Does time stop for a photon? Why is that a nonsensical question?

[JamieForum]

From this link http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/headlights.html I don't understand the following..."Does time stop for a photon?. . . It is really not possible to make sense of such questions and any attempt to do so is bound to lead to paradoxes. There are no inertial reference frames in which the photon is at rest so it is hopeless to try to imagine what it would be like in one."

In particular this statement "There are no inertial reference frames in which the photon is at rest". Can anyone explain that to me?

 

[ahaanomegas]

I believe the answer to your question is that light is defined as photons, so photons are, always, traveling the the speed of light (c). This is why photons are said to have 0 mass.

 

[Matterwave]

One of the postulates of Special Relativity is that light moves at the same speed, c, in vacuum, in all inertial reference frames. This postulate immediately says that there is no reference frame in which the light is NOT moving at c, and "at rest" is definitely not moving at c.

 

[tom.stoer]

In SR and GR there is a well-defined mathematical procedure to calculate proper time for moving objects along trajectoreis through spacetime (as measured by a co-moving clock). Applying this procedure to a light-like trajectory (along which photons move) the result is always zero, i.e. proper time for photons vanishes. In that sense photons are 'timeless'.

 

[DrGreg]

We have our own FAQ on this topic: Rest frame of a photon.

 

[atyy]

In particular this statement "There are no inertial reference frames in which the photon is at rest". Can anyone explain that to me?

Although one could possibly device some frame of reference in which a photon is at rest, that frame is not a Lorentz inertial system of spacetime coordinates (called "inertial reference frame" for short).

An inertial frame is one in which the principle of relativity holds - relative to an inertial frame, moving faster but at constant speed, the laws of physics take the "same form" as when one is not moving. This is due to a symmetry in the laws of physics called "Poincare invariance".

 

[Passionflower]

I do not like the argument that because we cannot make a frame of reference the question is nonsensical. Frames of reference have no physical significance as they do not exist in nature.

As tom.stoer pointed out one does not need a frame of reference to demonstrate that the total elapsed time for the path of a photon between two events is zero.

 

[atyy]

I do not like the argument that because we cannot make a frame of reference the question is nonsensical. Frames of reference have no physical significance as they do not exist in nature.

As tom.stoer pointed out one does not need a frame of reference to demonstrate that the total elapsed time for the path of a photon between two events is zero.

I don't agree fully with the first statement since with special relativity, inertial frames of reference are privileged because of Poincare symmetry, which is absolute.

I do agree with the second statement as providing a good meaning to "time stops for a photon".

 

[PeterDonis]

I don't agree fully with the first statement since with special relativity, inertial frames of reference are privileged because of Poincare symmetry, which is absolute.

I think a better way to put this might be that Lorentz boosts take timelike lines into *other* timelike lines, and spacelike lines into *other* spacelike lines, but they take null lines into themselves. So a "frame" constructed using null axes (two null and two spacelike axes for a standard set of null coordinates) will behave fundamentally differently under Lorentz boosts than an ordinary inertial frame constructed using one timelike and three spacelike axes. Since the "frame of a photon", in so far as one can construct one, would have to be constructed using null axes, it is a fundamentally different type of object than an ordinary inertial frame.

That's why saying that "time stops for a photon" is not really a good way, IMO, to convey the difference between null objects and timelike objects, since it invites the inference that a "photon frame" is just like an ordinary inertial frame, only "moving at c". Saying that the "length" of a photon's worldline is always zero between any two events on it is better, but calling that length "elapsed time" is still dodgy, IMO, because it again invites the erroneous inference. I would say that the concept of "proper time" or "elapsed time for the object" simply doesn't apply to objects that move on null worldlines. If amplification is needed, see my first paragraph above.

 

[atyy]

PeterDonis, I, of course, have no technical disagreement with what you say, only a poetic one - we shouldn't have to give up our favourite kludges, I think;)

 

[Passionflower]

I think a better way to put this might be that Lorentz boosts take timelike lines into *other* timelike lines, and spacelike lines into *other* spacelike lines, but they take null lines into themselves. So a "frame" constructed using null axes (two null and two spacelike axes for a standard set of null coordinates) will behave fundamentally differently under Lorentz boosts than an ordinary inertial frame constructed using one timelike and three spacelike axes. Since the "frame of a photon", in so far as one can construct one, would have to be constructed using null axes, it is a fundamentally different type of object than an ordinary inertial frame.

