the most relative to the observe background each year (have the largest proper motion) are also the stars that are very close and have some of the highest stellar parallaxes. Even though their observed tangential velocity is still (just) less than 1% of the speed of light, the transverse Doppler effect ([StackX overview) should still be measurable, but I'm not sure how astronomers do that (and I'd think measuring that would give useful information on radial velocity, but I could easily be wrong, not working the math). More to the point, the special relativistic effects similar to what you describe are definitely an important factor in eclipsing binaries, most famously in Ole Romer's first measurement of the speed of light by the moons of Jupiter. SamuelRiv (talk) 12:58, 16 August 2022 (UTC)

• (edit conflict) We have an article about aberration (astronomy), but it’s really verbose / dry. Tigraan^{Click here for my talk page ("private" contact)} 13:01, 16 August 2022 (UTC)

Yes, having established what we as observers think of as an inertial frame that we use to assign positions to objects, if we see an object at time t₀ at position p, and we know that the time it takes for the light to reach us equals Δt, we have observed that the object was at position p at time t₀ − Δt. If it is moving steadily and we know its velocity, we can extrapolate to obtain an estimate of its current position (in our frame). I don't think this is generally considered to be a relativistic effect.  --Lambiam 15:41, 16 August 2022 (UTC)

Sky atlases show the sky as it was. When the dot says Barnard's Star is here at 1900 and over 5/9ths of a degree towards this direction in 2100 and here in 1950 and 2000 and 2050 it doesn't account for the fact that it's really over 8 years ahead. Sagittarian Milky Way (talk) 05:40, 17 August 2022 (UTC)

Electron charge and speed

Does the charge of an electron decrease when it waits for a relativistic speed? For example as its mass was shown to increase. — Preceding unsigned comment added by Malypaet (talk • contribs) 14:18, 16 August 2022 (UTC)

I'm not sure what you mean for "waits for a relativistic speed"? Charge remains constant at all velocities in all frames of reference, though properties such as "charge density" and "specific charge" (which depend on volume and mass respectively) will change, because distances and masses are dependent on relative velocities. Be very careful on what you are measuring here. "Charge" is not the same as "specific charge" is not the same as "charge density". --Jayron32 14:33, 16 August 2022 (UTC)

sorry, reaches, not "waits".

How can you say that charge remains constant at all velocities in all frames of reference ?

What metric are you basing it on ? Malypaet (talk) 21:39, 16 August 2022 (UTC)

French attendre means "to wait", while atteindre means "to reach". A one-letter difference.  --Lambiam 07:41, 17 August 2022 (UTC)

You're getting into a very good question, which is essentially noting that gravitation and electromagnetism behave pretty darn similarly (true), but the concept of "charge" in gravitation (mass) and that in electromagnetism (electric charge) are very different in many respects, not least of which is that electric charge is Lorentz-invariant (which is experimentally verified, which is more or less how we know it's not not-true) (and rest mass is of course invariant). If you know your physics there's a really good explanation on StackX that sort of gets at how you could formulate a Lorentz-invariant mass-gravitation theory (I'm pretty sure this is gravitomagnetism, which is not nearly as useful as GR but can get around some nasty problems). I don't know that there's a useful non-Lorentz-invariant theory of electromagnetism, however (You might need to crank the exoticness of your particle theory up a notch for that). SamuelRiv (talk) 23:00, 16 August 2022 (UTC)

I wish people would finally drop that useless and harmful idea that mass should depend on centre-of-mass velocity. It doesn't explain anything and it leads to so many misunderstandings. Just let mass be rest mass, an intrinsic property of a body that is independent of reference frame. --Wrongfilter (talk) 08:37, 17 August 2022 (UTC)

Yes, but separating "rest mass" from "relativistic mass" doesn't take into account that an object's inertia is affected by its velocity. This feedback is much more mathematically convenient if you treat it as a mass. I'm sure there's other ways to slice that mathematical pie, but they are less efficient than treating the changes to inertia as a change to mass. --Jayron32 10:43, 17 August 2022 (UTC)

