Why is the speed of light absolute?

[PhizzicsPhan]

Special relativity upended our normal conceptions of space and time by postulating that the speed of light was absolute for all observers. So rather than the absolute space and time of Newton, Einstein postulated an absolute speed of light, which by necessity leads to a malleable space and time (speed is space/time so if the speed of light is absolute space and/or time must then become non-absolute).

I've never come across a good explanation of WHY the speed of light would be absolute, however, and it is obviously very counter-intuitive. All other speeds are non-absolute, so why is only the speed of light absolute? I'm curious if anyone can point me to any good philosophical/physical explanatios of Einstein's key postulate?

Or if anyone has any good ideas of their own as to why the speed of light would be absolute in an ontological sense, please let me know.

 

[danR]

Special relativity upended our normal conceptions of space and time by postulating that the speed of light was absolute for all observers. So rather than the absolute space and time of Newton, Einstein postulated an absolute speed of light, which by necessity leads to a malleable space and time (speed is space/time so if the speed of light is absolute space and/or time must then become non-absolute).

I've never come across a good explanation of WHY the speed of light would be absolute, however, and it is obviously very counter-intuitive. All other speeds are non-absolute, so why is only the speed of light absolute? I'm curious if anyone can point me to any good philosophical/physical explanatios of Einstein's key postulate?

Or if anyone has any good ideas of their own as to why the speed of light would be absolute in an ontological sense, please let me know.

It's about sticking to things that seem tangible and demoting things that seem derivative from those seemingly tangible things. Reinforcing that is the need for standard, fixed measures for gold, wheat, miles, kilograms, etc.

We've spent centuries, millennia, measuring length in a straightforward way, and have come to think of time somewhat later in a similar way.

So the idea that a certain distance/time should have a fixed value, no matter what our own d/t happens to be, is breaking down long habit.

 

[atyy]

Basically, the Lorentz symmetry is an experimental fact, and enters into Einstein's theory is a fundamental, unexplainable postulate.

Going beyond Einstein, scenarios for "explaining" Lorentz invariance are theories which are not Lorentz invariant at high energies.
http://arxiv.org/abs/0705.4652
http://arxiv.org/abs/1102.0835
http://arxiv.org/abs/1106.4501

No evidence for violation of Lorentz invariance has been found so far.
http://arxiv.org/abs/grqc/0502097

 

[bcrowell]

FAQ: Why is the speed of light the same in all frames of reference?

The first thing to worry about here is that when you ask someone for a satisfying answer to a "why" question, you have to define what you think would be satisfying. If you ask Euclid why the Pythagorean theorem is true, he'll show you a proof based on his five postulates. But it's also possible to form a logically equivalent system by replacing his parallel postulate with one that asserts the Pythagorean theorem to be true; in this case, we would say that the reason the "parallel theorem" is true is that we can prove it based on the "Pythagorean postulate."

Einstein's original 1905 postulates for special relativity went like this:

P1 - "The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems of co-ordinates in uniform translatory motion."

P2 - "Any ray of light moves in the 'stationary' system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body."

From the modern point of view, it was a mistake for Einstein to single out light for special treatment, and we imagine that the mistake was made because in 1905 the electromagnetic field was the only known fundamental field. Really, relativity is about space and time, not light. We could therefore replace P2 with:

P2* - "There exists a velocity c such that when something has that velocity, all observers agree on it."

And finally, there are completely different systems of axioms that are logically equivalent to Einstein's, and that do not take the frame-independence of c as a postulate (Ignatowsky 1911, Rindler 1979, Pal 2003). These systems take the symmetry properties of spacetime as their basic assumptions.

For someone who likes axioms P1+P2, the frame-independence of the speed of light is a postulate, so it can't be proved. The reason we pick it as a postulate is that it appears to be true based on observations such as the Michelson-Morley experiment.

If we prefer P1+P2* instead, then we actually don't know whether the speed of light is frame-independent. What we do know is that the empirical upper bound on the mass of the photon is extremely small (Lakes 1998), and we can prove that massless particles must move at the universal velocity c.

