Time traveling is just full of crap, innit?

[LostLocke]

What do you mean there is no global or objective time?

Replace time with distance.
There is no objective distance. I can't just say "Chicago is 20 miles away". I have to say "Chicago is 20 miles away from me", because for some people it will be less or greater.
That makes the distance relative.

[Edwardo Piet]

And yet 20 miles is still 20 miles and 20 minutes is still 20 minutes.

We talk of distance from ourselves and what time is it for us because... well,. that's how we make sense of our own experiences of the world and we can't know the world itself.

[The Valkyrie]

You don't die.

YAY!!!!!!!!

This makes me INVINCIBLE for the next few hours.

That's fun! I'm gonna go play in traffic now like I've always wanted to but my mommy wouldn't let me!

No, it doesn't make you invincible.  Anyone can be vinced.

It means you won't die for a number of hours.  It doesn't mean you can't be mortally injured and take that long to croak.

[Edwardo Piet]

YAY!!!!!!!!

This makes me INVINCIBLE for the next few hours.

That's fun! I'm gonna go play in traffic now like I've always wanted to but my mommy wouldn't let me!

No, it doesn't make you invincible.  Anyone can be vinced.

It means you won't die for a number of hours.  It doesn't mean you can't be mortally injured and take that long to croak.

Lol, I know.

I just love that part in Red Dwarf

KRYTEN
According to Cassandra, our future is decided and we four survive.
Therefore, while we're here, we cannot die. Regard:

[The Dwarfer pause in the corridor to attend KRYTEN. The mechanoid draws his
sidearm, places the barrel to his temple and pulls the trigger. The chamber
clicks empty. He points to and fires at KOCHANSKI, LISTER and CAT in turn,
and each time the chambers are empty. He levels the gun at RIMMER's head]

KRYTEN
Duck sir!

[KRYTEN fires, and a bullet richochet's off the wall behind RIMMER. The
bullet pings its way up and down the metal corridar in which they stand,
its noise first growing quieter, then steadily louder]

KRYTEN
Duck again, sir!

[RIMMER does so, just in time for the bullet to finally shatter against the
wall behind him]

KRYTEN
Just as i thought.

[KRYTEN deftly spins the pistol around his finger and drops it back into
his holster.]

CAT
So, in other words, if I...

[CAT gingerly plucks a large fire axe from the wall, and cracks LISTER
sharply across the back of the head with its long wooden handle. The others
flinch away in sympathy and LISTER clutches the back of head, rounding on
CAT, who grins unconcernedly]

LISTER
What was that for!?

CAT
You can't die!

LISTER
Yeah, but I can still feel pain, you smegger!

CAT
Oh, yeah...

[ErGingerbreadMandude]

It may seem to be 5 years from one point of reference and 50 years for another due to the way our measuring of time works.... but it's objectively the same amount of time that's passed...

That's the thing, it's not an objective amount. 5 years have passed for space guy, 50 years have passed for earth guy.
If it's objective then only 5 years or 50 years have passed. Which one is it?

Isn't it a bit like asking: "It's 4'o clock in UK", "It's 12'o clock in Dubai" : So which is the actual time? We both know that there is an objective "time" - the clock started ticking at the beginning of the universe, we just say it's 4'o clock in the UK to adjust with the different experience of sunset/sunrise. 

If 1 second can mean only 1 second to person A and 1 second can mean only 1 second to person B then time traveling would be pretty much impossible. Maybe the illusion of time traveling is possible but that's still just an illusion..?

---

There is another layer of subtlety to this that is barely ever discussed in popular treatments: travel speed itself is not an absolute observer-independent quantity in relativity, so there's no unique objective way to say who is faster and who is slower. This is why in two space ships passing each other, each could claim to be at rest while the other is fast, hence both could equally claim that their time runs faster. This seems paradoxical. The way the paradox is avoided in nature is very interesting: there is no way to compare the "time speed" of two observers unless 1. the two observers meet twice, or 2. if one observer prepares several synchronized clocks set up a distance apart, which are passed in succession by the other observer.

In the first case, the paradox is avoided bc one observer has to turn around and go back, breaking the symmetry of the situation. In the second case likewise, there is no paradox because the synchronicity of spatially separated clocks is not observer-independent either and the observer in the passing space ship will simply see the two clocks as not synchronous in the first place, explaining why they seemingly show more time passing even though they are the ones who are moving from his vantage point .

With regards to the first case, what happens if the two spaceships are traveling in huge circular routes, next to each other, but in opposite directions? Then they would technically pass each other multiple times without actually turning back?

Did I, or did I not just break the universe?

 ...
If you put the ships in a circular path but in opposite directions the ships would meet multiple times..yeah. You start your stop watch from the moment you see the ship and stop it when you see it again.. then you can do all your weird calculations and find the speed of the spaceship, the area of the circular path etc?

[Alex K]

There is another layer of subtlety to this that is barely ever discussed in popular treatments: travel speed itself is not an absolute observer-independent quantity in relativity, so there's no unique objective way to say who is faster and who is slower. This is why in two space ships passing each other, each could claim to be at rest while the other is fast, hence both could equally claim that their time runs faster. This seems paradoxical. The way the paradox is avoided in nature is very interesting: there is no way to compare the "time speed" of two observers unless 1. the two observers meet twice, or 2. if one observer prepares several synchronized clocks set up a distance apart, which are passed in succession by the other observer.

In the first case, the paradox is avoided bc one observer has to turn around and go back, breaking the symmetry of the situation. In the second case likewise, there is no paradox because the synchronicity of spatially separated clocks is not observer-independent either and the observer in the passing space ship will simply see the two clocks as not synchronous in the first place, explaining why they seemingly show more time passing even though they are the ones who are moving from his vantage point .

With regards to the first case, what happens if the two spaceships are traveling in huge circular routes, next to each other, but in opposite directions? Then they would technically pass each other multiple times without actually turning back?

Did I, or did I not just break the universe?

Yes, and it was replaced by an even weirder one in which Trump is president. But I digress.

In that case, I'm pretty certain that according to special relativity, they would measure the same time passing if they go round at the same speed in opposite directions, but if one goes round faster, its clock would start lagging behind at the rate given by the usual formulas, probably

sqrt(1-v1^2/c^2) / sqrt(1-v2^2/c^2)

where v1 and v2 are the two tangential velocities. But I'm only 90% sure about the formula