That's why saying that "time stops for a photon" is not really a good way, IMO, to convey the difference between null objects and timelike objects, since it invites the inference that a "photon frame" is just like an ordinary inertial frame, only "moving at c". Saying that the "length" of a photon's worldline is always zero between any two events on it is better, but calling that length "elapsed time" is still dodgy, IMO, because it again invites the erroneous inference. I would say that the concept of "proper time" or "elapsed time for the object" simply doesn't apply to objects that move on null worldlines. If amplification is needed, see my first paragraph above.

But Peter, don't you think a frame is just a mathematical object?

For instance asking "What would the rate of a clock be if we discount the light travel time of a given Doppler shift of an object which is in relative motion to us" is interesting for professors to ask students in a test but apart from that what is the scientific value of those questions, frames or planes of simultaneity do not really exist, or do you disagree?

 

[PeterDonis]

frames or planes of simultaneity do not really exist, or do you disagree?

The planes of simultaneity certainly exist, at least to the same extent that the spacetime as a whole exists. If you adopt the viewpoint that spacetime, as a whole, is a 4-dimensional geometric object, then obviously you can "cut" particular spacelike 3-surfaces out of that 4-dimensional object that are orthogonal to particular timelike worldlines at particular events. The worldlines and the 3-surfaces themselves are coordinate-independent geometric objects, and they are as "real" as the overall geometric object that they are parts of.

Labeling the coordinate-independent geometric objects with particular coordinates is arbitrary and doesn't affect the physics. So I would agree that "frames", in the sense of particular coordinate labelings, "do not really exist". But the things that the coordinates label do (at least in the same sense that spacetime itself does).

For instance asking "What would the rate of a clock be if we discount the light travel time of a given Doppler shift of an object which is in relative motion to us" is interesting for professors to ask students in a test but apart from that what is the scientific value of those questions

If the question you quoted in the above is equivalent to the question "How much proper time elapses along this timelike worldline between events A and B?", then that question seems to me to have a direct physical meaning, since the proper time in question is directly measurable by a clock traveling along the given worldline.

Questions about "proper length" and more generally about surfaces of simultaneity are more complicated to correlate to direct physical measurements, since you first have to talk about clock synchronization and the relativity of simultaneity. But it can still be done. Whether or not it is *useful* to do it depends on the problem. Over small distances it seems to me to be useful; for example, it's hard to talk about local inertial frames and what happens in them without talking about proper length measurements within those frames. But it can be problematic when people try to extend it out over large distances, such as the recent threads about what is happening "now" on Mars or in the Andromeda galaxy. In those cases I agree that trying to assign some sort of "real meaning" to a particular surface of simultaneity causes confusion and doesn't help with understanding the physics.

 

[Passionflower]

If the question you quoted in the above is equivalent to the question "How much proper time elapses along this timelike worldline between events A and B?", then that question seems to me to have a direct physical meaning, since the proper time in question is directly measurable by a clock traveling along the given worldline.

No of course it is not.
But even in this case another observer can calculate the total time on the other clock by observing the Doppler shift between the events. No such planes of simultaneity are neccesary.

I think that if we stick to relativistic Doppler shift, proper distance and proper velocity (celerity) special relativity becomes a lot simpler.

 

[PeterDonis]

No of course it is not.

Huh? Either I'm misunderstanding you or you misunderstood what I said. I said that the proper time elapsed along a given timelike worldline is directly measurable by a clock moving along that worldline. Are you disputing that? Or are you just saying that wasn't the same question you were describing with "What would the rate of a clock be if we discount the light travel time of a given Doppler shift of an object which is in relative motion to us"? If the latter, I'm not sure I understand what question you were describing.

 

[DaveC426913]

But Peter, don't you think a frame is just a mathematical object?

Every aspect of any model is a mathematical object. If you claim you want to eliminate them and only with what there is in nature, you would have empirical evidence and exactly zero theories to explain anything. "The sun is here at this time, the sun is here at this later time and here at this later time. We do not seek to explain why."

So, your attempt to dismiss mathematical objects would stop inquisitiveness in its tracks.

 

[Passionflower]

"Of course it is not equivalent."
That is what I meant.

There is a difference to me between how allegedly two clocks are running with a different rate when they are in relative motion and two clocks going between two events with a different path in spacetime. The first can never be proven while the second obviously can.

