B Is Time Experienced by Photons at the Speed of Light?

[YoungPhysicist]

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?

 

[Dale]

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.

 

[PeroK]

As far as I know, a object will experience time slower when its speed is close to the speed of light.

This statement is not accurate and is essentially a common misconception. Motion is relative and a massive object can only ever be said to be moving relative to something. There is no sense in Which an object can be said to be moving near the speed of light, or at any specific absolute speed.

What is true is that if one clock is moving relative to another clock, then the time they record is dilated as measured by the other clock. Or, more precisely, as measured in the rest frame of the other.

Note that this time dilation is symmetric, so neither can be said to be absolutely time dilated. Time dilation, like motion, is relative.

 

[haushofer]

How would you measure the time rate "experienced by" a photon?

 

[Demystifier]

photons do not have any proper time.

They do, but it's equal to zero.

 

[Dale]

They do, but it's equal to zero.

I actually prefer to say that proper time is only defined on timelike worldlines. So it has a spacetime interval which is equal to zero, but no proper time.

 

[PeterDonis]

I actually prefer to say that proper time is only defined on timelike worldlines. So it has a spacetime interval which is equal to zero, but no proper time.

I agree with this. The justification for interpreting the arc length along timelike worldlines as "proper time" is that you can use arc length as an affine parameter. You can't use arc length along a null worldline as an affine parameter, so the justification for interpreting arc length as proper time (of zero in the case of a null worldline) does not hold.

 

[PAllen]

A further observation is that if you try to view null interval as a limit of timelike interval , then it is also a limit of spacelike interval. So is it zero time or zero length? Neither seems the best answer to me.

 

[Ibix]

This has got quite technical.

@Young physicist - space and time form one 4d whole. You can measure "distance" along a path through spacetime, just as you can measure distance through space - and the "distance" along your path through spacetime turns out to be c times the elapsed time on your wristwatch.

An odd feature about the paths light follows is that this "distance" is zero. So time-for-a-pulse-of-light isn't a useful concept because it doesn't distinguish between points along the path. Advanced topic: you can use a slightly more complicated concept called an affine parameter to do the same mark-distance-along-the-path job as proper time with light. But these things don't confer a sense of time for light, really.

 

[YoungPhysicist]

This has got quite technical.

@Young physicist -
An odd feature about the paths light follows is that this "distance" is zero. So time-for-a-pulse-of-light isn't a useful concept because it doesn't distinguish between points along the path.

So do you mean that photons can’t tell if it’s moving or not?

I mean at its perspective.

 

[Dale]

I mean at its perspective.

A pulse of light doesn’t have a perspective, if by perspective you mean a rest frame.

 

[PeroK]

So do you mean that photons can’t tell if it’s moving or not?

I mean at its perspective.

If you really want to learn SR, you need a proper textbook. At the moment, you're just fishing in the dark. You need to get rid of these fundamental misconceptions.

For example, in post #3 I pointed out that all motion is relative, so nothing can "tell if it's moving or not". An insight, by the way, that goes back to Galileo.

 

[Ibix]

So do you mean that photons can’t tell if it’s moving or not?

I mean at its perspective.

No one can tell if they are moving or not. You can only tell whether or not you are moving relative to something (typically the surface of the Earth in every day life).

But what I mean is that you can't ask questions about the perspective of light. The question cannot be framed coherently. This kind of thing is very common as you move away from every day experience. Apparently perfectly reasonable questions are based on assumptions that don't apply. Asking about the perspective of light is like asking which direction is north - while standing at the north pole. The only possible answer is that there isn't one.

You will find people (notably Brian Greene) saying that time stops at the speed of light. He doesn't do it in his professional publications, I gather, only in his pop-sci stuff. So apparently even he doesn't think that's a useful answer...

 

[YoungPhysicist]

If you really want to learn SR, you need a proper textbook. At the moment, you're just fishing in the dark. You need to get rid of these fundamental misconceptions.

For example, in post #3 I pointed out that all motion is relative, so nothing can "tell if it's moving or not". An insight, by the way, that goes back to Galileo.

