7 ms·

"effectively traveling into your own past" Can you explain how traveling faster than light leads to that? Perhaps you meant "observing your own past?"

incompatible 5y ago
I'd guess there would be frames of reference (with relative high-speed motion) in which you'd appear at the destination before you've left the origin. The laws of physics are supposed to hold regardless of which frame of reference you use.
- javajosh 5y ago
  SR is 2 assertions, really: first, that the laws of physics are invariant in all reference frames, and second (and far stranger), that the speed of light is invariant in all reference frames. It's that second rule that must be violated for FTL to work - FTL implies a reference frame in which light can, at the very least, switch directions! That is, light from behind you looks like it's coming from the front (but still going c).
  - a1369209993 5y ago
    > SR is 2 assertions, really: first, that the laws of physics are invariant in all reference frames, and second (and far stranger), that the speed of light is invariant in all reference frames. Technically that's only one assertion (that is, the second is a strict subset of the first).
    - javajosh 5y ago
      No, it isn't. Light is unique in this way.
      - AnimalMuppet 5y ago
        Correct. In a bit more detail: Here's the grid coordinate system for the streets in my town. One intersection is (0, 0). But in the grid system used by the next town over, that same intersection is (100, 200). And that's fine - we're just shifting the origin, which changes all the numbers. The same thing happens with velocity. If I'm on the ground, "stationary" means that you're not moving with respect to the ground. If you're in an airplane, "stationary" means that you're sitting in your seat, and you see me on the ground moving toward the back of the plane at 550 MPH. And that's still fine - we just shifted the origin of velocity. Light is unique in that, when you shift the origin of velocity, it's speed is still the same number. No other velocity transforms that way. (Well... light isn't quite unique. The speed of light is unique in that way. But gravitational waves also travel at the speed of light.)
        drran 5y ago
        Moreover, gravitational waves are affecting speed of light, while EM waves are not affecting gravitation, so speed of gravitational waves is fundamental, while speed of light is not.
        danparsonson 5y ago
        > (Well... light isn't quite unique. The speed of light is unique in that way. But gravitational waves also travel at the speed of light.) Right - this is a subtle but important distinction! Photons don't have primacy, space-time does.
      - jlokier 5y ago
        No it isn't. The speed of light in SR is not really about light. It's just a good way to explain things. You can calculate the speed of light from other physical, measurable constants without using light (the permittivity and permeability of free space). Those you can measure with inductors and capacitors. So because those measurable physical behaviours are invariant in all reference frames under SR, it follows that the speed of light is also invariant.
        javajosh 5y ago
        c is defined. https://physics.nist.gov/cgi-bin/cuu/Value?c https://physics.nist.gov/cgi-bin/cuu/Value?c You don't have to measure c, or calculate it. The causal chain goes the other way - we assume c is invariant and define capacitance and inductance in terms of it (c, that is).
        a1369209993 5y ago
        No, the meter is defined (as c times a certain number of seconds).
        javajosh 5y ago
        No, the meter is defined in terms of c and the hyperfine transition frequency of Cs-133 is defined https://physics.nist.gov/cgi-bin/cuu/Value?nucs https://physics.nist.gov/cgi-bin/cuu/Value?nucs. If that link isn't good enough, you can check out the SI Mises en pratique on the subject: https://www.bipm.org/en/publications/mises-en-pratique https://www.bipm.org/en/publications/mises-en-pratique Edit: I'm totally this guy right now https://xkcd.com/386/ https://xkcd.com/386/
        a1369209993 5y ago
        > the meter is defined in terms of c That is what I just said: the meter is defined in terms of c, not vice versa.
        5y ago
      - danparsonson 5y ago
        Try this: https://www.youtube.com/watch?v=au0QJYISe4c https://www.youtube.com/watch?v=au0QJYISe4c c isn't defined as the speed of light - light travels at c, but so does everything else, when you factor in movement through time.
        javajosh 5y ago
