Why is light speed constant in all reference frames?

[catalyst0435]

Hey, this is my first post. I am a biology major so I know pretty much nothing about physics, yet sometimes it interests me way more than chromosomes do.

So, given that I know nothing about physics, this is probably going to sound like a stupid question. But I've always wondered how it is possible that a person A moving at speed observes light emitted from their frame of reference at c, and then a person B sitting still (relative to person A of course) sees that same light traveling also at c. It seems like that just shouldn't happen.

So I think I'm right when I say it has something to do with space and time being perceived differently depending on how fast you are going. I guess when you are moving quickly, a meter seems shorter and/or a second seems longer than when you are moving more slowly? And since c is m/s, well, that explains it right? But I really have no idea. Someone rescue me from my misconceptions and more fully explain to me why this is happening!

 

[Hootenanny]

Welcome to Physics Forums.

The invariance in the speed of light was postulated by Einstein in the early 20th century. Einstein deduced (for want of a better word) this from the lack of an aether (i.e. the fact that light doesn't need a medium to propagate through). This invariance has been confirmed by countless experiments since. Indeed, now the meter is actually defined in terms of the speed of light. It is defined as the distance traveled by light in around 3x10^-8 seconds.

You are quite correct that lengths and times varying according to your motion relative to the "thing" being measured (length contraction and time dilation respectively). However, these do not explain the invariance of the speed of light, rather they are a consequence of it.

It is interesting to note, that one can derive the "equations of special relativity" (i.e. the Lorentz transformations) without requiring that the speed of light is constant. The principle of causality (i.e. that an event cannot be caused by a future event), is enough to impose a maximum speed of transmission of information. It turns out that this coincides with the speed of light.

 

[catalyst0435]

So, I can sort of grasp why it's the case, but why precisely must time and length vary as a consequence of invariant speed? Is there some thought experiment that would make this clearer to me?

Also, then, what exactly is the reason for light speed's invariance? If I understand your post correctly, it's because some complex equations impose a maximum speed of things? Is there a particular reason why it is light that has the privilege of getting to be at this maximum natural speed? And is there a way to understand why there is a natural maximum speed of transmission of information without knowing some serious mathematics, or will I just have to be content with "the equations prevent stuff from going faster"?

 

[Hootenanny]

So, I can sort of grasp why it's the case, but why precisely must time and length vary as a consequence of invariant speed? Is there some thought experiment that would make this clearer to me?

First you must understand the Lorentz transformations: http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/ltrans.html#c2

Then you can take a look at length contraction & time dilation: http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/tdil.html

Also, then, what exactly is the reason for light speed's invariance? If I understand your post correctly, it's because some complex equations impose a maximum speed of things? Is there a particular reason why it is light that has the privilege of getting to be at this maximum natural speed? And is there a way to understand why there is a natural maximum speed of transmission of information without knowing some serious mathematics, or will I just have to be content with "the equations prevent stuff from going faster"?

Such explanations a notoriously difficult and one has to strike a balance between under and over simplification. Take a look at this page: http://www.phys.unsw.edu.au/einsteinlight/jw/module3_weird_logic.htm and in particular the section entitled "Galileo's principle of relativity and Einstein's principle of special relativity".

 

[catalyst0435]

Such explanations a notoriously difficult and one has to strike a balance between under and over simplification. Take a look at this page: http://www.phys.unsw.edu.au/einsteinlight/jw/module3_weird_logic.htm and in particular the section entitled "Galileo's principle of relativity and Einstein's principle of special relativity".

This page confuses me. Under that section you've listed, it makes sense that a scientist performing an EM experiment on a stationary platform would observe light from his frame of reference to be traveling at c. It also makes sense that a scientist performing an EM experiment on a moving train would observe the light from his frame of reference to also be traveling at c. This mirrors Galilean relativity, says the article. But I'm not understanding how this explains why a scientist on a stationary platform would observe his moving colleague's light to be traveling at c, while his colleague simultaneously observes his own experiment's light to be traveling at c.

I think by the very end of the article, an explanation is attempted. It says essentially, "how can this invariance of speed of light from the same source but at different frames of reference be explained?" But it says the answer is "time dilation." Which I thought we established is actually a consequence, not an explanation, of invariance of speed.

 

[Joel_GMG]

Time dilation is indeed a consequence. SR is the grand explanation.

will I just have to be content with "the equations prevent stuff from going faster"?

I am afraid so, unless you are willing to invest a lot of time furthering your understanding.

 

[A.T.]

but why precisely must time and length vary as a consequence of invariant speed? Is there some thought experiment that would make this clearer to me?

This might help:

Also, then, what exactly is the reason for light speed's invariance?

