# Does a photon age?

1. Mar 13, 2008

### GearsofWar

Does a photon age?

2. Mar 13, 2008

### GearsofWar

a related question:

Suppose a battery-operated clock flew at 99.999999999999999999999999999999999999% the speed of light.

When we decelerated the clock to rest relative to the lab frame, and took the batteries out so the hands would stop, would we all agree on the time read on the clock?

Does a photon age?

3. Mar 13, 2008

### JesseM

Yes, everyone agrees on local physical facts like this.
It's not really valid to apply the time dilation equation to the photon, but I don't think there's any meaningful sense in which it could be said to age, so I'd say no.

4. Mar 13, 2008

### pmb_phy

Sure. Why not? Since its meaningless to speak of time as measured in the photons rest frame we are left speaking only in terms of (coordinate) time and as such a photon can exists for any given amount of time. I suppose you can call that "aging."

Pete

5. Mar 13, 2008

### peter0302

The only meaningful definition of "age" in the context of a subatomic particle would be decay, since subatomic particles have no "moving parts" by which to measure their age, and photons do not decay.

Your other question about the clock - yes, we'd all agree on the time read on the clock. Not sure what your point is.

One of the clearest experimental verifications of time dilation is the decay of muons in the atmosphere. Muons last only a short time in a lab, but those coming from space, travelling at relativistic speeds, actually last long enough to be detected on the surface. That wouldn't be possible unless they slowed down their "aging." A photon, travelling at 'c', would have its "aging" slowed down to zero, so we could never know what would happen to the photon if it ever did "age."

Last edited: Mar 13, 2008
6. Mar 13, 2008

### GearsofWar

Protons don't decay either.

Does that mean that time stops at the speed of protons?

7. Mar 13, 2008

### JesseM

It's actually unknown whether they decay, there are some theories which say they should. But in any case, a larger clock moving alongside a proton would still tick as seen in any frame, so even if they don't decay, it's not a consequence of time dilation.

8. Mar 13, 2008

### GearsofWar

So does a photon age?

Does a clock which approaches the speed of light tick faster or slower?

9. Mar 13, 2008

### JesseM

A clock which is approaching the speed of light relative to us will tick slower, approaching a rate of zero, in our frame (in the clock's own rest frame at any given moment, it is our clocks which are ticking slower). But it's impossible to accelerate a clock to exactly the speed of light (it would require infinite energy), and the Lorentz transformation gives meaningless answers if you try to plug in v=c to calculate the "frame" of a photon.

10. Mar 13, 2008

### GearsofWar

How about a clock which is approachingthe speed of light not relative to us?

Would it tick slower or faster?

Also, if a clock is going

99.99999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999% the speed of light, would time pass slower or faster for it?

11. Mar 13, 2008

### JesseM

Do you understand that there is no objective truth about the rate a clock is ticking in relativity, that the answer will be different in different reference frames, and no frame is physically preferred over any other? The details of how fast the clock is ticking at different points depend on what frame you choose. Regardless of what frame you choose, if the clock's speed is v at a given moment in that frame, then its rate of ticking at that moment will be slowed down by a factor of $$\sqrt{1 - v^2/c^2}$$ in that frame. But of course, different frames will disagree about the clock's speed at any moment along its journey.
There is also no objective truth about speed in relativity, except for light which has the same speed in every frame. In our rest frame, we may have a speed of zero while the clock is moving at 99.99999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999% the speed of light and is ticking extremely slowly in our frame, but in the clock's own rest frame it has a speed of zero and we have a speed of 99.99999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999% the speed of light, and it is our clocks which are ticking extremely slowly in this frame.

12. Mar 13, 2008

### GearsofWar

so what you're saying is that after an atomic clock is flown about the world, and the plane lands, they will see that our clock has ticked slower and we will see that their clock has ticked slower.

13. Mar 13, 2008

### GearsofWar

I think you need to update this wikipedia entry and tell them that their experiments were false: " Velocity and gravitational time dilation combined-effect tests
Hafele and Keating, in 1971, flew cesium atomic clocks east and west around the Earth in commercial airliners, to compare the elapsed time against that of a clock that remained at the US Naval Observatory. Two opposite effects came into play. The clocks were expected to age more quickly (show a larger elapsed time) than the reference clock, since they were in a higher (weaker) gravitational potential for most of the trip (c.f. Pound, Rebka). But also, contrastingly, the moving clocks were expected to age more slowly because of the speed of their travel. The gravitational effect was the larger, and the clocks suffered a net gain in elapsed time. To within experimental error, the net gain was consistent with the difference between the predicted gravitational gain and the predicted velocity time loss. In 2005, the National Physical Laboratory in the United Kingdom reported their limited replication of this experiment.[1] The NPL experiment differed from the original in that the cesium clocks were sent on a shorter trip (London–Washington D.C. return), but the clocks were more accurate. The reported results are within 4% of the predictions of relativity." http://en.wikipedia.org/wiki/Time_dilation

