# Do photons experience time?

I know you can't go at C, because of the mass. I'm just asking a what if question. What if you were traveling at C, what would the clock show. I'm not planning on travelling at C anytime soon ibix.

But it seems it's not important "what if" question because you will never reach C.

It is NOT about the mass, and your question still can't be answered. This is the most fundamental thing to understand about SR, and denying it will not help.

jbriggs444
Homework Helper
2019 Award
Dale and m4r35n357
PAllen
2019 Award
So if I was traveling at the speed of light my clock would not be moving relative to me? I would be frozen in time? I wouldn't notice it but to others that's how it looks like?
No. A material body can’t travel at the speed of light, and a reference frame at light speed is impossible in SR. See:

PeterDonis
Mentor
2019 Award
I'm just asking a what if question
You can't ask even a what if question based on a self-contradictory premise.

What if you were traveling at C, what would the clock show.
A clock can't travel at c either, so again, the question is based on a self-contradictory premise and is meaningless. As @ZapperZ has already pointed out, there are plenty of previous threads, plus a FAQ article, here on PF explaining this.

jbriggs444
vanhees71
Gold Member
2019 Award
That looks quite good, but there are some questionable statements in there. For example:

"Einstein wrote two theories of relativity; the 1905 work is known as “special relativity” because it deals only with the
special case of uniform (i.e. non-accelerating) motion."

Which is not right. You can study any motion in SR, accelerated or not. What SR does not deal with is Gravity and curved spacetime.

So, I'd be careful with that pdf. There are a few things I saw like that that made me raise an eyebrow.

If you want a free source, I would recommend:

http://www.lightandmatter.com/sr/

If you want to buy a book, I would recommend:

It's also using the outdated concept of "relativistic mass". Here's my attempt for a modern introduction to SRT, starting right away with the covariant formalism a la Minkowski (however with a real "metric" as it should be):

https://th.physik.uni-frankfurt.de/~hees/pf-faq/srt.pdf

weirdoguy and YoungPhysicist
Mister T
Gold Member
As far as I know, a object will experience time slower when its speed is close to the speed of light.
That can't possibly be true. It would provide a way to distinguish between a state of rest and a state of uniform motion, violating the foundation upon which the entire theory is based.

What's confusing here is that when an object moves relative to you, time passes more slowly for that object relative to you.

Likewise, when you move relative to an object, time passes more slowly for you relative to that object.

Note that because of this symmetry, there is no way to claim whether it's you, the object, or both, that are in motion.

Another part of the foundation is that a beam of light in a vacuum will always travel at the same speed ##c## relative relative to both you and that object. Imagine trying to chase after that beam. No matter how fast you travel in your attempt to catch it, it will always recede from you at speed ##c##. Thus you can never reach speed ##c##.

YoungPhysicist
From what PAllen said that a photon of light does not have a reference frame and another Forum I read saying that the concept of space or time don't apply to a photon, then why is the concept of quantum entanglement so mysterious?

PeroK
Homework Helper
Gold Member
From what PAllen said that a photon of light does not have a reference frame and another Forum I read saying that the concept of space or time don't apply to a photon, then why is the concept of quantum entanglement so mysterious?
Quantum entanglement applies to all types of particle.

Ibix
ZapperZ
Staff Emeritus
From what PAllen said that a photon of light does not have a reference frame and another Forum I read saying that the concept of space or time don't apply to a photon, then why is the concept of quantum entanglement so mysterious?
What does "... a photon of light does not have a reference frame and another Forum I read saying that the concept of space or time don't apply to a photon ... " have anything to do with "... quantum entanglement... " that makes it so "mysterious"?

Zz.

What field of physics deals with what a photon experiences as far as it comes to time and such. Like two photons traveling side by side, or having some kind of interaction. Is that the realm of quantum mechanics?

Nugatory
Mentor
What field of physics deals with what a photon experiences as far as it comes to time and such. Like two photons traveling side by side, or having some kind of interaction. Is that the realm of quantum mechanics?
No field of physics deals with what a photon experiences or with two photons travelling side by side, for the same reason that no field of biochemistry deals with the metabolism of pink unicorns - these concepts make no sense.

Photon-photon interactions do happen, and they are described by quantum electrodynamics.

vanhees71 and PAllen
What field of physics deals with what a photon experiences as far as it comes to time and such.

phinds and vanhees71
The Lorentz equations cannot be applied to photons. Their time dilation is not zero, it is either undefined or meaningless.

vanhees71
Gold Member
2019 Award
What do you mean by "Lorentz equations"? A photon is described by the relativistic QFT named QED, which is completely compatible with special relativity (as are the classical Maxwell equations).

In the Netherlands, on the Wikipedia page of the lemma 'foton', part of the physics section, it says: "Volgens de speciale relativiteitstheorie staat de lokale tijd van een lichtdeeltje stil." Translation: "According to Special Relativity the local time of a photon stands still." It's been there for fifteen years or more and apparently no one seems to care. I have always thought (i.e. known) it couldn't be right, but I'm not gonna edit Wikipedia pages... but it shows as a nice example of the reliability of Wikipedia and such!

vanhees71
Gold Member
2019 Award
Well in Wikipedia there are some people allowed to write and they quite often don't write in completely correct terms. I've no clue what a local time might be. If they mean "proper time" it's of course nonsense, because there's no way to define proper time of a photon.

