Is the Andromeda Paradox accepted in physics?

[Herman Trivilino]

I know it's a small bit off topic, but is it possible that your 'now' is simultaneous with your own past? Say if you went very far away from earth, on a far away planet, and moved so it made it so you were simultaneous with Earth 5 years ago, back when you were on earth, would you be simultaneous with your own past?

One way to answer this is to consider events. One event is my leaving Earth, the other event is my arrival at the distant planet. Since I was present at both events, those two events cannot be simultaneous.

Do a search for spacelike, lightlike, and timelike intervals.

 

[1977ub]

Light coming different distances (as measured in our rest frame) takes different lengths of time to get to us (as measured by us) but all light takes the same (zero) time to get anywhere from light's own point of reference.

Is this effect actually visable in reality? My physics teacher stated that relative simultaneity is just a mathematical abstraction, and thus when someone uses different coordinate systems, it is just a mathematical abstraction. Is this correct?

The thing to focus on is that simultaneity itself - for events spatially separated - is a "mathematical abstraction".

 

[Ibix]

but all light takes the same (zero) time to get anywhere from light's own point of reference.

Light does not have a reference frame, and time is not defined along null worldlines. So this statement is wrong.

 

[1977ub]

Light does not have a reference frame, and time is not defined along null worldlines. So this statement is wrong.

in the limit then, i suppose.

 

[PeterDonis]

in the limit then, i suppose.

There is no limit involved. You were describing light. Light is not the limit of anything, it's just light. @Ibix makes a valid point.

 

[1977ub]

There is no limit involved. You were describing light. Light is not the limit of anything, it's just light. @Ibix makes a valid point.

the faster something goes from A to B approaching the speed of light, the closer the perceived increment approaches zero.

 

[Ibix]

the faster something goes from A to B approaching the speed of light, the closer the perceived increment approaches zero.

...and you can never reach light speed this way and all inertial frames are equivalent, so "approaching the speed of light" is also "stationary". So there is no valid limit as you approach the speed of light, because you aren't doing so in any meaningful sense.

Proper time is a parameter that indexes the point along a worldline. Unfortunately, it's degenerate for null worldlines and you have to use a different approach.

 

[TheQuestionGuy14]

It's the mathematical abstraction you use to interpret your clock readings.

Ultimately it's like using a map. Does the world actually look like a map? No. We use maps instead of photographs to navigate because they remove the observation point dependence of the photo, and maps are an incredibly useful way to describe and communicate about terrain.

The relativity of simultaneity is analogous to your freedom to draw a map with north pointing in any direction you like. Length contraction and time dilation are consequences of that choice - analogous to a building being square or diamond-shaped, depending on your choice of north.

None of the relativistic effects are directly visible for the same reason that the world doesn't look like Google Maps. But they are very real in the same sense that a building that is square on one map is a diamond on another.

Say though if an astronaut was in a spaceship going .95c to the right, and a lightbulb turned on activating two clocks, both equal distances from the bulb. To the astronaut, the clocks are turned on simultaneously. But to a stationary observer, the clock at the back turns on first, and the clock at the front second. Both disagree on the clock readings then.

If the astronaut slowed back down and flew back to the stationary observer and got out and took both clocks with him, would they both agree on the clock readings?

 

[Nugatory]

If the astronaut slowed back down and flew back to the stationary observer and got out and took both clocks with him, would they both agree on the clock readings?

In general, no. The clocks have followed slightly different paths through spacetime. You must also consider exactly how the turnaround is done: do both clocks reverse their direction of travel at the same time using the ship frame or the stationary observer frame?

 

[Herman Trivilino]

Both disagree on the clock readings then.

No, they don't. Suppose Clock A reads noon. One observer will claim that when Clock A reads noon, Clock B also reads noon. The other observer will claim that it's necessary to wait some time after Clock A reads noon before Clock B reads noon.

Both observers agree that both clocks read noon.

They do disagree on the interpretation of those clock readings.

 

[PAllen]

the faster something goes from A to B approaching the speed of light, the closer the perceived increment approaches zero.

But is the zero limit a time or a distance? It depends how you take the limit. Evidence that the limit cannot be just 'zero time'.