Why is the speed of light independent of a frame of reference?

[drudkh]

I understand the Michelson–Morley experiment and its result; but what I don't know yet is the REASON.
Example:
A torch in free space is moving at a velocity [v] w.r.t me. Considering the material nature of light, shouldn't the speed of photons emitted from the torch be [v+c] w.r.t ME?
According to the experiment, it's not so.
I know how relativistic velocities are formulated (Lorentz transformation, Einstein's Addition, etc.). But all these calculations are based on the accepted norm that light speed in invariant. WHY? What's the scientific explanation of this (if any)? Is it still a mystery?

I've seen other posts regarding the speed of light, but couldn't go through them all. So, apologies if this topic already exists.

 

[Ich]

"Why" is hard to answer, if you expect something tangible. Like the name of the responsible god.
One can prove from relatively basic axioms/postulates that there must be an invariant velocity, which is the maximum velocity also. This velocity could be infinite, then you'd get Newtonian mechanics. But it happens to be finite. Don't know why.
Light simply goes as fast as possible, as it has no mass. That's why the speed of light is the invariant speed.

 

[jtbell]

In physics, the answers to all "why?" questions have more "why?" questions lurking behind them.

For example, one answer to "why is the speed of light invariant?" is "because spacetime has Lorentz symmetry." But that simply begs the question, "why does spacetime have Lorentz symmetry?" Nobody knows, at least not yet. Maybe someday someone will come up with an answer "X" that becomes generally accepted. But then everybody will ask, "But why is X?"

 

[stever]

Doesn't somebody have some ideas about how light stays at c in different frames? Or is it just a given?

 

[LostConjugate]

The speed c, distance over time, is the tangent of the furthest angle from both the space and time orthogonal basis vectors. The 45 degree angle.

Since these basis vectors transform in a way that they collapse in on each other, this tangent never changes in any set of coordinates (reference frame).

See attached picture.

 

[CosmicVoyager]

The speed c, distance over time, is the tangent of the furthest angle from both the space and time orthogonal basis vectors. The 45 degree angle.

Since these basis vectors transform in a way that they collapse in on each other, this tangent never changes in any set of coordinates (reference frame).

See attached picture.

Greetings,

It seems like you have an explanation, but I don't quite understand it

Could you elaborate?

Thanks

 

[LostConjugate]

Speed is distance over time and distance and time are not constant.

Time for example, is a measurement of the motion of some reference object. Some people use pendulums, the sun, digital clocks, computers, etc. All these objects are one and the same, another object in motion.

This means time is once again a measurement of distance over time itself. So distance over distance over time means time is relative based on your reference object's reference frame (relative velocity).

So when you accelerate from one reference frame to the other (increase your relative velocity) the measurement of the speed of light, distance over relative time, will be exactly the same.

Time is something defined by humans, not a physical thing, and it is simply the comparison of objects that move. Because of this it is relative, and because there is nothing to use as a reference object that moves faster than the speed of light, the relative time will never allow any reference object (clock) to measure light moving any faster than c from any reference frame.

The number c itself has no special meaning, it is just a conversion factor. For another alien race the number c would be a different number based on their decision of units for measuring space and time. In proper units c = 1 and one unit of space is roughly 300 million meters.

 

[drudkh]

So this still leaves the reason behind the invariance of c, a mystery. I guess it's better to leave it that way.. for now.
Another question: Is there any solid experimental evidence to prove that mass increases with velocity or the phenomenon of time dilation?

 

[jtbell]

Check out the sticky thread at the top of this forum: "FAQ: Experimental Basis of Special Relativity."

 

[LostConjugate]

It is not a mystery, c is exactly the tangent of the 45 degree angle between the space and time basis vectors (which is always 1 y/x), any transformation will leave this tangent the same. After completing a special relativity course (which can be done in a 1 day cram) it can be fully understood. The big factor is that speed is a derivative of space.

