Why is the speed of light the same everywhere in the Universe?

[Rick16]

I have always wanted to ask this question, because there is something that I am missing.

I think it should be conceivable that the speed of light is affected by masses, so that its value within the solar system would be different from its value in interstellar space and even more so in intergalactic space. I am aware that the current theories exclude this. But Newton's theory of gravitation also excludes that light be bent by the mass of a star, and yet it happens. General relativity has shown that Newton's theory does not tell the whole story. Why should general relativity tell the whole story?

I suppose that there is some other reason why the speed of light is taken to be the same everywhere in the universe, but what is it?

 

[PeroK]

Why should general relativity tell the whole story?

It doesn't. An active topic of current research is to look for an underlying theory of quantum gravity.

I suppose that there is some other reason why the speed of light is taken to be the same everywhere in the universe, but what is it?

It's effectively an assumption that light moves along null geodescics. That implies that the locally measured speed of light is invariant. The available experimental evidence supports this. And that's the way physics works.

 

[Rick16]

It doesn't. An active topic of current research is to look for an underlying theory of quantum gravity.

It's effectively an assumption that light moves along null geodescics. That implies that the locally measured speed of light is invariant. The available experimental evidence supports this. And that's the way physics works.

If I understand correctly, you are saying that experimental evidence supports that the locally measured speed of light is invariant. But how can we have experimental evidence about the speed of light in other locations?

 

[PeroK]

If I understand correctly, you are saying that experimental evidence supports that the locally measured speed of light is invariant. But how can we have experimental evidence about the speed of light in other locations?

There's no evidence that the laws of physics vary from location to location (or over time). That's the default position unless and until there is evidence to the contrary. There is no need to postulate that the laws of physics are different in the Andromeda Galaxy in order to explain what we observe. Similiarly, when we look back to the early universe, we can apply the laws of physics as we know them to make sense of the last scattering surface, for example.

There are, of course, at least two significant unexplained phenomena. The accelerated expansion of the universe, which seems to be driven by vacuum (or dark) energy. And, the anomalous galaxy rotation curves that suggest that about 90% of a galaxy's mass is of some unknown (dark) matter. There are potentially alternative theories to explain this: most notably MOND (Modified Newtonian Dynamics).

This is the way physics works: you try to explain the observed phenomena. There is no real progress to be made from simply hypothesising that our current theories might be wrong in some unknown way. Unless and until you provide some justification for an alternative theory, it's of no real use.

Moreover, it's not enough to say something like: perhaps spacetime is not homogeneous. That's just a wishful thought. Instead, you need a mathematically sound model that can be tested against experiment - and some specific justification.

All that said, the speed of light itself is not a fundamental constant of physics. Instead, you need to look at the fine structure constant. If you could explain some unexplained phenomena by varying this, then you'd be getting somewhere. But, anyone with any knowledge of physics (and without putting pen to paper of doing any actual work or study) can hypothesise that the fine structure constant may be changing over time. Even if there is no evidence for it.

 

[Drakkith]

If I understand correctly, you are saying that experimental evidence supports that the locally measured speed of light is invariant. But how can we have experimental evidence about the speed of light in other locations?

Because we can observe things in other locations. We can see billions of galaxies in our telescopes and can see that they behave identically to our own local area (or at least behave similarly enough to our own area that we can't see any difference). Spectroscopy lets us see the compositions of objects, and the spectral fingerprints of all the elements and chemicals are all identical, something which wouldn't happen if the laws of physics were different in faraway galaxies. Galaxies and star clusters all look similar to each other in a broad sense, which wouldn't be the case if gravity behaved wildly differently in different areas. Dust in one galaxy blocks light and emits IR just like dust in a distant galaxy does.

None of this would be the case if the laws of physics varied substantially from place to place. We should see wildly different results in spectroscopy, or be able to see through dust in one galaxy but not another, or see that all the galaxies in one direction are shaped differently than all the galaxies in another direction. But we don't see any of these things.

 

[Dale]

If I understand correctly, you are saying that experimental evidence supports that the locally measured speed of light is invariant. But how can we have experimental evidence about the speed of light in other locations?

This is just a fine point that was already made above but bears reiteration. The locally measured speed of light is guaranteed to be constant when measured in SI units.