As far SR is concerned, this is simply postulated based on experimental evidence.

 

[harrylin]

[..] So I think I'm right when I say it has something to do with space and time being perceived differently depending on how fast you are going. I guess when you are moving quickly, a meter seems shorter and/or a second seems longer than when you are moving more slowly? And since c is m/s, well, that explains it right? [..]

So, I can sort of grasp why it's the case, but why precisely must time and length vary as a consequence of invariant speed? Is there some thought experiment that would make this clearer to me?

Also, then, what exactly is the reason for light speed's invariance? If I understand your post correctly, it's because some complex equations impose a maximum speed of things? Is there a particular reason why it is light that has the privilege of getting to be at this maximum natural speed? And is there a way to understand why there is a natural maximum speed of transmission of information without knowing some serious mathematics, or will I just have to be content with "the equations prevent stuff from going faster"?

Yes, that's roughly correct, but of course equations are man-made and can't do anything (no Sim Sala Bim magic in physics!).

The same thing can be "explained" in many ways, and it depends on your thinking which one you find most satisfying. Some people like a more mathematical explanation (it's because of the invariance of c that lengths and times are measured differently at different speeds) or a more physical explanation (it's because speed affects the measurements of lengths and times in the right way that c is invariant). So what one person regards to be a consequence, another person will regard as cause - it's more of an interplay of phenomena that are consistent with each other.
Another way is to say that all matter has wave properties and at very high speed everything becomes more and more wave-like (and thus more light-like). And again another way is to say that conservation of energy and momentum makes that the speed of light is invariant. And so on.

Then what is special about "light": it just happens that light, just like radio waves, propagates at the speed limit c because it has no rest mass (as far as we know).

 

[harrylin]

This page confuses me. Under that section you've listed, it makes sense that a scientist performing an EM experiment on a stationary platform would observe light from his frame of reference to be traveling at c. It also makes sense that a scientist performing an EM experiment on a moving train would observe the light from his frame of reference to also be traveling at c. This mirrors Galilean relativity, says the article. But I'm not understanding how this explains why a scientist on a stationary platform would observe his moving colleague's light to be traveling at c, while his colleague simultaneously observes his own experiment's light to be traveling at c.

I think by the very end of the article, an explanation is attempted. It says essentially, "how can this invariance of speed of light from the same source but at different frames of reference be explained?" But it says the answer is "time dilation." Which I thought we established is actually a consequence, not an explanation, of invariance of speed.

If you account for the different synchronization of clocks as well as time dilation and length contraction, then you will find that according to the "stationary" scientist the "moving" colleague will measure the speed of a light ray in vacuum to be c relative to his platform, if he uses that platform as independent reference system.

A clarification is at its place, to reduce the mystery: if the "moving" colleague is for example in a space lab and he uses the reference system of his colleague on Earth as the "resting" reference system, then he will measure the speed of the same light ray relative to his moving lab, as measured with the Earth's standards, to be different from c. The basic rules of logic and mathematics apply as they should.

Perhaps it is useful to consider the example (just a simple example by way of illustration!) of a light clock.
en.wikipedia.org/wiki/Time_dilation#Simple_inference_of_time_dilation_due_to_relative_velocity
If every clock on the "moving" platform ticks slower by the same amount as the light clock, then an observer on that platform will measure that bouncing light ray to propagate at c.

Does that illustration help?

 

[whosapopstar?]

Please regard spaceship 2 and spaceship 3.
All this question is a asked only from the POV of spaceship 2!
Velocity x is different from velocity y.

If in some kind of transformation, spaceship 2 measures light speed detection at spaceship 3 as not being 300000kps, how come it measures by its own detectors, the speed of light as 300000kps? It is the same beam of light that spaceship 3 measures in between the detectors of spaceship 2! Shouldn't a transformation that measures anything but 300000kps for spaceship 3 as well, change the results of spaceship 2 detection itself?

 

[ghwellsjr]

Please regard spaceship 2 and spaceship 3.
All this question is a asked only from the POV of spaceship 2!
Velocity x is different from velocity y.

If in some kind of transformation, spaceship 2 measures light speed detection at spaceship 3 as not being 300000kps, how come it measures by its own detectors, the speed of light as 300000kps? It is the same beam of light that spaceship 3 measures in between the detectors of spaceship 2! Shouldn't a transformation that measures anything but 300000kps for spaceship 3 as well, change the results of spaceship 2 detection itself?

You are having four different pairs of detectors traveling at different speeds "measure" the one-way speed of the same light beam and you're wondering why scientists say that they all come up with the same "measurement" of c? Correct? Is that your question?

