5 ms·

Nice explanation. I don't get the problem with traveling to stars though. If I could move at the speed of light through space, then I'd be not moving through ti

Nice explanation. I don't get the problem with traveling to stars though. If I could move at the speed of light through space, then I'd be not moving through time, which means the time to travel anywhere, as measured by me would be exactly zero. So I conclude I could reach the center of our galaxy. Is it correct reasoning? However, the ones that would stay on the Earth would never see me arrive at my destination, as they would have to wait those millions of years. Is it correct? But then another thing I don't understand in this model is the twin paradox. If I returned back to Earth what would be the Earth time and my local time? Would that agree? Or would I arrive millions years in the future?

deleted 4y ago
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ml_more 4y ago
Think of it from the earth frame of reference. They watch you travel for a million years there and a million years back. When you arrive two million years have passed in the earth frame of reference. You were traveling at some large fraction of the speed of light so from your frame of reference you traveled for a year there and a year back. You age 2 years Earth ages 2 Million years
hotpotamus 4y ago
> I don't get the problem with traveling to stars though. If I could move at the speed of light through space Well, to be a stickler for the laws of physics, you can't move through space at the speed of light. In principal, you can get moving pretty quick though, but what if there's a space pebble between you and those stars? If you're going at half the speed of light, the collision force would be in the atomic bomb range. So that's a potential problem.
- vc8f6vVV 4y ago
  If a large pebble (or a large piece of space junk) collides with ISS at 16 km/s ISS will be destroyed. That doesn't prevent ISS for being where it is though.
  - hotpotamus 4y ago
    The ISS routinely eats micrometeorites or other tiny debris and also maneuvers to avoid larger trackable junk on occasion. If it was travelling at half the speed of light, it would be a bit more problematic.
    - vc8f6vVV 4y ago
      We don't know how much of this stuff exists in the interstellar space. It's possible that a probability to hit something there is much less than ISS hitting something big enough to destroy it, even with all radars and maneuvers. So I wouldn't consider it as an argument against interstellar travel, not yet.
      - hotpotamus 4y ago
        I'd say it's one of many potential problems. We've already observed a couple interstellar comets, right? So we actually already know that there are objects out there. Certainly it wouldn't hurt to send some laser accelerated probes out there and see what we find, but human travel seems like pure sci fi.
jfengel 4y ago
You're absolutely correct. You can get anywhere you want, in whatever (nonzero) time you would like. You can reach the center of the galaxy in a millisecond, without violating relativity. And yes, if you returned home, you'd find that tens of thousands of years had passed. (Not millions. It's not that far.) This explanation is designed to ask you to see if from the point of view of spacetime, rather than asking what it looks like from any particular point of view. You can use the conversion factors to translate. What those factors tell you is that if they could "see" you, they'd see you moving very, very slowly on your spaceship, because it's always moving at very near the speed of light. They'd see you aging very slowly, and arriving at the center of the galaxy (a thousand generations later) having aged barely at all. Then they'd see you coming back -- again aging very slowly -- and arriving back another thousand generations later. When you stop your ship at earth, they'd watch a clock on your ship suddenly start ticking at a normal rate.
- vc8f6vVV 4y ago
  Suddenly is the wrong word. You will need to decelerate, and that can't be done suddenly. If you decide to fly by your clock will still be ticking at a slow rate (from the point of view of an observer on Earth of course).
  - FrontierPsych 4y ago
    If someone can figure out how to go the speed of light, then they certainly can figure out how to decelerate "suddenly." This is all just funsies anyways, right?
- pkolaczk 4y ago
  Thank you, this is very useful. But I have some more doubts. What is that space-time actually? Is there a global single space-time or does everyone has its own? Let's assume the starting point of my travel have coordinates (x: 0, y: 0, z: 0, t: 0). Then I go at near the speed of an light to the center of the Galaxy and come back to the same point a second later (as measured by my local clock). So I arrive at coordinates (0,0,0,1). But for the ones who stayed on Earth this point has a different coordinate (0,0,0,<very large number>). So it looks like it is not a single spacetime and this analogy kinda breaks :(
vc8f6vVV 4y ago
It's slightly more complicated. The main idea of special relativity theory is that you can't compare "time" at two separate points of "space". E.g. it makes absolutely no sense to compare age of a twin left on Earth with the age of a twin on a moving space ship as long as they are not at the same point in space. So if you really want to compare age you will have to turn your space ship around and return to Earth. Now, that part is covered not by special relativity, but general relativity. Here we need to consider effects of a gravitational field (any acceleration is equal to gravitation). The most interesting question in the twin paradox is what's the difference between space ship and Earth. Why it's the ship which is accelerating, but not the Earth. Why Earth is considered an inertial system (according to the Newton's first law), but a space ship making U-turn is not.