If they mean the eikonal approximation for free electromagnetic fields it doesn't make sense either, because light-like curves do not admit the definition of a uniquely defined proper time. All you can do is to use some parameter to parametrize the light-like ray. You can even use an affine parameter, but that doesn't define a uniquely defined proper time either. The proper time is well-defined for worldlines of massive particles, which are time-like. Then it's the affine parameter ##\tau## which is defined uniquely by the equation
$$\frac{\mathrm{d} x^{\mu}}{\mathrm{d} \tau} \frac{\mathrm{d} x_{\mu}}{\mathrm{d} \tau}=c^2.$$

In short, the Wikipedia statement is (a) inaccurate using undefined non-standard terms ("local time") and if with the best will you try to understand what they might mean, it doesn't make sense either.

Well, fundamental particles don’t really experience anything. However, the time that human beings experience (and clocks and other animals and ...) is called “proper time”, and photons do not have any proper time.
The reference system of the “fast” object Measures a velocity c respect to the photon. It’s time is relented only if watched by another reference system, but it’s own time “proper time” is unchanged and its “perceptions” are unchanged. Given that a photon don’t have a reference system, we can’t say anything about the time flow in the life of a photon.
Nevertheless the photon has a lifespan and a frequency if viewed from any inertial observer. And a frequency is a vibration of something in the time of observer. The faster the observer flies towards the photons’ source, the higher will be the observed frequency. The faster the observer would run away from the photon source, the lower will be the observed frequency. In this case we can go to the limit: if the object goes away from the source at c-ɛ, as ɛ → 0 it will observe a frequency f → 0. But this is still the time of a reference system, not the photon’s time. So, we can’t claim that nothing oscillates from the point of view of the photon.

PeterDonis
Mentor
2019 Award
The Lorentz equations cannot be applied to photons.
It depends on what you mean by "the Lorentz equations". You can describe the motion of a photon in any inertial frame just fine, and the description transforms between inertial frames according to the Lorentz transformation just fine. What you can't do is define an inertial frame in which the photon is at rest.

So if I was traveling at the speed of light my clock would not be moving relative to me? I would be frozen in time? I wouldn't notice it but to others that's how it looks like?
If you were travelling near the speed of light time would be just fine, your clocks running, to an observer you'd be literally glowing and no way to read out the clock at that speed!

Nugatory
Mentor
If you were travelling near the speed of light time would be just fine, your clocks running....
Yes
to an observer you'd be literally glowing
If you are approaching the observer, yes. But not if you were moving away, then Doppler would red-shift the light coming from you.

.Scott
Homework Helper
As far as I know, a object will experience time slower when its speed is close to the speed of light.
But photons themselves moves at the speed of light, does that mean that they experience no time?
Notwithstanding the very valid issues already posted, it is possible to answer this question in the spirit in which it was asked.

Although you cannot be accelerated to the speed of light, you can, in theory, be accelerated to speeds which approach the speed of light.
So we can talk about what would happen as you reach speeds of about 0.999999999c (relative to Earth). From a Earth-bound observer, your clock will have essentially stopped and your relativistic mass would have increased enormously. If your destination was 1000 light-years away, you will be observed to arrive there in roughly 1000 years, but your clock will have advanced by only a couple of weeks.

From your perspective, you will have completed the journey in only weeks. But wouldn't that make it seem as though, from your point of view, you were travelling at a speed much faster than light? It won't because from your point of view, the entire universe would appear to be foreshortened. By your measurement, your distance traveled would be on the order of the diameter of the solar system.

So what would a photon see?
First, it doesn't have time to see anything. Even if it had a clock, it would never tick. This is why neutrinos, that seem to be able to change while travelling, are determined to be travelling at something less than the speed of light.
Second, from the photons perspective, the starting and ending points are coincident. It travels a distance of zero in zero time.
Third, photons don't really travel like that. They follow quantum mechanical rules that defy the notion of a straight path from a source point to a destination point.

Last edited:
The proper time is well-defined for worldlines of massive particles, which are time-like. Then it's the affine parameter ##\tau## which is defined uniquely by the equation
$$\frac{\mathrm{d} x^{\mu}}{\mathrm{d} \tau} \frac{\mathrm{d} x_{\mu}}{\mathrm{d} \tau}=c^2.$$
Is there any online reference that explains this equation in more detail? What is ##\tau##, x##\mu##, and x##\mu##? Thanks.

and your mass would have increased enormously.
Relativistic mass which is not what is meant nowadays when physicists say 'mass' (as been pointed out zilion times, even in this thread...).

Second, from the photons perspective
This whole thread is about the fact that there is no such thing as "photons perspective", so why you write things like this?

phinds
.Scott
Homework Helper
Relativistic mass which is not what is meant nowadays when physicists say 'mass' (as been pointed out zilion times, even in this thread...).
The sentence started "From an Earth-bound observer". I don't need to explicitly say "relativistic".

This whole thread is about the fact that there is no such thing as "photons perspective", so why you write things like this?
And my point is that we can still look at the situation as the limit is approached. I think I did a very good job in explaining some of the problems with the notion of the "photon's perspective".

YoungPhysicist