Why is the speed of light... erm... the speed of light?by Vectus Tags: finite, photon, spacetime, speed of light 

#1
Jul1509, 04:05 PM

P: 37

I'm not entirely sure if this is the right place to ask this question, but here goes:
I have a question that's been plaguing me for a while. I understand why it is pointless to think in terms of a photon's rest frame, as it is either impossible or meaningless to define said frame. I also understand why the speed of light should be finite and constant, given the permittivity and permeability of free space, and the aspects of relativity that state it should be constant in every frame of reference. However, I cannot think intuitively to reconcile these bits of knowledge. How is it that to us, a photon takes some finite amount of time to travel any given distance, but the photon does not experience time, and is emitted and absorbed at the same instant? 



#2
Jul1509, 04:18 PM

PF Gold
P: 4,081

Does a rock experience time ? Or anything ? Photons are inanimate ( if they exist at all ) so I don't see the point of your question. When a photon is created, travels somewhere and is absorbed  time has passed between those events.
What's to reconcile ? You're just confusing yourself with naive literalism which you mistake for insight. The title of this post is misleading because your question has nothing to do with the speed of light. 



#3
Jul1509, 04:58 PM

P: 37

Well of course I do not mean 'experience' as in the photon somehow is conscious and can take in information about its surroundings. I use it in a figurative sense. I thought that much would be apparent.
I do not appreciate the tone you take for me asking about something that puzzles me. I get that I may not accurately have expressed what it is I wish to. That does not warrant mocking me. Do you have any suggestions for what I should change the title to? 



#4
Jul1509, 07:29 PM

Emeritus
Sci Advisor
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P: 9,018

Why is the speed of light... erm... the speed of light? 



#5
Jul1509, 07:30 PM

PF Gold
P: 4,081

It is a postulate of SR that for timelike ( by which I mean subluminal ) worldlines the proper time may be understood to mean the elapsed time on clocks on the worldline. This postulate does not include null geodesics. So SR does not say that the time experienced by a photon is zero, even if you admit to the concept of 'photon experiencing time' which I do not. I'm sorry about the tone. Forget the title. Maybe the above will help. 



#6
Jul1509, 09:56 PM

P: 37

I see now.
Thanks for the answers. That really was something simple I was overlooking. 



#7
Jul1509, 10:04 PM

P: 15,325

In 4 dimensional spacetime, a photon is a line whose two end points intersect two electrons. There is no "passage" of time; it is simply a static, straight line that connects two points.
For an analogy, read this. 



#8
Jul1609, 02:33 AM

Mentor
P: 16,488

It seems like we haven't had one of those in a while. 



#9
Jul1609, 03:15 AM

P: 128

I was thinking about this at age 15 or so, certain that I had just stumbled on the explanation for 'spooky actions at a distance'  that two entangled photons shared behavior without delay because the information was being transferred 'backward' up one photonpath and then down the other photonpath, all in the same 'timeslice'. It broke my heart when I learned that entanglement occurred with sublight particles, too, as I was looking forward to that Nobel prize :) 



#10
Jul1709, 06:05 PM

P: 44

As I see it all the problem is of philosophical nature in particular epistemological nature.
Physics relate the world of physical phenomena with a certain world of discourse that in this case is a mathematical discourse that have the task to make predictions. Unfortunately there is no guarantee that there is a 11 mapping of the physical phenomena to the discourse or even that all the predictions made by the discourse have a physical phenomenon related. In fact the leap from the physical world to the physical discourse (the converse is no so controversial) have no recipes (although some people think that actually there is a SCIENTIFIC METHOD). So if you think that the time for a photon "does't pass" you think that only because Relativity (the Discourse) says (through the mathematical machinery) that arc length over a null geodesic with the Minkowski metric is always zero. But that's only a mathematical result, a prediction of the Discourse. There's no Physics on it (no more than the physics that is implied by the axioms of relativity), then you have to pass from the Discourse to the Physical World. 



#11
Jul1709, 09:43 PM

P: 15,325





#12
Jul1809, 12:57 AM

P: 37

Causality doesn't exist? Spacetime is static? That's going a bit far I think. What makes you say this? 



#13
Jul1809, 07:31 AM

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P: 16,488




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