        Good video! Thanks. c is still defined as "the speed of light" by NIST, even if it's not measured, and even if it's more universal than light. But I get your point (via the video) - photons are particles that move only in space, and not at all in time. Kind of makes your mind bend when you consider the cosmic background radiation hasn't aged a day since the beginning of the universe...and since it comes from everywhere equally, we really are the center of the universe. And so is everwhere else! Fun.
- lamontcg 5y ago
  You could break the problem of closed timelike loops by having a preferred reference frame for the Universe where nothing could travel backwards in time relative to that reference frame. And the CMBR rest frame might be an artifact of that preferred reference frame existing in the early Universe.
gary_0 5y ago
No, not "observing your own past". Instead, your past self could observe your future self. And then your future self could turn their FTL ship around and run you over, killing their past self and creating a paradox. https://old.reddit.com/r/askscience/comments/l9sba/why_does_ftl_travelinformation_break_causality/ https://old.reddit.com/r/askscience/comments/l9sba/why_does_...
- px43 5y ago
  > turn their FTL ship around I feel like this is the part that would probabally be hard (ie not possible) when going faster than light, assuming such a thing is possible. Also whatever magical space warp it is that lets you go that fast could presumably also obscure your ability to see yourself in the distance.
  - gary_0 5y ago
    "Turning around" is just stopping at some point (ie. some inertial reference frame) and then accelerating in the opposite direction with your FTL drive. You have all the time in the world to do this maneuver because you're already effectively in the past: https://physics.stackexchange.com/a/54242 https://physics.stackexchange.com/a/54242
    - nine_k 5y ago
      There's no absolute coordinate system in space, so there's no way to "stop". You can only change your velocity relative to some other object, e.g. your past self in the example. What counts is acceleration / deceleration. Slowing down to reach the speed of light might be as hard as speeding up to reach it; I remember it played a role in the theory of tachyons, hypothetical FTL elementary particles which could not be slowed down.
      - jchrisa 5y ago
        > acceleration / deceleration Another question for the thread: Anyone here feel like they can feel the Coriolis effect if they focus enough? I haven't travelled across the equator enough to know if the north/south jetlag is different from east/west.
nwallin 5y ago
Being able to send messages faster than the speed of light grants the ability to send messages to yourself in the past. [0] If you can send messages to yourself in the past all manner of causality goes out the window. First of all, it gives you the ability to solve NP-complete problems trivially. Step 1: you receive a message from yourself from the future. It explains the problem, gives you a potential solution. Let's say you ask yourself to solve boolean satisfiability, gives you the problem statement, and then a potential solution. You check the solution: If the possible solution is correct, you send yourself an identical message. If the solution is not correct, you send the next possible permutation of the answer. So if you sent yourself the possible solution TTFTFTTTFTFFTTTFTF, you send yourself TTFTFTTTFTFFTTTFTT. If there are no further possible permutations, then you send yourself a statement saying that the problem is not solvable. You've created a time paradox; a future in which you send yourself an incorrectly solution is logically inconsistent. The only consistent universes are the ones where you send yourself the correct solution or not-solvable, and all other universes (starting with the one where you sent yourself the message FFFFFFFFFFFFFFFFFF) collapse into the valid one. https://en.wikipedia.org/wiki/Tachyonic_antitelephone https://en.wikipedia.org/wiki/Tachyonic_antitelephone
- drran 5y ago
  > Being able to send messages faster than the speed of light grants the ability to send messages to yourself in the past. No, it doesn't. Even if we will be able to communicate instantly (0 delay = infinite faster than speed of light), we will not be able to send a message to the past (negative delay).
  - nl 5y ago
    It does. Read the numerical example on https://en.m.wikipedia.org/wiki/Tachyonic_antitelephone https://en.m.wikipedia.org/wiki/Tachyonic_antitelephone
    - drran 5y ago
      1) It's not applicable for this kind of drive. Isn't? 2) It's an unsolved problem, which waits for a correct solution. 3) Next time, please read what you are posting, please.