 

[Passionflower]

Every aspect of any model is a mathematical object. If you claim you want to eliminate them and only with what there is in nature, you would have empirical evidence and exactly zero theories to explain anything. "The sun is here at this time, the sun is here at this later time and here at this later time. We do not seek to explain why."

So, your attempt to dismiss mathematical objects would stop inquisitiveness in its tracks.

I really do not think you understand what I was saying.

 

[atyy]

Every aspect of any model is a mathematical object. If you claim you want to eliminate them and only with what there is in nature, you would have empirical evidence and exactly zero theories to explain anything. "The sun is here at this time, the sun is here at this later time and here at this later time. We do not seek to explain why."

So, your attempt to dismiss mathematical objects would stop inquisitiveness in its tracks.

Surely "sun" and "here" are also mathematical models. I'm a brain in a vat:)

 

[atyy]

SR predicts that if you try to experimentally set-up a global inertial frame, you can succeed. GR predicts that you will fail. So the existence of global inertial frames is a prediction of SR (which has been falsified by GR). So inertial frames are very important in SR.

 

[DaveC426913]

I really do not think you understand what I was saying.

I think I do. It is quite a simple concept.

You take for granted most mathematical constructs in science. This one rubs you the wrong way so you want to weaken it by claiming it doesn't exist in nature. Well, neither does relativity or spatial curvature or geodesics.

I would be interested to see you have any further meaningful discussion in this thread (let alone on PF) without resorting to some aspect of a model that does not exist in nature.

 

[DaveC426913]

Surely "sun" and "here" are also mathematical models. I'm a brain in a vat:)

but no. Even as a brain is a vat, you can still report what your senses tell you.

 

[Passionflower]

You take for granted most mathematical constructs in science.

Mathematical constructs are useful if they can be used to predict experiments.

What does a plane of simultaneity predict?

 

[PeterDonis]

"Of course it is not equivalent."
That is what I meant.

There is a difference to me between how allegedly two clocks are running with a different rate when they are in relative motion and two clocks going between two events with a different path in spacetime. The first can never be proven while the second obviously can.

Ah, ok. So your question was describing #1 and mine was describing #2. I agree that #2 is a direct observable but #1 is not.

 

[Passionflower]

Ah, ok. So your question was describing #1 and mine was describing #2. I agree that #2 is a direct observable but #1 is not.

Yes, and in my opinion educators spend an undeserved disproportionate amount of time on #1 issues.

Indeed matters such as 'seeing different times (or whatever grandiose ways of describing it)' on Earth for observers on Andromeda walking a stroller versus standing still are rather useless compared to doing a calculation with for instance Doppler shifts.

 

[atyy]

I don't think #2 is a "direct observable". It requires an ideal clock. But an ideal clock is defined as one that measures elapsed proper time. So before you observe it, you have to make sure you have an ideal clock. How do you make sure you have such a clock?

 

[Passionflower]

I don't think #2 is a "direct observable". It requires an ideal clock. But an ideal clock is defined as one that measures elapsed proper time. So before you observe it, you have to make sure you have an ideal clock. How do you make sure you have such a clock?

There several experiments done with clocks aboard planes that clearly show the effects of time dilation.

 

[atyy]

There several experiments done with clocks aboard planes that clearly show the effects of time dilation.

Did they calibrate the clocks without setting up inertial frames?

 

[PeterDonis]

I don't think #2 is a "direct observable". It requires an ideal clock. But an ideal clock is defined as one that measures elapsed proper time. So before you observe it, you have to make sure you have an ideal clock. How do you make sure you have such a clock?

Any "twin paradox" scenario clearly shows that the elapsed proper time between two given events depends on the path taken, i.e., the specific worldline followed. The Hafele-Keating type experiments that Passionflower mentions are examples of such scenarios, though they do require GR to accurately predict the results so they're not as simple as the standard twin paradox scenario. Those experiments also establish that we can build actual physical clocks whose readings correspond very closely to the calculated readings for the worldlines they follow. If all this doesn't meet your definition of "direct observable", then at that point I think the issue is one of language, not physics.

 

[atyy]

Any "twin paradox" scenario clearly shows that the elapsed proper time between two given events depends on the path taken, i.e., the specific worldline followed. The Hafele-Keating type experiments that Passionflower mentions are examples of such scenarios, though they do require GR to accurately predict the results so they're not as simple as the standard twin paradox scenario. Those experiments also establish that we can build actual physical clocks whose readings correspond very closely to the calculated readings for the worldlines they follow. If all this doesn't meet your definition of "direct observable", then at that point I think the issue is one of language, not physics.

