4 ms·

Warp drive that somehow doesn't shatter causality is fantasy.

by maxerickson 3y ago

throwaway60707 3y ago
Is that because of the currently accepted equations for space-time, or is there some experimental proof of that?
- maxerickson 3y ago
  It's just mechanistic. If you travel somewhere faster than the light from where you left, you can watch yourself leave. Now do it twice.
  - throwaway60707 3y ago
    Ok, so I'm watching myself watching myself leave. And so what? My "past selves" are just reflections of myself, I am not there anymore, it's just light that looks like me. The light is not going to change anything about the past, it's going to follow me to my next location until I stop and let it catch up. Maybe I'm completely misunderstanding you or the problem - feel free to correct me, please. I'd really like to understand how causality is broken by FTL but could never grok it.
    - btilly 3y ago
      Yeah, that explanation was dead wrong. Here is the real one. The phrase "at the same time" means different things in different inertial reference frames. So what is FTL to one observer, is traveling backwards in time to another. If you can warp from any reference frame, you can therefore travel backwards in time. As for why "at the same time" changes meanings, let's adapt Einstein's example. I'm in the middle of a moving train. You're on the train station. As we pass, lightning strikes right between us. Will the light from the lightning arrive at both ends of the train at the same time? According to me, obviously. In my reference frame the train is not moving. Light has a constant speed. It will reach both ends at the same time. According to you, obviously not. Light travels at constant speed, but the train is moving. Therefore the back of the train is moving towards the light, and the front is moving away. So the light gets to the back of the train before the front. So when the person on the train says that an event in front and behind happened at the same time, the person not on the train says that the event behind happened before the event in front. Now consider this. I'm on a spaceship with a warp drive. This is a great warp drive, travels instantly in whatever reference frame I'm in.. I warp forward a long distance - like a light year. I accelerate forward. I warp backwards and arrive back a bit behind where I started at. I decelerate, and I'm in my own past! That's the causality problem.
      - maxerickson 3y ago
        Your second to last paragraph is just a more detailed version of what I said.
        btilly 3y ago
        No, it is really not. You left out the "accelerate forward" bit. That changes your reference frame. Without changing your reference frame, you don't get FTL to turn into time travel. If you have FTL but on a time scale determined by one reference frame, then as I pointed out in https://news.ycombinator.com/item?id=37012594 https://news.ycombinator.com/item?id=37012594, no causality issues exist. You need FTL in 2 reference frames to create a closed time-like loop.
        maxerickson 3y ago
        Does the machine have to be the source of the acceleration, or would ~any gravitational force count?
        btilly 3y ago
        When you bring in gravity, you bring in general relativity. And now the phrase "at the same time as" becomes an entirely arbitrary choice of coordinate systems. So it becomes complicated. But as long as FTL goes forward in time by some unique global definition of time, there is no causality problem.
btilly 3y ago
Only if it works in every reference frame. A warp drive that only works against the fixed reference frame of the distant stars introduces no causality issues.
- SideburnsOfDoom 3y ago
  A warp drive that only works if _nobody is moving too fast_ is somewhat useless. Physics doesn't have privileged frames of reference.
  - btilly 3y ago
    I have no idea why you think that being able to warp from here to another star in a day would be somewhat useless. It is unlikely to be possible, but it would be very useful. As for not having privileged frames of reference, that doesn't stop interactions with your environment from mattering. For a trivial example, "at rest" is a privileged frame of reference for you. If you start moving in a different frame of reference you'll encounter weird things like "wind". For a non-trivial example, the Standard Model says that there is no privileged ratio of masses between the electron and the proton in the laws of physics. The actual ratio of masses is set by an interaction with the part of the environment known as the Higgs field. Astronomy shows that the fixed reference frame of the distant stars is environmentally privileged. It is also somewhat privileged physically as well. For example Mach's principle (see https://en.wikipedia.org/wiki/Mach%27s_principle https://en.wikipedia.org/wiki/Mach%27s_principle if you don't know it) says that we notice we are rotating when we are rotating relative to that reference frame. We know of no way to react to this feature of our environment and get from here to there faster than the speed of light. But there is no fundamental principle of physics that we know of which says it is not possible - we just have no clue how it might happen.
    - SideburnsOfDoom 3y ago
      > I have no idea why you think that being able to warp from here to another star in a day would be somewhat useless. And I have no idea why you don't notice that this is moving fast. That's the irony.
DennisP 3y ago
FTL between two planets with minimal relative velocity doesn't have any effects on causality. It's pretty much how Star Trek depicts. Of course there are other scenarios where you can get backwards signaling. But some recent papers showed that in these cases, were they possible, then events would arrange themselves to be consistent overall, even though some events have causes from their futures.