 

[nismaratwork]

There needs to be a sticky explaining the difference between "why" and "how" as questions addressed to physics, and what physics actually attempts to answer.

 

[LostConjugate]

Questions like why is c the number that it is,
and why do charges accelerate each other,
and why is the mass of a proton the number that it is,
might not have an exact answer.

but with the question of why is the speed of light the same in all reference frames, it can be answered and has been since 1905.

 

[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 (Rindler 1979).

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 a system such as Rindler's, 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 the existence of a universal speed c is a property of spacetime that must exist because spacetime has certain properties (basically, it has some symmetries, and it doesn't have universal simultaneity).

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

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

 

[yuiop]

...
A torch in free space is moving at a velocity [v] w.r.t me. Considering the material nature of light, shouldn't the speed of photons emitted from the torch be [v+c] w.r.t ME?
...

If your question is about what is different between relativity theory and Newtonian/Galilean physics that allows something (not traveling with infinite velocity) to be measured as having the same velocity in any reference frame, then I can perhaps offer a physically satisfying answer in terms of LET. If on the other hand your question is about why is the universe the way it is, then one possible answer is the anthropic principle.

 

[drudkh]

c is exactly the tangent of the 45 degree angle between the space and time basis vectors

But this is again experimentally shown isn't it? Is there any available derivation?

 

[drudkh]

@bcrowell

empirical upper bound on the mass of the photon is extremely small

But this again is experimental right?

And I'm sorry but my knowledge in this field is yet shallow, so you lost me on the universal simultaneity thingy.

Also, what I inferred from you post is that a particle cannot be accelerated to light speed with such high a force due to some unexplained barriers of spacetime which are it's property.
But consider an universe in which there exists only one body and even if it's traveling at a velocity larger than c, we wouldn't know because there's nothing else to compare it to. So we can assume as well that the body is at rest, so the phenomenon is feasible. But in reality it's not. So does that mean that a body can not be accelerated to light speed but it can well be traveling at light speed (or maybe a large fraction of it) by default?

 

[matheinste]

But this is again experimentally shown isn't it? Is there any available derivation?

It is just a consequence of our choice of axes. We usually choose the time axis to be perpendicular to the space axis with two dimensions supressed. The space axis units are chosen to be one meter and the time axis units are chosen to be one light meter. With this choice of axes the light path referred to these axes bisects the angle between these axes, that is forty five degrees to both axes.

Whatever the speed of light happened to be we could always set up the axes so that the light path makes such an angle with the axes.

Matheinste.

 

[stevmg]

As a simpleton I would state that all we know is that c is constant, is measurable by various means and, so far, has held up under all observations (now, I am referring to 30,000,000 m/sec in a vacuum - light does slow down when it passes though a medium such as water or oil.)

It seems to logically fit into other observations about the so-called "tangent" as described above as well other "symmetries" of which I have no clue how they were derived, but actually, t is an observed "assumption."

To wit - geometry as taught in high school is Euclidean, and all the assumptions or postulates were just that - the most basic elements of this geometry so far never disproven. Along comes non-Euclidean geometry which forces a change in basics (for example, parallel lines do meet in a finite distance. This helps in the understanding of General Relativity, which Einstein and Hilbert "derived” (and Eddington demonstrated in 1919 and the Aussies did in 1922-3.)

Look at the parable of the two travelers who live in a two-dimensional world but that two dimensions is the surface of a sphere. As they move "north" they approach each other as if there were a "force" of attraction. Their unstoppable move "north" is the result of the unstoppable and ever persistent advance of time which extends the world lines. The worldlines should be straight in two dimensions but because the two dimensions are on a sphere, the world lines bend towards each other which means acceleration or force.

Likewise with c - the speed of light. It is possible that we are a speck on some greater system in which c "holds its own" at 30M m/sec. It is possible that this could change, too. After all, Newton was right with the world as he knew it. His speed, time, distance formulas, to use the British phrase, were "spot on." But the observable world as he knew it and as common sense tells us was merely a speck on a greater "reality" which Maxwell, Lorentz , Einstein, Hilbert and Eddington uncovered.

