# Question from the FAQ on Rest Frame of a Photon

#### DonB

In a previous thread someone pointed me to DH's FAQ article on the Rest Frame of a Photon. In the article DH states, "Time and length cease to have meaning in the limit v→c. In that limit, all time and length intervals shrink to zero," and I've been thinking about the implications of that thought. The ramifications of this statement suggest (to me) that since light travels at c, when it leaves a distant star it is instantly visible to us on our planet (since the distance traveled at c is zero); the term "light year" becomes meaningless (since time at c is zero); and even the idea of "speed of light" has no meaning at all (since speed is a factor of both distance and time -- and both are zero at c). Any thoughts?

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#### Dale

Mentor
the term "light year" becomes meaningless (since time at c is zero); and even the idea of "speed of light" has no meaning at all
Yes, it is all meaningless in the hypothetical rest frame of a photon. Therefore such a frame doesn't exist.

#### DonB

Yes, it is all meaningless in the hypothetical rest frame of a photon. Therefore such a frame doesn't exist.
Okay. But the light going at c isn't hypothetical. Must there be a "frame" for DH's principles to apply? If so, why?

#### Dale

Mentor
Yes, light going at c is not hypothetical in any inertial frame.

The mere fact that you can string together some words does not imply that they represent a self-consistent concept. In the case of the words "inertial rest frame of a photon" there is a self-contradiction since "inertial" implies v=c and "rest" implies v=0. That is why it is nonsense. There is nothing deeper to be learned, and the self-contradiction of a photon's frame does not imply anything amiss for self-consistent concepts.

#### DonB

The mere fact that you can string together some words does not imply that they represent a self-consistent concept. In the case of the words "inertial rest frame of a photon" there is a self-contradiction since "inertial" implies v=c and "rest" implies v=0. That is why it is nonsense. There is nothing deeper to be learned, and the self-contradiction of a photon's frame does not imply anything amiss for self-consistent concepts.
DaleSpam, I'm not sure what you are upset about. I did a search for your quoted phrase ("inertial rest frame of a photon") and could not find it used anywhere else in this thread. And the closest thing to it came from DH's article, not my thoughts. If you have a problem with it, I suppose you need to address it to the folks in charge here at PF that decided to use his stuff as the authority on the subject. But I don't think the phrase is mine, and I'm not sure how to field your complaint.

#### Pengwuino

Gold Member
Ok here's the deal.

You CAN assume the rest frame of a photon exists. However, IF you do, everything ELSE stops making sense. We KNOW Maxwell's Equations work in all frames of reference. They've never been shown to fail, ever. IF you were to create this frame where a photon is at rest, in other words an inertial rest frame, you would be looking at a static electromagnetic field. Such a field is in contradiction to Maxwell's Equations, such a field is NOT a solution to the equations. That means Maxwell's Equations are wrong or the photon must not have a inertial rest frame one can observe from. What's more likely? The photon has no inertial rest frame, consistent with our experimental observations, or that maxwell's equations are wrong , completely contrary to over a century of millions if not billions of experiments showing it to be true?

#### Dale

Mentor
DaleSpam, I'm not sure what you are upset about.
I don't know what would make you think I am upset.

I did a search for your quoted phrase ("inertial rest frame of a photon") and could not find it used anywhere else in this thread. And the closest thing to it came from DH's article, not my thoughts. If you have a problem with it, I suppose you need to address it to the folks in charge here at PF that decided to use his stuff as the authority on the subject. But I don't think the phrase is mine, and I'm not sure how to field your complaint.
I thought you understood that when we say "Bob's frame" it is short hand for the more correct but cumbersome "inertial rest frame of Bob" or "inertial frame where Bob is at rest".

Similarly, when you say "photon's frame" it is short hand for "inertial rest frame of the photon". If you are interested in non-inertial frames then a whole different set of problems arise, and if you are interested in frames where the photon is not at rest then it isn't the photon's frame.

#### DonB

Ok here's the deal.