The answer is that it is not possible to make a measurement of the one-way speed of light until you have previously defined the one-way speed of light. In Einstein's Special Relativity, that definition is c. So when you then make the "measurement", what do you get? Obviously, c.

 

[whosapopstar?]

Please disregard Earth and the big spaceship.
I am asking regarding posts that mentioned in some contexts that under a transformation, the speed of light is not necessarily 300000kps. Is it out of context regarding my question?

 

[jtbell]

To get back to the original question, we have an FAQ about this in the FAQ section at the top of this very forum:

https://www.physicsforums.com/showthread.php?t=534862

 

[ghwellsjr]

OK, we'll just look at spaceship2 and spaceship3.

Neither one of those spaceships can measure the one-way speed of light until they independently define how long it takes for the light to traverse the distance from their individual detector1 to detector2. They can each individually measure the distance between their own two detectors using their own rulers, correct? Of course, the way you've drawn it, spaceship2 will measure a much greater distance than spaceship3, correct? But that won't matter because spaceship2 will measure a greater time interval and so the calculation when they each divide the distance by the time interval will come out the same, correct?

Now in order for each spaceship to measure how long it takes for the light to traverse from the first detector to the second detector, they have to send signals from both detectors to a timer that starts when the light gets to detector1 and stops when the light gets to detector2. They could place this timer anywhere and it shouldn't make any difference, correct? Well let's just say they put the timer next to detector2 and they will send a signal from detector1 to the timer to start the timer. So let's say they run a cable from detector 1 to the timer and let's say that the electrical signal travels in the cable at the speed of light. They also have a very short cable from detector2 to the timer to stop it.

Now they start the experiment. The flashlight comes on and some unknown time later the light hits detector1 which sends an electrical signal down the cable at the speed of light to the timer. But this signal is tracking right along with the light coming from the flashlight so the signal starts the timer at the exact same time that detector2 tells the timer to stop so the measured time interval is zero and they calculate the speed of light to be some distance divided by zero which is infinite.

Well that doesn't seem right so you rearrange the experiment and put the timer next to detector1 with the short cable and use the long cable from detector2 to bring the stop signal back to the timer. Now when you do the experiment, it will take however long the round-trip measurement of the speed of light would take for the measured time interval and so you get an answer that is presummably half what it should be.

Do you see the problem? Depending on how you set up your experiment you will get an answer for the measured one-way speed of light that is somewhere between c/2 and infinity.

There is no way to avoid or resolve this problem base purely on measurement. You might say that you should put the timer half way between the two detectors and make sure the cables are equal length but now you are merely claiming that the one-way speed of light is equal to the two-way speed of light but you haven't actually measured it.

So what Einstein proposed is that each spaceship puts the timer half way between their own two detectors and that will be the agreed upon definition of the one-way speed of light but separately for each spaceship.

You ask about transformation. What that boils down to is if spaceship2 uses his definition for the one-way speed of light to measure what happens on spaceship3, he will determine that spaceship3's measurement of the one-way speed of light is incorrect but rather is somewhere else between c/2 and infinity. The same is true for spaceship3 using his definition of the one-way speed of light to determine spaceship2's measurement.

There will never be any problem as long as you use one definition for all measurements on all spaceships. And it can be any consistent definition, even one where there is no spaceship. What we are really talking about here is the definition of a Frame of Reference which applies to all objects in the scenario. You can then use the Lorentz Transform to switch to a different definition (a different Frame of Reference) and apply that new definition to all the same spaceships or objects that you had in the first FoR.

 

[whosapopstar?]

I think that I understand your explanation.
Thanks.

 

[bobc2]

...But I've always wondered how it is possible that a person A moving at speed observes light emitted from their frame of reference at c, and then a person B sitting still (relative to person A of course) sees that same light traveling also at c. It seems like that just shouldn't happen.

...and more fully explain to me why this is happening!

catalyst0435, there is a more fundamental reason for this. However, to grasp the reason you would have to wrap your brain around a couple of significant fundamental aspects of special relativity.

First of all, you must envision the universe as 4-dimensional, populated by 4-dimensional objects. Particles of matter are filament-like objects strung out predominently along the 4th dimension, including your bundles of neurons in your brain. Your bundle of brain fibers may be strung out along your 4th dimension for multiple billions of miles.

Next, you would have to understand that people moving at different speeds, at a given instant, live in different 3-dimensional cross-sections of the 4-dimensional universe. For a person moving at relativistic speed with respect to some "rest frame", his 4th spatial dimension coordinate (X4) rotates clockwise relative to the rest 4th dimension while his normal spatial dimension (X1) rotates counter clockwise.