In the symmetry-based systems, the existence of a universal velocity c is proved rather than assumed, and the behavior of photons is related empirically to c in the same way as for P1+P2*. We then have a satisfying answer to the "why" question, which is that existence of a universal speed c is a property of spacetime that must exist because spacetime has certain other properties.

W.v.Ignatowsky, Phys. Zeits. 11 (1911) 972

Rindler, Essential Relativity: Special, General, and Cosmological, 1979, p. 51

Palash B. Pal, "Nothing but Relativity," http://arxiv.org/abs/physics/0302045v1

R.S. Lakes, "Experimental limits on the photon mass and cosmic magnetic vector potential", Physical Review Letters 80 (1998) 1826, http://silver.neep.wisc.edu/~lakes/mu.html

 

[PhizzicsPhan]

Ben, thanks for this wealth of information from the FAQ. I'll delve into some of this to be sure.

However, it doesn't look like any of it goes to my essential question: why would the speed of light (or even your less specific c from P*) be absolute?

Just as Einstein explained Brownian motion in a satisfying way in his 1905 paper (the same year as his special relativity paper) by appealing to the existence of otherwise-undetectable molecules in water buffeting small particles floating on the surface, I'm looking for a physical explanation as to why any speed would be absolute and independent of one's frame of reference.

The positivist trend is generally fading, due in no small part to Einstein himself, so I'm personally not satisfied with explanations that simply appeal to the data (which can be interpreted in many different ways). Rather, I'm looking for a physically satisfying answer that goes beyond the surface data.

Any further thoughts?

 

[DaveC426913]

However, it doesn't look like any of it goes to my essential question: why would the speed of light (or even your less specific c from P*) be absolute?

Just to be clear, c is the speed limit of the universe - and all its forms of energy. Light is just one thing of many that travels at it.

The question might be more illuminating if turned on its head, to-wit: since the speed limit of the universe is c, and all forms of EM energy travel at that speed, what is it about things with mass that cause them to slow down to less than c?

The positivist trend is generally fading, due in no small part to Einstein himself, so I'm Rather, I'm looking for a physically satisfying answer that goes beyond the surface data.

Look up vacuum permittivity. It is one of the properties of free space that seems to determine the value of c.

 

[bcrowell]

However, it doesn't look like any of it goes to my essential question: why would the speed of light (or even your less specific c from P*) be absolute?

My FAQ gives three ways of looking at it:

1) P1+P2
2) P1+P2*
3) http://arxiv.org/abs/physics/0302045v1

1 and 2 don't explain why c is frame-independent. 3 does, but the explanation is in the paper, not in the FAQ itself.

 

[bcrowell]

Look up vacuum permittivity. It is one of the properties of free space that seems to determine the value of c.

I don't see how that's relevant. The OP isn't asking why c has the numerical value it does (which is just a matter of choosing a system of units), s/he is asking why it's frame-independent.

 

[DaveC426913]

I don't see how that's relevant. The OP isn't asking why c has the numerical value it does (which is just a matter of choosing a system of units)...

Nor am I answering that question.

... s/he is asking why it's frame-independent.

However, I see your point. When he used the term 'absolute', I thought he was asking why c is 'the absolute limit of the universe'.

 

[danR]

However, it doesn't look like any of it goes to my essential question: why would the speed of light (or even your less specific c from P*) be absolute?

Any further thoughts?

Why not? Why should linear measure be fixed?

You're just used to it, that's all. Why should the universe be by held hostage to dumb, parochial habits of beings who have only been around since 1 second to twelve o'clock?

 

[rede96]

s/he is asking why it's frame-independent.

I've read (or started to read) the paper explaining why c is frame dependent. To be honest, for a very interested but simple layman such as myself, it goes over my head.

I know I will never understand SR/GR properly until I understand the equations, however I do have a fundamental belief that we should be able to explain most principles without the need for math. After all, math is just a way of representing reality, which helps make predictions.