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ml_more 4y ago

Think of it from the earth frame of reference. They watch you travel for a million years there and a million years back. When you arrive two million years have passed in the earth frame of reference. You were traveling at some large fraction of the speed of light so from your frame of reference you traveled for a year there and a year back. You age 2 years Earth ages 2 Million years

hotpotamus 4y ago

> I don't get the problem with traveling to stars though. If I could move at the speed of light through space Well, to be a stickler for the laws of physics, you can't move through space at the speed of light. In principal, you can get moving pretty quick though, but what if there's a space pebble between you and those stars? If you're going at half the speed of light, the collision force would be in the atomic bomb range. So that's a potential problem.

vc8f6vVV 4y ago
If a large pebble (or a large piece of space junk) collides with ISS at 16 km/s ISS will be destroyed. That doesn't prevent ISS for being where it is though.
- hotpotamus 4y ago
  The ISS routinely eats micrometeorites or other tiny debris and also maneuvers to avoid larger trackable junk on occasion. If it was travelling at half the speed of light, it would be a bit more problematic.
  - vc8f6vVV 4y ago
    We don't know how much of this stuff exists in the interstellar space. It's possible that a probability to hit something there is much less than ISS hitting something big enough to destroy it, even with all radars and maneuvers. So I wouldn't consider it as an argument against interstellar travel, not yet.
    - hotpotamus 4y ago
      I'd say it's one of many potential problems. We've already observed a couple interstellar comets, right? So we actually already know that there are objects out there. Certainly it wouldn't hurt to send some laser accelerated probes out there and see what we find, but human travel seems like pure sci fi.

jfengel 4y ago

You're absolutely correct. You can get anywhere you want, in whatever (nonzero) time you would like. You can reach the center of the galaxy in a millisecond, without violating relativity. And yes, if you returned home, you'd find that tens of thousands of years had passed. (Not millions. It's not that far.) This explanation is designed to ask you to see if from the point of view of spacetime, rather than asking what it looks like from any particular point of view. You can use the conversion factors to translate. What those factors tell you is that if they could "see" you, they'd see you moving very, very slowly on your spaceship, because it's always moving at very near the speed of light. They'd see you aging very slowly, and arriving at the center of the galaxy (a thousand generations later) having aged barely at all. Then they'd see you coming back -- again aging very slowly -- and arriving back another thousand generations later. When you stop your ship at earth, they'd watch a clock on your ship suddenly start ticking at a normal rate.

vc8f6vVV 4y ago
Suddenly is the wrong word. You will need to decelerate, and that can't be done suddenly. If you decide to fly by your clock will still be ticking at a slow rate (from the point of view of an observer on Earth of course).
- FrontierPsych 4y ago
  If someone can figure out how to go the speed of light, then they certainly can figure out how to decelerate "suddenly." This is all just funsies anyways, right?
pkolaczk 4y ago
Thank you, this is very useful. But I have some more doubts. What is that space-time actually? Is there a global single space-time or does everyone has its own? Let's assume the starting point of my travel have coordinates (x: 0, y: 0, z: 0, t: 0). Then I go at near the speed of an light to the center of the Galaxy and come back to the same point a second later (as measured by my local clock). So I arrive at coordinates (0,0,0,1). But for the ones who stayed on Earth this point has a different coordinate (0,0,0,<very large number>). So it looks like it is not a single spacetime and this analogy kinda breaks :(

vc8f6vVV 4y ago

It's slightly more complicated. The main idea of special relativity theory is that you can't compare "time" at two separate points of "space". E.g. it makes absolutely no sense to compare age of a twin left on Earth with the age of a twin on a moving space ship as long as they are not at the same point in space. So if you really want to compare age you will have to turn your space ship around and return to Earth. Now, that part is covered not by special relativity, but general relativity. Here we need to consider effects of a gravitational field (any acceleration is equal to gravitation). The most interesting question in the twin paradox is what's the difference between space ship and Earth. Why it's the ship which is accelerating, but not the Earth. Why Earth is considered an inertial system (according to the Newton's first law), but a space ship making U-turn is not.