Light & Time

[spacecadet2563]

Not a great question but I heard someone say that time does not move for a photon. T=0.
So therefore both its Emission &Absorption are simultaneous events. I say no. If both of these events are supposed to be considered different events than there would need to be a time interval between them. Is that correct? Thanks for responses. I will have to think about the previous ones...no way

 

[robphy]

Not a great question but I heard someone say that time does not move for a photon. T=0.
So therefore both its Emission &Absorption are simultaneous events. I say no. If both of these events are supposed to be considered different events than there would need to be a time interval between them. Is that correct? Thanks for responses. I will have to think about the previous ones...no way

"Proper time" does not exist for the photon.

However, there must be a sense of causality,
The Emission-event occurs before the Absorption-event.
They are distinct-events with a nonzero-displacement-4-vector, even though that displacement vector is lightlike (null, has square-magnitude zero).

 

[Ibix]

Not a great question but I heard someone say that time does not move for a photon.

No. The time experienced along a null path is not zero, rather it is not defined.

If both of these events are supposed to be considered different events than there would need to be a time interval between them.

No. Or, more precisely, you need to define whether you are talking about the interval along the path between two distinct events (which may be zero, but isn't "time" in that case) or the coordinate time somebody defines, which will be non-zero between emission and absorbtion events.

These two concepts are distinct in relativity but not in Newtonian physics. Failing to grasp this is one reason people end up with contradictory ideas about relatvity.

 

[.Scott]

I don't think there's any simple way to describe how a photon moves. It is not at all like a regular missile.
The information that is involved in it's trajectory comes from a much wider region than you would expect.

Consider this Mach-Zehnder interferometer experiment:
1) Each photon reaches the first half-silvered mirror and effectively follows both paths.
2) Using mirrors, the photon paths are rejoined at another half-silver mirror where an interference pattern forms.
3) In parts of that pattern, destructive interference keeps photons from reaching those parts.
4) Now block one of the two paths ...
5) Photons will now reach those previous dark zones.

The photons that light those zones are photons that did not encounter the block. But because they could have, they were able to reach the dark zone.

It's called a counterfactual - something that didn't happen but affected the final outcome because it could have.

 

[DaveC426913]

I say no.

For future reference:
PF does not allow personal theories. PF supports established, peer-reviewed standard model science.

That does not mean you have to know everything about current established, peer-reviewed standard model science; it simply means you have to not declare it's wrong.

If you do not like what our standard model of physics says about things like spacetime and relativity, it is not appropriate here to describe your own personal ideas. The appropriate thing to do is to ask where you are misunderstanding our current models.

This is the PF way.

 

[spacecadet2563]

For future reference:
PF does not allow personal theories. PF supports established, peer-reviewed standard model science.

That does not mean you have to know everything about current established, peer-reviewed standard model science; it simply means you have to not declare it's wrong.

If you do not like what our standard model of physics says about things like spacetime and relativity, it is not appropriate here to describe your own personal ideas. The appropriate thing to do is to ask where you are misunderstanding our current models.

This is the PF way.

Never declaring anything wrong.

 

[Dale]

Not a great question but I heard someone say that time does not move for a photon. T=0.

There are two types of time in relativity: proper time and coordinate time. This statement is not true for either.

So therefore both its Emission &Absorption are simultaneous events. I say no.

Indeed, emission and absorption are not simultaneous events. This statement is a statement about coordinate time since the proper time between emission and absorption is not defined.

If both of these events are supposed to be considered different events than there would need to be a time interval between them. Is that correct?

Yes, there is an interval of coordinate time between emission and absorption.

 

[DaveC426913]

Never declaring anything wrong.

:

I say no.

It would be better to just read what physics does say about photons, than to you make your own assertions.

 

[.Scott]

Not a great question but I heard someone say that time does not move for a photon. T=0.

If you want to simulate what a photon does, then at the moment that the "emission" occurs, you need to capture all the information about what that photon can encounter in every possible direction it could be following. So, using that Mach-Zehnder interferometer experiment I mentioned before, what is important is not whether the block was there at the moment of emission, but whether it would be there when the photon would reach that point in any of its potential trajectories.
It's as if: 1) all of that information is collected at the moment of emission; 2) the interference pattern is calculated; and 3) a random destination is picked weighted by the values in the interference pattern.
But, in all cases, the absorption will occur an amount of time after the emission that is determined by the time it takes light to follow that randomly selected path.

Part of @Dale response to you (@spacecadet2563 ) was this:

This statement is a statement about coordinate time since the proper time between emission and absorption is not defined.

In light of @Dale's statement, let's consider the speed of neutrinos. It was originally thought that neutrinos were massless and that they therefore must travel at the speed of light. But, about 25 years ago, experiments by Takaaki Kajita and Arthur B. McDonald showed that neutrinos change during flight. They "oscillate" among their three possible neutrino flavors as they cruise through the universe. Well, they can't do that without proper time. Proper time must be defined for them - and holding to the commonly held presumption that they are not interacting with anything in flight, that would mean those neutrinos must have non-zero mass.
So the fact that photons have no proper time is a more significant statement than it might first appear. It means that photons can't change in flight. The end result is as if they follow those superposition of paths and ultimately land on some select destination - but that entire process isn't an evolution of the photon - it is just a single emission/absorption process.

I would guess that if someone said that "time does not move for a photon", they were likely alluding to this characteristic of massless particles. But as you can see, it's really not that simple. And trying to describe it is also not simple because our language is tuned for describing how classical objects move through Newtonian space and time.

 

[spacecadet2563]

If you want to simulate what a photon does, then at the moment that the "emission" occurs, you need to capture all the information about what that photon can encounter in every possible direction it could be following. So, using that Mach-Zehnder interferometer experiment I mentioned before, what is important is not whether the block was there at the moment of emission, but whether it would be there when the photon would reach that point in any of its potential trajectories.
It's as if: 1) all of that information is collected at the moment of emission; 2) the interference pattern is calculated; and 3) a random destination is picked weighted by the values in the interference pattern.
But, in all cases, the absorption will occur an amount of time after the emission that is determined by the time it takes light to follow that randomly selected path.

Part of @Dale response to you (@spacecadet2563 ) was this:

In light of @Dale's statement, let's consider the speed of neutrinos. It was originally thought that neutrinos were massless and that they therefore must travel at the speed of light. But, about 25 years ago, experiments by Takaaki Kajita and Arthur B. McDonald showed that neutrinos change during flight. They "oscillate" among their three possible neutrino flavors as they cruise through the universe. Well, they can't do that without proper time. Proper time must be defined for them - and holding to the commonly held presumption that they are not interacting with anything in flight, that would mean those neutrinos must have non-zero mass.
So the fact that photons have no proper time is a more significant statement than it might first appear. It means that photons can't change in flight. The end result is as if they follow those superposition of paths and ultimately land on some select destination - but that entire process isn't an evolution of the photon - it is just a single emission/absorption process.

I would guess that if someone said that "time does not move for a photon", they were likely alluding to this characteristic of massless particles. But as you can see, it's really not that simple. And trying to describe it is also not simple because our language is tuned for describing how classical objects move through Newtonian space and time.

Its all really one event not two would be a safe assumption.

 

[Dale]

Its all really one event not two would be a safe assumption.

Event has a technical meaning in relativity. It is not one event, but it is one process that does not involve an evolution of the photon itself.