Warp drive that somehow doesn't shatter causality is fantasy.

throwaway60707 3y ago

Is that because of the currently accepted equations for space-time, or is there some experimental proof of that?

maxerickson 3y ago
It's just mechanistic. If you travel somewhere faster than the light from where you left, you can watch yourself leave. Now do it twice.
- throwaway60707 3y ago
  Ok, so I'm watching myself watching myself leave. And so what? My "past selves" are just reflections of myself, I am not there anymore, it's just light that looks like me. The light is not going to change anything about the past, it's going to follow me to my next location until I stop and let it catch up. Maybe I'm completely misunderstanding you or the problem - feel free to correct me, please. I'd really like to understand how causality is broken by FTL but could never grok it.
  - btilly 3y ago
    Yeah, that explanation was dead wrong. Here is the real one. The phrase "at the same time" means different things in different inertial reference frames. So what is FTL to one observer, is traveling backwards in time to another. If you can warp from any reference frame, you can therefore travel backwards in time. As for why "at the same time" changes meanings, let's adapt Einstein's example. I'm in the middle of a moving train. You're on the train station. As we pass, lightning strikes right between us. Will the light from the lightning arrive at both ends of the train at the same time? According to me, obviously. In my reference frame the train is not moving. Light has a constant speed. It will reach both ends at the same time. According to you, obviously not. Light travels at constant speed, but the train is moving. Therefore the back of the train is moving towards the light, and the front is moving away. So the light gets to the back of the train before the front. So when the person on the train says that an event in front and behind happened at the same time, the person not on the train says that the event behind happened before the event in front. Now consider this. I'm on a spaceship with a warp drive. This is a great warp drive, travels instantly in whatever reference frame I'm in.. I warp forward a long distance - like a light year. I accelerate forward. I warp backwards and arrive back a bit behind where I started at. I decelerate, and I'm in my own past! That's the causality problem.
    - maxerickson 3y ago
      Your second to last paragraph is just a more detailed version of what I said.
      - btilly 3y ago
        No, it is really not. You left out the "accelerate forward" bit. That changes your reference frame. Without changing your reference frame, you don't get FTL to turn into time travel. If you have FTL but on a time scale determined by one reference frame, then as I pointed out in https://news.ycombinator.com/item?id=37012594 https://news.ycombinator.com/item?id=37012594, no causality issues exist. You need FTL in 2 reference frames to create a closed time-like loop.
        maxerickson 3y ago
        Does the machine have to be the source of the acceleration, or would ~any gravitational force count?
        btilly 3y ago
        When you bring in gravity, you bring in general relativity. And now the phrase "at the same time as" becomes an entirely arbitrary choice of coordinate systems. So it becomes complicated. But as long as FTL goes forward in time by some unique global definition of time, there is no causality problem.

btilly 3y ago

Only if it works in every reference frame. A warp drive that only works against the fixed reference frame of the distant stars introduces no causality issues.

SideburnsOfDoom 3y ago
A warp drive that only works if _nobody is moving too fast_ is somewhat useless. Physics doesn't have privileged frames of reference.
- btilly 3y ago
  I have no idea why you think that being able to warp from here to another star in a day would be somewhat useless. It is unlikely to be possible, but it would be very useful. As for not having privileged frames of reference, that doesn't stop interactions with your environment from mattering. For a trivial example, "at rest" is a privileged frame of reference for you. If you start moving in a different frame of reference you'll encounter weird things like "wind". For a non-trivial example, the Standard Model says that there is no privileged ratio of masses between the electron and the proton in the laws of physics. The actual ratio of masses is set by an interaction with the part of the environment known as the Higgs field. Astronomy shows that the fixed reference frame of the distant stars is environmentally privileged. It is also somewhat privileged physically as well. For example Mach's principle (see https://en.wikipedia.org/wiki/Mach%27s_principle https://en.wikipedia.org/wiki/Mach%27s_principle if you don't know it) says that we notice we are rotating when we are rotating relative to that reference frame. We know of no way to react to this feature of our environment and get from here to there faster than the speed of light. But there is no fundamental principle of physics that we know of which says it is not possible - we just have no clue how it might happen.
  - SideburnsOfDoom 3y ago
    > I have no idea why you think that being able to warp from here to another star in a day would be somewhat useless. And I have no idea why you don't notice that this is moving fast. That's the irony.

DennisP 3y ago

FTL between two planets with minimal relative velocity doesn't have any effects on causality. It's pretty much how Star Trek depicts. Of course there are other scenarios where you can get backwards signaling. But some recent papers showed that in these cases, were they possible, then events would arrange themselves to be consistent overall, even though some events have causes from their futures.