You CAN assume the rest frame of a photon exists. However, IF you do, everything ELSE stops making sense.
Pengwuino, I am not (knowingly) assuming anything about the photon rest frame. Relativity holds that a photon has speed (c) even without having having a rest frame, right? And DH's article states that as something approaches c certain characteristics in time and length/distance are noted. Therefore, I put the two together and get the ponderings of my original post -- with or without photon rest frame. So where have I screwed up?

#### DonB

I don't know what would make you think I am upset.
Okay, in the absence of voice inflection and the like it is easy to misinterpret the tone of written conversation. If that is the case here, my apologies.

I thought you understood that when we say "Bob's frame" it is short hand for the more correct but cumbersome "inertial rest frame of Bob" or "inertial frame where Bob is at rest".... The concept is self-contradictory nonsense.
Yes, I understand the nomenclature. But what I don't understand is why you're addressing these issues with me -- they are not my words. The folks here at PF have set this article (with its specific terminology) as authoritative, not me; and if you believe the wording to be self-contradictory I don't think I'm the one that can do anything about it.

#### ghwellsjr

Gold Member
Yes, I understand the nomenclature. But what I don't understand is why you're addressing these issues with me -- they are not my words. The folks here at PF have set this article (with its specific terminology) as authoritative, not me; and if you believe the wording to be self-contradictory I don't think I'm the one that can do anything about it.
You don't understand this nomenclature which you quoted from the FAQ:
Time and length cease to have meaning in the limit v→c. In that limit, all time and length intervals shrink to zero.​
What that means is as a velocity approaches the speed of light, but never getting there, the time and length intervals approach zero, but they never get there. The whole purpose of the FAQ is to address the meaninglessness of the concept of a rest frame for a photon. But instead of leaving it at that, you say:
The ramifications of this statement suggest (to me) that since light travels at c, when it leaves a distant star it is instantly visible to us on our planet (since the distance traveled at c is zero); the term "light year" becomes meaningless (since time at c is zero); and even the idea of "speed of light" has no meaning at all (since speed is a factor of both distance and time -- and both are zero at c). Any thoughts?​
Why do you persist in thinking about meaningless ideas and asking us to think about them too?

#### nitsuj

Pengwuino, I am not (knowingly) assuming anything about the photon rest frame. Relativity holds that a photon has speed (c) even without having having a rest frame, right? And DH's article states that as something approaches c certain characteristics in time and length/distance are noted. Therefore, I put the two together and get the ponderings of my original post -- with or without photon rest frame. So where have I screwed up?
I wouldn't say you've "screwed up".

"And DH's article states that as something approaches c certain characteristics in time and length/distance are noted."

I'm just someone who likes this stuff too.

DH's comment about time and distance characteristics changing as something accelerates to c is critical in perceiving why a photon is not a frame for reference. Photons don't exist in 3D. They exist in the time dimension.

The time dimension is not able to provide a reference for motion, like objects in the material dimensions can.

As you accelerate to c, you are accelerating into the time dimension, leaving the 3D world behind so bye bye reference frames once you "enter" the time dimension entirely like a photon.

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#### pervect

Staff Emeritus
What was originally said by DH is equivalent to this:

True statement: by going sufficiently fast, a traveller can travel an arbitrarily long distance (measured in the lab frame) in an arbitrarily short time (as measured by the traveler's watch).

Dr Don seem to be leaping from that to the following incorrect conclusion:

False statement: a traveler can travel an arbitrarily large distance(as measured in lab frame) by the laboratory watch.

The usual short version of this is "time is relative".

Because it will probably come up later, I'll mention another false statement that doesn't follow:

False statement: A traveller can travel an arbitrarily long distance (as measured in the traveller's frame) in an arbitrarily short time (as measured by the traveller's watch).

Short version: distance is relative too.

