Why is the Speed of Light Constant in All Inertial Frames?

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The discussion centers on the constancy of the speed of light across all inertial frames, a principle established by Einstein's theory of special relativity. Participants highlight that while the speed of light is consistently measured at approximately 299,792,458 meters per second, the underlying reasons for this constancy remain elusive. Some suggest that this constancy may be linked to fundamental properties of space, such as permittivity and permeability, as described in Maxwell's equations. Others question whether the speed of light could vary over time or under different gravitational conditions, though current experiments have not confirmed such variations. Ultimately, the conversation reflects ongoing curiosity and debate about the foundational principles of physics and the nature of light.
  • #31
No, your meterstick is one meter.

Right. No matter how I move the meterstick, no matter what reference frame I put the meter stick in, its proper length is always one meter.


If the speed of light varied, and the length of a meter was defined by the speed of light, then we would measure the proper length of the meterstick as something other than a meter.


In order to prove that simultaneity is absolute, one must show that for any Length formula which is a function of speed, a contradiction will be arrived at, not just the Lorentz formula.

I don't see how that follows either.


The most important axiom of state theory, is that for any state x, and any state y, not (x before y and y before x)

Relativity of simultaneity doesn't contradict that axiom. In each reference frame, defining "before" in the classical way obeys this axiom. If you're doing what you "should" be doing relativistically and say that if two events, x and y, are space-like seperated, then it is false that x is before y and it is false that y is before x, thus the axiom holds.

Relativistic causality is a partial order, not a total order.

(IOW, in terms of relativistic causality, "x before y" only if x and y have a timelike separation... though this could probably be safely extended to lightlike separations as well)


Yes, didn't like it, alpha and beta seem fudgable to me, seems to make conclusions relative.

Well, you need to learn to tolerate it; no experiment can prove anything in the mathematical sense... you can only prove things to certain levels of accuracy and confidence.

(to be pedantic, the same is true about mathematical proofs; e.g. there's always the remote possibility that everyone in the history of mathematics overlooked a subtle mistake in the calculation that 2 + 3 = 5)
 
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  • #32
Originally posted by chroot
I guess you've got your very own universe over there. This one seems indifferent to your beliefs.

- Warren
I believe that the Eagles are going to the Super Bowl. I believe the universe agrees with me.

I also believe that you (TheAthiestKing) are allowing your beliefs and desires (the way you WANT the universe to work) cloud your ability to objectively interpret the evidence presented to you.

You have to let it all go - your fear, doubt, and disbelief. Free your mind. [/Morpheus]
the daily GPS results that do not detect the velocity of the Earth relative to local space. That is - we do not detect speed when both the transmitter and receiver are comoving...
GPS transmitters and recievers are NOT comoving and the observed relativistic effects of their independent motions fit with the theory to a high degree of accuracy.
 
  • #33
There is a comoving aspect with respect to space - both the GPS satellite and the receiver are carried along with the Earth and adjusted for height and velocity relative to the Earth centered reference frame - but the Earth is also moving relative to space, yet there is no variation in the measured time between signals that travel from the GPS satellite to the Earth bound receiver that depend upon the direction of the Earth's motion vector relative to space. The receiver on the Earth does move with respect to the GPS satellite (due to the Earth's's rotation) and this change in the position of the receiver from the time the signal is transmitted must be corrected for - and of course the receiver and GPS satellite are not comoving in that context... so the change in the receivers position during the transit time must be compensated for... but not the change in the direction of the transmission relative to the Earth's motion with respect to the Sun or the CBR or Milky Way, whatever. I call this the comoving frame since it moves with the Earth - this is the significance of the GPS as a test of "one way isotropy" ... an issue that eluded satisfactory experimental confirmation for many years.
 
  • #34
Originally posted by yogi
...but the Earth is also moving relative to space, yet there is no variation in the measured time between signals that travel from the GPS satellite to the Earth bound receiver that depend upon the direction of the Earth's motion vector relative to space.
Thus we conclude there is no ether.
 
