# Do photons travel in time?

1. Oct 24, 2006

### chandubaba

if photons travel in the speed of light why dont they travel in time

Last edited: Oct 24, 2006
2. Oct 24, 2006

### DaveC426913

For starteres, photons are not particles like atoms. They do not exist in the same physical sense, so material laws do not apply to them in the same way.

Because photons travel at c, they do not experience time at all. The entire universe of their existence happens simultaneously.

3. Oct 24, 2006

### ZapperZ

Staff Emeritus
Who said they don't?

We know that it takes light about 8 minutes to get to us from the sun. Don't you think that is a "travel in time"? Or are you make a faulty application of SR's time dilation effects?

Zz.

4. Oct 24, 2006

### swain1

I have read and been told that photons do not travel through time. If they are travelling at the maximum speed through space they cannot travel through time. Therefore photons don't age. Obviously from our view they are travelling through time just as we do.

5. Oct 24, 2006

### DaveC426913

Yeah, Zz, I think we can assume he means "photons don't experience time".

6. Oct 24, 2006

### ZapperZ

Staff Emeritus
This is where we have to define what is meant by "travel in time". If something that is separated in space, and we measure zero time for the thing to go from Point A to Point B, then yes, we can say that it did not travel in time, for the obvious reason that it didn't use up any measured time to go in between those two points.

Now, this is DIFFERENT than observing that a clock in the reference frame moving with the photon doesn't move. In our frame, we notice that the photon's clock didn't change at all when it went from Point A to Point B. It is from our perspective that the photon's clock didn't register any time change when it went from Point A to Point B. However, we certainly measure, in OUR frame, that the photon did take a time period to go between the two points.

I have always tried to emphasize that when one starts dealing with SR and QM, the question itself becomes an issue that needs to be clearly defined. So when one asks "Did something travels through time?", my first inclination would be to see if it DOES takes time to go from one point to another. If it does, then yes, it did travel through time. This is how we describe a dynamical system. We do not, however (at least not very often), describe the dynamical system from the frame of reference of the object that is moving. So when I ask "how long did it take the muon to travel from the upper atmosphere to the earth", you wouldn't instinctly give me the time lasped from the point of view of the muon, would you?

Thus, when such a question pops up, I always answer from my (our) reference frame, because this is what WE measure.

Zz.

7. Oct 24, 2006

### DaveC426913

One way, the question is so trivial as to be nonsensical, causing you to question if that's really what he meant, leading us to: the other way - an extremely common concept that trips up and confuses almost every new student to SR. You don't think, even it retrospect, that it should have been obvious which?

Anyway, on with the question - well, unless it's already been answered....

8. Oct 24, 2006

### rcgldr

What I don't get is photons have a frequency, yet it's stated that they don' experience time. As previously mentioned in other posts, my thoughts on this is that the rules for sub-light speed objects with mass are different for light speed photons.

9. Oct 25, 2006

### ZapperZ

Staff Emeritus
I've been on here long enough to know NOT to make "guesses" on what some people mean.

The OP hasn't gotten back on this. This could easily be an "ask and run" type of thing. So we could easily be worked up over nothing. In any case, I still think that it is a worthwhile effort to make people THINK about the question itself, and how things we take for granted to be obvious really need to be considered carefully when we start dealing with SR and QM. To me, that is as important of a realization as the understanding of what QM and SR say.

Zz.

Last edited: Oct 25, 2006
10. Oct 25, 2006

### billiards

I agree with ZapperZ, when I first saw the title of the thread I was going to write something like: they do travel in time, but they (photons) don't know it.

Although I must admit I don't really understand GR so apologies if that's wrong.

11. Dec 8, 2006

### chandubaba

by travel in time I mean going to the future before any other observer reaches .If photons can be considered particles then they will travel in time(front or back)

12. Dec 8, 2006

### daniel_i_l

What does that mean? reaches what?

13. Dec 8, 2006

### vanesch

Staff Emeritus
The word "time" is loaded with many different but related meanings and if one isn't precise about what meaning exactly one is talking about, then it is the best way to get a confused discussion.

One way of defining time is: the event coordinate usually taken in an observer rest frame corresponding to that observer's clock. With such a definition, it is obvious that photons travel in time - as Zz pointed out: in our reference frame, they travel about 8 minutes from the sun to here.
Another way to define time is "eigentime". It is the invariant interval measured along a thing's world line. As photons travel on light-like worldlines, by definition, their eigentime is zero. But given that no observer can be on a lightlike world line, there are no existential problems for a potential "observer" travelling with a photon: he has no reference frame in which he is at rest (which is a requirement to be an observer).

Other definitions of time are of a more philosophical nature and I won't go into them here.

14. Dec 8, 2006

### chandubaba

1Vanesh Sir what is eigentime and world line?When we are standing on earth then we are at rest w.r.t. the photons,isn't it ?please clarify.
2Daniel sir,I mean the photons reaches some other time other than a stationary observer (w.r.t. to the photons),say a day later .Dont you think the light we see from the sun is a day old.please clarify if this is a misconception.Do we have to consider that photons are not particles but waves as dave sir pointed out in post #2,but they are considered BOTH isn't it?