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Does time stop for a photon? Why is that a nonsensical question? 
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#1
Nov2011, 12:03 AM

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From this link http://math.ucr.edu/home/baez/physic...eadlights.html I don't understand the following..."Does time stop for a photon?. . . It is really not possible to make sense of such questions and any attempt to do so is bound to lead to paradoxes. There are no inertial reference frames in which the photon is at rest so it is hopeless to try to imagine what it would be like in one."
In particular this statement "There are no inertial reference frames in which the photon is at rest". Can anyone explain that to me? 


#2
Nov2011, 12:19 AM

P: 27

I believe the answer to your question is that light is defined as photons, so photons are, always, traveling the the speed of light ([itex]c[/itex]). This is why photons are said to have [itex]0[/itex] mass.



#3
Nov2011, 12:55 AM

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One of the postulates of Special Relativity is that light moves at the same speed, c, in vacuum, in all inertial reference frames. This postulate immediately says that there is no reference frame in which the light is NOT moving at c, and "at rest" is definitely not moving at c.



#4
Nov2011, 03:30 AM

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Does time stop for a photon? Why is that a nonsensical question?
In SR and GR there is a welldefined mathematical procedure to calculate proper time for moving objects along trajectoreis through spacetime (as measured by a comoving clock). Applying this procedure to a lightlike trajectory (along which photons move) the result is always zero, i.e. proper time for photons vanishes. In that sense photons are 'timeless'.



#5
Nov2011, 09:42 AM

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PF Gold
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We have our own FAQ on this topic: Rest frame of a photon.



#6
Nov2011, 10:08 AM

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An inertial frame is one in which the principle of relativity holds  relative to an inertial frame, moving faster but at constant speed, the laws of physics take the "same form" as when one is not moving. This is due to a symmetry in the laws of physics called "Poincare invariance". 


#7
Nov2011, 10:38 AM

P: 1,555

I do not like the argument that because we cannot make a frame of reference the question is nonsensical. Frames of reference have no physical significance as they do not exist in nature.
As tom.stoer pointed out one does not need a frame of reference to demonstrate that the total elapsed time for the path of a photon between two events is zero. 


#8
Nov2011, 10:48 AM

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I do agree with the second statement as providing a good meaning to "time stops for a photon". 


#9
Nov2011, 02:17 PM

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That's why saying that "time stops for a photon" is not really a good way, IMO, to convey the difference between null objects and timelike objects, since it invites the inference that a "photon frame" is just like an ordinary inertial frame, only "moving at c". Saying that the "length" of a photon's worldline is always zero between any two events on it is better, but calling that length "elapsed time" is still dodgy, IMO, because it again invites the erroneous inference. I would say that the concept of "proper time" or "elapsed time for the object" simply doesn't apply to objects that move on null worldlines. If amplification is needed, see my first paragraph above. 


#10
Nov2011, 02:34 PM

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PeterDonis, I, of course, have no technical disagreement with what you say, only a poetic one  we shouldn't have to give up our favourite kludges, I think;)



#11
Nov2011, 02:36 PM

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For instance asking "What would the rate of a clock be if we discount the light travel time of a given Doppler shift of an object which is in relative motion to us" is interesting for professors to ask students in a test but apart from that what is the scientific value of those questions, frames or planes of simultaneity do not really exist, or do you disagree? 


#12
Nov2011, 04:55 PM

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Labeling the coordinateindependent geometric objects with particular coordinates is arbitrary and doesn't affect the physics. So I would agree that "frames", in the sense of particular coordinate labelings, "do not really exist". But the things that the coordinates label do (at least in the same sense that spacetime itself does). Questions about "proper length" and more generally about surfaces of simultaneity are more complicated to correlate to direct physical measurements, since you first have to talk about clock synchronization and the relativity of simultaneity. But it can still be done. Whether or not it is *useful* to do it depends on the problem. Over small distances it seems to me to be useful; for example, it's hard to talk about local inertial frames and what happens in them without talking about proper length measurements within those frames. But it can be problematic when people try to extend it out over large distances, such as the recent threads about what is happening "now" on Mars or in the Andromeda galaxy. In those cases I agree that trying to assign some sort of "real meaning" to a particular surface of simultaneity causes confusion and doesn't help with understanding the physics. 


#13
Nov2011, 07:31 PM

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But even in this case another observer can calculate the total time on the other clock by observing the Doppler shift between the events. No such planes of simultaneity are neccesary. I think that if we stick to relativistic Doppler shift, proper distance and proper velocity (celerity) special relativity becomes a lot simpler. 


#14
Nov2011, 08:06 PM

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#15
Nov2011, 08:13 PM

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So, your attempt to dismiss mathematical objects would stop inquisitiveness in its tracks. 


#16
Nov2011, 08:14 PM

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"Of course it is not equivalent."
That is what I meant. There is a difference to me between how allegedly two clocks are running with a different rate when they are in relative motion and two clocks going between two events with a different path in spacetime. The first can never be proven while the second obviously can. 


#17
Nov2011, 08:15 PM

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#18
Nov2011, 08:19 PM

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