Anyway, I have a different way of looking at it. I must say up front that this is not based on any tangible physics, just my view based on what I have read so far, without using any math!

So please forgive me if what I have written below is completely wrong! However…

I think of this problem as having something to do with why any 'particle' with no mass will appear to move at the same speed to all observers.

My thoughts are that it might have something to do with our 4-dimentional space-time and objects with mass will travel through all 4 dimensions, (as they can never reach the speed c.) however objects without mass might only be able to travel through just 3 dimensions. EG, maybe they don't travel through 'time'

So in order to measure a difference in relative speeds, we need time. As light (or any mass-less particle) does not have a time element, all frames must measure it to be a constant.

Another way I imagine it is for example, presume that we live in a 3d world. For all of us a cube has length, height and depth. In this example, imagine that the height and length don't change between frames but different frames can have cubes of different depths. Thus we can measure the differences in depths relative to our frame.

Now imagine that 'light' would only travel through 2 dimensions, height and length. So people in a 3D world can only see its length and height.

As length and height don’t change between frames, everyone must always measure the light to be the same. (I.e. they can only see the ‘square’, which never changes.)

Now this is just my interpretation of why all observers will measure light to be the same and I do not make any claims that this is reality. It would just be nice if it was!

 

[bcrowell]

I've read (or started to read) the paper explaining why c is frame dependent. To be honest, for a very interested but simple layman such as myself, it goes over my head.

For a nonmathematical treatment of the same ideas, try:
http://www.lightandmatter.com/html_books/0sn/ch07/ch07.html

 

[rede96]

For a nonmathematical treatment of the same ideas, try:
http://www.lightandmatter.com/html_books/0sn/ch07/ch07.html

Thanks for that. I'll need to read it a few times until it sinks in! :)

However, IMO it still does not answer mine or the OP’s question as to what natural events happen to cause c to be consistent in all frames.

It uses math and logic to show how it must be so, but doesn't really offer a satisfying answer as to what is happening.

I know my example is probably wrong, but I was looking for something like that as an answer. Most answers come in mathematical form or just simple statements like 'that is just the way it is...'

 

[bcrowell]

It uses math and logic to show how it must be so, but doesn't really offer a satisfying answer as to what is happening.

I don't see how we could have a satisfying answer without math and logic, or what would be required in addition to math and logic to provide a satisfying answer.

 

[rede96]

I don't see how we could have a satisfying answer without math and logic, or what would be required in addition to math and logic to provide a satisfying answer.

Unfortunately, you are probably right.

I suppose it would be a bit like asking the question, how do I know my car is traveling at 70mph.

An answer might be that we draw a grid on the floor with increments of ‘x’, when the car reached x1 I start my clock, when the car reaches x10, stop my clock. By using the time and distance I can prove that the car was traveling at 70mph.

The answer I would be looking for might be something along the lines of:

When the driver depressed the pedal, it pulls on a wire that allows fuel into a chamber. A spark ignites the fuel and pushes up a piston that is connected to a shaft. This in turn connects through a series of cogs and drive shaft to the reel wheels of the car. This causes the wheel to turn and speed ‘x’. Due the size of the wheels, this means that the turn y amount of times every minute which must propel the car at 70mph…

So we all know that light travels at c, and that it is the same for all observes. We can prove this a number of ways. However, there must be physical reason for why this is.

So far I have not read anything that really explains what is happening.

The nearest to an explanation I have read here was around the 4 dimensions. I can’t remember who posted it, but if I find it I’ll link to it.

I would also like to say that even though I have not found a ‘satisfying’ answer, the answers that all the kind people post are very much appreciated and have helped my understanding tremendously.

 

[nitsuj]

I don't see how we could have a satisfying answer without math and logic, or what would be required in addition to math and logic to provide a satisfying answer.

That's a great way to put it bcrowell.

