**Ask a monkey a physics question thread**

[OngBonga]

Ok, I'm cool with intertial frames of reference.

Let's say we observe a photon that originated from the big bang and moves at a constant velocity of exactly c. That's an intertial frame of reference, right? The photon, as far as it is concerned, is motionless, because it is not accelerating. So it does not experience time dilation, rather everything moving relative to it experiences time dilation, at least from its pov.

So how old is the photon, from its pov? How about from our pov? How old does the photon think we are?

So the question begs... exactly how old is the photon? Is the photon right, or are we right, or are we both right?

[MadMojoMonkey]

Ok, I'm cool with intertial frames of reference.

Let's say we observe a photon that originated from the big bang and moves at a constant velocity of exactly c. That's an intertial frame of reference, right?

Unfortunately, no. Newton's laws break down when infinities are involved.

By definition, if Newton's laws don't work, then it's not an inertial reference frame. The two ideas are inseparably linked. Newton's first says, "this is an inertial reference frame," The other laws are, "In an inertial reference frame..."

The photon, as far as it is concerned, is motionless, because it is not accelerating. So it does not experience time dilation, rather everything moving relative to it experiences time dilation, at least from its pov.

*nods*

So how old is the photon, from its pov?

If assigning a number even makes sense, that number is 0 [time units].

How about from our pov?

If we can determine the source position in a distance measure, d, in an inertial reference frame, then we know that light moves at c, so the time traveled in our reference frame is d/c.

If we can determine the time, t, it has traveled in our reference frame, then we can calculate the distance d = ct.

How old does the photon think we are?

Is this a koan?

It is in the location of its creation and its annihilation at the same moment in its reference frame.

So the question begs... exactly how old is the photon? Is the photon right, or are we right, or are we both right?

I'm going with both right. Time is relative.

***
Photons have highly well-defined velocity and wavelength - perhaps infinitely well-defined, even. This means the only room for uncertainty is in position, which is not defined, let alone well-defined. The best we can do to find a solvable (converging) position function is put some limits on space.

Say we stipulate that we have a system which contains a photon in a region of space (particle in a box, so to speak). Still, the photon has very well-defined velocity and wavelength. (Whether or not we observe it; these are intrinsic properties.) Now we can force the position function to tell us something... which is that the position is equally probable everywhere in the box. We gained nothing. The position is not defined any better than our original stipulation that there was a photon in that region of space.


*not a real word