Can't I build a "clock" that travels on a null geodesic and returns a null reading? Say the clock is two beams of light of different frequencies. The "elapsed time" is the change in phase between them from the initial phase.

 

[yoron]

Jamie, it's a tricky question. If I can measure a photon that I know has been sent out by a source like some laser, and finds that to take a 'time' according to my clock, can that photon at the same time be 'time less'? We do have a propagation for it, 'c'.

And somehow 'photons propagates' inside our measure of time. On the other hand, if now 'photons' isn't 'time less', how can they still be existing there in the Cosmic Microwave Background radiation? That is approximately 13.7 billion years ago as we understands it today.

Most of the astrophysics would have to be redefined if it was wrong, as I think. And the models we have make good sense even though there isn't a 'theory of everything' so far.

How Old is the Universe?

 

[Passionflower]

Did they calibrate the clocks without setting up inertial frames?

What is the difficulty?

If we have an experiment where two clocks go between two events using different paths in spacetime then both at the start and end event they are obviously co-located. So at the start we just make sure they show the same time and at the end we check their times again.

No frames or planes of simultaneity are necessary.

 

[robphy]

Mathematical constructs are useful if they can be used to predict experiments.

What does a plane of simultaneity predict?

The "initial value problem" often has data specified on a plane of simultaneity.

For example, given the electromagnetic field at a "given time" [i.e. on a plane of simultaneity], use maxwell's equations to determine the electromagnetic field at events in the future.

Another example is the wave equation
http://en.wikipedia.org/wiki/Wave_equation

 

[Passionflower]

Jamie, it's a tricky question. If I can measure a photon that I know has been sent out by a source like some laser, and finds that to take a 'time' according to my clock, can that photon at the same time be 'time less'? We do have a propagation for it, 'c'.

And somehow 'photons propagates' inside our measure of time. On the other hand, if now 'photons' isn't 'time less', how can they still be existing there in the Cosmic Background radiation? That is approximately 13.7 billion years ago as we understands it today.

Most of the astrophysics would have to be redefined if it was wrong, as I think. And the models we have make good sense even though there isn't a 'theory of everything' so far.

Actually there are two ways of thinking about time dilation:

If we take a situation where we have two clocks going between events A and B on different spacetime paths we could say that :

"One clock ran slower than the other".

but we could also say that:

"Both clocks run at the same rate however, one clock took a path in spacetime that took simply less time as, by analogy, some roads from city A to B take less miles".

We cannot prove one is more valid than the other, however my personal preference is for the latter explanation.

In the interpretation of the latter explanation we can say that it is actually not true that time freezes for photons, time goes the same for everything in the universe including photons however, the paths that photons take in spacetime simply take no time whatsoever.

 

[atyy]

What is the difficulty?

If we have an experiment where two clocks go between two events using different paths in spacetime then both at the start and end event they are obviously co-located. So at the start we just make sure they show the same time and at the end we check their times again.

No frames or planes of simultaneity are necessary.

OK, let's say you did the experiment and the SR prediction is not verified. Were the clocks broken or is SR wrong?

 

[Passionflower]

OK, let's say you did the experiment and the SR prediction is not verified. Were the clocks broken or is SR wrong?

Well that would be a problem.

But guess what?
We are lucky, as the experiments confirm SR!

 

[robphy]

Getting back to the OP's question

From this link http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/headlights.html I don't understand the following..."Does time stop for a photon?. . . It is really not possible to make sense of such questions and any attempt to do so is bound to lead to paradoxes. There are no inertial reference frames in which the photon is at rest so it is hopeless to try to imagine what it would be like in one."

In particular this statement "There are no inertial reference frames in which the photon is at rest". Can anyone explain that to me?

I offer an old post of mine:
www.physicsforums.com/showthread.php?p=899778#post899778
which tries to directly address this question
by first trying to
DEFINE what one might mean by a reference frame.

 

[yoron]

Yes Passion flower, it makes sense :)

It will be our measurements, relative our clocks, that defines a SpaceTime path for each one of us measuring some other. But, for the one moving 'faster' relative the other, the Lorentz contraction he measures will make his path 'shorter' than the 'slower ones' measure of it.

