If a photon had a mass, time travels would be possible

[jtbell]

As I understand it neutrinos don't travel at c.

Correct, they don't, precisely because their mass is non-zero.

 

[phyti]

At the speed of light, clocks stop, there is no time!
You don't travel in time, back or forth.

 

[Frame Dragger]

At the speed of light, clocks stop, there is no time!
You don't travel in time, back or forth.

That is the same as asking how a photon experiences time... and the answer is... who knows?

 

[piersdad]

At the speed of light, clocks stop, there is no time!
You don't travel in time, back or forth.

this explains why we can see light from billions of years ago.
the photon leaving the far galaxy experiences no time so arrives here instantly unless slowed by gravity of a massive object

 

[Temporal]

this explains why we can see light from billions of years ago.
the photon leaving the far galaxy experiences no time so arrives here instantly unless slowed by gravity of a massive object

I believe you have this a bit confused. Photons travel at light speed and therefore can't possibly travel instantaneously to Earth from a far galaxy. You see the light from the far galaxy because the photons have been traveling for, in your example at least, billions of years.

Since light is not instantaneous, when you look at the night sky you are seeing things as they appeared at some point in the past (how far into the past depends on how distant the object is which you are observing).

 

[Frame Dragger]

this explains why we can see light from billions of years ago.
the photon leaving the far galaxy experiences no time so arrives here instantly unless slowed by gravity of a massive object

Ok, One, do NOT teach your kid physics, because you have no idea what you're talking about. Two, the notion that gravity imposes a light speed barrier is just... silly. Gravity is about the geometry of spacetime, and even in a black hole light isn't "slowed" below its local speed in that medium.

Distance in ly = years it takes for light from that ditance to reach the observer. There is nothing instantaneous about it.

 

[piersdad]

gravity bends light

a recent observation of a very old gamma ray burst lasted several seconds or more because of some effect that stretched the waves over time and distance .
what i wanted to decribe is
that if you were a- photon -travelling over distance you will not experience any time even to us as an observer billions of years go by.

 

[Frame Dragger]

gravity bends light

a recent observation of a very old gamma ray burst lasted several seconds or more because of some effect that stretched the waves over time and distance .
what i wanted to decribe is
that if you were a- photon -travelling over distance you will not experience any time even to us as an observer billions of years go by.

You need to do some research on Gravitional Lensing, The Doppler Effect, Lightspeed, and as for the rest, no one knows what the experience of time (or not) would be for a photon.

 

[ia_]

If you wanted to consider a lagrangian with a nonzero photon mass in e&m, you could modify the lagrangian density from

$$L = - \frac{1}{16 \pi} F_{\alpha \beta} F^{\alpha \beta} - \frac{1}{c} J_{\alpha} A^{\beta}$$

by just adding a mass term:

$$L = - \frac{1}{16 \pi} F_{\alpha \beta} F^{\alpha \beta} - \frac{1}{c} J_{\alpha} A^{\beta} + \frac{\mu^{2}}{8 \pi} A_{\alpha} A^{\alpha}$$

where $$\mu = m_{\gamma} c /\hbar$$.

The new eq of motion becomes $$\partial ^{\beta} F_{\alpha \beta} + \mu^{2} A_{\alpha} = \frac{4 \pi}{c} J_{\alpha}$$.

The potential A is actually observable, because of the nonzero photon mass.

If you have a point charge at rest here, then the static potential will look like a Yukawa potential instead of just 1/r:
$$\phi(x) = q \frac{e^{-\mu r}}{r}$$. This is why potentials would decay more rapidly with distance if photons had mass.

In reply to piersdad. You made an oversight in your statement. Maybe this will help: In a reference frame moving at speed c, time will still progress at a normal rate for an observer in that frame. Also, an observer in an inertial frame moving at speed c will still see that light will travel at velocity c (this just comes from normal sr velocity addition). However, due to the effects of time dilation, an observer on the Earth will see that time in the other frame progresses at the rate $$\Delta t' = \frac{\Delta t}{\sqrt(1-v^2/c^2)}$$. As the relative velocity $$v \rightarrow c$$, this time dilation becomes infinite. But this doesn't mean that time in the other reference frame stops relative to an observer in that frame.

 

[piersdad]

i always question any established science
gravity to me is an outward acceleration -- not an inwards pull.
while centripetal force is an inward acceleration not an out ward pull.

so a photon in our frame or observation has time, but itself, from its frame, has no time -- and no mass-- so it can not experience time.
would this explan why light can travel for billions of years to our frame of time, when for it, there is no time.