A photon of light is oriented at a 45-degree angle relative to the rest frame. So, for any position along the 4-dimensional photon filament, its position along the X4 axis always has the same displacement value as its displacement position along the X1 axis. Thus, it is the orientation of the X4 axis together with the symmetric orientation of the X1 axis that results in the ratio X1/X4 = 1.0 for anyone, no matter what their speed. And all observers move along their own X4 at speed c. That means that for all observers, their displacement change along their 4th dimension can be calculated using a clock: X4 = ct. The reason clocks work is because t = X4/c. Caution: That does not mean time is the 4th dimension--no more than the path along an interstate is a time dimension just because you can keep track of your progress along the highway by watching your clock on the dashboard.

You should google spacetime diagram and also "block universe" to help with the understanding of these concepts.

So, even though the discovery that "the speed of light is constant" motivated the discovery of relativity, the constant speed of light is not the reason for special relativity. Rather, the constant speed of light is a consequence of this very strange way nature has chosen to work: A 4-dimensional universe, and 3-D cross-section views of the universe are different for observers moving at different speeds--and the views are oriented so that the 45-degree orientation of a 4-D filament (photon of light) will always bisect the angle between X4 and X1, no matter what the orientation (slope--angle, etc.) of the X4 coordinate.

See sketches below illustrating the concept of the symmetric rotation of coordinates for observers moving at different speeds. The lower right box emphasizes how this results in the constant speed of light for all observers. This is really the fundamental answer to your original question.

Another very significant aspect of the rotated coordinates is that the laws of physics are the same for all coordinates rotated in this scheme of symmetric X4 and X1.

Now, the fundamental question becomes, "How did nature manage to come up with the fabric of the universe woven in such a way as this, with weaving rules that give us physics?"

 

[rbj]

Also, then, what exactly is the reason for light speed's invariance?

it's because the laws of nature are invariant for all inertial frames of reference. let's say that you and i are in our own separate space crafts, neither are being accelerated and we fly by each other at a relative speed that is very fast (say c/2). since we're not accelerated, our velocity is constant, both of us look at our own reference frame as being "stationary". we both think it's the other guy who's moving at a speed of c/2 and that our own spacecraft is not moving.

so who's right?

turns out we both are.

now if both of us have equal claim to being "at rest", there is no aether that marks a frame of "rest", then there is no reason for the laws of physics (such as Maxwell's equations) in your inertial frame of reference to be any different from the laws of physics in my inertial frame of reference. we both have the same $\epsilon_0$ and $\mu_0$, so we should have the same c.

r b-j

 

[TGlad]

But that doesn't explain why light is privileged in being one of the laws of nature that must be invariant, whereas the speed of sound and the speed of a train aren't invariant.

I would describe it that a universe needs some maximum speed of communication for it to be causal. If it didn't have a maximum speed then you would get circular dependencies and the universe wouldn't really work, at least not anything like the one we're in.
Given that there is a maximum communication speed, the Lorentz equations in relativity are the simplest ones which provide it consistently, without requiring a frame of reference.

The reason that light happens to go at this speed-of-communication is probably just because it has 0 rest mass, and is a very very simple phenomenon, like the tiniest unit of communication.

Last question, why is c the value that it is?
I don't know, well I suppose it had to be some value.

 

[bobc2]

...Last question, why is c the value that it is?
I don't know, well I suppose it had to be some value.

Because the 4-dimensional photon filiment is oriented so as to bisect the angle between X4 and X1 for all observers, and all observers move along their 4th dimension (X4) at approximately 186,000 mi/sec.

 

[jtbell]

why is c the value that it is?

From the Frequently Asked Questions section at the top of this forum:

https://www.physicsforums.com/showthread.php?t=511385

 

[bobc2]

From the Frequently Asked Questions section at the top of this forum:

https://www.physicsforums.com/showthread.php?t=511385

That is an excellent presentation of the logical development of special relativity. Thanks very much for the FAQ.

I was just trying to present a concept embraced by many special relativity physicists that nature has somehow presented a 4-dimensional spatial universe that is all there at once, the so-called "block universe" (the earliest usage I've found is with Kurt Godel in the late '40s or early '50s. In this view the whole 4-dimensional universe fabric is there with orientations of the 4-dimensional filaments (particles--material objects) woven in a way that conforms to certain rules, which result in the laws of physics. And in particular, those rules yield laws that are the same in all Lorentz frames. Further, a natural outcome is that of the special filament (photon) oriented at a 45-degree angle in all rest frames (or bisects the angle between X4 and X1 for all observers).

Subjectively, I do not like the picture at all--because of some bizaare implications. But, I don't know of any other picture that has been advanced, which can resolve the problems manifest with the special relativity theory.

