3 ms·

With a ship going the speed of light you could travel from galaxy to galaxy instantaneously. Zero distance. Though time on the other hand... If you travel to a

With a ship going the speed of light you could travel from galaxy to galaxy instantaneously. Zero distance. Though time on the other hand... If you travel to a star 4 light years away and then back, what would be no time for you, everyone on Earth would be 8 years older.

CydeWeys 5y ago
It ends up being worse than that once you plug in general relativity. You're only using special relativity, but that doesn't hold when acceleration is involved (as would be required to go from 0 to 0.99999999c).
- tshaddox 5y ago
  It’s SR pretty much fine for accelerations of fairly small masses (like a spaceship that’s much smaller than, say, a planet)?
- perl4ever 5y ago
  "It's a common misconception that special relativity cannot handle accelerating objects or accelerating reference frames. Sometimes it's claimed that general relativity is required for these situations, the reason being given that special relativity only applies to inertial frames. This is not true. Special relativity treats accelerating frames differently from inertial frames, but can still deal with accelerating frames. And accelerating objects can be dealt with without even calling upon accelerating frames. This idea that special relativity cannot handle acceleration or accelerated frames often comes up in the context of the twin paradox, when people claim that it can only be resolved in general relativity because of the acceleration present. Their claim is wrong." https://math.ucr.edu/home/baez/physics/Relativity/SR/acceleration.html https://math.ucr.edu/home/baez/physics/Relativity/SR/acceler...
  - CydeWeys 5y ago
    Interesting, but the point remains that the answer is different from the 8 years mentioned given that you do have to accelerate up to near-c (at gees the human body can survive in) and then decelerate from it. You can't just instantly go up to near-c and then back down to zero. Whether it's special/general relativity that's necessary for this calculation, it's a materially different result.

CydeWeys 5y ago

It ends up being worse than that once you plug in general relativity. You're only using special relativity, but that doesn't hold when acceleration is involved (as would be required to go from 0 to 0.99999999c).

tshaddox 5y ago
It’s SR pretty much fine for accelerations of fairly small masses (like a spaceship that’s much smaller than, say, a planet)?
perl4ever 5y ago
"It's a common misconception that special relativity cannot handle accelerating objects or accelerating reference frames. Sometimes it's claimed that general relativity is required for these situations, the reason being given that special relativity only applies to inertial frames. This is not true. Special relativity treats accelerating frames differently from inertial frames, but can still deal with accelerating frames. And accelerating objects can be dealt with without even calling upon accelerating frames. This idea that special relativity cannot handle acceleration or accelerated frames often comes up in the context of the twin paradox, when people claim that it can only be resolved in general relativity because of the acceleration present. Their claim is wrong." https://math.ucr.edu/home/baez/physics/Relativity/SR/acceleration.html https://math.ucr.edu/home/baez/physics/Relativity/SR/acceler...
- CydeWeys 5y ago
  Interesting, but the point remains that the answer is different from the 8 years mentioned given that you do have to accelerate up to near-c (at gees the human body can survive in) and then decelerate from it. You can't just instantly go up to near-c and then back down to zero. Whether it's special/general relativity that's necessary for this calculation, it's a materially different result.