An object's inertia is not affected by its velocity. What does that even mean? There is no such thing as absolute velocity that could influence inertia. Relativity is a theory of space-time, and it describes how different observers see the object. Which of the many possible observers do you think determines the inertia of the object? --Wrongfilter (talk) 11:09, 17 August 2022 (UTC)

So, what you are saying is that the work needed to accelerate an object moving from 98% the speed of light to 99% the speed of light is the same as the work needed to accelerate the object moving from 1% the speed of light 2% the speed of light? Interesting. Please tell me more. --Jayron32 11:52, 17 August 2022 (UTC)

98% of the speed of light with respect to what? The difficulty of accelerating with respect to some reference frame is due to relativistic velocity addition (space-time!) not to some change in the properties of the object. I know people like this mechanistic argument and they imagine they understand something, but it really goes against what relativity is all about. --Wrongfilter (talk) 12:10, 17 August 2022 (UTC)

The point is that, while you can do formulations both with and without relativistic mass as a concept, that doesn't mean that relativistic mass is not a useful concept. When you put it in the formulations that use it, and turn the crank, you get the same right answers as when you reformulate those equations to avoid using it. There's an unfounded prejudice among some people that because things can be re-written to avoid using it, that it isn't a "thing". It's a fine concept. It works. Physics doesn't need to eliminate it. It doesn't need to have it either, but that is not a reason to pretend like it's an abomination. If you can use it and always get the correct predictions, have at it. --Jayron32 15:06, 17 August 2022 (UTC)

You could have fooled me. Some people think that relativity is just a theory that has little or no practical application. But this is not so. In the design of cathode ray tubes (until recent years used as the display device in TV receivers and lab instruments), the relativistic mass and consequent inertia has to be taken into account. In typical CRT's, a high voltage (circa 15 to 30 KV) is used to accelerate electrons in a very narrow beam to hit a phosphor screen with enough energy to cause light emission. Electromagnets or electrically charged plates were used to deflect the beam left and right and up and down. The amount of current or voltage, respectively, to deflect the beam any given angle is always slightly greater than electron rest mass would indicate. This is because the electrons travel at an appreciable fraction of the speed of light and so the electron inertia is increased a little. Dionne Court (talk) 12:11, 17 August 2022 (UTC)

The phenomenon can be explained by Lorentz transformations (space-time) on the four-momentum (energy + three-momentum) without invoking mass increase. --Wrongfilter (talk) 12:15, 17 August 2022 (UTC)

As Jayron said, it is mathematically convenient to calculate on the basis of a mass increase. And it works - you get what you can measure.

Jayron asked is the energy needed to go from 0.98c t0 0.99c the same as for 0.01c to 0.02c. And your answer is? A simple direct answer now, is it the same or is it different? If it is only different under certain circumstances, what are those circumstances? Dionne Court (talk) 14:44, 17 August 2022 (UTC)

Of course it's harder for a spaceship to go from 0.98c to 0.99c relative to Earth. The question is why is that? Your answer is that the rocket is getting more massive. Put yourself on the spaceship and your answer will be Earth is getting more massive. I'm sitting on Earth and I dispute that it is getting more massive just because something somewhere is moving fast relative to Earth. Of course you can define $\gamma m_{0}$ to be some quantity $m$ and you'll get the same answer. That is not my point. My point is about the interpretation of the theory as a theory of the geometry of spacetime. I could dig out John Archibald Wheeler's take on "relativistic mass" and the importance of invariant quantities (which rest mass is and mass ought to be) but I'm tired... --Wrongfilter (talk) 15:29, 17 August 2022 (UTC)

Of course invariant quantities are important, to bring this back to the original question, which is why I reminded the OP about the difference between charge (an invariant quantity that everyone will measure the same in all reference frames) and charge density or specific charge, which people in different reference frames would measure differently. Take specific charge for example: the differences between specific charge as measured in different reference frames can be calculated using differences in mass (i.e. relativistic mass) or by doing some complex bit of mathematics using Lorentz transformations and the like. They both give the same right answer; and some people prefer the more efficient explanation using relativistic mass. Merely because relativistic mass is not an invariant doesn't make it useless or verboten. Understanding invariants is useful for deep theoretical foundations, but it isn't required to build sound models that produce accurate results that match reality. --Jayron32 16:03, 17 August 2022 (UTC)