So what you are actually interested in is the fine structure constant. We can calculate how changes in the fine structure constant would change the spectral lines of stars. Then we can look ant distant and see if those lines are changed.

 

[Vanadium 50]

Well first, the speed of light is not the same everywhere in the universe. I am looking at a piece of glass now, and the speed of light is measurably slower in it.

The speed of light in vacuum is the sane everywhere, just as the speed of light in glass is the same everywhere. Why wouldn't it be? What would make vacuum here be different than vacuum there?

 

[Rick16]

There's no evidence that the laws of physics vary from location to location (or over time). That's the default position unless and until there is evidence to the contrary. There is no need to postulate that the laws of physics are different in the Andromeda Galaxy in order to explain what we observe. Similiarly, when we look back to the early universe, we can apply the laws of physics as we know them to make sense of the last scattering surface, for example.

There are, of course, at least two significant unexplained phenomena. The accelerated expansion of the universe, which seems to be driven by vacuum (or dark) energy. And, the anomalous galaxy rotation curves that suggest that about 90% of a galaxy's mass is of some unknown (dark) matter. There are potentially alternative theories to explain this: most notably MOND (Modified Newtonian Dynamics).

This is the way physics works: you try to explain the observed phenomena. There is no real progress to be made from simply hypothesising that our current theories might be wrong in some unknown way. Unless and until you provide some justification for an alternative theory, it's of no real use.

Moreover, it's not enough to say something like: perhaps spacetime is not homogeneous. That's just a wishful thought. Instead, you need a mathematically sound model that can be tested against experiment - and some specific justification.

All that said, the speed of light itself is not a fundamental constant of physics. Instead, you need to look at the fine structure constant. If you could explain some unexplained phenomena by varying this, then you'd be getting somewhere. But, anyone with any knowledge of physics (and without putting pen to paper of doing any actual work or study) can hypothesise that the fine structure constant may be changing over time. Even if there is no evidence for it.

I don't want to push any hypotheses or alternative theories, I just want to understand the current situation.

Let me try to recapitulate what I have understood so far: You wrote earlier "It's effectively an assumption that light moves along null geodescics." I understand this means that the idea of the speed of light being the same everywhere has been inferred from general relativity. This idea was then locally verified by experiment, and since the laws of physics should be the same everywhere in the universe, it can be inferred that the situation should be same everywhere. And unless some as yet unknown phenomena are discovered, there is no reason to believe otherwise. This still leaves the possibility open that new discoveries may change the situation, but speculating about undiscovered phenomena is not physics. Also, hypothetical undiscovered phenomena may change many things, and it is pointless to talk about them as long as no evidence exists. Is that about it?

 

[Rick16]

Well first, tyhe speed of light is not the same everywhere in the universe. I am looking at a piece of glass now, and the speed of light is measurably slower in it.

The speed of light in vacuum is the sane everywhere, just as the speed of light in glass is the same everywhere. Why wouldn't it be? What would make vacuum here be different than vacuum there?

I would say that vacuum is different in different places, because some places contain big masses and others don't. An asteroid flying through open space behaves differently than an asteroid being caught in the gravitational field of the earth. And since the trajectory of light is also altered by a big mass, one wonders if a big mass might not also alter the speed of light. Although in relativity a light beam passing a star just takes the shortest possible path, when I look at the situation from a force perspective, then the star effectively accelerates the light beam. Why would this acceleration only act perpendicularly to the passing light beam and not in its direction of propagation? Because general relativity says so. But doesn't the fact that a star deflects light mean that there is some interaction between gravitation and electromagnetism? I understand that I am entering the realm of speculation with this, but I find this deflection of light very intriguing from a force/field perspective, and it seems to suggest that something is going on between the star and the light beam.

 

[PeroK]

I would say that vacuum is different in different places, because some places contain big masses and others don't. An asteroid flying through open space behaves differently than an asteroid being caught in the gravitational field of the earth. And since the trajectory of light is also altered by a big mass, one wonders if a big mass might not also alter the speed of light. Although in relativity a light beam passing a star just takes the shortest possible path, when I look at the situation from a force perspective, then the star effectively accelerates the light beam.

There is no force or (proper) acceleration for light. In fact, a particle with mass experiences no force or acceleration when in free fall in a gravitational field. In GR locally (in a small enough region of spacetime), we have SR. And, in SR, the invariance of the speed of light is a consequence of the Lorentz Transformation. This behaviour translates to light following null worldlines.

