I Photon states should not evolve?

[Tio Barnabe]

Since proper time for photons doesn't change, i.e. in their reference frame time doesn't change, then it should be that photons don't change their quantum mechanical state, or the equivalent in Maxwell's theory.

One could say, well they don't experience time, but we do. Okay, but since their reference frame is inertial, as well as ours (if we properly choose so), so the laws of nature should be the same in their reference frame as well as in our reference frame. Therefore, if they don't experience time and thus, in their reference frame their state never evolve, so we should observe the same thing. Why is it not so? May be because Rules of General Relativity applies only for "objects" traveling at a speed less than that of light?

 

[Dale]

Since proper time for photons doesn't change, i.e. in their reference frame

There is no such thing as the reference frame of a photon.

since their reference frame is inertial, as well as ours

It is not.

 

[Tio Barnabe]

There is no such thing as the reference frame of a photon.

So should I have said "point of view" of a photon instead?

 

[Tio Barnabe]

Let's not get stuck in the technical details. I think everyone reading my opening post understands what I am asking.

 

[Dale]

So should I have said "point of view" of a photon instead?

In relativity "point of view" is a colloquialism for "reference frame". Either way, it is the same thing, and a photon doesn't have one

 

[Dale]

Let's not get stuck in the tecnical details.

You are asking a technical question. The technical details are essential.

Your question is based on a faulty assumption. So the proper answer to the question is to point out the inconsistency

 

[Tio Barnabe]

You are asking a technical question. The technical details are essential.

Your question is based on a faulty assumption. So the proper answer to the question is to point out the inconsistency

Then focus on the main point of my question, please.

 

[Ibix]

Talking of the reference frame of a photon is contradicting yourself. You'll never get a sensible answer from that starting point, so it's not surprising that you are struggling trying to make sense of the consequence of your initial mistake. The solution is to start somewhere else in your reasoning.

 

[Tio Barnabe]

Okay. So should we always analyse a photon from a reference frame where time evolves?

 

[Ibix]

Okay. So should we always analyse a photon from a reference frame where time evolves?

There is no reference frame in which "time stands still", so you don't really have an option. Your initial mistake was in trying to define one.

 

[jerromyjon]

Since proper time for photons doesn't change

Photons only move at c in a vacuum...

 

[Ibix]

Photons only move at c in a vacuum...

Please don't get into photons in a medium in this thread. You need quantum field theory to do it justice and naive assumptions about "photons slowing down" are pretty much wrong, in my limited understanding.

 

[martinbn]

Let's not get stuck in the technical details. I think everyone reading my opening post understands what I am asking.

I don't. I find it very unclear and confused.

 

[jerromyjon]

Please don't get into photons in a medium in this thread.

I wasn't trying to, I was just pointing out a fact that "the point of view of a photon" only makes some kind of sense when that is all there is in empty space.

naive assumptions about "photons slowing down" are pretty much wrong, in my limited understanding.

I'd bet you know much more than me about it, but I do agree photons "slowing down" is just a pop-science attention grabber.

 

[Dale]

Okay. So should we always analyse a photon from a reference frame where time evolves?

As @Ibix mentioned, there are no other kinds of reference frames!

Then focus on the main point of my question, please.

I did. The main part of your question is based on a faulty premise. The question is simply wrong and cannot be answered

 

[Tio Barnabe]

Is it also wrong to say that photons don't experience time?

 

[jtbell]

What does it mean for a photon to "experience" something?

 

[jerromyjon]

Is it also wrong to say that photons don't experience time?

Photons don't lose energy with time. The cosmic microwave background has been traveling for over 13 billion years and the frequency has been redshifted by expansion of space but it is still coming at us since the recombination epoch.

 

[Tio Barnabe]

Photons don't lose energy with time

This statement seems to contradict this one

the frequency has been redshifted by expansion of space

What does it mean for a photon to "experience" something?

I mean in the sense that things like energy, position, etc evolve with time.

 

[phinds]

I wasn't trying to, I was just pointing out a fact that "the point of view of a photon" only makes some kind of sense when that is all there is in empty space.

No, "the point of view of a photon" NEVER makes sense.

 

[phinds]

I mean in the sense that things like energy, position, etc evolve with time.

Which is something WE observe. The photon does not "observe" or "experience" anything.

 

[Dale]

Is it also wrong to say that photons don't experience time?

Without anthropomorphising, photons are governed by the laws of QED, which do include the way a photon's state evolves over time. Whether that implies they "experience" time is more a question of semantics than physics

 

[Carrock]

In special relativity, you can make the the frequency or momentum etc of a photon any arbitrary finite value you want, by choosing an appropriate reference inertial frame from which to measure it.

Similarly, in GR, any apparent bending, doppler shift etc is a result of curved space etc and using a non-local reference frame to calculate the apparently changing state of the photon.It would be possible, by using suitable different local inertial frames near different points on the photon's path, to calculate the photon's state to be unchanging.

i.e. by using 'correct' inertial frames, photons' state can be calculated to never evolve.

It's perhaps worth mentioning that the changing state of moving neutrinos is regarded as strong evidence they have mass.