14. Mar 13, 2008

### JesseM

No, I'm not. The time dilation equation only works equally well in different inertial frames, i.e. the frames of observers with constant speed and direction. You can't use the time dilation equation in the frame of an observer moving in a circle (not to mention that when moving around the Earth, you have to go beyond special relativity where the time dilation applies, and take into account the curvature of spacetime predicted in the neighborhood of the Earth by general relativity).

15. Mar 13, 2008

### JesseM

No, I don't. Perhaps you should ask questions and try to understand what I'm saying before jumping to conclusions that I'm contradicting established physics and trying to "taunt" me about it. The idea that all inertial reference frames are equally valid physically, and that they differ on questions of who is moving faster or whose clock is ticking slower, is one of the most basic ideas of special relativity, any introductory text on the subject will explain this.

16. Mar 13, 2008

### GearsofWar

Yes, but the experimentalists took both the time dilation due to gravity, acceleration, and velcoity into account.

The experiment showed that relativistic time dilation is a physical effect.

Why are you denying the experimental results?

The experiment showed that relativistic time dilation is a physical effect.

Why are you denying the experimental results?

17. Mar 13, 2008

### GearsofWar

I'm not taunting you.

I'm just asking you why you're denying the physical results of the physical experiment that demonstarted the physical reality of relativistic time dilation for moving clocks, which have been physically shown, by physical experiment, to physically tick slower when they're physically moving:

http://en.wikipedia.org/wiki/Time_dilation
"The gravitational effect was the larger, and the clocks suffered a net gain in elapsed time. To within experimental error, the net gain was consistent with the difference between the predicted gravitational gain and the predicted velocity time loss. In 2005, the National Physical Laboratory in the United Kingdom reported their limited replication of this experiment.[1] The NPL experiment differed from the original in that the cesium clocks were sent on a shorter trip (London–Washington D.C. return), but the clocks were more accurate. The reported results are within 4% of the predictions of relativity." http://en.wikipedia.org/wiki/Time_dilation

18. Mar 13, 2008

### Staff: Mentor

Please ease up on the attitude. What experimental results do you think JesseM is "denying"?

19. Mar 13, 2008

### GearsofWar

I'm just asking you why Jessie is denying the physical results of the physical experiment that demonstarted the physical reality of relativistic time dilation for moving clocks, which have been physically shown, by physical experiment, to physically tick slower when they're physically moving:

http://en.wikipedia.org/wiki/Time_dilation
"The gravitational effect was the larger, and the clocks suffered a net gain in elapsed time. To within experimental error, the net gain was consistent with the difference between the predicted gravitational gain and the predicted velocity time loss. In 2005, the National Physical Laboratory in the United Kingdom reported their limited replication of this experiment.[1] The NPL experiment differed from the original in that the cesium clocks were sent on a shorter trip (London–Washington D.C. return), but the clocks were more accurate. The reported results are within 4% of the predictions of relativity." http://en.wikipedia.org/wiki/Time_dilation

20. Mar 13, 2008

### JesseM

Of course it is. Even if you do an experiment like this in flat spacetime with no gravity, it's still true that all frames will agree that a clock moving in a circle around a center which is moving inertially will age less than a clock which is at a fixed position on that circle. They will disagree about the circular clock's speed as a function of time v(t) as it moves around the circle, and also disagree about its rate of ticking as a function of time $$\sqrt{1 - v^2/c^2}$$, but when they integrate $$\int \sqrt{1 - v(t)/c^2} \, dt$$ for one complete circle to find the total time elapsed on the circular clock between two meetings with the clock at a fixed position on the circle, they will all end up with exactly the same answer for the elapsed time, and all agree it is less than the elapsed time on the other clock. In general, when different frames calculate how much time elapses on two clocks between two successive meetings of these clocks, they will always get the same answers regardless of the motion of the clocks--that's just a nice feature of the mathematics of special relativity.
Sigh. The fact that all inertial frames are on equal footing is relativity 101, GoW, I think physicists would have noticed if it conflicted with the Hafele-Keating experiment. All you're doing here is boldly displaying your own ignorance of the basics of the subject.