The ball (information) is in OP's court now. And up to him'her to become familiar with it, until those pesky "why is c inveriant" questions subdue.

I still cannot clearly discern proper scientific questions from those of the philosophical type.

But because of your bull-stance on the appearent clear difference, I'm learning to keep my mouth shut

 

[Passionflower]

The question might be more illuminating if turned on its head, to-wit: since the speed limit of the universe is c, and all forms of EM energy travel at that speed, what is it about things with mass that cause them to slow down to less than c?

Good point!

And something many perhaps do not realize is that the constituents of a mass are not really sitting still either. Which makes the term rest mass a bit of a misnomer because quantum mechanically no particle can be at rest wrt to another particle.

 

[PhizzicsPhan]

Why not? Why should linear measure be fixed?

You're just used to it, that's all. Why should the universe be by held hostage to dumb, parochial habits of beings who have only been around since 1 second to twelve o'clock?

Dan, because every other speed in the universe that we know of is non-absolute. That is, speeds change with respect to the observer. C ostensibly doesn't. And this is why relativity theory has been acknowledged from its inception to be very counter-intuitive (and brilliant).

 

[PhizzicsPhan]

I still need to review the papers posted thus far (though I did look over the Pal paper already and it doesn't seem to suggest any physical explanation, focusing instead on mathematical explanations), but here's a thought, inspired by Einstein himself.

Einstein promoted the idea of a "new ether," or "total field" or "spacetime," for most of the second half of his career, even though this history is largely ignored today. Einstein was always careful to stress that his "new ether" was relativistic and that he wasn't promoting a Lorentzian non-relativistic ether or the 19th Century luminiferous ether.

If we view spacetime itself, the fabric of our universe, as not simply a void, then it raises the question: what does this non-void consist of?

But more specifically for present purposes it seems that spacetime itself may simply be a medium through which information moves and that c is the ultimate speed of this information. That is, the physical structure of spacetime simply can't support any information speed higher than c.

Once folks respond to this suggestion, I'll delve into the intriguing problems raised for this idea by quantum non-locality and the many Bell experiments that show some type of influence does in fact move faster than c (Salart, et al., have shown at least 10,000 times c).

 

[bcrowell]

If we view spacetime itself, the fabric of our universe, as not simply a void, then it raises the question: what does this non-void consist of?

But more specifically for present purposes it seems that spacetime itself may simply be a medium through which information moves and that c is the ultimate speed of this information. That is, the physical structure of spacetime simply can't support any information speed higher than c.

Once folks respond to this suggestion, I'll delve into the intriguing problems raised for this idea by quantum non-locality and the many Bell experiments that show some type of influence does in fact move faster than c (Salart, et al., have shown at least 10,000 times c).

I would suggest that you first review PF's rules https://www.physicsforums.com/showthread.php?t=414380 about overly speculative posts, and then, if you think what you're talking about is within the rules, start a separate thread for it, and include references to peer-reviewed papers.

Einstein promoted the idea of a "new ether," or "total field" or "spacetime," for most of the second half of his career, even though this history is largely ignored today. Einstein was always careful to stress that his "new ether" was relativistic and that he wasn't promoting a Lorentzian non-relativistic ether or the 19th Century luminiferous ether.

I think you may be misinterpreting the history of this topic. The following may be relevant.

FAQ: Didn't Einstein say that general relativity was an aether theory? Is general relativity compatible with an aether?

No, Einstein didn't say that general relativity was an aether theory. Einstein wrote a 1924 paper in which he made the philosophical point that although relativity killed off the luminiferous aether as the supposed medium of electromagnetic vibrations, it still imbued the vacuum with specific physical characteristics, such as curvature and energy. The basic point of the paper is that we can't decide, purely based on philophical ideas like Mach's principle, whether the vacuum has its own properties; we actually have to go through the usual scientific cycle of theory and experiment in order to find out the answer. Internet kooks love to misinterpret and overinterpret this paper, or to misrepresent it by saying that Einstein referred to GR in general, throughout his career, as an aether theory.

