Photon Timelessness: Does It Exist?

[YASH AWasthi]

I have read that photons do not experience time...
If that's the case then if a particular photon is emitted by a body then that should exist in every time relative to us i.e that same photon should be there at exactly the same point forever.

 

[phinds]

I have read that photons do not experience time...

You have not read that in a physics text, it is pop-sci nonsense.

It is one of the most common misconceptions in pop-sci presentations and we even have a FAQ on it here somewhere on the forum but I don't have it bookmarked so can't point you to it. Try a forum search. The links at the bottom of this page are a good place to start.

 

[Dale]

I have read that photons do not experience time...

What is the specific source where you read this? It is wrong for several reasons. As @phinds mentioned, it is probably not a valid source

If that's the case then if a particular photon is emitted by a body then that should exist in every time relative to us i.e that same photon should be there at exactly the same point forever.

This doesn't follow. We still experience time regardless.

 

[Ibix]

It's not that photons don't experience time so much as photons don't experience full stop. It's impossible to describe a photon's point of view in relativity, so any attempt to work backwards from what you think that experience is fails even before it starts.

 

[PeterDonis]

This FAQ is relevant:

https://www.physicsforums.com/threads/rest-frame-of-a-photon.511170/

 

[YASH AWasthi]

You have not read that in a physics text, it is pop-sci nonsense.

It is one of the most common misconceptions in pop-sci presentations and we even have a FAQ on it here somewhere on the forum but I don't have it bookmarked so can't point you to it. Try a forum search. The links at the bottom of this page are a good place to start.

https://www.quora.com/Does-a-photon-experience-time
see the post of a guy named Jerzy Michał Pawlak

 

[Nugatory]

https://www.quora.com/Does-a-photon-experience-time
see the post of a guy named Jerzy Michał Pawlak

And stuff like that is the reason Physics Forums has the rule that acceptable sources are appropriate peer-reviewed journals and books.

 

[phinds]

https://www.quora.com/Does-a-photon-experience-time
see the post of a guy named Jerzy Michał Pawlak

As I said, pop-sci nonsense. He talks about the "point of view of a photon". There is no such thing.

 

[YASH AWasthi]

As I said, pop-sci nonsense. He talks about the "point of view of a photon". There is no such thing.

sorry to ask a stupid question.Can't rely on phd...

 

[PeroK]

https://www.quora.com/Does-a-photon-experience-time
see the post of a guy named Jerzy Michał Pawlak

Here is Jerzy's post:

The only way particles can "experience time" is by having their internal state evolve. The easiest case to observe is probably particle decay into other particles. Photons don't decay (nor do any other massless particles). They also don't spontaneously change any of their observable properties, like energy or polarisation - this would be violation of energy/momentum/angular momentum conservation. So, to the best of our knowledge, they don't "experience time".

There is no mention of a photon's point of view that I can see.

 

[phinds]

Here is Jerzy's post:

The only way particles can "experience time" is by having their internal state evolve. The easiest case to observe is probably particle decay into other particles. Photons don't decay (nor do any other massless particles). They also don't spontaneously change any of their observable properties, like energy or polarisation - this would be violation of energy/momentum/angular momentum conservation. So, to the best of our knowledge, they don't "experience time".

There is no mention of a photon's point of view that I can see.

Here it is

Photons are the only things that can travel at the speed of light (photons). They are massless. Relativity shows us that as objects approach the speed of light they experience increased relative mass and time dilation. But since a photon is massless, and can travel at the speed of light, is their time dilation infinite--meaning does time "pass" for a photon? The implications of this are interesting and absurd--it would mean that a photon would reach your eye at exactly the same time that it left the star (from the photon's point of view, of course).

But to be fair, I now see that he is saying this would be absurd. I just scanned it prior to my previous post and was struck by the "photon's point of view".

 

[PeroK]

Here it isBut to be fair, I now see that he is saying this would be absurd. I just scanned it prior to my previous post and was struck by the "photon's point of view".

To be even fairer that quotation is not attributable to Jerzy Michal Pawlak. It's not clear who is the originator of that particular entry.

 

[vanhees71]

I don't understand what the authors of this popular-"science" book want to express with the phrase "a particle experiences time". It's an undefined empty phrase. Forget popular-science books and read real physics books. It's very difficult to write a good popular-science book, way more difficult than to write a good physics textbook, because in the former you are not allowed to use the adequate language to describe physics, which is mathematics. On the other hand, making a statement like "a particle experiences time" is, in fact, utter nonsense even in a popular-science-book context! I'm pretty sure they explain somewhere that photons are massless particles, implying a "billard-ball picture" of "particles". That's as misleading as you can be.

 

[Herman Trivilino]

sorry to ask a stupid question.Can't rely on phd...

No, you absolutely cannot rely on something just because it was written by someone with a Ph.D. in the discipline. For example, suppose two of them disagree, as is often the case? Most if not all of the physicists who have replied to you here have Ph.D.'s in physics or closely related disciplines!

In the early 1990's there was a problem known as the solar neutrino problem. The number of neutrinos from the sun hitting detectors on Earth was less than what theorists predicted based on the nuclear reactions that they calculated must be occurring to produce the amount of heat and light being output by the sun. In other words, there was no way to explain how the sun could be so bright and so hot without these nuclear reactions, and no way to explain the nuclear reactions without a sufficient number of neutrinos being produced. But when experimentalists used their detectors to look for these neutrinos, a significant fraction of them were missing!

One possible explanation, that later turned out to be right, is that the missing neutrinos changed flavor on their way from the sun to Earth.