And it is true in one way more. You won't ever find your own 'local clock' to differ depending on motion or mass, only others. That's the most interesting one to me too.

 

[atyy]

I offer an old post of mine:
www.physicsforums.com/showthread.php?p=899778#post899778
which tries to directly address this question
by first trying to
DEFINE what one might mean by a reference frame.

How about saying that time doesn't pass for light because for two light waves in the same direction, they always maintain the same phase, so if you are situated at the peak of one light wave, the peaks and troughs of the other light wave never pass you, so time is frozen for you if you use the other wave as a clock. (This is my "physical interpretation" of the null wave vector.)

 

[PeterDonis]

Can't I build a "clock" that travels on a null geodesic and returns a null reading? Say the clock is two beams of light of different frequencies. The "elapsed time" is the change in phase between them from the initial phase.

Well, then the "elapsed time" by this definition wouldn't return a null result, would it? If the beams started in phase at the emitter, then in general they would not be in phase at the detector (unless the detector were located very precisely relative to the emitter, so that an exactly integral number of wave fronts of both waves spanned the distance between them).

EDIT: I see I misunderstood what you were visualizing. What I said just above is still true, but it's not really a response to your question. See post #41 below for a better response.

 

[yoron]

A photon doesn't exist in its 'propagation' Jamie. It only has a 'source' (laser), and its 'sink' (eye). But it isn't 'there' for you in the same way a moving ball is. Physics enjoy different interpretations about that 'propagation'. Some want to use the fact that all photons are 'identical', to define a path from measuring those identical photons at different positions and time, and so create a 'average path'.

It all depends on where you stand and look at that fact.

Most of what a photon does, as its 'recoil' is explained through conservation laws, not 'classically' as we explain that ball in form of its acceleration. What we know is that it has a constant invariant 'c' though.

 

[PeterDonis]

How about saying that time doesn't pass for light because for two light waves in the same direction, they always maintain the same phase, so if you are situated at the peak of one light wave, the peaks and troughs of the other light wave never pass you, so time is frozen for you if you use the other wave as a clock. (This is my "physical interpretation" of the null wave vector.)

Einstein tried this thought experiment, and he realized that a "standing" electromagnetic wave like this violates Maxwell's Equations. This was one of the lines of reasoning that led him to SR. However, that still does leave the question of what exactly is wrong with the plausible-seeming picture you present. The resolution is that the wave fronts of each light beam, the surfaces of constant phase that you are calling "peaks and troughs", are not spacelike surfaces; they are null surfaces (because they are orthogonal to the null wave vectors, and a surface orthogonal to a null vector must itself be null). But your reasoning in the quote above (and the reasoning that shows that a "standing electromagnetic wave" violates Maxwell's Equations) depends implicitly on the wave fronts being spacelike; otherwise there is no physical meaning to "being situated at the peak of one light wave", because the "peak" is not a spatial location.

 

[atyy]

Einstein tried this thought experiment, and he realized that a "standing" electromagnetic wave like this violates Maxwell's Equations. This was one of the lines of reasoning that led him to SR. However, that still does leave the question of what exactly is wrong with the plausible-seeming picture you present. The resolution is that the wave fronts of each light beam, the surfaces of constant phase that you are calling "peaks and troughs", are not spacelike surfaces; they are null surfaces (because they are orthogonal to the null wave vectors, and a surface orthogonal to a null vector must itself be null). But your reasoning in the quote above (and the reasoning that shows that a "standing electromagnetic wave" violates Maxwell's Equations) depends implicitly on the wave fronts being spacelike; otherwise there is no physical meaning to "being situated at the peak of one light wave", because the "peak" is not a spatial location.

How about just considering a normal plane wave solution of Maxwell's equations. Coordinatize it in an inertial frame. The relative phase of two plane waves does not change.

I don't think I want to use light front coordinates since it those coordinates, "light cone time" passes for a photon.

The basic kludge I'd like to keep is that neutrino oscillations indicate they have mass, since they couldn't oscillate if time did not pass for them. Something like http://physics.stackexchange.com/qu...no-oscillations-imply-nonzero-neutrino-masses or http://m-Newton.ex.ac.uk/teaching/resources/eh/phy3135/lecture11.pdf . (I swear this was taught to me twice as an undergrad - doesn't mean it's right, but that's how I was brainwashed - the same guys loved relativistic mass!)

 

[PeterDonis]

How about just considering a normal plane wave solution of Maxwell's equations. Coordinatize it in an inertial frame. The relative phase of two plane waves does not change.