 

[jerromyjon]

Photons don't lose energy with time.

This statement seems to contradict this one

the frequency has been redshifted by expansion of space

I thought the same thing when I was thinking of what to type, but when you look at comparatively short distances and humanly observable time frames any loss of energy would be irrelevant and minuscule. The fact that they do lose energy to expansion should give you the clue that photon states HAVE to evolve to account for this.

 

[Carrock]

The fact that they do lose energy to expansion should give you the clue that photon states HAVE to evolve to account for this.

You're assuming the energy of a photon must be defined by a comoving inertial frame.
Unless you can justify that, then you can choose a local frame where the photon has not lost energy or evolved.

 

[jerromyjon]

You're assuming the energy of a photon must be defined by a comoving inertial frame.

Yes, I'm specifying that. Many things affect the wavelength of photons, time is not one of them.

 

[zonde]

Since proper time for photons doesn't change, i.e. in their reference frame time doesn't change, then it should be that photons don't change their quantum mechanical state, or the equivalent in Maxwell's theory.

Photons change their quantum mechanical state in Schrödinger picture. But you can adopt Heisenberg picture for photons and leave Schrödinger picture for massive particles. This should resolve your question.

 

[rrogers]

In relativity "point of view" is a colloquialism for "reference frame". Either way, it is the same thing, and a photon doesn't have one

I don't know what "reference frame" means but in GR, coordinate systems using null geodesics are used for certain purposes. Photons, in GR, have "direction" and an associated foliation.

 

[Dale]

I don't know what "reference frame" means but in GR, coordinate systems using null geodesics are used for certain purposes. Photons, in GR, have "direction" and an associated foliation.

Although often we use the term "reference frame" to mean "coordinate system", this is one case where the difference becomes important. The technical term for a reference frame is a tetrad. It consists of a set of four vector fields covering some section of spacetime. Three of the vector fields are spacelike and one is timelike and they are orthonormal. So even though you can define coordinate systems with null basis vectors, the basis vectors of those coordinates do not form a tetrad or reference frame

 

[martinbn]

Technically they (the null vectors) do form a tetrad, but not a reference frame.

 

[Dale]

Technically they (the null vectors) do form a tetrad, but not a reference frame.

No, part of the definition of a tetrad is that one is timelike and three are spacelike. So none can be null.

 

[martinbn]

This is a matter of terminology, but I believe that the standard terminology doesn't require the tetrad to be one time-like and three space-like. For example null tertrads are often used and called tetrads, for instance in the Newman-Penrose formalism.

 

[Dale]

Interesting. I thought that the standard terminology did require 1 timelike and 3 space like.

https://en.m.wikipedia.org/wiki/Tetrad_(general_relativity)

 

[martinbn]

It seems there is no consistency. In the same article, the link for tetrad, defines it as four linearly independent vectors, no restriction of one being timelike and the others spacelike. I thought by the name itself that it just means a basis and nothing more. Anyway this is going far off the topic (although more interesting in my opinion.

 

[Dale]

Yes, I guess that we would need to get more authoritative references than Wikipedia!

 

[vanhees71]

Although often we use the term "reference frame" to mean "coordinate system", this is one case where the difference becomes important. The technical term for a reference frame is a tetrad. It consists of a set of four vector fields covering some section of spacetime. Three of the vector fields are spacelike and one is timelike and they are orthonormal. So even though you can define coordinate systems with null basis vectors, the basis vectors of those coordinates do not form a tetrad or reference frame

Well, we had this debate some time ago, and I still stand to what I said: A reference frame is something realizable by an experimentalist. The most simple example we have in everyday life is a clock at the wall of my office and one corner of the office with three (orthogonal) edges, is a realization of a reference frame. This is a pretty accurate realization of @Dale's tetrades. I don't think that Wikipedia is so bad in this case (it's even pretty good).

Of course, I can use any coordinates that please me most in my problem, e.g., light-cone coordinates. These coordinates, however, are not necessarily realizable as a (local) reference frame, and light-cone coordinates are an example. In other words there are more coordinate systems than real-world (local) reference frames :-).

 

[martinbn]

Yes, I guess that we would need to get more authoritative references than Wikipedia!

Now I think that even the serious sources don't follow the same convention.

 

[vanhees71]

Of course you can work with other bases of the tangent spaces than the tetrades described by @Dale and the Wikipedia article, but as I argued, the usual more narrow definition of tetrades are at least in principle realizable with real-world material in the lab :-).

 

[nitsuj]

Photons only move at c in a vacuum...

is there atmospheric influence at such a scale..if its not a vacuum then there is stuff a photon may interact with. A photon always goes c. Some parts of it faster !

 

[PeterDonis]

A photon always goes c. Some parts of it faster !

What are you referring to here?

 

[jerromyjon]

A photon always goes c. Some parts of it faster !

I would like to believe photons always travel at c but there is currently no way to prove that (that I've heard of). By some parts do you mean the phase velocity? That I have heard can exceed "c" in some sense, but it would require multiple photons to have a phase, wouldn't it?

 

[Dale]

I would like to believe photons always travel at c but there is currently no way to prove that (that I've heard of).