A more subtle question is what kinds of aether theories can be constructed, and how they relate to (or don't relate to) general relativity. Philosophers and historians of scientists have debated whether any real aether theory ever actually existed, and what that would mean. Earman (1989) investigates earlier work by Trautman (1966), and concludes: "[A]bsolute space in the sense of a distinguished reference frame is a suspect notion, not because armchair philosophical reflections reveal that it is somehow metaphysically absurd, but because it has no unproblematic instantiations in examples that are physically interesting and that conform even approximately to historical reality." Debate on this philosophical and historical issue continues,[Rynasiewicz 2003] but one should keep in mind that this discussion is all about theories that have been falsified by observation since the Michelson-Morley experiment. Jacobson (2008) has investigated a theory in which Lorentz invariance is approximate, and is broken in the gravity sector at large Lorentz boost velocities. This theory includes phenomena like aether dust settling onto a planet and giving it an aether charge. Jacobson's model has two adjustable parameters which, if nonzero, differentiate it from general relativity, and which are constrained by astrophysical observations. It is important to note that the model is not compatible with Galilean relativity, and it predicts all the same counterintuitive phenomena as standard relativity, including, e.g., the twin paradox, length contraction, and black holes.

A. Einstein, "Über den Äther," Schweizerische naturforschende Gesellschaft 105 (1924) 85

original text - http://www.wikilivres.info/wiki/Über_den_Äther

English translation of [Einstein 1924]- http://www.oe.eclipse.co.uk/nom/aether.htm

commentary by John Baez on [Einstein 1924] - http://web.archive.org/web/20070204022629/http://math.ucr.edu/home/baez/RelWWW/wrong.html

A. Trautman, in B. Hoffmann (editor) Perspectives in Geometry and Relativity, Bloomington, 1966, p. 413.

J. Earman, World Enough and Space-Time, Absolute versus Relational Theories of Space and Time. Cambridge, 1989, MIT.

Rynasiewicz, "Field Unification in the Maxwell-Lorentz Theory with Absolute Space," Philosophy of Science 70 (2003) 1063, available at http://philsci-archive.pitt.edu/archive/00001096/ . Rynasiewicz starts by summarizing two important earlier papers that are now difficult to obtain: Trautman 1966 and Earman 1989.

Ted Jacobson, "Einstein-aether gravity: a status report," 2008, http://arxiv.org/abs/0801.1547

 

[Dale]

Dan, because every other speed in the universe that we know of is non-absolute. That is, speeds change with respect to the observer. C ostensibly doesn't.

Actually, you can show from first principles that there must be one (and only one) invariant speed. So there are only two possibilities: either it is finite (Einstein) or it is infinite (Galileo).

Experiments show that it is finite. Once it is determined to be finite its actual value is simply a matter of the choice of units.

 

[Zac Einstein]

The speed of light

Einstein said, the speed of light does not change, why is that?

[Merged this thread-starting post into the present thread, since it's the same topic. -bcrowell]

 

[ghwellsjr]

Einstein said, the speed of light does not change, why is that?

He did not exactly say that.

What was already determined prior to Einstein, was that if a person is inertial, that is, not accelerating, and he attempts to measure the speed of light, he will always get the same answer, c. But any experiment to measure the speed of light must necessarily involve a round trip for the light. In Einstein's 1905 paper, he describes a typical experiment where you have a timer located at the source of light and a reflector some measured distance away. You start the timer when you set off a flash of light and you stop the timer when you see the reflected light return back to you. Then you calculate the average round-trip speed of light by dividing double the measured distance by the measured time interval. You can read where he says this near the end of article 1 of his 1905 paper.

But no experiment can measure the one-way speed of light without making some assumptions. So Einstein's assumption (his second postulate) is that the one-way speed of light equals the average measured two-way speed of light. He mentions his postulate in his introduction and describes this process in detail in his first two articles. In his second article, he actually uses the phrase "ray of light" to emphasize that it is the one-way speed of light that he was defining earlier. So what he said was that he will assume that the light takes the same amount of time to get to the mirror as it does to get back from the mirror. That is what he said.