Careful. As I said in an earlier post, if you consider two plane waves with different frequencies/wavenumbers, between a source and a detector, the "relative phase" does change in the sense that if the waves are in phase at the source, they will be out of phase at the detector unless the distance from the source to the detector is chosen very carefully (to include an integral number of wave fronts of both waves). So you can't just say categorically that the relative phase does not change; you have to be more specific about what you mean by "relative phase", and being more specific makes it clear that the surfaces of constant phase are null, not spacelike.

I don't think I want to use light front coordinates since it those coordinates, "light cone time" passes for a photon.

It's not a question of coordinates. The wave vectors are null vectors, therefore the wave front surfaces orthogonal to them are also null. That's a coordinate-independent geometric statement; it's true even if you use an ordinary timelike-spacelike coordinate system to describe the vectors and wave fronts.

The basic kludge I'd like to keep is that neutrino oscillations indicate they have mass, since they couldn't oscillate if time did not pass for them. Something like http://physics.stackexchange.com/qu...no-oscillations-imply-nonzero-neutrino-masses .

I don't think anything I've said is inconsistent with this, and I agree that neutrino oscillations imply a non-zero neutrino rest mass. However, as Frank H's response on the stackexchange page makes clear, it's not really because "time must pass for neutrinos in order for them to oscillate". The key is that oscillations require that the mass eigenstates must be unequal (so at least two of the three mass eigenstates must be nonzero--normally all three are taken to be nonzero), and also that the mass eigenstates must be different than the flavor eigenstates.

 

[atyy]

Careful. As I said in an earlier post, if you consider two plane waves with different frequencies/wavenumbers, between a source and a detector, the "relative phase" does change in the sense that if the waves are in phase at the source, they will be out of phase at the detector unless the distance from the source to the detector is chosen very carefully (to include an integral number of wave fronts of both waves). So you can't just say categorically that the relative phase does not change; you have to be more specific about what you mean by "relative phase", and being more specific makes it clear that the surfaces of constant phase are null, not spacelike.

Yes, that's right.

It's not a question of coordinates. The wave vectors are null vectors, therefore the wave front surfaces orthogonal to them are also null. That's a coordinate-independent geometric statement; it's true even if you use an ordinary timelike-spacelike coordinate system to describe the vectors and wave fronts.

So that seems to leave this possibility. What's wrong with it if we already allow that here the "time" direction is null? It seems ok to me, except that now this seems to prove that time does pass for photons.

Let me sketch out my initial thoughts on this. Evidently some of it doesn't work, but let's see if it can be corrected:

Time does not pass for photons -> photons travel on null lines -> photons have null wave vectors -> null wave vectors mean no dispersion -> the shape of a wave composed of different wavelengths does not change as it travels -> Time does not pass for photons.

I don't think anything I've said is inconsistent with this, and I agree that neutrino oscillations imply a non-zero neutrino rest mass. However, as Frank H's response on the stackexchange page makes clear, it's not really because "time must pass for neutrinos in order for them to oscillate". The key is that oscillations require that the mass eigenstates must be unequal (so at least two of the three mass eigenstates must be nonzero--normally all three are taken to be nonzero), and also that the mass eigenstates must be different than the flavor eigenstates.

Yes to the mass eigenstate explanation, but what about the handwavy explanation? Do we chuck that out completely? A quick google shows it's not an uncommon handwave.

 

[harrylin]

From this link http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/headlights.html I don't understand the following..."Does time stop for a photon?. . . It is really not possible to make sense of such questions and any attempt to do so is bound to lead to paradoxes. There are no inertial reference frames in which the photon is at rest so it is hopeless to try to imagine what it would be like in one."

In particular this statement "There are no inertial reference frames in which the photon is at rest". Can anyone explain that to me?

Hi Jamie welcome at physicsforums.

I'm afraid that the replies may have become overly technical. What that statement means in practice, is that a clock that would go at the speed of light would not only stand still, but also have zero(!) length, and infinite inertia(mass). Such a clock cannot exist.

Apart of that, the authors there differ in opinion from other authors such as Einstein who (in 1905) nevertheless thought it useful to discuss this limit speed as follows:

"For v=c all moving objects—viewed from the “stationary” system—shrivel up into plane figures. For velocities greater than that of light our deliberations become meaningless; we shall, however, find in what follows, that the velocity of light in our theory plays the part, physically, of an infinitely great velocity."