  • #35
Original quote TheAtheistKing:
--------------------------------------------------------------------------------
The most important axiom of state theory, is that for any state x, and any state y, not (x before y and y before x)
--------------------------------------------------------------------------------

Quote: Hurkyl

Relativity of simultaneity doesn't contradict that axiom. In each reference frame, defining "before" in the classical way obeys this axiom. If you're doing what you "should" be doing relativistically and say that if two events, x and y, are space-like seperated, then it is false that x is before y and it is false that y is before x, thus the axiom holds.

Relativistic causality is a partial order, not a total order.

(IOW, in terms of relativistic causality, "x before y" only if x and y have a timelike separation... though this could probably be safely extended to lightlike separations as well)

Yes relativity does contradict that, and showing this to be true is the purpose of the primary thread I made. And what on Earth is the classical way of defining 'before'? I explained the operational definition of 'before' somewhere.
 
  • #36
R Waters - yes - GPS dealt a blow to traditional ether theory as a detectable particle wind - but not necessarily a fatel blow to all ether concepts - there are still possibilities that satisfy this result - Selleri transformations and Modified Lorentz Ether Theory both comport with one-way isotrophy - and as we have discussed previously, there is the idea that mass conditions space so as to create a local comoving inertial frame... What is needed is a theory that explains all aspects of space - why it has a characteristic impedance, a capacitance, an inductance, and why it can be modified by matter as per GR.
 
  • #37
Fatal blow to all aether theories?

Originally posted by yogi
R Waters - yes - GPS dealt a blow to traditional ether theory as a detectable particle wind - but not necessarily a fatel blow to all ether concepts - there are still possibilities that satisfy this result - Selleri transformations and Modified Lorentz Ether Theory both comport with one-way isotrophy - and as we have discussed previously, there is the idea that mass conditions space so as to create a local comoving inertial frame... What is needed is a theory that explains all aspects of space - why it has a characteristic impedance, a capacitance, an inductance, and why it can be modified by matter as per GR.
What experiments would be fatal for the remaining aether theories?

Presumably only those conducted with small spacecraft from from massive bodies like the Earth, or are there others?
 
  • #38
Good question Nereid - I read everything I can on this subject - but the analogy to anything physical that we are familiar with always seems unattainable - as Einstein said - there is an ether - but the idea of motion cannot be applied to it - obviously it has properties, but its make up is "sui generis" When we get a model that is testable - then perhaps experiments can be devised that will confirm or demolish competing theories... at least to the satisfaction of rational thinkers.
 
  • #39
Originally posted by yogi
as Einstein said - there is an ether - but the idea of motion cannot be applied to it
The quote is used a lot by people favoring an ether, but if motion can't be applied to it (as E said) then it isn't the same "ether" as is generally discussed. Its not the same "ether" that M&M searched unsuccessfully for. Its unfortunate that he didn't use a different word.

In any case, I liken the various ether theories to the attempts to keep a geocentric universe (and indeed that's one of the goals some people have with ether). But how many epicycles on epicycles on epicycles does it take before it has reached the point of absurdity and should be discarded? I think ether has passed that point.
 
  • #40
R. Waters -- The word ether has been given bad press - space is something - your analogy to Geocentricity is unfounded - before you throw out the baby with the bath water, perhaps you should ponder how "nothing" can be so rich in properties - how do you explain CBR dipole anisotrophy, both globally and yearly variations, stellar aberration, spatial dimensional changes due to matter, spatial permeability and permittivity, the characteristic spatial impedance, the manifestation of fleeting particles from the vacuum, the inertial reaction that is exhibited by every mass when it is accelerated relative to space, the attraction of two closely spaced parallel plates, and so on. To give up is easy - to pursue the real nature of space, call it whatever you want, is probably the greatest challenge facing 21 century physics - In order to understand the universe, we must comprehend space and time - for that is the stuff of which it is made (My own quote).
 
  • #41
yogi,

Just a guess - if there exists an ether effect, it is too small to account for almost all, if not all, of the familiar phenonema you mention. What standard velocity do their measurements yield for the ether and why, more simply than relativity?
 
  • #42
Originally posted by yogi
...before you throw out the baby with the bath water, perhaps you should ponder how "nothing" can be so rich in properties.
This is the crux of the problem. No one has ever said that space is devoid of properties. But that does NOT make it a medium for the transmission of waves in the manner of air transmitting sound.