#### DonB

Why do you persist in thinking about meaningless ideas and asking us to think about them too?
Because without asking, I can not understand what is meaningless and what is not. I am an admitted newbie in the field, and we infants have a way of putting everything into our mouth. Without a mentor to point the way I have no means of differentiating between the cheerios on the table, the match on the bar, and the dog droppings on the carpet. Many on this forum have been so kind as to hold my hand as I struggle with my early steps -- and I am thankful for that kindness. If such babysitting is not your cup of tea, please feel no obligation to read or respond to my baby drool.

#### DonB

What was originally said by DH is equivalent to this:

True statement: by going sufficiently fast, a traveller can travel an arbitrarily long distance (measured in the lab frame) in an arbitrarily short time (as measured by the traveler's watch).

Dr Don seem to be leaping from that to the following incorrect conclusion:

False statement: a traveler can travel an arbitrarily large distance(as measured in lab frame) by the laboratory watch.

The usual short version of this is "time is relative".

Because it will probably come up later, I'll mention another false statement that doesn't follow:

False statement: A traveller can travel an arbitrarily long distance (as measured in the traveller's frame) in an arbitrarily short time (as measured by the traveller's watch).

Short version: distance is relative too.
By Jove..., I think you're right (in identifying my problem), at least partly.

Okay, so "light year" and "speed of light" do have meaning because they are measurements from my frame of ref, which is not traveling at c and thus not warping time or distance. I'm on board with that.

But the part about the length of time that it takes for light to reach me from a distant star..., that question still isn't settled in my mind. Yes, from my ref. frame I can see how I would perceive it taking a super long time. However, (with or without its own frame of ref.) that light beam leaves the star traveling at c (a relativity given), and according to my understanding of DH's article, that makes both time and distance equal zero in its "travel". Thus, the light instantly covers the non-distance in zero-time for what from my ref. frame is measured to be a very vast distance and a vast time.

My first reaction is to say that this doesn't match with experimental evidence which measures the speed of light. But as I think about it, those experiments are conducted on "our side" of the picture, and thus may not reflect what happens on the light's side.

#### ghwellsjr

Gold Member
Let me repeat your statement from the first post:
I've been thinking about the implications of that thought. The ramifications of this statement suggest (to me) that since light travels at c, when it leaves a distant star it is instantly visible to us on our planet (since the distance traveled at c is zero); the term "light year" becomes meaningless (since time at c is zero); and even the idea of "speed of light" has no meaning at all (since speed is a factor of both distance and time -- and both are zero at c). Any thoughts?​
You don't have any trouble knowing that these are meaningless ideas. It's not an issue of your not knowing which ideas are meaningful and which are meaningless, you've identified them as meaningless. My question is: why do you want to continue to think about meaningless ideas after you have put them in your mouth and been told by your babysitters that they are meaningless and to spit them out and don't put them back in your mouth? Nobody's complaining about your asking questions, just why do you reject the answers?

#### ghwellsjr

Gold Member
By Jove..., I think you're right (in identifying my problem), at least partly.

Okay, so "light year" and "speed of light" do have meaning because they are measurements from my frame of ref, which is not traveling at c and thus not warping time or distance. I'm on board with that.

But the part about the length of time that it takes for light to reach me from a distant star..., that question still isn't settled in my mind. Yes, from my ref. frame I can see how I would perceive it taking a super long time. However, (with or without its own frame of ref.) that light beam leaves the star traveling at c (a relativity given), and according to my understanding of DH's article, that makes both time and distance equal zero in its "travel". Thus, the light instantly covers the non-distance in zero-time for what from my ref. frame is measured to be a very vast distance and a vast time.

My first reaction is to say that this doesn't match with experimental evidence which measures the speed of light. But as I think about it, those experiments are conducted on "our side" of the picture, and thus may not reflect what happens on the light's side.
DH's article does not say that the time and distance for a light beam are equal to zero. It says that's a meaningless idea. The light doesn't have a side. That's also a meaningless idea.