I agree with his way of saying it, as long as it is well understood that objects can never fully reach that speed.

Harald

 

[DaveC426913]

I'm afraid that the replies may have become overly technical. What that statement means in practice, is that a clock that would go at the speed of light would not only stand still, but also have zero(!) length, and infinite inertia(mass). Such a clock cannot exist.

By suggesting "what would happen", you open the door to people coming up with flawed counter-suggestions about how to mitigate those results. It happens all the time.

It is more than just such a clock cannot exist; it is that the universe won't let it.

What you need to do is talk about what happens to the clock as it approaches the speed of light, and then it becomes obvious to the OP why it can never get there. An easy one is the approaches infinite mass property. As it approaches infinite mass, it also requires an amount of energy approaching infinity to accelerate it further. There simply isn't that amount of energy.

 

[PeterDonis]

So that seems to leave this possibility. What's wrong with it if we already allow that here the "time" direction is null?

Because a null direction is not a timelike direction. They're fundamentally different physically, and calling a null direction "the time direction of a photon" does not make them the same, nor does it mean that "time passes for a photon".

Time does not pass for photons -> photons travel on null lines -> photons have null wave vectors -> null wave vectors mean no dispersion -> the shape of a wave composed of different wavelengths does not change as it travels -> Time does not pass for photons.

Except for the first and last sentences, this looks OK. The problem with saying that "time does not pass for photons" is that it invites further inferences which are not justified. Saying that "the concept of time passing does not apply to photons" is better because it makes it clear that there is not just a binary choice, "does time pass or not?"; rather, more complex physics is involved. See below.

Yes to the mass eigenstate explanation, but what about the handwavy explanation? Do we chuck that out completely? A quick google shows it's not an uncommon handwave.

The problem I have with the handwave as it stands (i.e., if it is not supplemented with the more accurate explanation) is that it invites the view that a photon is "frozen", which invites the view that "everything is simultaneous to a photon", or "to a photon, the entire universe looks like a point", and so on. I realize these are popular "handwaves" too, but that doesn't make them right. IMO it's better to state right up front that objects moving on null worldlines are fundamentally different, physically, from objects moving on timelike worldlines, and some concepts, like "proper time" and "simultaneity", that apply to the latter simply don't apply to the former.

 

[harrylin]

[..] As it approaches infinite mass, it also requires an amount of energy approaching infinity to accelerate it further. There simply isn't that amount of energy.

Yes, exactly - thanks for the elaboration!

 

[atyy]

Because a null direction is not a timelike direction. They're fundamentally different physically, and calling a null direction "the time direction of a photon" does not make them the same, nor does it mean that "time passes for a photon".

I see. Your view is that we have some notion of time already, and it doesn't apply. I agree with that, except that I then say, well, anyway, time is just what we define it to be, and we can have many definitions of time. So can we find another definition in which the sentence is true? So I have no problems with calling the null direction the "light front time". I think it's nice because it makes it clear that with this definition, time does pass for a photon. Anyway, no physics disagreement.

Except for the first and last sentences, this looks OK. The problem with saying that "time does not pass for photons" is that it invites further inferences which are not justified. Saying that "the concept of time passing does not apply to photons" is better because it makes it clear that there is not just a binary choice, "does time pass or not?"; rather, more complex physics is involved. See below.

OK, again, no physics disagreement.

The problem I have with the handwave as it stands (i.e., if it is not supplemented with the more accurate explanation) is that it invites the view that a photon is "frozen", which invites the view that "everything is simultaneous to a photon", or "to a photon, the entire universe looks like a point", and so on. I realize these are popular "handwaves" too, but that doesn't make them right. IMO it's better to state right up front that objects moving on null worldlines are fundamentally different, physically, from objects moving on timelike worldlines, and some concepts, like "proper time" and "simultaneity", that apply to the latter simply don't apply to the former.

What I was trying to do with the null wave vector is to bring the idea of dispersion forward (ie. photons are dispersionless), since the neutrino oscillation explanation uses the massive dispersion relation to get different masses to travel at different speeds - and in this way link the handwavy "timeless" idea with the dispersion idea which is sound.

 

[atyy]

The culprit: http://books.google.com/books?id=P_...tions+time+stands+still&source=gbs_navlinks_s, p131, "particles moving at the speed of light experience no lapse of time".