The "ether" Einstein was talking about is space having properties. No one disagrees.

The "ether" relevant to M&M and light transmission experiments is a transmission medium. It does not exist.

Two different ethers. Or maybe, one ether with some properties that exist and some that don't.
 
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  • #43
Loren - Russ - Let me put it this way - we all agree that space has a lot of properties - and we all agree that we have not been able to make an experiment that (when fairly evaluated by most of the physics community) reveals the motion of the Earth relative thereto (at least there appears to be no motion relative to the local ether). The 64 dollar question is why? If relativity is totally kinametic, it is puzzling how these many properties of space are obscured in our velocity experiments.
 
  • #44
Originally posted by yogi
If relativity is totally kinametic, it is puzzling how these many properties of space are obscured in our velocity experiments.
Why? Why do these properties of space have to have some connection to motion?
 
  • #45
Russ - if space in any sense is energy - whether it be in the form of fields or in the stress tensor of Einstein that is postulated to condition space in consequence of local matter - then - stress - in its traditional understanding, is a form of energy - energy is mass - and therefore why is motion with respect to this mass containing medium not revealed. In SR we accept the idea of observational measurements made on other masses relative to an inertial frame - so why should we not expect to ascertain motion relative to a mass endowed medium?
 
  • #46
Spacetime itself has mass? I don't think I've ever heard that before.
 
  • #47
Russ - Einstein's stress energy tensor implies what it says - stress-energy. Whether there is a separate spatial stress apart from that induced by matter is debatable - but as you well know, there is energy in the field of a gravitational body - the energy density is inversely proportional to the 4th power of the distance - so when you integrate over the entire volume of the universe for every particle of mass - you get some average spatial energy density. Feynman once analysed the charging of a capacitor in terms of the field energy flowing into the device - as opposed to the electrons flowing to the plates via the wires that supplied the charging potential. Its been urged by some that all energy lies in the field and not in the objective particle.

Of course - the very fact that the spatial energy tends to be uniformly distributed - or nearly so - may be the very thing that prevents detection - hmm - maybe that answers may query
 
  • #48
Originally posted by Kannan Kailas
Hi All,

It is Experimentally very evident that the speed of light in any any frame of reference is constant. But is there any explanation for why this happens.

Velocity of light seems to be like zero in mathematics. Any thing multiplied my zero results in zero irrespective of the number you are using for multiplication.

If there is any explanation for the consistancy of speed of light,can anyone please explain me?

Thanks,
Kannan


Why this happens has to do with the location of the photons and the location of an atomic clock used to measure the speed of the photons at that particular clock. Einstein made this clear in his 1911 gravitational redshift paper.

An atomic clock slows down in a stronger gravitational potential, and so does the speed of light. So, when local photon speed is measured in a certain gravitational potential by an atomic clock in that same gravitational potential, the speed is measured as being “c”. But when the photon speed is measured by an atomic clock that is located in a different gravitational potential, the speed of the photons relative to that second clock is not “c”. It can be more or less than “c”. So the original 1905 concept of “the constancy of the speed of light” is not correct. The updated 1911 version seems to be correct, but the speed of the photons must be locally measured by a local atomic clock resting on the surface of an astronomical body right where the speed of the photons are measured, ie, at that particular atomic clock.
 
  • #49
Originally posted by yogi
Good question Nereid - I read everything I can on this subject - but the analogy to anything physical that we are familiar with always seems unattainable - as Einstein said - there is an ether - but the idea of motion cannot be applied to it - obviously it has properties, but its make up is "sui generis" When we get a model that is testable - then perhaps experiments can be devised that will confirm or demolish competing theories... at least to the satisfaction of rational thinkers.

In 1920 he didn’t attribute any “motion” to it, but I think that was a premature statement based on the incorrect information he had then about the universe being “static” and non-expanding and all the starts were pretty much “fixed” in space, with only small motions being attributed to the planets, while large motions were not attributed to the stars or galaxies in 1920.