Why would this acceleration only act perpendicularly to the passing light beam and not in its direction of propagation?

You can't mix and match Newtonian concepts of force and acceleration within GR. This question is, therefore, not meaningful.

 

[Drakkith]

I would say that vacuum is different in different places, because some places contain big masses and others don't.

Doesn't matter. Light still travels at c locally in these places.

And since the trajectory of light is also altered by a big mass, one wonders if a big mass might not also alter the speed of light.

It is not. What does happen is that the light takes a longer path from point A to point B than it would otherwise take if the large mass isn't there.

Why would this acceleration only act perpendicularly to the passing light beam and not in its direction of propagation? Because general relativity says so.

It's not an acceleration and it doesn't only act perpendicularly to the direction of propagation. The longitudinal effect is a redshift or blueshift of the light instead of a deflection.

But doesn't the fact that a star deflects light mean that there is some interaction between gravitation and electromagnetism?

Certainly. All forces and interactions take place on the backdrop of spacetime.

 

[Dale]

This still leaves the possibility open that new discoveries may change the situation, but speculating about undiscovered phenomena is not physics. Also, hypothetical undiscovered phenomena may change many things, and it is pointless to talk about them as long as no evidence exists

This is the general idea. Let me add a little detail.

Although we do not have evidence against some of these simplifying assumptions, the professional scientific literature is full of papers investigating how our models would have to change to accommodate these effects, and what observations we would expect. So these concepts are not unexplored or ignored.

However, we have to keep in mind that the goal of science is to predict new observations, not just fit existing observations. Suppose we have two models, one which has an additional free parameter modeling some effect and the other which does not. The model with the free parameter will always fit the data better, but it will often predict new data worse. Until the evidence for the effect is strong enough that the model with the extra parameter predicts new data better, we stay with the simpler and more predictive model.

 

[Rick16]

I think I will leave it at that. I asked this question in the hope to see a little clearer. I have received answers with many details that I will have to think about. Thank you everyone for taking the time to answer.

 

[geordief]

Is it the case that the Lorentz transformations ,which are the only possible transformations between moving reference frames lead to the relativistic addition of velocities that only permit an invariant speed for c?

I also think that the speed of light in a vacuum has been experimentally measured to be equal to c.

I hope this is correct.

Edit I just saw the OP's last post and that s/he is satisfied with the responsesPerhaps my post is unnecessary in the circumstances and the thread is now closed?

 

[jbriggs444]

Let me try to recapitulate what I have understood so far: You wrote earlier "It's effectively an assumption that light moves along null geodescics."

This is discussing whether the speed of light matches the universal invariant speed, $c$. The best tests for this are bounds on the photon mass.

Separately, we have the question of whether the universe has a universal invariant speed for light to match. This is about the correctness of general relativity.

Separately, we have the question of what units we could adopt to test the constancy of the speed of light. because the speed of light is currently buried in our choice of units. This is the bit about the fine structure constant.

 

[Rick16]

This is discussing whether the speed of light matches the universal invariant speed, $c$. The best tests for this are bounds on the photon mass.

Separately, we have the question of whether the universe has a universal invariant speed for light to match. This is about the correctness of general relativity.

Separately, we have the question of what units we could adopt to test the constancy of the speed of light. because the speed of light is currently buried in our choice of units. This is the bit about the fine structure constant.

My question was about point 2, whether the universe has a universal invariant speed for light. But I did not want to question the correctness of general relativity, I wanted to understand why the universe has this universal speed of light (and if perhaps there is a possibility that this might not be the case).

 

[Rick16]

Is it the case that the Lorentz transformations ,which are the only possible transformations between moving reference frames lead to the relativistic addition of velocities that only permit an invariant speed for c?

I also think that the speed of light in a vacuum has been experimentally measured to be equal to c.

I hope this is correct.

Edit I just saw the OP's last post and that s/he is satisfied with the responsesPerhaps my post is unnecessary in the circumstances and the thread is now closed?

It is never unnecessary. I am grateful for every response.

 

[jbriggs444]

My question was about point 2, whether the universe has a universal invariant speed for light. But I did not want to question the correctness of general relativity, I wanted to understand why the universe has this universal speed of light (and if perhaps there is a possibility that this might not be the case).