#### DonB

You don't have any trouble knowing that these are meaningless ideas. It's not an issue of your not knowing which ideas are meaningful and which are meaningless, you've identified them as meaningless. My question is: why do you want to continue to think about meaningless ideas after you have put them in your mouth and been told by your babysitters that they are meaningless and to spit them out and don't put them back in your mouth? Nobody's complaining about your asking questions, just why do you reject the answers?
Man, the interaction helps me know if the tentative hypotheses have merit. If they do, fine; if they don't those more knowledgeable can help me see the errors. While you have complained about my approach, Pervect has been gracious enough to walk me through what I needed to see (see post #14), and my search of understand has made a significant step forward.

I do not reject the answers; however, my drive is to understand the answers, and that pushes for deeper questions as those initial answers are given to me. Just that simple.

#### nitsuj

Because without asking, I can not understand what is meaningless and what is not. I am an admitted newbie in the field, and we infants have a way of putting everything into our mouth. Without a mentor to point the way I have no means of differentiating between the cheerios on the table, the match on the bar, and the dog droppings on the carpet. Many on this forum have been so kind as to hold my hand as I struggle with my early steps -- and I am thankful for that kindness. If such babysitting is not your cup of tea, please feel no obligation to read or respond to my baby drool.
Very well said,
Building an intuition for this stuff doesn't happen just because you are told something is "meaningless". You need to "see" for your self why.

You've been told it's poop, but you have to "put it in your mouth" to be certain that it's not good.

#### ghwellsjr

Gold Member
I do not reject the answers; however, my drive is to understand the answers, and that pushes for deeper questions as those initial answers are given to me. Just that simple.
DrGreg, your response brings up a tangent question I hope you (or others) can answer -- a question I don't recall thinking of before. Accepting that I, as an at-rest observer (but "at rest" relative to what?), see a light beam approaching me at c, if I were to towards or away from that beam, would I still see it at c, or at c plus/minus my own velocity?
Have you ever thought about what it means to "see a light beam approaching" you and how you would measure its speed? Has it occurred to you that you cannot see the beam, all you can see is the portion of it when it reaches you? If you're thinking about seeing a beam from a search light, for example, the only reason why you can see it as a beam is because there are dust particles in the air that are illuminated by the beam itself and then scattered in all directions and you're seeing the ones that are scattered in your direction. But if you were in the vacuum of space, you would not see any beam; you would just see the spot where it hits your eye or your detector. It may look like it is coming from a source far away, but you cannot see the image of that source until the light from it reaches you, correct?

So now, how do you measure its speed since you cannot see it until it reaches you? First off, it doesn't matter if you are "at rest" relative to anything, just that you are not accelerating. Secondly, you don't have to worry about any frame of reference or any theory about relativity or ether or anything else. Thirdly, you don't have to worry about the source of light, just that it is not accelerating, which just means that it is not changing.

Now in order to actually measure the speed of light coming from a fixed distant source, you need to have some equipment. Since speed is length divided time, you need to have a rigid measuring stick of a known fixed length and a timing device. Since the beam is coming at you all the time, you also need a shutter so that you can start and stop the beam to give you something to observe and know when the light has traveled a certain distance. One last thing you need is a mirror. So you fix your shutter at one end of your measuring stick along with your timer which you start together. The pulse of light now travels to the other end of your measuring stick where it reflects off the mirror and back to you where you have a light detector that stops the timer. Then you calculate the speed of light as twice the distance between your shutter/detector and your mirror, divided by the time interval. The value that you get will be c.

Now you fire your rockets and head toward the light source until you reach whatever speed you want. You turn off your thrusters and you repeat your measurement exactly as you did before and you get the exact same value for the speed of light.

Now you turn your rocket around and head away from the light source until you are going in the other direction from your first measurement and go as fast as you want. You stop, turn around and repeat the measurement. You get the same answer.