If you merely say that Einstein’s 1920 “ether” is the local group of “fields” (magnetic, electric, and gravitational) that are generated by astronomical bodies and that travel through space with the bodies, then there is the local “ether” which regulates the speed of light locally, at the bodies. Local areas of this “ether” travel through space with the bodies. Local light speed then would be measured at an by an atomic clock that is resting on the surface of an individual astronomical body, as the light photons pass that particular clock.
 
  • #50
Originally posted by russ_watters
If it slowed down or sped up due to gravity, it would be noticeable in the GPS system.

Not necessarily. It depends on which atomic clock you use to measure its speed and where the photons are located when you measure their speed with that particular atomic clock. Apparently, atomic clocks slow down and speed up at different gravitational potentials in the same amount that light slows down and speeds up at those same potentials.

question - I have never been able to find an unequivocal proof as to whether gravity simply bends light or slows it - seems it depends a lot on the interpreter

I think it does both. A beam of light passing the sun acts like a plane wave. The area of the wave nearest the sun is slowed down, while the area of the wave fartherest from the sun is not slowed down as much. Thus, the light beam bends toward the sun. This is very much like a light beam being bent as it enters water from an angle. The part of the plane wave hitting the water first slows down first. This results in the bending of the plane wave when it enters water.

But, there is something else to consider about light passing the sun. If its speed while nearest the sun is measured by atomic clocks at that same place, the gravitational potential at that place slows down the atomic clocks, so they measure the speed of “c” for the passing light waves. The light slows down at those clocks but the rate of the clocks slows down too, so those clocks measure the local speed of light at “c”. The light would appear to be slowed down when the speed of the photons at the sun are measured by an atomic clock located at the earth.

These various phenomena have led to a lot of confusion about what really happens when light passes the sun. It does slow down relative to the Earth and earth-based atomic clocks, but relative to atomic clocks at the sun, it doesn’t slow down. However, once the photons leave that area of the sun and move into deeper space, they speed up when their speeds are measured by the atomic clocks that are located at the sun. So, the sun-based atomic clocks measure the deep-space speed of light to be greater than “c”, while they measure the speed of the passing photons, passing right at the clocks, to be “c”.
 
  • #51
David - your views are very much in conformity with the MLET template - not traditional SR. The Modified Loretz Ether Theory does give a physical explanation of the observed phenomena - but you are going to get a lot of static on these boards.
 
  • #52


Originally posted by David
An atomic clock slows down in a stronger gravitational potential, and so does the speed of light. So, when local photon speed is measured in a certain gravitational potential by an atomic clock in that same gravitational potential, the speed is measured as being “c”. But when the photon speed is measured by an atomic clock that is located in a different gravitational potential, the speed of the photons relative to that second clock is not “c”. It can be more or less than “c”.
If I understand you correctly, you are saying that reltive to a clock in its reference frame a photon travels at C and relative to a clock in a different frame, that photon doesn't travel at C? Thats one of those paradoxes that isn't a paradox. Its covered by SR/GR and its quite simple: you can't mix reference frames in that way.
Not necessarily. It depends on which atomic clock you use to measure its speed and where the photons are located when you measure their speed with that particular atomic clock. Apparently, atomic clocks slow down and speed up at different gravitational potentials in the same amount that light slows down and speeds up at those same potentials.
I've heard this argument before and it boils down to "the laws of the universe are conspiring against us to make it APPEAR that the speed of light is constant." Scintists sometimes try to make convoluted explanations like that, but its generally because they don't like the implications of what their own data/theories tell them.

It appears you are ok with the implication that time isn't constant, but are not ok with the implication that DISTANCE is not constant. Thats unusual because the distance is the easier one to picture as it is also not a constant in Newtonian physics. That makes it easier for most people to accept.

The speed of light is constant. It is always measured by all observers, everywhere, to be C.
 
  • #53


Originally posted by russ_watters
If I understand you correctly, you are saying that reltive to a clock in its reference frame a photon travels at C and relative to a clock in a different frame, that photon doesn't travel at C? Thats one of those paradoxes that isn't a paradox. Its covered by SR/GR and its quite simple: you can't mix reference frames in that way.