 

[TCS]

I understand the Michelson–Morley experiment and its result; but what I don't know yet is the REASON.
Example:
A torch in free space is moving at a velocity [v] w.r.t me. Considering the material nature of light, shouldn't the speed of photons emitted from the torch be [v+c] w.r.t ME?
According to the experiment, it's not so.
I know how relativistic velocities are formulated (Lorentz transformation, Einstein's Addition, etc.). But all these calculations are based on the accepted norm that light speed in invariant. WHY? What's the scientific explanation of this (if any)? Is it still a mystery?

I've seen other posts regarding the speed of light, but couldn't go through them all. So, apologies if this topic already exists.

Because all reference frames are the same. If the speed of light varied according to reference frame then there would have to be some ultimate reference frame.

 

[drudkh]

Newton was right with the world as he knew it. His speed, time, distance formulas, to use the British phrase, were "spot on." But the observable world as he knew it and as common sense tells us was merely a speck on a greater "reality" which Maxwell, Lorentz , Einstein, Hilbert and Eddington uncovered.

I know what you mean. I understand the concept of higher dimensions and other realms of our universe (beyond our perception), but honestly, seems I need to study deeper to get it clear.

Because all reference frames are the same. If the speed of light varied according to reference frame then there would have to be some ultimate reference frame.

Well light has to originate somewhere, what if I let that 'somewhere' be the reference object (say, the sun).

@bcrowell What I inferred from you post is that a particle cannot be accelerated to light speed with such high a force due to some unexplained barriers of spacetime which are it's property.
But consider an universe in which there exists only one body and even if it's traveling at a velocity larger than c, we wouldn't know because there's nothing else to compare it to. So we can assume as well that the body is at rest, so the phenomenon is feasible. But in reality it's not. So does that mean that a body can not be accelerated to light speed but it can well be traveling at light speed (or maybe a large fraction of it) by default?

 

[ssbbkk]

I guess that the all frames of relativity is the only one multi-einsteinself frame.
so that relativity is einstein one man show so that c is always c.
I think so.

 

[Austin0]

Both statements work for predictions, but are unlikely to be the whole story. Spacetime is a model which conforms well to reality (usually), and the QM vacuum is the same for "empty" space. These are just current theories which are useful for application and furthering the quest for better and more descriptive theories. I don't think that people accept either of the premises you mention, they accept the theories which describe the observed behavior, and wait or search for better theories.

Isn't this somewhat a semantic distinction? Spacetime (geometric), QM quantum field and spacetime "fabric" all describe a space that has qualities.
Is not just nothingness, pure vacuum.

Einstein said roughly,,, Space has no existence outside of matter.

This can be interpreted as; space is brought into existence as an emergent consequence of matter. It could still have actual qualities as a background for the behavior and motion of matter and EM propagation.

I agree that space must be filled with waves, EM , Gravity (either as propagated "gravitons " or as a geometric reality) etc. That this alone constitutes a background of sorts , with possible consequences , besides what ever emergant or intrinsic properties it might possibly have.

 

[nismaratwork]

Isn't this somewhat a semantic distinction? Spacetime (geometric), QM quantum field and spacetime "fabric" all describe a space that has qualities.
Is not just nothingness, pure vacuum.

Einstein said roughly,,, Space has no existence outside of matter.

This can be interpreted as; space is brought into existence as an emergent consequence of matter. It could still have actual qualities as a background for the behavior and motion of matter and EM propagation.

I agree that space must be filled with waves, EM , Gravity (either as propagated "gravitons " or as a geometric reality) etc. That this alone constitutes a background of sorts , with possible consequences , besides what ever emergant or intrinsic properties it might possibly have.

This sounds a bit like an argument for an Ether...

 

[TCS]

This sounds a bit like an argument for an Ether...