Do you accept this as a factual statement of what would really happen if you could carry out this experiment?
And you responded:
Forgive me, for I have taken the lazy route and not stated myself as specifically as I suppose I should have. I was not particularly interested in all the mechanics. Rather, I simply wanted to know that as light approached me at c (relative to me) while I am "at rest", would that relative-to-me speed change if I were no longer at rest (e.g., if I were walking towards or away from the beam). What I really saw, or did not see, was really a mute point. My apologies if my casual description conveyed the wrong idea.
You rejected my answer, made no attempt to understand my answer and did not even respond to my question.

You are asking similar questions on this thread and you're not going to understand the answers until you quit being lazy and concentrate on the answers that you are given and then answer the questions that you are asked.

#### harrylin

[..] since light travels at c, when it leaves a distant star it is instantly visible to us on our planet (since the distance traveled at c is zero); the term "light year" becomes meaningless (since time at c is zero); and even the idea of "speed of light" has no meaning at all (since speed is a factor of both distance and time -- and both are zero at c). Any thoughts?
I'm not sure if this has actually been pointed out to you [edit: now I see that it's been done], but those issues only come up if you use a (physically impossible) reference system in which the light ray is in rest. The purpose of that FAQ is probably to explain why you should not try to use such a nonsensical reference system.
That has no bearing on the meaningful reference systems that we use, and all definitions relate to those systems.

But the part about the length of time that it takes for light to reach me from a distant star..., that question still isn't settled in my mind. Yes, from my ref. frame I can see how I would perceive it taking a super long time. However, (with or without its own frame of ref.) that light beam leaves the star traveling at c (a relativity given), and according to my understanding of DH's article, that makes both time and distance equal zero in its "travel". Thus, the light instantly covers the non-distance in zero-time for what from my ref. frame is measured to be a very vast distance and a vast time.
Hmm, perhaps it was not sufficiently elaborated?!
There is no "photon frame". As a matter of fact, no frame "belongs" to anything, nor do we need a physical "frame". Many people simply use as a shorthand description "so-and-so's frame" for "reference system in which so-and-so is in rest. However, for a description of the photon trajectory(or any other trajectory) as function of time, you can freely choose any physical or imaginary (but physically possible!) reference system that is in uniform linear motion. As no clocks or rulers can move at the speed of light, and measurements with such a system would be meaningless (like 0/0), it doesn't make sense to choose such a system.

Best,
Harald

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#### nitsuj

And you responded:

You rejected my answer, made no attempt to understand my answer and did not even respond to my question.

You are asking similar questions on this thread and you're not going to understand the answers until you quit being lazy and concentrate on the answers that you are given and then answer the questions that you are asked.
Well me too ghwellsjr. I think my response were clear and accurate enough to correct Drdon's understanding. But it is Penguino's delivery that Drdon can understand.

Your explinations in other areas helped me. My understanding of spacetime was that it is very much one and the same. years of that being reinforced was finaly broke in that coordinate / proper time thread. You helped hammer that difference home and once that happened the other concepts started to fall into place for me, and that was pretty cool!

#### ghwellsjr

Gold Member
Well me too ghwellsjr. I think my response were clear and accurate enough to correct Drdon's understanding. But it is Penguino's delivery that Drdon can understand.

Your explinations in other areas helped me. My understanding of spacetime was that it is very much one and the same. years of that being reinforced was finaly broke in that coordinate / proper time thread. You helped hammer that difference home and once that happened the other concepts started to fall into place for me, and that was pretty cool!
Well thanks very much for that feedback.

But, it was pervect, not pengwuino, that DrDon credited with identifying part of his problem.

#### Pengwuino

Gold Member
However, (with or without its own frame of ref.) that light beam leaves the star traveling at c (a relativity given), and according to my understanding of DH's article, that makes both time and distance equal zero in its "travel". Thus, the light instantly covers the non-distance in zero-time for what from my ref. frame is measured to be a very vast distance and a vast time.
Yes, and this is why it makes no sense to talk about the rest frame of a photon. Along with contradicting maxwell's equations, you are unable to take measurements and do physics in this 'photon rest frame'.