I was in attendance at a seminar for physicists at the University of Houston when the Nobel prize-winning physicist Sheldon Glashow, who was the invited guest and speaker, stated that if neutrinos are massless then they travel at the speed of light and, loosely speaking, can't experience time so therefore can't oscillate (change) from one flavor to another.

Of course we now know that neutrinos are not massless, that they therefore travel at speeds less than the speed of light, and that they do indeed oscillate.

So why the caveat "loosely speaking"? The other posters are trying to get you to understand why. The moral of the story is information processing. You can't simply take something you hear or read at face value without trying to understand why it was said or written. If you do you run the risk of drawing a false conclusion from it, as you have done here in your original post.

 

[whatif]

It's impossible to describe a photon's point of view in relativity

However, to check my understanding, I write the following. Something can be said about a fast-moving object as measured in a particular inertial frame. As an object’s measured speed approaches the speed of light, any clock moving with the object is measured to run slower than a clock which is stationary for that frame. The speed of the object can, in principle, be made arbitrarily close to the speed of light, and the slower clock can be made to run arbitrarily slow. It is just wrong to conclude from this that a photon is at the same point forever, at all places in the direction of travel all the time, travels any distance in zero time or that time stands still for the photon.

 

[PeroK]

However, to check my understanding, I write the following. Something can be said about a fast-moving object as measured in a particular inertial frame. As an object’s measured speed approaches the speed of light, any clock moving with the object is measured to run slower than a clock which is stationary for that frame. The speed of the object can, in principle, be made arbitrarily close to the speed of light, and the slower clock can be made to run arbitrarily slow. It is just wrong to conclude from this that a photon is at the same point forever, at all places in the direction of travel all the time, travels any distance in zero time or that time stands still for the photon.

There is no guarantee that the limit of a process is a valid physical or mathematical thing. For example, if you take a sequence of spheres of increasing radius, then what is the limit of this sequence? There is none.

Likewise, $\lim_{v \rightarrow c} \frac{1}{\sqrt{1 - v^2/c^2}}$ does not exist. And, therefore, there is no Lorentz transformation to a frame traveling at the speed of light.

It's tempting to take a direct formula, such as $\tau = t/\gamma$, which you have done, and conclude that because the limit:

$\lim_{v \rightarrow c} \tau = 0$

Then, you have a valid physical quantity at the limit point itself, $v = c$.

But, mathematically, $\gamma$ and $\tau$ are simply not defined in the limit $v = c$. In particular, you cannot say that a photon experiences zero proper time. It is much more precise to say that proper time is not defined for a photon.

 

[whatif]

It's tempting to take a direct formula, such as τ=t/γτ=t/γ\tau = t/\gamma, which you have done

Thank you for your response. However, with respect, you seem to be confirming my understanding:

It is just wrong to conclude from this that a photon is at the same point forever, at all places in the direction of travel all the time, travels any distance in zero time or that time stands still for the photon.

 

[Ibix]

Thank you for your response. However, with respect, you seem to be confirming my understanding:

Yes. The easiest way to see this is to consider the perspective of the "fast moving" object. It can regard itself as stationary, so still has 3×10⁸m/s to accelerate.

The speed of light in relativity behaves rather like infinite speed in Newtonian physics - you can't get there no matter how hard you try, and talking about the perspective of something traveling at that speed is nonsensical.

 

[phinds]

Thank you for your response. However, with respect, you seem to be confirming my understanding:

Those are all correct except the last one. It is not right or wrong to say that "time stands still for a photon", it is only correct to say that time is UNDEFINED for a photon.

 

[whatif]

Those are all correct except the last one. It is not right or wrong to say that "time stands still for a photon", it is only correct to say that time is UNDEFINED for a photon.

Thank you, but I cannot make sense of that, except as a theoretical formality. That is, I can make sense of saying that it is undefined, but not sense in saying that it is neither right nor wrong.

 

[phinds]

Thank you, but I cannot make sense of that, except as a theoretical formality. That is, I can make sense of saying that it is undefined, but not sense in saying that it is neither right nor wrong.

Well, just leave it that it is undefined.

 

[PeterDonis]

I can make sense of saying that it is undefined, but not sense in saying that it is neither right nor wrong.

It's neither right nor wrong in the same sense that a statement like "the number three is blue" is neither right nor wrong. If time is undefined for a photon, there is no way to even assign a meaning to the question of whether or not "time stands still" for a photon, just as there is no way to even assign a meaning to the question of what color the number three is. Contrast this with, for example, "the number three is greater than the number four", which is wrong--but to know that it's wrong, you have to be able to assign a meaning to the question of which of two numbers is greater.

 

[whatif]

but to know that it's wrong, you have to be able to assign a meaning to the question

That is a moot point. "the number three is blue" is wrong on the face of it. However, it depends on context. We try to avoid context by defining things as precisely as we can. If it is necessarily meaningless then it is wrong in the sense that it has no application.

Well, just leave it that it is undefined.

I am happy with that.

 

[PeterDonis]

If it is necessarily meaningless then it is wrong in the sense that it has no application.

So basically your preference for how to use the word "wrong" differs from mine. That's fine, as long as we're clear about what we each mean by it.

 

[whatif]

That's fine, as long as we're clear about what we each mean by it.

Yes. I am not trying to be difficult. I appreciate the responses.

 

[Herman Trivilino]

It is just wrong to conclude from this that a photon is at the same point forever, at all places in the direction of travel all the time, travels any distance in zero time or that time stands still for the photon.

That is correct. Everything else you wrote in that post is also correct.

 

[whatif]

That is correct. Everything else you wrote in that post is also correct.

Thank you.

 

[PeterDonis]

After moderator review, the thread will remain closed as it has run its course.