I'm not against an "ether" as long as you are really careful about how you define it. If you define the ether as having a zero velocity for all observers and if you say that all particles are distributed throughtout the entire universe and the thickness of the ether is the sum of the the thicknesses of all particles at the spot, then I would accept the existence of a type of ether. Although such an ether wouldn't have the qualites of the 19th century ether and you couldn't think of it as the oscilatory medium of em waves in the 19th century sense.

However, you would want to think of the ether as expanding with time so that the particles would have a Minkowski (hyberbolic in time) type metric and the expansion as being constrained by the thickness of the ether so that the particles would have a GR type metric (the local degree of hyberbolicity would be determined by the energy density)..

 

[matheinste]

I'm not against an "ether" as long as you are really careful about how you define it. If you define the ether as having a zero velocity for all observers and if you say that all particles are distributed throughtout the entire universe and the thickness of the ether is the sum of the the thicknesses of all particles at the spot, then I would accept the existence of a type of ether. Although such an ether wouldn't have the qualites of the 19th century ether and you couldn't think of it as the oscilatory medium of em waves in the 19th century sense.

However, you would want to think of the ether as expanding with time so that the particles would have a Minkowski (hyberbolic in time) type metric and the expansion as being constrained by the thickness of the ether so that the particles would have a GR type metric (the degree of hyberbolicity would be determined by the energy density)..

It must also be undetectable.

Matheinste.

 

[nismaratwork]

It must also be undetectable.

Matheinste.

Yeah, and that is a bit of rub for any theory; makes the search for magnetic monoples and stranglets look downright hopeful.

 

[TCS]

It is kind of like saying a bed sheet isn't detectable. Even though we only interact with threads, we could infer the existence of a bed sheet from the behaviour of the threads. Likewise, you could say that the metric that defines the distance between vectors in space time implies the existence of the "ether". Although, there may be other explanations for those metrics.

 

[stevmg]

I just don't believe in empty space. I think that there is energy density everywhere and that the wavelength of the photon is dependant upon the enrgy density of the space it is passing through.

I think that is true - that there is "energy density" but that would not stop packets of energy either.

Who the hell knows?

 

[Dale]

If you define the ether as having a zero velocity for all observers

Then it is not related to the usual aether concept in any fashion other than its name. We already get into enough problems re-using words, e.g. "spin" and "particle" and even "mass". Labeling what you described "aether" is a bad idea.

 

[stevmg]

Then it is not related to the usual aether concept in any fashion other than its name.

DaleSpam -

If there were energy density, and if the universe were expanding, wouldn't this "density" "bleed off" to zero - eventually?

Or, if the universe expands to a point then recontracts, I guess that would be an alternative scenario.

 

[TCS]

DaleSpam -

If there were energy density, and if the universe were expanding, wouldn't this "density" "bleed off" to zero - eventually?

Or, if the universe expands to a point then recontracts, I guess that would be an alternative scenario.

I'm thinking that if the energy density gradient is high enough or you have enough energy with respect to the local neighborhood, like in a black hole, then instead of just constraining the expansion (hyberbolicity) of space, the space becomes eliptical so that the energy crunches in on itself until it reaches a point of maximum focus where the energy is redistributed into a flat distribution(once the energy or all of the vector segments are contracted to minimum size or a point then there is no longer an energy density gradieint so there would no or very little gravity) and then starts to spread and form a new fabric with it's own time and that the universe allows for infnite levels of rescaling, but that it just speculation.

 

[stevmg]

In classical physics, if energy level is uniform, though NOT zero, no work can be done. It takes a difference in energy level to get work done:

Sadi Carnot, 1828

Q = (T₂ - T₁)/T₂

where Q = efficiency, T₂ = higher temperature, T₁ = lower temperature.

Of course this is thermodynamics but I think the concept is more universal.

By the way, to those of you who know me... I taught Sadi Carnot

 

[drudkh]

I agree with TCS and Austin on one point. I think it could very well be a medium which we 'perceive' as space. It doesn't have to have a flow as was the basis of the M-M experiment. And matter could be the perceivable 'complement' of space.