No, not exactly. I’m saying that if you want to arrive at a local speed of “c” for light photons, their speed must be measured locally by an atomic clock resting at a certain place on the surface of an astronomical body, and the photons have to pass by that particular clock, at very close range to the clock. And then that clock measures “c” for the photons at that clock. But if you use that same clock to measure the speed of photons that are traveling someplace else, you will measure greater or less than “c” for the speed of the photons.

This is not a paradox. The tick rates of atomic clocks go down at the same places the speed of the photons go down, and apparently in the same amount. This is not covered by SR at all. It’s not even mentioned in SR, because gravitational potential and acceleration are not mentioned in SR.

The reason I refer to an atomic clock is because it’s local rate is tied to the local speed of photons. Not all clocks slow down when and where atomic clocks slow down, or speed up when and where atomic clocks speed up.

In a real world you can mix reference frames any way you want to. But you have to know where the photons are that you are measuring, and where the atomic clock that you are using for a time reference is located , because an atomic clock resting in one place (such as at the earth), will not measure “c” for the speed of photons that are moving some place else (such as past the sun).
 
  • #54
Originally posted by yogi
David - your views are very much in conformity with the MLET template - not traditional SR. The Modified Loretz Ether Theory does give a physical explanation of the observed phenomena - but you are going to get a lot of static on these boards.

Thanks for the info. I’ve never heard of the MLET Template, but I just looked it up, and of course that is correct. The “ether” that controls the speed of light locally is apparently the fields that are generated by astronomical bodies and that move through space with the bodies.

Lorentz’s “ether” was the incorrect 19th Century version of this, but with only one large “ether” filling all of space inside a “fixed” non-expanding universe filled with “fixed” stars.

The Michelson Morley experiment was designed to detect the 18.6 mps “ether wind” that 19th Century physicists thought was present at the surface of the earth. But, it turns out that since the Earth carried its own local “ether” through space with it, their experiment was like raising a flag up a flagpole inside their laboratory and being surprised that the flag didn’t flap around in the breeze that was blowing outdoors, outside their laboratory.

The most ironic thing is that, although Einstein in his 1905 paper claimed there was no “ether”, he used two of them in his theory. Each one was fixed inside each of his two relatively moving “frames”. That’s why he measured “c” for the speed of light in each frame. He literally postulated the “local ether” concept into existence in that theory. Of course, one error he made was that he had “length contraction” being cause for no particular real physics reason. That’s why he had to print a retraction in one of his 1907 papers saying that the “geometrical” shape of a relatively moving body does not contract at all, not because of “relative motion”. Of course, Lorentz had his “length contraction” due to the motion of a mass “through” his “ether”, with the “ether” putting up a resistance to the motion of the mass, thus “squeezing” it somewhat in the direction of its motion.
 
  • #55


Originally posted by russ_watters
I've heard this argument before and it boils down to "the laws of the universe are conspiring against us to make it APPEAR that the speed of light is constant."


The “laws of the universe” don’t “conspire”. They do what they’re supposed to do, and if we can’t figure out exactly what they do, then we are just plain dumb.

I suppose you think that the color “red” is where you see it? Look around your room for something that is red. See it? Ok, where is the “red”?

It’s in your brain. It’s not at the place where you see it. Electromagnetic waves have no color, just like compression and vacuum waves in air have no “sound” until that wave phenomena reaches deep inside your brain in the form of electrical impulses.

Originally posted by russ_watters
It appears you are ok with the implication that time isn't constant, but are not ok with the implication that DISTANCE is not constant. Thats unusual because the distance is the easier one to picture as it is also not a constant in Newtonian physics. That makes it easier for most people to accept.



“Time” depends on how fast things move, vibrate, or oscillate, when compared to the motion, vibration, or oscillation rate of something else. Time is a kinetic energy/motion phenomenon, and different kinds of “clocks” speed up and slow down at different rates in the same places and under the same conditions. Anything that moves, vibrates, oscillates, or hums is a “clock”. An atomic clock doesn't represent all of "time". It only represents internally vibrating atom rates.
 
  • #56


Originally posted by russ_watters
The speed of light is constant. It is always measured by all observers, everywhere, to be C.

