# Stationary Photons

1. Mar 31, 2004

### Swiss Army

I know that the speed of light is constant no matter the speed of the observer, my question deals with the effects of time dilation on a photon moving at light speed. A photon at light speed would experience 100 percent time dilation due to it's velocity, therefore, shouldn't it appear stationary? Or is it wrong to assume that just because time stopped for the photon, that it isn't moving?

2. Apr 1, 2004

### Staff: Mentor

Good start to your thought process. Clearly, since light travels at C, time cannot exist for a photon. This may explain some of the observed quantum weirdness such as photons "knowing" what path to take before the path exists (opening or closing a slit in the two-slit experiment after the photon has passed it changes the outcome). If a photon exists without time, "before" is irrelevant. The path just is.

3. Apr 1, 2004

### Swiss Army

hmmm...

Ohhh, I see. Since time doesn't exist for the photon, but how would time dilation effect it's motion as viewed by an observer. I've read of many examples of where an object such a person falls into a black hole and experiences time dilation so severe that an onlooker would see the falling person to be stationary at the event horizon of the black hole because of this time dilation due to gravity. So why do we still witness a photon to be moving although it also experiences 100 percent time dilation as well.

4. Apr 1, 2004

### one_raven

Have you ever actually witnessed a photon moving?

5. Apr 1, 2004

### DrMatrix

No, you've got it backwards. A stationary object is in the same place at different times. An object in different places at the same time is moving infinitely fast. So I'd say from the photon's POV, it is moving infinitely fast, if only I could ignore the inconvient fact that a photon has no POV.
It is the photon that is experiencing the time dialation (or would be if it could experience anything), not us. Time flows for us.

6. Apr 1, 2004

### MathematicalPhysicist

i dont think it can explain the "knowing of what path to take" because also electrons have this feature of "knowing what path to take" and they (as far as i know) dont travel at speed c.

7. Mar 22, 2010

### Harry Mac

electrons travel at close to the speed of light as beta radiation yet only at 0.024 cm/s with a 10 amp supply in copper wire.

8. Mar 22, 2010

### jfy4

I have received an excellent reply to a similar question by Fredrick. Ill paraphrase his/her response.

It doesnt make any sense to assume that a photon would experience no time, or that the universe would have no length in its reference frame. Specifically we derived the length contraction and time dilation formulas assuming we were not light! We Arrived at those conclusions using the speed of light as constant, and actually making sure that we were talking about objects with mass. Taking the limit as v->c in those formulas holds good for when we are talking about massive objects. Thus while it might seem clever to assume that light would experience these effects in its frame of reference, it makes little sense when you think about it specifically because of the nature with which our conclusions were derived.

I hope this provides some insight.

9. Oct 20, 2010

### physics pfan

I think the best explanation of this I have read is in Klevgard's book [ http://www.einsteinsmethod.com ] "Einstein's Method: A Fresh Approach to Quantum Mechanics and Relativity" especially pp. 46, and 127. I can try to summarize how it applies to your question.

Matter quanta stationary in space (emphasis: for some observer) have an extension dimension (space) and a progression dimension (time). An "extension" dimension for a quantum is that dimension in which the quantum: 1) extends and 2) is located relative to some arbitrary reference. We as material observers share the same extension and progression dimensions as matter quanta (particles). Hence when the stationary matter quantum starts to move we measure its velocity in distance traveled in our extension dimension divided by the time interval in our progression dimension. But things are not so straightforward for radiation quanta.

Energy quanta (radiation photons) are not stationary in space, they are stationary in time. Photons also extend in time and in theory you could designate one photon as reference and assign a location to other photons by their (static) temporal separation from the reference. All of this is to say that photons are the exact opposite of space-stationary matter quanta. A photon's progression dimension is space and its extension dimension is time. Time for radiation is a fixed set of temporal intervals separating energy quanta; to repeat, time for the photon is an extension, it is not a progression.

And so when we try to measure the velocity of radiation we are forced to impose our (progressing) time measure upon energy quanta whose innate time measure is an extension and doesn't progress.

So the answer to your question is that yes the photon is stationary in its extension dimension (time) just as the matter quanta/particle is at rest in its extension dimension (space). If we ignore projectile motion we can say that all quanta are at rest in their extension dimension and definitely not at rest in their progression dimension.

Projectiles may have "motion" in space and in time, but at-rest matter and photons have "motion" (progression) in only one dimension and they differ on which dimension that is.

Hope this helps.

Last edited: Oct 20, 2010