B Does Light Phase Change During Travel?

[Bob Dylan]

If a photon doesn't travel through time (if there is no internal change by one definition of time) then I would expect a photon not to have a changing phase as that seems to count as a timeable internal change. Is this the case?

 

[lekh2003]

If a photon doesn't travel through time (if there is no internal change by one definition of time) then I would expect a photon not to have a changing phase as that seems to count as a timeable internal change. Is this the case?

What do you mean by phase of light? Why would light change phase?

 

[Bob Dylan]

I mean the phase in the quantum sense of a wavefunction.

 

[lekh2003]

I mean the phase in the quantum sense of a wavefunction.

You mean it's probability and wave function. That's not how the wave function works. It's not a phase changing. It's a probability.

Think about probabilities. Time becomes irrelevant and so does your question. Think about it, or read up on the wave function.

 

[Ibix]

If a photon doesn't travel through time (if there is no internal change by one definition of time)

There is no coherent definition of "a photon's perspective", and from any other perspective a photon clearly does travel through time.

 

[Bob Dylan]

Well, if youll permit me to revise the question a bit, Ill ask about minowki spacetime. When we say that a particle doesn't pass through time, are we really saying that it doesn't pass through spacetime. I am confused about how that's being defined. For example if a photon had a change in time of one and a change in x of one, then would it be traveling at C because in Minowski Spacetime the time is negative to space and therefore cancels to 0.

 

[Bob Dylan]

The reason I add that is that ultimately I am trying to understand why change in space seems to equate to change in time in the Dirac equation.

 

[Ibix]

I mean the phase in the quantum sense of a wavefunction.

I'm not at all sure this question is clearly posed. Again, properties of a photon clearly can evolve with time (single-photon diffraction patterns look like classical ones, and vary with distance - and hence flight time - from the diffracting object).

 

[Bob Dylan]

It is confusing to me because if there were theoreticaly (though impossibly) a massless fermion in the Dirac equation then seemingly it would be traveling at C and through time and phase by my clearly flawed understanding.

 

[Ibix]

When we say that a particle doesn't pass through time

I challenge you to find a non popsci source that says anything of the sort.

For example if a photon had a change in time of one and a change in x of one, then would it be traveling at C

A particle moving a distance $\Delta x$ in time $\Delta t$ has speed $\Delta x/\Delta t$. In the example you give, that is 1. Whether or not $c=1$ depends on your choice of units. Since you are saying that this particle is a photon, you are apparently using such a scheme.

because in Minowski Spacetime the time is negative to space and therefore cancels to 0.

You don't use the interval to measure speed.

 

[Ibix]

The reason I add that is that ultimately I am trying to understand why change in space seems to equate to change in time in the Dirac equation.

Is your problem just not realising that if you pick units where c=1 then you can drop the c?

It is confusing to me because if there were theoreticaly (though impossibly) a massless fermion in the Dirac equation then seemingly it would be traveling at C and through time and phase by my clearly flawed understanding.

I don't know enough quantum field theory to comment - others here do. I will just observe that you seem to be struggling with special relativity here, and you will need to be confident with that before you can follow relativistic field theory. Also, if you are trying to use a theory to understand something that theory says is impossible you are bound to end up with problems.

 

[Bob Dylan]

Alright, I've seen natural units used many times, but I've always found that confusing because I've heard that photons don't travel through time, but apparently I was just being confused by the popsci.Thanks for the answers.

 

[vanhees71]

Don't start thinking about photons before you have understood classical physics. To really understand photons you need relativistic quantum field theory. Photons are the most non-classical objects we observe in everyday life, and there is no other way to understand what they really are than to learn relativistic QFT. For sure, you have a wrong picture, even on a qualitative level, if you think of them as minature billard balls, which they are clearly not.

In everyday life they usually appear as coherent states of quite high intensity, which are well approximated by classical electrodynamics. My suggestion is to first study relativistic classical mechanics of point particles. This you can find in my FAQ article on SR:

https://th.physik.uni-frankfurt.de/~hees/pf-faq/srt.pdf

and then classical electrodynamics, which treats the subject relativistically like, e.g.,

Landau, Lifshitz vol 2 or Schwartz, Principles of Electrodynamics

 

[Dale]

When we say that a particle doesn't pass through time

Anybody who says this is talking nonsense

Im confused about how that's being defined

It is not defined. That is why it is nonsense.

All you can do is ignore it. You cannot gain any meaningful information by struggling to understand nonsense like this.

 

[pervect]

SR is a classical theory, so it can be (and probably should be) considered without dealing with photons, but rather with pulses of light. With any of the classical definitions of the phase of a wave, the phase of the wave will propagate at the speed of light. . I.e. if you pick out a point on a plane wave that has a known value of the E-field (which is the part of the wave that I assume you're measuring the phase of) then whatever the value the E-field and it's phase has will propagate with the speed of light. Thus the phase will be constant along the worldline of the light pulse . If we consider a light pulse propagating in the +x direction, then if the phase is zero at t=0 and x=0, the phase will be zero at t=1sec, x=c*1 sec, or t=2 sec and x=c*2sec, in general at t=a and x=c*a the phase will be the same as it was at t=0 and x=0.