See sections 3.3 and 3.4

http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html

 

[jerromyjon]

See sections 3.3 and 3.4

http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html

That's the speed in a vacuum, I meant in matter, where the refractive index "slows" light.

 

[NoTe]

I was wondering about the possibility of decay of photons, but read that this could not occur because the photon's time stood still. Since this argument isn't valid at all, - it leaves my question: is decay of photons possible? This is not just a question; if decay of photons is possible, we may see the universe quite different from what it really IS! (Because the photons arising from far away objects have decayed in the mean time) Let's say a mean decay time of days or so - this doesnot have any influence on our solar system.

 

[PeterDonis]

I was wondering about the possibility of decay of photons, but read that this could not occur because the photon's time stood still.

Which, as should be evident from this thread, is a pop science misconception.

is decay of photons possible?

What does "decay" mean? A pair of photons of sufficient energy can produce a particle-antiparticle pair. Does that count as "decay"?

If you mean "decay" of a single photon in free space, no, that's not possible.

 

[Carrock]

I was wondering about the possibility of decay of photons, but read that this could not occur because the photon's time stood still. Since this argument isn't valid at all, - it leaves my question: is decay of photons possible? This is not just a question; if decay of photons is possible, we may see the universe quite different from what it really IS! (Because the photons arising from far away objects have decayed in the mean time) Let's say a mean decay time of days or so - this doesnot have any influence on our solar system.

The general view, I think, is that time isn't an applicable concept for a photon, since a reference frame 'moving with' a photon isn't possible.
To go from this to saying that photons are timeless (i.e. without defined time) is OK IMO. Whether or not their 'time' 'stands still' is semantics rather than physics.
No one seems to have directly disagreed with my previous post

In special relativity, you can make the the frequency or momentum etc of a photon any arbitrary finite value you want, by choosing an appropriate reference inertial frame from which to measure it.

Similarly, in GR, any apparent bending, doppler shift etc is a result of curved space etc and using a non-local reference frame to calculate the apparently changing state of the photon.It would be possible, by using suitable different local inertial frames near different points on the photon's path, to calculate the photon's state to be unchanging.

i.e. by using 'correct' inertial frames, photons' state can be calculated to never evolve.

It's perhaps worth mentioning that the changing state of moving neutrinos is regarded as strong evidence they have mass.

except

Yes, I'm specifying that [the energy of a photon must be defined by a comoving inertial frame]. Many things affect the wavelength of photons, time is not one of them.

which is simply an assertion.

 

[PeterDonis]

The general view, I think, is that time isn't an applicable concept for a photon, since a reference frame 'moving with' a photon isn't possible.

Yes.

To go from this to saying that photons are timeless (i.e. without defined time) is OK IMO.

Unfortunately, this doesn't seem to work in practice; as soon as you say photons are "timeless" instead of saying that the concept of "time" does not apply to them, people start drawing incorrect inferences.

 

[PeterDonis]

No one seems to have directly disagreed with my previous post

Your statements in that post are not incorrect, but they're not really useful either. When we're talking about the energy of a photon, what we really care about is its energy relative to some observer or object that it is interacting with. That energy is a Lorentz scalar: you take the inner product of the photon's 4-momentum vector with the 4-velocity of the object or observer, and inner products of vectors are scalars, independent of any choice of frame.

So, for example, if we say a particular photon is Doppler shifted, what we mean is that its energy relative to some observer or detector is different from its energy relative to its source--which means that the inner product of the photon's 4-momentum with the detector's 4-velocity is different from the inner product of the photon's 4-momentum with the source's 4-velocity. That is a statement about a comparison between scalars, and is independent of any choice of frame.

The only other technical point involved in all this is how the photon's 4-momentum gets "propagated" from the source to the detector; but the answer to that is simply that the 4-momentum gets parallel transported along the photon's worldline. That is also independent of any choice of frame.

 

[Carrock]

...Unfortunately, [saying that photons are timeless (i.e. without defined time)] doesn't seem to work in practice; as soon as you say photons are "timeless" instead of saying that the concept of "time" does not apply to them, people start drawing incorrect inferences...

'Saying that the concept of "time" does not apply to them' also often leads to incorrect inferences; I subjectively find the latter more annoying.

Your statements in that post are not incorrect, but they're not really useful either. When we're talking about the energy of a photon, what we really care about is its energy relative to some observer or object that it is interacting with...

I was simply trying to indicate that it's never necessary to have photons' energy, momentum etc change.

Without anthropomorphising, photons are governed by the laws of QED, which do include the way a photon's state evolves over time. Whether that implies they "experience" time is more a question of semantics than physics

It's often useful to think the photon is changing during its (unobservable) flight. Since an object can always be chosen such that the absorbed photon is in the same state as when it was emitted, and emission and absorption are the only times a photon's state can be observed, there is no requirement for a photon's state to evolve over time.
A particle's state 'really' evolving during flight, like a neutrino, is generally regarded as proof mass is associated with it.

 

[nitsuj]

That's the speed in a vacuum, I meant in matter, where the refractive index "slows" light.

what do you mean "slows light"