It is "universal invariant speed". Full stop. That speed may or may not be the speed of light in a vacuum. Evidence to date suggests that light in vacuum does move at the universal invariant speed. Technically, the letter $c$ denotes the universal speed, not the speed of light in a vacuum, even though we believe that the two are the same.

We do not have a [satisfactory] deeper theory that explains general relativity. Such a theory would let us justify why general relativity is as it. Such a theory might let us anticipate conditions where the Einstein field equations might fail to hold

In the absence of such an underlying theory, our best explanation for general relativity is "because the universe is that way".

We could ask why we have adopted a theory where there is a universal invariant speed. That one we can answer. Because Occam's razor. Scientists prefer theories with fewer adjustable parameters over theories with more parameters. Making the speed of light adjustable would be adding a parameter for no good reason.

 

[Rick16]

You can't mix and match Newtonian concepts of force and acceleration within GR. This question is, therefore, not meaningful.

Perfectly obvious. According to the Newtonian model, deflection of light by a star can't happen at all. Consequently, I cannot use this model to describe it. And yet, this is exactly what I tried to do. Amazing. How many stupid mistakes does a man have to make in his life? Does somebody have the answer to this?

 

[PeroK]

You could assume that light was attracted to a mass with the same gravitational acceleration as any other particle. And, in fact, if you do that then the deflection of light under that Newtonian model is half what it is under GR - in the case of light being deflected by the Sun, for example. That difference was quite important back in 1919 when Eddington did the famous experiment.

https://en.m.wikipedia.org/wiki/Eddington_experiment

 

[Rick16]

You could assume that light was attracted to a mass with the same gravitational acceleration as any other particle. And, in fact, if you do that then the deflection of light under that Newtonian model is half what it is under GR - in the case of light being deflected by the Sun, for example. That difference was quite important back in 1919 when Eddington did the famous experiment.

https://en.m.wikipedia.org/wiki/Eddington_experiment

I could assume this, but it would not be correct, would it? Light would have to have mass in order to be attracted to a mass with the Newtonian gravitational acceleration.

 

[PeroK]

I could assume this, but it would not be correct, would it? Light would have to have mass in order to be attracted to a mass with the Newtonian gravitational acceleration.

Newtonian gravity says nothing about light. But if the calculations were the same, I doubt 1919 would have been seen as conclusive evidence of GR!

 

[jbriggs444]

I could assume this, but it would not be correct, would it? Light would have to have mass in order to be attracted to a mass with the Newtonian gravitational acceleration.

It is not that simple.

If we grant the correctness of $F=\frac{Gm_Sm_l}{r^2}$ and $F=m_la$ then we arrive at $a=\frac{Gm_s}{r^2}\frac{m_l}{m_l}$. Cancelling zeroes is not allowed.

If $m_l=0$ then that expression involves the indeterminate $\frac{m_l}{m_l} = \frac{0}{0}$.

So it can be argued that Newtonian mechanics makes no prediction for the trajectory of light under the effect of gravity. [You do not even need the mathematical legerdemain. If the mass of light is zero then any acceleration whatsoever is consistent with $F=ma$, a zero force and a zero mass]

However, it would be quite natural to evaluate the indeterminate as $\lim_{m_l \to 0}\frac{m_l}{m_l} = 1$ and arrive at a definite prediction.

 

[Rick16]

However, it would be quite natural to evaluate that indeterminate as limml→0mlml=1 and arrive at a definite prediction.

I understand the mathematics, but I don't see how I can make physical sense of it.

 

[Dale]

I could assume this, but it would not be correct, would it? Light would have to have mass in order to be attracted to a mass with the Newtonian gravitational acceleration.

The acceleration of gravity is independent of the mass. There is no reason that light would have to have nonzero mass.

Also, Newtonian gravity can be described in terms of curved space.

 

[Rick16]

The acceleration of gravity is independent of the mass. There is no reason that light would have to have nonzero mass.

Now you are seriously confusing me.

 

[Dale]

Now you are seriously confusing me.

When you remove the air a bowling ball falls at the same rate as a feather. Both accelerate at $g$. The acceleration is independent of the mass. That is the key observation that leads to the form of the gravitational force.

 

[Rick16]

When you remove the air a bowling ball falls at the same rate as a feather. Both accelerate at $g$. The acceleration is independent of the mass. That is the key observation that leads to the form of the gravitational force.

