Determinism, Free Will and Paradox

[Alex K]

It is maybe mathematically more consistent to talk about the proper time (squared), which can be calculated for any trajectory without encountering any infinities. It corresponds to the total time elapsing for the traveller when having gone through this trajectory. For photons, this is exactly zero.

Finally.

So, it's clear that what we all here already know about relativity-- i.e. that it's relative-- leads us to something like a paradox.

There's no paradox...

While that photon was traveling, did time pass or didn't it? Yes it did, and no it didn't. (and this is directed at Pickup more than you, btw)

There is not the time in relativity, so the question is meaningless unless you prescribe which time you are interested in.

The difference isn't the reality of changes in state in a universal sense, but rather their relation to an "observer" (and I use this word loosely here).

I have literally no idea what you mean by any of that.

So am I wrong in stating that for a travelling photon which never encounters another body, no time passes between that photon's release from a body and the end of the universe?

It would be the standard interpretation to say that no time passes when one follows the trajectory of the photon. This is consistent with the fact that arbitrarily little time passes for observers moving between two points if they are travelling arbitrarily close to the speed of light. As a limiting case, you have light speed with no time passing.

As an arbitrary numerical example, if you have two stars which are one lightyear apart as seen in their rest frame, a traveller on a spaceship moving at 0.9999999999999995 times the speed of light will pass by these two stars within little more than one second as measured by her on the spaceship. Of course, the pilot of the spaceship will see the two stars moving past her with only 0.9999999999999995 times the speed of light, and they will look as if they are merely 300000 km apart instead of a light year. (So from the venture point of a photon, all of the universe is in one place and it takes no time to travel from A to B. You might call this a paradox, but it merely tells you that going to the rest frame of a photon is a limiting case, a pathological choice of coordinates.)

However you want to interpret that, fact is that the outside observer in the spaceship example will have measured one year passing during this trip, while the pilot has measured one second.