 

[harrylin]

[..] I was just trying to present a concept embraced by many special relativity physicists [..] the so-called "block universe" [..]
Subjectively, I do not like the picture at all--because of some bizaare implications. But, I don't know of any other picture that can resolve the problems manifest with the special relativity theory.

I'm flabbergasted, you presented it with so much conviction that I thought that you like it.
Don't you know the Lorentzian picture?

 

[bobc2]

I'm flabbergasted, you presented it with so much conviction that I thought that you like it.
Don't you know the Lorentzian picture?

When I try to convey the block universe concept to someone, I try to give it my best shot and not include personal bias. But, it remains a deep mystery for me personally. Many physicists who discuss it feel that this model is directly manifest by the Lorentz frames. I have tried and tried for many years to find material that effectively counters it, without success. I've tried to think up scenarios without success. I can't counter the argument for the block universe, but at the same time I just don't see how we can reconcile it when you consider the bizarre implications. Take your choice between zombies or a consciousness that is present over the entire neuronal world line. Einstein actually seemed to favor the block universe with consciousness(es) present over the entire world line (his letter to Besso's wife upon the death of his close friend and colleague, Besso).

harrylin, I've been impressed with the quality of your posts and consider you one of the reliable physicists here. So, I'm intrigued by your comment about the "Lorentzian picture." I have always felt that the block universe model is the most direct picture of the Lorentz frames that could be envisioned. Thanks for your comments.

You may have noticed that I've posted comments based on the block universe from time to time. I always hope for someone who can post counters to the model, but I have decided that the subject is not too interesting to forum members (or perhaps they feel it is too close to the edge of philosophy--or maybe way over the line and far too deeply into philosophy).

 

[rbj]

it's because the laws of nature are invariant for all inertial frames of reference.
...
now if both of us have equal claim to being "at rest", there is no aether that marks a frame of "rest", then there is no reason for the laws of physics (such as Maxwell's equations) in your inertial frame of reference to be any different from the laws of physics in my inertial frame of reference. we both have the same $\epsilon_0$ and $\mu_0$, so we should have the same c.

But that doesn't explain why light is privileged in being one of the laws of nature that must be invariant, whereas the speed of sound and the speed of a train aren't invariant.

it's not just the speed of light (which is the speed of the EM interaction), it's the speed of all of the "instantaneous" interactions. it's a property of space and time, not of any particular interaction; EM, gravity, strong.

I would describe it that a universe needs some maximum speed of communication for it to be causal.

i think you got the cause and effect sort of backwards. the speed of communication is limited to c because the fundamental interactions are not really instantaneous and have their effect speed limited.

imagine you and i are standing some distance apart and facing each other. you're holding some big negative charge and I'm holding a big positive charge. we are both restricting movement of our charges to a plane that is perpendicular to the line connecting us. i move my charge up and you allow your charge to follow it up. i move my charge to my right and yours follows to your left (since we're facing each other). my charge is taking the role of transmitting antenna and your charge is taking the role of a receiving antenna.

if i move my charge back-and-forth a million times per second, you could tune that in on an AM radio. if i do it 100 million times a second, you can tune it in on FM radio. if i do it 600 trillion times a second, it would look like green light to you. i think there are receptors in our retina with charge that resonates to around this frequency.

so we can communicate to each other with these charges held in our hands. i can send Morse code to you.

now, imagine instead, that we are big as gods and that we're both holding planets instead of charges. we could also communicate with each other by perturbing the planet we hold (and feel for a perturbation resulting from motion of the other). but whether the force is EM or gravity, when viewed by a third observer who is equal distant from you and me, this observer sees the perturbation of my charge or planet, and then will see your charge respond at a later time (and, i know, gravity isn't a force as such but a disturbance in the curvature of spacetime from a moving mass has the same finite speed of propagation quality). and the time interval between cause and effect is proportional to the length between the two and that speed is a property of spacetime (not so much of which interaction it is).

and, if every inertial frame of reference has equal claim to being stationary and each has the same laws of physics, this means both qualitative and quantitative, then each inertial frame of reference should measure that property of spacetime identically. there is no reason for one inertial observer to measure $\epsilon_0$ or $\mu_0$ or c or G or $\hbar$ differently than another inertial observer, even if the other one is in a different inertial frame and moving relative to the first.

it's because the fundamental interactions are not really instantaneous with effect limited to c, that communication is not instantaneous (and is limited to c). we communicate by poking each other somehow.

Last question, why is c the value that it is?
I don't know, well I suppose it had to be some value.

it has to be real, positive, and finite. other than that, it doesn't really matter. it really becomes just a matter of units and Nature doesn't give a damn about what units humans (or the aliens on the planet Zog) choose to use.

the only pertinent or "operational" questions about quantitative values are those about dimensionless values, like that of the fine-structure constant.

the speed of light (which is the speed of the other instantaneous interactions) will always be 1 Planck length per Planck time. if we answer (with dimensionless numbers) questions like why are there about 10³⁵ Planck lengths in a meter and why there are about 10⁴⁴ Planck times in a second, then we'll have an idea why c is about 10⁹ meters per second.