 

[drudkh]

The speed of sound through water doesn't depend upon the speed of the boat emitting it or the submarine listening to it. It does depend upon the relative velocity of the submarine to the water...

Ok so, two situations:
1] Both the boat and the submarine is at rest
2] The boat is moving in the -x direction and the submarine is moving in the +x direction.

Wouldn't the speed of sound be different in the two cases?

 

[Austin0]

Ok so, two situations:
1] Both the boat and the submarine is at rest
2] The boat is moving in the -x direction and the submarine is moving in the +x direction.

Wouldn't the speed of sound be different in the two cases?

Between 1) and 2) I would say yes

Case 2) Boat and submarine could disagree on measured speed.

 

[stevmg]

Then it is not related to the usual aether concept in any fashion other than its name. We already get into enough problems re-using words, e.g. "spin" and "particle" and even "mass". Labeling what you described "aether" is a bad idea.

DaleSpam -

We have a criss-crossed chain of posts here which has messed up this "blog."

It Just happened 1255 EST or 1755 GMT.

Stevmg

 

[rod_worth]

Back when Michelson and Morley conducted their experiments it wasn't 'completely' known whether or not the speed of light was truly constant, or if it could vary a little, which is what their experiment would have told them if this was true. Their 'null' result needed an explanation. 'Part' of the reason Einstein became so famous was that he was bold enough to propose that the speed of light was a constant; special relativity you could kind of say 'fell out of' that assumption, if you will (I say that very loosely). As more and more experiments were conducted to test special relativity, the experiments all pointed towards Einstein's bold proposal as being correct. Today, the speed of light is considered a well respected 'constant of nature'. This means 'c is c' NOT 'v + c'.

I personally like to reserve the 'why' questions for the philosophers; physicists are 'generally' more concerned with 'how' and 'what'. If you want to know 'why' the speed of light is what it is then I recommend consulting __________ (insert religious figure here). There is a saying, and I can't remember where I heard/read it that goes, 'nature is what it is, does what it does, and who are we to say otherwise!?'

 

[Sybren]

there's no reason, it's an assumption

 

[TCS]

there's no reason, it's an assumption

If a photon always travels the entire path length from it's creation to it's observation within the receiving particle, then all receiving particles can have the same C because the recceiving particle is the medium of oscilation.

 

[Quantum-lept]

what about red/blue shift?

 

[TCS]

what about red/blue shift?

Since the particle extends through space, it will be expanding with time and that expansion will provide the red shift. Also, there will be relative motion during the brief period of interaction.

If you think of an interaction as a shear in the time energy dimension, then the situation is analogous to an Earth quake. The energy of the interaction travels though each layer at the layers own speed regardless of the properties of the other stratigraphic layers, although pressure impossed by one layer can affect another. Even if a river is flowing over a stratigraphic layer, it won't change the Earth quake wave speed through the layer. However, if the layer has been bent by stratigraphic forces, some of it could be bent deep into the Earth and very it's properties through that region. Accordingly, if particles could were like stratigraphic layers, then they would all see light at C as a constant.

The ocean would be an even better example. Each particle is like a separte layer of ocean flowing at it's own speed in it's own direction in an ocean that is heating up.

Although, in reality, it couldn't be quite as simple as my anologies. It would have to be the quantum mechanical possibilities that travel through the particle.

 

[stevmg]

I understand the Michelson–Morley experiment and its result; but what I don't know yet is the REASON.
Example:
A torch in free space is moving at a velocity [v] w.r.t me. Considering the material nature of light, shouldn't the speed of photons emitted from the torch be [v+c] w.r.t ME?
According to the experiment, it's not so.
I know how relativistic velocities are formulated (Lorentz transformation, Einstein's Addition, etc.). But all these calculations are based on the accepted norm that light speed in invariant. WHY? What's the scientific explanation of this (if any)? Is it still a mystery?