Do you mean things like this?

https://web.stanford.edu/~oas/SI/SRGR/notes/srHarris.pdf

 

[m4r35n357]

Do you mean things like this?

This is simpler.

 

[YoungPhysicist]

I mean this part:

If you really want to learn SR, you need a proper textbook. At the moment, you're just fishing in the dark. You need to get rid of these fundamental misconceptions..

And yeah, that part too.

 

[m4r35n357]

OK you got me, but IMO Galileo is much underrated ;) I see a lot of questions here that were answered comprehensively 500 years ago.

As for textbooks, I learned my stuff from the WWW . . . The Wikipedia page here is not too bad, but the shortest canned overview of SR that I know of is here (it is not strictly a textbook, but describes special relativity in a modern way, derives the Lorentz transform from Einstein's postulates, and from it the spacetime interval - you know, the one that is zero for light).

Both of these contrast SR with Newtonian/Galilean relativity.

 

[YoungPhysicist]

Thanks. I’ll check it out.

 

[PeroK]

Do you mean things like this?

https://web.stanford.edu/~oas/SI/SRGR/notes/srHarris.pdf

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:

https://www.goodreads.com/book/show/6453378-special-relativity

 

[sqljunkey]

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?

 

[PeterDonis]

So if I was traveling at the speed of light

You can't travel at the speed of light. Only light (and other massless things) can.

 

[Dale]

So if I was traveling at the speed of light my clock would not be moving relative to me?

Don’t worry, you won’t be traveling at c no matter what, so the question never arises.

 

[Ibix]

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?

It isn't possible to describe what it looks like when traveling at c, any more than it is possible to describe which way is north at the north pole. The question makes no sense.

 

[m4r35n357]

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?

You can't. Thinking about that (at age 16) was what got Einstein started towards SR in the first place.

 

[ZapperZ]

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?

1. You can travel at ANY speed with your clock, and your clock will not be moving relative to you. That's the definition of things being in the same reference frame. Presumably, the device that you typed with to post on this forum wasn't moving with respect to you while you were using it. So why not the clock?

2. There is a fundamental problem with the statement "... if I was traveling at the speed of light..." and then, applying Special relativity concepts of time and space to conclude what will happen. The problem comes in with the postulates of SR. These are the starting points of SR, whereby all other consequences, such as time dilation and length contraction, are derived from. One of the postulates is that the speed of light is a constant in all inertial reference frame. This means that no matter what reference frame that you choose, the speed of light remains at "c".

So already your scenario has a problem, because if you are traveling at the speed of light, then light must be having a speed of zero in your reference frame. If this is so, you cannot use the equations derived from SR, because they are not valid! It breaks one of the postulates of SR to begin with. So your concept of time (i.e. your ability to measure time in your frame and in another frame) is unknown and undefined. You will have to use another concept of time from a theory that has yet to be consistently formulated AND verified that goes beyond SR.

This type of question has appeared repeatedly in this forum, and it might be educational to do a search on it, because all the responses that we are providing here are almost repetition of what have already been mentioned several times.

Zz.

 

[sqljunkey]

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 traveling at C anytime soon ibix.

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

 

[m4r35n357]

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]

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.

Anything, everything and nothing.
https://en.wikipedia.org/wiki/Principle_of_explosion

 

[PAllen]

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:

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

 

[PeterDonis]

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.

 

[vanhees71]

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:

https://www.goodreads.com/book/show/6453378-special-relativity

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

 

[Herman Trivilino]

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 $c$.

 

[Ratman101]

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]

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.

 

[ZapperZ]

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.

 

[sqljunkey]

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]

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 traveling 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.

 

[weirdoguy]

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

Did you even read what was said in this thread?

 

[Chris Miller]

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

 

[vanhees71]

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).

 

[NoTe]

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 going to edit Wikipedia pages... but it shows as a nice example of the reliability of Wikipedia and such!

 

[vanhees71]

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.

 

[Alfredo Tifi]

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]

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.

 

[Tom Atkinson]

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 traveling 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]

If you were traveling 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]

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 traveling 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 traveling 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.

 

[kurt101]

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.

 

[weirdoguy]

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?

 

[.Scott]

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".