 

[bobc2]

...it has to be real, positive, and finite. other than that, it doesn't really matter. it really becomes just a matter of units and Nature doesn't give a damn about what units humans (or the aliens on the planet Zog) choose to use.

Thanks for the nice post, rbj. Also, from the block universe model the speed of light is special and has the value that it does for three reasons:

1) Nature presents her physics to us using a continuous sequence of 3-D Lorentz frames (cross-sections of a 4-D universe--they are different for differently moving observers).

2) The world line of the photon bisects the angle between the X4 and X1 axis for all observers.

3) All observers move along their respective world lines (respective X4 dimensions) at approximately 186,000 mi/sec. The unitless speed would be 1.0 since that's the distance traveled along X1 divided by the corresponding distance traveled along X4 as depicted in the sketch below.

 

[yoron]

So, given that I know nothing about physics, this is probably going to sound like a stupid question. But I've always wondered how it is possible that a person A moving at speed observes light emitted from their frame of reference at c, and then a person B sitting still (relative to person A of course) sees that same light traveling also at c. It seems like that just shouldn't happen.

So I think I'm right when I say it has something to do with space and time being perceived differently depending on how fast you are going.

"The Michelson-Morley experiment (MMX) was intended to measure the velocity of the Earth relative to the “lumeniferous aether” which was at the time presumed to carry electromagnetic phenomena. The failure of it and the other early experiments to actually observe the Earth's motion through the aether became significant in promoting the acceptance of Einstein's theory of Special Relativity, as it was appreciated from early on that Einstein's approach (via symmetry) was more elegant and parsimonious of assumptions than were other approaches (e.g. those of Maxwell, Hertz, Stokes, Fresnel, Lorentz, Ritz, and Abraham)."

The idea was that as Earth is seen to move with the solarsystem as well as around the sun you would find a different speed of light relative what motion you measured that speed in. That as the aether was thought of as some absolute 'frame of reference' which all bodies moved relative. But the light was measured the same speed everywhere (approximately, as good as could be done by that time.). And that was a headache because even if you assumed no aether, you still had to explain why that light speed didn't change with Earths motion, against or with it.

Michelson–Morley experiment.

To make sense of that, and later even more refined experiments, came two assumptions, that light was a 'constant' and that the reason why it constantly gave the same speed ignoring the motion of the object it rested on, like earth, must be that something happened with the measuring equipment, a Lorentz contraction. Learning about this Poincaré suggested that this local time, as indicated by clocks 'moving in the aether', could be synchronized under the assumption of a constant light speed.

It is said that Einstein based his assumptions not so much on this, but instead on Maxwell's equations, that also gave radiation a absolute speed. But he must have known of those results and ideas too. And as far I know all experiments done so far has validated his definitions. The thing with it is that you start from those experimental results, and then you keep on building. That was what Einstein did when he later presented GR, in where 'gravity' became equivalent to a uniform constant acceleration.

As all 'clocks' differ with their motion, as defined relative some observer, he now could bind how clocks differed in a motion to gravity. And all of this came from one postulate, as I see it. That lights speed in a vacuum was a constant 'c', no matter from where it was sent, if in motion relative the observer or not. It's a whole 'frame work', and I all to often put it all together :) but it all works out experimentally, and what it comes down to for me is 'c'.

To assume differently you need to give a definition to those experiments already done, where you prove how light can be a variable at the same time as it always give you a constant 'c' when measured locally as I see it. And that one should be tricky, especially as gravity and 'clocks' works out beautifully in Einsteins definitions, and also fits very well with Special relativity following his definitions.

experimental basis of Special Relativity.

 

[harrylin]

When I try to convey the block universe concept to someone, I try to give it my best shot and not include personal bias. But, it remains a deep mystery for me personally. Many physicists who discuss it feel that this model is directly manifest by the Lorentz frames. I have tried and tried for many years to find material that effectively counters it, without success. I've tried to think up scenarios without success. I can't counter the argument for the block universe, but at the same time I just don't see how we can reconcile it when you consider the bizarre implications. Take your choice between zombies or a consciousness that is present over the entire neuronal world line. Einstein actually seemed to favor the block universe with consciousness(es) present over the entire world line (his letter to Besso's wife upon the death of his close friend and colleague, Besso).

harrylin, I've been impressed with the quality of your posts and consider you one of the reliable physicists here. So, I'm intrigued by your comment about the "Lorentzian picture." I have always felt that the block universe model is the most direct picture of the Lorentz frames that could be envisioned. Thanks for your comments.