I've seen other posts regarding the speed of light, but couldn't go through them all. So, apologies if this topic already exists.

I do believe that when drudkh asks "why" he doesn't mean the philosphical "why." I believe he is referring to the nuts and bolts "why." These questions were addressed by the many posts on this topic.

I believe the Michelson-Morley series of experiments were the major scientific approaches to this subject along with Einstein's generalized conjecture which, so far, has not been disproven.

When I first studies physics and that was many years after Einstein they still discussed the "ether" or something like it. The Aristotlean view or Newtonian view had not been cleared from the thinking of secondary school or basic entry college physics instructors. That was the 1950s and early 60s.

 

[stevmg]

I understand the Michelson–Morley experiment and its result; but what I don't know yet is the REASON.
Example:
A torch in free space is moving at a velocity [v] w.r.t me. Considering the material nature of light, shouldn't the speed of photons emitted from the torch be [v+c] w.r.t ME?

To elucidate on this point in particular, the invariance of the speed of light in his situation is taken up by time dilation and length contraction. They do work opposite to each other but one offsets the other. What should be an increase in closure speed as you posit is enhanced by the length contraction yet offset by the time dilation so that, relative to you, things "slow down" back to light speed.

Am I clear? I hope I got it correct. It took me a long time to figure this one out. Chapter 2 of Spacetime Physics, the first edition in the 1960s by Taylor/Wheeler goes into it (by discussion the maintenance of momentum/energy between different frames of reference which means different underlying velocities) but you have to read it slowly and over, over and over again and work the problems, which they give.

stevmg

 

[TCS]

When I first studies physics and that was many years after Einstein they still discussed the "ether" or something like it. The Aristotlean view or Newtonian view had not been cleared from the thinking of secondary school or basic entry college physics instructors. That was the 1950s and early 60s.

To Aristotle, Metaphysics was just the chapter in his book that came after physics. He described what he could of the operation of the universe using physics and he used metaphysics for the rest. Over time, as physicists have devloped better models, much of what was in the metaphysics chapter has shifted into the physics chapter. I think that it is a valid goal of physicists to try and make the metaphysics chapter as small as possible and to explain everything with physics.

 

[stevmg]

Ha! Ha!

 

[TCS]

Why is the speed of light independent of a frame of reference?

It might be because, the photon will always follow the geodesic (the shortest path) and the shortest path to you is the geodesic through a medium traveling in your reference frame (which would be expanding with time along with the rest of space), since the space is hyberbolic.

 

[Austin0]

To elucidate on this point in particular, the invariance of the speed of light in his situation is taken up by time dilation and length contraction. They do work opposite to each other but one offsets the other. What should be an increase in closure speed as you posit is enhanced by the length contraction yet offset by the time dilation so that, relative to you, things "slow down" back to light speed.

. Chapter 2 of Spacetime Physics, the first edition in the 1960s by Taylor/Wheeler goes into it (by discussion the maintenance of momentum/energy between different frames of reference which means different underlying velocities) but you have to read it slowly and over, over and over again and work the problems, which they give.

stevmg

Hi ...As length contraction is the same in both directions (-x) and (+x)
and dilation is of course without direction ,,how would this explanation work with a light coming from behind (c-v)

 

[maritz]

do not convince me yet pal, maybe a mathematical proof will work

 

[Red_CCF]

Wait so if there's a light source shining light in space, and I'm traveling at exactly the speed of light away from the source (in space), then the light from the light source will still have a velocity of c with respect to me (assuming light is shined in the direction I'm travelling)?

 

[Bussani]

Wait so if there's a light source shining light in space, and I'm traveling at exactly the speed of light away from the source (in space), then the light from the light source will still have a velocity of c with respect to me (assuming light is shined in the direction I'm travelling)?

This seems a little obvious to point out, but you can't travel at exactly the speed of light. If you picked a speed like 0.999c instead, then yes, the light would still be c with respect to you.