 

[danR]

Originally Posted by danR
Why not? Why should linear measure be fixed?

You're just used to it, that's all. Why should the universe be by held hostage to dumb, parochial habits of beings who have only been around since 1 second to twelve o'clock?

​

Dan, because every other speed in the universe that we know of is non-absolute. That is, speeds change with respect to the observer. C ostensibly doesn't. And this is why relativity theory has been acknowledged from its inception to be very counter-intuitive (and brilliant).

Yes, but by the same token, there are no fixed measurements by observation of any linear measure, even by classical 3-space geometry. Any attempt to compare lengths requires a transformation, either by a literal movement and superposition of end-points, or a mathematical function requiring a complete description of the relevant measures. Otherwise 2 sticks of different length can appear to be the same length, and conversely. We don't think about these transformations because we are used to doing them.

On the other hand, the idea that there exists a d/L that is fixed with respect to any other *d/L is mysterious (I suspect it would seem perfectly natural to a being with 4-space perceptions), but not very counterintuitive. I don't find Euler's equivalence counterintuitive, but I find it very mysterious, much more so than the second postulate.

Edit: sorry, d/t.

 

[Integral]

I see some need for a historical perspective in this thread.

First of all Einstein did not postulate a constant speed of light out of the blue. The physicists of the 19th century had this forced on them by Maxwell. In 1867 C. Maxwell gathered the results of the previous 100yrs of electrical observations into a single set of equations; we now call them Maxwell's equations. Maxwell was able to cast these relations into a form recognized as the PDE describing wave motion. From this expression he found the speed of electro-magnetic waves to be given as,
$$\sqrt {\frac {1} {\epsilon_0 \mu_0 }}$$ .

When he computed this, much to his surprise he found himself looking at a number which agreed with the measured value of the speed of light (try it, it still works!). Note that this gives the speed of light as a combination of fundamental constants of space, so is therefore a constant. Even Maxwell was uncertain about this, and the community of Physicists saw it as "his problem". They felt that he would soon find the "error" in his work.

There was no error, and this is the world A. Einstein grew up in. The Speed of light was recognized as a constant. His work finally reunited the world of EM with the rest of Physics.

 

[bcrowell]

He was able to cast these relations into a form recognized as the PDE describing wave motion. From this expression he found the speed of electro-magnetic waves to be given as,
$$\sqrt {\frac {1} {\epsilon_0 \mu_0 }}$$ .

When he computed this, much to his surprise he found himself looking at a number which agreed with the measured value of the speed of light (try it, it still works!).

You've got the history wrong. All of this was done long before Einstein.

[EDIT] Oops -- completely misread Integral's post! Never mind!

 

[ghwellsjr]

You've got the history wrong. All of this was done long before Einstein.

I think he was talking about Maxwell.

 

[bcrowell]

I think he was talking about Maxwell.

Ah, I see -- sorry, Integral, I completely misunderstood your post!

 

[Integral]

Ah, I see -- sorry, Integral, I completely misunderstood your post!

Humm, guess that means I need to give it a re-read and make it clear. Sorry about that.

 

[SinghRP]

Sorry, I couldn't read the whole thing. But...
Q: Why is the speed of light absolute? in all frames?
A: Photons, the quanta of light have zero rest mass and spin 1 (zero orientation).
That is, a photon has no inertial mass. So, the poor thing can't be slowed down or speeded up.
Don't worry about frames, they are for us observers to take measurements.
If its spin orientation is not zero then it's not Lorentz invariant.
They say light slows down in material medium such as water. Actually when light passes water, it is gone by interactions and new waves are generated, which propagate. This process takes a little time, so it appears light slwed down.
I hope this helps you as a physicist.

 

[bcrowell]

Sorry, I couldn't read the whole thing. But...

It would be more constructive if you would read the previous posts before posting.