You may have noticed that I've posted comments based on the block universe from time to time. I always hope for someone who can post counters to the model, but I have decided that the subject is not too interesting to forum members (or perhaps they feel it is too close to the edge of philosophy--or maybe way over the line and far too deeply into philosophy).

Yes, it's very close to philosophy - but then, "why" is very close to philosophy, and physics started out as a branch of philosophy ("PhD" isn't an error). Anyway, since you presented the 4D interpretation, it's only fair to also mention the Lorentzian 1+3D interpretation.

That interpretation treats Minkowski space-time as a mathematical model or method for calculating physical events (that was at least around 1920 also Einstein's position; only he was rather ambiguous about his interpretation). There is no reason to assume that a calculation method should itself have a 1-to-1 correspondence to physical reality; in general that is also not the case in physics.

If you are familiar with Newton's absolutist interpretation of classical relativity, then SR is easy to understand as a refinement, or correction, of Newtonian mechanics. Time and space are still regarded as concepts that refer to physically very different things. The relativistic effects are explained as effects from absolute motion, just as Newton explained the effects from rotation as due to absolute motion.* Absolute motion has absolute effects, but those effects don't allow us to detect an "absolute frame", even if there is one. That this should not be regarded as a "conspiracy" can be made plausible by for example arguing that everything, even matter, is made up of fields and radiation, or that the conservation laws demand this.

So, it's a trade-off: the block universe model suggests a 1-to-1 correspondence to what physically "really" happens, but with rather bizarre implications; while the absolute motion model suggests that we may retain some of our intuition about "space" and "time", but the downside is that the beautiful symmetry is only in the phenomena.

Harald

PS: The answer to why c has the value that it has, is from that point of view that it is the propagation constant of space.

* This was elaborated in Langevin's rather long-winding expose on The evolution of space and time (1911):
http://en.wikisource.org/wiki/The_Evolution_of_Space_and_Time

 

[bobc2]

Yes, it's very close to philosophy - but then, "why" is very close to philosophy, and physics started out as a branch of philosophy ("PhD" isn't an error). Anyway, since you presented the 4D interpretation, it's only fair to also mention the Lorentzian 1+3D interpretation.

That interpretation treats Minkowski space-time as a mathematical model or method for calculating physical events (that was at least around 1920 also Einstein's position; only he was rather ambiguous about his interpretation). There is no reason to assume that a calculation method should itself have a 1-to-1 correspondence to physical reality; in general that is also not the case in physics.

If you are familiar with Newton's absolutist interpretation of classical relativity, then SR is easy to understand as a refinement, or correction, of Newtonian mechanics. Time and space are still regarded as concepts that refer to physically very different things. The relativistic effects are explained as effects from absolute motion, just as Newton explained the effects from rotation as due to absolute motion.* Absolute motion has absolute effects, but those effects don't allow us to detect an "absolute frame", even if there is one. That this should not be regarded as a "conspiracy" can be made plausible by for example arguing that everything, even matter, is made up of fields and radiation, or that the conservation laws demand this.

So, it's a trade-off: the block universe model suggests a 1-to-1 correspondence to what physically "really" happens, but with rather bizarre implications; while the absolute motion model suggests that we may retain some of our intuition about "space" and "time", but the downside is that the beautiful symmetry is only in the phenomena.

Harald

PS: The answer to why c has the value that it has, is from that point of view that it is the propagation constant of space.

* This was elaborated in Langevin's rather long-winding expose on The evolution of space and time (1911):
http://en.wikisource.org/wiki/The_Evolution_of_Space_and_Time

Great response. Thanks a lot.

 

[rbj]

The answer to why c has the value that it has, is from that point of view that it is the propagation constant of space.

and that value can be whatever you want, as long as it's real, positive, and finite. otherwise it doesn't matter and the question is sort of meaningless.

 

[bobc2]

and that value can be whatever you want, as long as it's real, positive, and finite. otherwise it doesn't matter and the question is sort of meaningless.

I think I know what you mean, rbj. But, could you clarify whether you are referring mainly to

a) The particular numerical value and units associated with the speed (186,000 mi/sec, ... etc.) or,

b) The speed of light is the same for all observers.

 

[ZealScience]

I think it is based on the basic assumption of relativity, which is the equivalence of natural laws of physics are the same in all inertial reference frames. Then light must have the same speed in all frames, since speed of light is derived from Maxwell's equations which is a valid law of nature. And this principle is quite tenable, as there is really no distinction between different inertial reference frames.