I didn't get an answer to my question

A constant velocity ion with two electrons missing, caught by an electron it captures:

- Its charge decreases

- Its mass increases

- Its speed increases

So what experiment shows that the electric charge of an electron is independent of its speed ? Malypaet (talk) 21:18, 17 August 2022 (UTC)

Of course early 20th century scientists discovered the electron, its properties and charge conservation. More specifically, the constancy of charge with respect to its speed is empirically supported by and consistent with hundreds of thousands particle accelerator experiments, List of accelerators in particle physics. --Modocc (talk) 22:59, 17 August 2022 (UTC)

I can find many papers giving the charge/mass ratio of an electron according to its velocity and deducing the mass based on the assertion of the constancy of charge, but no experiments measuring charge alone.

Apparently no one is able to name a single one here too!

And as for load constancy, in my example, it doesn't apply at the macro level. Malypaet (talk) 08:50, 18 August 2022 (UTC)

Of note, the electrons' energies do change w/ accelerations because their significant kinetic energies are delivered to various targets that are struck within the atom smashers with predictable results, yielding the well-known and established relativistic relationship between their kinetic energies and their speeds (without involving their electric charge). Given mass-energy equivalence, charge conservation is therefore verified empirically by measuring an electron's mass-to-charge ratio since its speed is measured such that its energy and mass can be calculated. Modocc (talk) 13:21, 18 August 2022 (UTC)

Sources?

Experiment with publication peer reviewed ? Malypaet (talk) 22:13, 18 August 2022 (UTC)

I'm not really visualizing the capture process you're describing in this experiment, but I don't think it matters. The measurement process for the charge of an individual ion (say deflection in a magnetic field) would show the same result regardless of velocity relative to the observer: so if you took an ion through a cloud chamber in a magnetic field (clouds, the observer (you), the field generator, are all fixed) the ion would behave as predicted by the Lorentz force with constant charge regardless of the speed you throw it at, and that's confirmed by experiment. That's actually how they did all this back in the day, and there's another thing that binds experiment and modern theory (and even special relativity) into a nice bundle (if you accept one of the things as true, at least, then the rest are self-evident): CPT symmetry (Gentle introduction from Nave if you start at the beginning of the lecture). Fair warning: if you were worried about the sanctity of charge, I'd suggest not asking angular momentum where it's been all weekend. SamuelRiv (talk) 03:32, 18 August 2022 (UTC)

There are all the same many experiments which give a mass and a charge to the photon, except the acceleration of an electron is only done by the addition of electromagnetic energy, therefore of photons.

Could this be an explanation for the increase in the mass/charge ratio of an accelerated electron, as in my example with the ION? Malypaet (talk) 09:00, 18 August 2022 (UTC)

For visualizing, imagine an elementary liquid battery ! Malypaet (talk) 09:02, 18 August 2022 (UTC)

This thread on Stack Exchange may be useful here, it asks (and answers) what I think is roughly the same question you have about proving charge invariance. --Jayron32 13:18, 18 August 2022 (UTC)

No, this thread didn't give experiment with peer reviewed publication. Just assertions. Malypaet (talk) 22:17, 18 August 2022 (UTC)

I still am not visualizing what's happening with your example (I'm guessing it's just simply "positive ion + electron go in separately, neutral (or less-positive) ion comes out with appropriate momentum." As was mentioned above, the apparent (relativistic) mass is still subject to Lorentz contraction as usual, so the mass/charge ratio should change as you'd expect, but apparent charge would be fixed on that ion as the same as if it were fixed. So I don't know why you need to talk about the addition of some kind of energy here. If you're still (understandably) trying to nail the concept of relativistic vs rest mass down you should look into some of the Mass in special relativity sections. SamuelRiv (talk) 15:01, 20 August 2022 (UTC)