3 ms·

> I'm assuming this affects clocks (and things) but not time, right? What's the difference? > otherwise we would be surrounded by time travellers by now I'm

by pdonis 2y ago

> I'm assuming this affects clocks (and things) but not time, right? What's the difference? > otherwise we would be surrounded by time travellers by now I'm not sure what your reasoning is here. > where does this cancels out? I don't understand the question. What is supposed to cancel out? > someone observing it from a third equidistant point would see both lasers blinking at the same rate. No, they wouldn't. > From earth, the moon clock ticks slower compared to earth clocks No, it ticks faster. That was my point.

alganet 2y ago
Consider this scenario: - I get into a spacecraft, go into orbit around earth. - Once in orbit, I start accelerating until my clock ticked faster for long enough to be 1s ahead of earth. - Once I'm 1s ahead of earth, I capture transmissions my buddy sent to me of the stock exchange rates. - My buddy, on the ground, has a telescope and is ready to invest or sell stocks depending on whether I stop or keep accelerating. - My buddy now has knowledge about the future, I land and share the money we stole from the future with him. My intuition says this should be impossible, but it seems that I got the "why" wrong. Or maybe everything wrong.
- pdonis 2y ago
  > Consider this scenario Let me re-describe your scenario properly: Your buddy sends you information about stock exchange rates when his clock reads 12 noon exactly. You are one light-second away from him, so his message is received by you at 12 noon + 1 second, his time. That is also 12 noon + 2 seconds, your time, but that doesn't matter; it's still information about rates when his clock said 12 noon, not rates when his clock said 12 noon + 1 second, 1 second after he sent the message. In other words, the fact that your clock ticks faster and gets "ahead" of your buddy's does not mean you receive information from your buddy's future.
  - alganet 2y ago
    Your example makes sense, the distance for light to arrive makes it impossible (also, the distance for the light from the craft to reach the telescope in step 4, it's a round trip). Going further than this is way above my skillset :D Thanks for the time to indulge my curiosity, I'll study more and try to develop a better intuition.
    - pdonis 2y ago
      > the distance for light to arrive makes it impossible If you mean the 1 second light travel time is the same as the 1 second clock difference, you can increase the clock difference by just spending longer in orbit. For example, you could wait until the clock difference was 1 hour. Then, if your buddy sent you a light signal at 12 noon by his clock, it would arrive at 1 pm + 1 second (1 second light travel time) by your clock. But it would still be telling you stock exchange rates at 12 noon by your buddy's clock, not 1 pm by your buddy's clock.
- pdonis 2y ago
  > Once in orbit, I start accelerating until my clock ticked faster Note that this is wrong: you don't have to accelerate to make your clock tick faster. You just have to be in orbit at a high enough altitude for the speedup due to altitude to outweigh the slowdown due to your free-fall orbital speed.
  - alganet 2y ago
    I was under the impression that the speed in orbit matters, therefore accelerating matters. From the paper: > Φ0 is the effective gravitational potential in the rotating frame, which is the sum of the static gravitational potential of the Earth, and a centripetal contribution This is just so I can catch up with a specific desired dilation relative to earth. I don't want to make my orbit higher, on the contrary, the less distance the better so communication is faster.
    - pdonis 2y ago
      > I was under the impression that the speed in orbit matters It does. > therefore accelerating matters. No, it doesn't. In a free-fall orbit, proper acceleration is zero; you are weightless. "Accelerate" would mean firing your rockets to change your orbit. You don't want or need to do that. It is true that there is a coordinate acceleration for a body in a circular orbit, in coordinates centered on the Earth, but coordinate acceleration is irrelevant to what we are discussing. > This is just so I can catch up with a specific desired dilation relative to earth. As I said, you do that by staying in an appropriate free-fall orbit for a long enough time. You don't need or want to fire your rockets. > I don't want to make my orbit higher, on the contrary, the less distance the better so communication is faster. But the lower your orbit, the less your clock speeds up relative to Earth clocks. And if your orbit is low enough, your clock will actually run slow compared to Earth clocks (because the altitude effect no longer outweighs the effect of your orbital speed). For example, clocks on the ISS run slow compared to Earth clocks.
      - alganet 2y ago
        > "Accelerate" would mean firing your rockets to change your orbit That's almost what I meant! Spacecraft, acceleration to pick up speed (not to go higher), stock-exchange cheaters. I don't get the "don't need" or "don't want". It is part of my scenario. I also don't want a twin falling into a black hole, but it is a thought experiment that helps put things in perspective, specially for layman like me. I always heard of the scenario of the twin at the speed of light that remains younger. I am introducing an element of communication (originally, clocks with laser beams then stock exchange rates) into that scenario and trying to understand what the offsets mean. This is all above my paygrade, I know, so don't worry! I know I'm far from getting it and I don't want to bother :)