 

[Petyab]

How do you put on your shoes? Doesn't the light between your hands and your shoes change wavelength and frequency. v= lambda*frequency. HMMMMM It even says in some physics books blue light travels faster than red light. But don't be sure about that either. DERRRRRRR AT ALL LIGHT IS MASSLESS AND MOVES AT V=C.

 

[harrylin]

Great response. Thanks a lot.

You're welcome.

I forgot to mention that Langevin's "absolutist" argument starts at p.47, with the "twin" scenario as illustration.

And perhaps an elaboration is at its place:

- The answer to why c has the value that it has, is from that point of view that it is the propagation constant of space;
- An intuitive answer to why light speed is constant in all reference frames, is from that point of view that apparently everything, even matter, is made up of fields and radiation, so that everything is affected the same;
- The invariance of the speed of light between reference frames is imposed by the conservation laws (indeed, the modern PoR has even been derived based on those laws). That argument is independent of the physical model that one uses.

Harald

 

[bobc2]

You're welcome.

I forgot to mention that Langevin's "absolutist" argument starts at p.47, with the "twin" scenario as illustration.

And perhaps an elaboration is at its place:

- The answer to why c has the value that it has, is from that point of view that it is the propagation constant of space;
- An intuitive answer to why light speed is constant in all reference frames, is from that point of view that apparently everything, even matter, is made up of fields and radiation, so that everything is affected the same;
- The invariance of the speed of light between reference frames is imposed by the conservation laws (indeed, the modern PoR has even been derived based on those laws). That argument is independent of the physical model that one uses.

Harald

Thanks again for the comments and reference. The reference you gave may not have been the one you were remembering. There are only 16 pages in this reference. Although he did use an illustration analogous to the twin scenario, he did not actually present the twin paradox in this paper.

Also, there was no discussion of the interpretation of special relativity. From what you say, I assume he takes the position of logical positivists, wherein physics should no longer be concerned with external objective reality--or models of an assumed external material world. This is in contrast to Einstein's comment,

"The belief in an external world independent of the perceiving subject is the basis of all natural science." (from "Clerk Maxwell's Influence on the Evolution of the Idea of Physical Reality", from Einstein's "The World As I See It").

Also, Einstein seems to imply from some of his writings and lectures that there is a three-fold distinction between an "external world", the observer's "perception of that external world", and "our notions of it."

In another comment in an Address at Columbia University, "Behind the tireless efforts of the investigator there lurks a stronger, more mysterious drive: it is existence and reality that one wishes to comprehend."

The main point I would make in this context (in conjunction with our discussion of the "Block Universe") is that physicists such as Langevin and others, who reject the reality of a physical 4-dimensional universe, do it without proposing any other concept of an external physical world. And the reason is simple: Special relativity has no description of an external physical world other than a 4-dimensional space in which observers have different cross-section views of that 4-dimensional world. And in any case they do not embrace any external world (that is, an external world as implied in Einstein's comments above).

Einstein himself was careful to not force such a model on his audience, as can be seen with his comment that followed immediately after the above comment in his Columbia University address: "But one shrinks from the use of such words, for one soon gets into difficulties when one has to explain what is really meant by 'reality' and by 'comprehend' in such a general statement." Of course Einstein was well read and schooled in philosophical writings and understood quite well the problem of reality.

But my primary point here is, again, that a physicist chooses one of three stances: 1) physics pursues the comprehension of an external physical world (which from special relativity directly implies a a 4-dimensional world), 2) the logical positivist or operational view, in which physics should only be interested in predicting the outcome of experiments, making measurements and advancing mathematical models that agree with experiments (the models are mathematically symbolic only and have no implications about an actual external physical reality), or finally 3) One may simply take the stance of no committment to either 1) or 2).

 

[PhilDSP]

it's not just the speed of light (which is the speed of the EM interaction), it's the speed of all of the "instantaneous" interactions. it's a property of space and time, not of any particular interaction; EM, gravity, strong.

i think you got the cause and effect sort of backwards. the speed of communication is limited to c because the fundamental interactions are not really instantaneous and have their effect speed limited.

It seems TGLAD was evaluating the universe from a designer's perspective - which is intriguing and probably educational but potentially deceptive (if such a design isn't constrained by human imagination).

it has to be real, positive, and finite. other than that, it doesn't really matter. it really becomes just a matter of units and Nature doesn't give a damn about what units humans (or the aliens on the planet Zog) choose to use.

Curious. What is the motivation for that restriction? The mathematics behind Quantum Mechanics pretty much tells us that when dealing with more than one dimension, energy translation must be considered with regard to complex values, even hyper-complex values when more than two dimensions are involved. Though I can understand that SR is in some sense limited in application to a single spatial dimension (a linear path between emitter and absorber).