There is an analogy in thermodynamics that might be worth mentioning. One of the things physicists will tell you is that any substance can not reach absolute zero temperature. Now, you can do your thermodynamics and find various properties of systems as a function of temperature. So you ask what happens to these properties when you allow the temperature to go to 0. Well, near absolute 0, quantum mechanics describes the interaction of particles. One of the basic postulates of quantum mechanics is that you can't know a particles exact position and exact momentum simultaneously. So when you look at your theory and look at what happens when you set T = 0, you'll get non-sense out. This means absolute 0 is impossible to attain.

So this means either 1) your theory is incomplete or 2) your assumption that T = 0 (akin to assuming the photon has a rest frame) is nonsensical and cannot be used to consistently describe anything. We have absolutely 0 evidence to believe either quantum mechanics or special relativity are incomplete or incorrect in any major way. Thus it only makes sense to assume absolute 0 and the speed of light are unobtainable.

My first reaction is to say that this doesn't match with experimental evidence which measures the speed of light. But as I think about it, those experiments are conducted on "our side" of the picture, and thus may not reflect what happens on the light's side.
May not. Does not. It is not possible to, as Einstein would put it, travel on a lightwave and do a physics experiment. This is not just because an experimental inability, it's theoretically impossible and nonsensical to do such a thing.

#### DonB

I'm not sure if this has actually been pointed out to you [edit: now I see that it's been done], but those issues only come up if you use a (physically impossible) reference system in which the light ray is in rest. The purpose of that FAQ is probably to explain why you should not try to use such a nonsensical reference system.
That has no bearing on the meaningful reference systems that we use, and all definitions relate to those systems.

Hmm, perhaps it was not sufficiently elaborated?!
There is no "photon frame". As a matter of fact, no frame "belongs" to anything, nor do we need a physical "frame". Many people simply use as a shorthand description "so-and-so's frame" for "reference system in which so-and-so is in rest. However, for a description of the photon trajectory(or any other trajectory) as function of time, you can freely choose any physical or imaginary (but physically possible!) reference system that is in uniform linear motion. As no clocks or rulers can move at the speed of light, and measurements with such a system would be meaningless (like 0/0), it doesn't make sense to choose such a system.

Best,
Harald
Thanks Harald. I know some think I'm just being obstinate or intentionally thick-headed..., but really I'm just trying to understand.

From what you say, it seems that I'm attempting to make a ref. frame (for making time and distance measurements) for the photon when that is theoretically impossible. And even in supposedly ignoring the question of ref. frames, when I then try to make time or distance measurements for light traveling at c, that attempt in itself is ultimately an effort to construct another ref. frame. Would that be a fair statement?

Accepting (at least for the moment) that theoretically relativity disallows such, I'm still stumped in the practical realm. In the real world (universe), light has to travel from Star X to my eye here on earth. (I know I'm asking for what is theoretically undefined, but hope that you can follow my material-universe perspective for at least a moment...) If the photon is 'born' when it is emitted at the Star, how old is it when I see it (since it -- traveling at c -- ages at a different rate than I at rest perceive the same time period to last)?

Thanks again for the help.

#### Pengwuino

Gold Member
Accepting (at least for the moment) that theoretically relativity disallows such, I'm still stumped in the practical realm. In the real world (universe), light has to travel from Star X to my eye here on earth. (I know I'm asking for what is theoretically undefined, but hope that you can follow my material-universe perspective for at least a moment...) If the photon is 'born' when it is emitted at the Star, how old is it when I see it (since it -- traveling at c -- ages at a different rate than I at rest perceive the same time period to last)?
No, time does NOT go at a different rate for a photon. Time and length have no meaning to photons. It is like asking "what does a rock think about the stock market". Rocks can't think. Thinking is something only possible with sentient beings. However, we humans can think about rocks, rocks can exist, etc etc, but that doesn't mean rocks can think. We can measure the time it takes for photons to do certain things, we can measure how far of a distance it takes to do these things, but that in no way means that they have to be able to do these things themselves.

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