by pdonis 2y ago

alganet 2y ago

Consider this scenario: - I get into a spacecraft, go into orbit around earth. - Once in orbit, I start accelerating until my clock ticked faster for long enough to be 1s ahead of earth. - Once I'm 1s ahead of earth, I capture transmissions my buddy sent to me of the stock exchange rates. - My buddy, on the ground, has a telescope and is ready to invest or sell stocks depending on whether I stop or keep accelerating. - My buddy now has knowledge about the future, I land and share the money we stole from the future with him. My intuition says this should be impossible, but it seems that I got the "why" wrong. Or maybe everything wrong.

pdonis 2y ago
> Consider this scenario Let me re-describe your scenario properly: Your buddy sends you information about stock exchange rates when his clock reads 12 noon exactly. You are one light-second away from him, so his message is received by you at 12 noon + 1 second, his time. That is also 12 noon + 2 seconds, your time, but that doesn't matter; it's still information about rates when his clock said 12 noon, not rates when his clock said 12 noon + 1 second, 1 second after he sent the message. In other words, the fact that your clock ticks faster and gets "ahead" of your buddy's does not mean you receive information from your buddy's future.
- alganet 2y ago
  Your example makes sense, the distance for light to arrive makes it impossible (also, the distance for the light from the craft to reach the telescope in step 4, it's a round trip). Going further than this is way above my skillset :D Thanks for the time to indulge my curiosity, I'll study more and try to develop a better intuition.
  - pdonis 2y ago
    > the distance for light to arrive makes it impossible If you mean the 1 second light travel time is the same as the 1 second clock difference, you can increase the clock difference by just spending longer in orbit. For example, you could wait until the clock difference was 1 hour. Then, if your buddy sent you a light signal at 12 noon by his clock, it would arrive at 1 pm + 1 second (1 second light travel time) by your clock. But it would still be telling you stock exchange rates at 12 noon by your buddy's clock, not 1 pm by your buddy's clock.
pdonis 2y ago
> Once in orbit, I start accelerating until my clock ticked faster Note that this is wrong: you don't have to accelerate to make your clock tick faster. You just have to be in orbit at a high enough altitude for the speedup due to altitude to outweigh the slowdown due to your free-fall orbital speed.
- alganet 2y ago
  I was under the impression that the speed in orbit matters, therefore accelerating matters. From the paper: > Φ0 is the effective gravitational potential in the rotating frame, which is the sum of the static gravitational potential of the Earth, and a centripetal contribution This is just so I can catch up with a specific desired dilation relative to earth. I don't want to make my orbit higher, on the contrary, the less distance the better so communication is faster.
  - pdonis 2y ago
    > I was under the impression that the speed in orbit matters It does. > therefore accelerating matters. No, it doesn't. In a free-fall orbit, proper acceleration is zero; you are weightless. "Accelerate" would mean firing your rockets to change your orbit. You don't want or need to do that. It is true that there is a coordinate acceleration for a body in a circular orbit, in coordinates centered on the Earth, but coordinate acceleration is irrelevant to what we are discussing. > This is just so I can catch up with a specific desired dilation relative to earth. As I said, you do that by staying in an appropriate free-fall orbit for a long enough time. You don't need or want to fire your rockets. > I don't want to make my orbit higher, on the contrary, the less distance the better so communication is faster. But the lower your orbit, the less your clock speeds up relative to Earth clocks. And if your orbit is low enough, your clock will actually run slow compared to Earth clocks (because the altitude effect no longer outweighs the effect of your orbital speed). For example, clocks on the ISS run slow compared to Earth clocks.
    - alganet 2y ago
      > "Accelerate" would mean firing your rockets to change your orbit That's almost what I meant! Spacecraft, acceleration to pick up speed (not to go higher), stock-exchange cheaters. I don't get the "don't need" or "don't want". It is part of my scenario. I also don't want a twin falling into a black hole, but it is a thought experiment that helps put things in perspective, specially for layman like me. I always heard of the scenario of the twin at the speed of light that remains younger. I am introducing an element of communication (originally, clocks with laser beams then stock exchange rates) into that scenario and trying to understand what the offsets mean. This is all above my paygrade, I know, so don't worry! I know I'm far from getting it and I don't want to bother :)