So, you mean that it is not the missing mass that keeps light from being attracted by other masses? This seems to suggest that the Newtonian model cannot be used at all to account for this fact. On the other hand, if light had nonzero mass, then it should be attracted by other masses according to the Newtonian model, but since it does not have nonzero mass and it is not attracted, one could say that everything is in line with the Newtonian model and that the Newtonian model explains the situation correctly. $F=-\frac {Gm_1m_2} {r^2}=0$ when one of the masses is zero.

 

[Dale]

This seems to suggest that the Newtonian model cannot be used at all to account for this fact.

Why not? The Newtonian acceleration is independent of the mass. So it is easy to account for the acceleration in the model. Why would you say it cannot be used?

then it should be attracted by other masses according to the Newtonian model, but since it does not have nonzero mass and it is not attracted

It is attracted. The Newtonian model is wrong because it incorrectly predicts the quantitative amount of attraction. But the model easily predicts that it is attracted.

The question for Newtonian gravity is whether the acceleration is fundamental and the force derived or vice versa. If the acceleration is fundamental then the acceleration is given by $$a = \frac{GM}{r^2}$$ where $M$ is the gravitating mass and then the force is derived by applying Newton's 2nd law $F=ma=0$ where $m=0$. If the force is fundamental then $$F=\frac{GMm}{r^2}=0$$ and then the acceleration is derived by applying Newton's 2nd law $a=F/m=undefined$. In the absence of experimental evidence to the contrary it is reasonable to take the former approach since the latter approach is undefined. There is nothing in the physical model that requires the force to be considered the primary thing, and there are many instances where as a practical matter the force is derived from the motion rather than vice versa.

 

[Rick16]

It is attracted. The Newtonian model is wrong because it incorrectly predicts the quantitative amount of attraction. But the model easily predicts that it is attracted.

It is attracted? That is new to me, except in the case of a black hole. If light is attracted by the star that sends it out, wouldn't that imply that it speeds up the farther away it gets from the star since the attraction decreases with distance?

 

[jbriggs444]

It is attracted? That is new to me, except in the case of a black hole. If light is attracted by the star that sends it out, wouldn't that imply that it speeds up the farther away it gets from the star since the attraction decreases with distance?

The attraction is most easily seen as a deflection for light from a distant star passing by the sun on its way to a telescope on Earth. [This is the Eddington experiment mentioned by @PeroK in #20].

We see a resulting change in the apparent direction to the far away star when the sun passes nearby and an eclipse allows us to see.

Since light always moves at $c$ in vacuum, attraction in the radial direction does not manifest as a slow down. Instead, it manifests as a red shift for light climbing up or a blue shift for light falling down.

The red shift or blue shift is cumulative, of course. It reflects the difference in gravitational potential. A sort of integral of the tangential component of gravitational acceleration along the trajectory.

 

[Dale]

It is attracted? That is new to me, except in the case of a black hole. If light is attracted by the star that sends it out, wouldn't that imply that it speeds up the farther away it gets from the star since the attraction decreases with distance?

For Newtonian gravity, the speed of light can indeed change. This is another quantitative demonstration of the deficiency of Newtonian gravity for relativistically moving objects. However, it would not speed up as it gets further away, it would just decelerate less.

For general relativity, the local speed is unchanged, even in the case of a black hole. Only the direction changes. Both the fact that in GR the speed is unchanged and also the quantitative amount of the change in direction are consistent with experiment.

 

[Rick16]

Why not? The Newtonian acceleration is independent of the mass. So it is easy to account for the acceleration in the model. Why would you say it cannot be used?

I would say it like this: The Newtonian model is correct for nonzero masses. Therefore it applies to nonzero masses. But it is not correct for zero mass. Therefore it does not apply to zero mass. What is wrong with this reasoning?

 

[renormalize]

But it is not correct for zero mass. Therefore it does not apply to zero mass. What is wrong with this reasoning?

What's wrong is that it is just an assertion without proof. Can you show mathematically that the Newtonian model has a discontinuous limit for $m\rightarrow0$?

 

[Rick16]

Since Newton's law of gravitation predicts that light would speed up (or decelerate less) the farther away it gets from a star, and this prediction is generally taken to be incorrect, isn't this proof enough, i.e. proof by counterexample?