This is not true. How can you make such a bold erroneous statement? Einstein disagrees with you. He said in 1911 that the speed of light slows down when it passes near the sun. It speeds up again when the photons move away from the sun. That’s what causes a light ray to bend when it passes near the sun. You need to study his actual papers and not popular media reports about them. The media reporters just don't understand much of what he said, and they're always getting things mixed up.
 
  • #57


Originally posted by David
This is not true. How can you make such a bold erroneous statement? Einstein disagrees with you. He said in 1911 that the speed of light slows down when it passes near the sun. It speeds up again when the photons move away from the sun. That’s what causes a light ray to bend when it passes near the sun. You need to study his actual papers and not popular media reports about them. The media reporters just don't understand much of what he said, and they're always getting things mixed up.
No, sorry, russ is correct. Perhaps you need to take your own advice? russ actually knows quite a bit about relativity!

Every observer who builds an apparatus at rest with respect to him, and allows light to go through it, will always measure the speed of light as c.

If you built an apparatus near the sun, the apparatus would indicate that the light is traveling at c. Light always travels at c locally. When you say "light slows down near the Sun," you mean that an Earth-bound observer would say it took longer than it would've on Earth. A Sun-bound observer, however, would say it took just as long as it should've.

- Warren
 
  • #58


Originally posted by chroot
No, sorry, russ is correct.

Well, now, let’s see. Russ said:

“The speed of light is constant. It is always measured by all observers, everywhere, to be C.”

That’s not quite the same as what you said.

You said:

”Every observer who builds an apparatus at rest with respect to him, and allows light to go through it, will always measure the speed of light as c.”

And that’s pretty much what I said earlier:

”So, when local photon speed is measured in a certain gravitational potential by an atomic clock in that same gravitational potential, the speed is measured as being “c”.”

See? You are referring to a “local” observer observing the photons’ speed “locally”, and that is also what I was referring to. I added a specific kind of clock which we can use to time the local speed of the local photons. But Russ’ less specific statement could include “local” observers observing the motion of distant photons moving from one distant place to another.


Originally posted by chroot
If you built an apparatus near the sun, the apparatus would indicate that the light is traveling at c. Light always travels at c locally.

That's what I said, but you've got to specify what type of clock you are using for the measurement, and to get "c", an atomic clock is requried.

Originally posted by chroot
When you say "light slows down near the Sun," you mean that an Earth-bound observer would say it took longer than it would've on Earth. A Sun-bound observer, however, would say it took just as long as it should've.

Right, if both observers are using resting atomic clocks where they are located.

So, Russ’ statement is not quite correct, and it's misleading:

“The speed of light is constant. It is always measured by all observers, everywhere, to be C.”

Because a local observer at the Earth will measure a slowdown in the speed of light as it passes near the sun, so we can’t say “It is always measured by all observers, everywhere, to be C”, since the Earth observer would tell the sun observer, “The light slowed down when it passed near to you,” while the sun observer would say to the Earth observer, “It traveled at ‘c’ when it passed me, but now it’s on the way to you traveling faster than ‘c’, relative to me.”
 
  • #59


Here is what Einstein said about the changing speed of light in his 1911 paper:

Einstein said in 1911

“If we call the velocity of light at the origin of co-ordinates Co, then the velocity of light C at a place with the gravitational potential Φ will be given by the relation C = Co (1 + (Φ/C^2))”

It’s surprising how many people who talk about “relativity” have never read that paper and have no idea what that equation means.
 
  • #60
Yes, I think it's just a matter of semantics then. You can't really measure a photon's speed unless that photon goes through an apparatus at rest w.r.t. you -- and if it goes through an apparatus at rest w.r.t. you, you'll always measure it going at c.

In other words, for every photon you actually measure with your own apparatus, you'll always get the answer c.

When you're talking about observing a photon at another place (near the Sun, for example), you're not really measuring the speed of that photon directly, since it never comes through a local apparatus. You're inferring speed via other events, but not measuring it directly. If or when you do measure it directly, however, you can bet it'll be going c.

Seems we're all on the same page, it's just a matter of wording.

- Warren
 

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