incompatible 5y ago

I'd guess there would be frames of reference (with relative high-speed motion) in which you'd appear at the destination before you've left the origin. The laws of physics are supposed to hold regardless of which frame of reference you use.

javajosh 5y ago
SR is 2 assertions, really: first, that the laws of physics are invariant in all reference frames, and second (and far stranger), that the speed of light is invariant in all reference frames. It's that second rule that must be violated for FTL to work - FTL implies a reference frame in which light can, at the very least, switch directions! That is, light from behind you looks like it's coming from the front (but still going c).
- a1369209993 5y ago
  > SR is 2 assertions, really: first, that the laws of physics are invariant in all reference frames, and second (and far stranger), that the speed of light is invariant in all reference frames. Technically that's only one assertion (that is, the second is a strict subset of the first).
  - javajosh 5y ago
    No, it isn't. Light is unique in this way.
    - AnimalMuppet 5y ago
      Correct. In a bit more detail: Here's the grid coordinate system for the streets in my town. One intersection is (0, 0). But in the grid system used by the next town over, that same intersection is (100, 200). And that's fine - we're just shifting the origin, which changes all the numbers. The same thing happens with velocity. If I'm on the ground, "stationary" means that you're not moving with respect to the ground. If you're in an airplane, "stationary" means that you're sitting in your seat, and you see me on the ground moving toward the back of the plane at 550 MPH. And that's still fine - we just shifted the origin of velocity. Light is unique in that, when you shift the origin of velocity, it's speed is still the same number. No other velocity transforms that way. (Well... light isn't quite unique. The speed of light is unique in that way. But gravitational waves also travel at the speed of light.)
      - drran 5y ago
        Moreover, gravitational waves are affecting speed of light, while EM waves are not affecting gravitation, so speed of gravitational waves is fundamental, while speed of light is not.
      - danparsonson 5y ago
        > (Well... light isn't quite unique. The speed of light is unique in that way. But gravitational waves also travel at the speed of light.) Right - this is a subtle but important distinction! Photons don't have primacy, space-time does.
    - jlokier 5y ago
      No it isn't. The speed of light in SR is not really about light. It's just a good way to explain things. You can calculate the speed of light from other physical, measurable constants without using light (the permittivity and permeability of free space). Those you can measure with inductors and capacitors. So because those measurable physical behaviours are invariant in all reference frames under SR, it follows that the speed of light is also invariant.
      - javajosh 5y ago
        c is defined. https://physics.nist.gov/cgi-bin/cuu/Value?c https://physics.nist.gov/cgi-bin/cuu/Value?c You don't have to measure c, or calculate it. The causal chain goes the other way - we assume c is invariant and define capacitance and inductance in terms of it (c, that is).
        a1369209993 5y ago
        No, the meter is defined (as c times a certain number of seconds).
        javajosh 5y ago
        No, the meter is defined in terms of c and the hyperfine transition frequency of Cs-133 is defined https://physics.nist.gov/cgi-bin/cuu/Value?nucs https://physics.nist.gov/cgi-bin/cuu/Value?nucs. If that link isn't good enough, you can check out the SI Mises en pratique on the subject: https://www.bipm.org/en/publications/mises-en-pratique https://www.bipm.org/en/publications/mises-en-pratique Edit: I'm totally this guy right now https://xkcd.com/386/ https://xkcd.com/386/
        a1369209993 5y ago
        > the meter is defined in terms of c That is what I just said: the meter is defined in terms of c, not vice versa.
        5y ago
    - danparsonson 5y ago
      Try this: https://www.youtube.com/watch?v=au0QJYISe4c https://www.youtube.com/watch?v=au0QJYISe4c c isn't defined as the speed of light - light travels at c, but so does everything else, when you factor in movement through time.
      - javajosh 5y ago
        Good video! Thanks. c is still defined as "the speed of light" by NIST, even if it's not measured, and even if it's more universal than light. But I get your point (via the video) - photons are particles that move only in space, and not at all in time. Kind of makes your mind bend when you consider the cosmic background radiation hasn't aged a day since the beginning of the universe...and since it comes from everywhere equally, we really are the center of the universe. And so is everwhere else! Fun.
lamontcg 5y ago
You could break the problem of closed timelike loops by having a preferred reference frame for the Universe where nothing could travel backwards in time relative to that reference frame. And the CMBR rest frame might be an artifact of that preferred reference frame existing in the early Universe.

gary_0 5y ago

No, not "observing your own past". Instead, your past self could observe your future self. And then your future self could turn their FTL ship around and run you over, killing their past self and creating a paradox. https://old.reddit.com/r/askscience/comments/l9sba/why_does_ftl_travelinformation_break_causality/ https://old.reddit.com/r/askscience/comments/l9sba/why_does_...

px43 5y ago
> turn their FTL ship around I feel like this is the part that would probabally be hard (ie not possible) when going faster than light, assuming such a thing is possible. Also whatever magical space warp it is that lets you go that fast could presumably also obscure your ability to see yourself in the distance.
- gary_0 5y ago
  "Turning around" is just stopping at some point (ie. some inertial reference frame) and then accelerating in the opposite direction with your FTL drive. You have all the time in the world to do this maneuver because you're already effectively in the past: https://physics.stackexchange.com/a/54242 https://physics.stackexchange.com/a/54242
  - nine_k 5y ago
    There's no absolute coordinate system in space, so there's no way to "stop". You can only change your velocity relative to some other object, e.g. your past self in the example. What counts is acceleration / deceleration. Slowing down to reach the speed of light might be as hard as speeding up to reach it; I remember it played a role in the theory of tachyons, hypothetical FTL elementary particles which could not be slowed down.
    - jchrisa 5y ago
      > acceleration / deceleration Another question for the thread: Anyone here feel like they can feel the Coriolis effect if they focus enough? I haven't travelled across the equator enough to know if the north/south jetlag is different from east/west.

nwallin 5y ago

Being able to send messages faster than the speed of light grants the ability to send messages to yourself in the past. [0] If you can send messages to yourself in the past all manner of causality goes out the window. First of all, it gives you the ability to solve NP-complete problems trivially. Step 1: you receive a message from yourself from the future. It explains the problem, gives you a potential solution. Let's say you ask yourself to solve boolean satisfiability, gives you the problem statement, and then a potential solution. You check the solution: If the possible solution is correct, you send yourself an identical message. If the solution is not correct, you send the next possible permutation of the answer. So if you sent yourself the possible solution TTFTFTTTFTFFTTTFTF, you send yourself TTFTFTTTFTFFTTTFTT. If there are no further possible permutations, then you send yourself a statement saying that the problem is not solvable. You've created a time paradox; a future in which you send yourself an incorrectly solution is logically inconsistent. The only consistent universes are the ones where you send yourself the correct solution or not-solvable, and all other universes (starting with the one where you sent yourself the message FFFFFFFFFFFFFFFFFF) collapse into the valid one. https://en.wikipedia.org/wiki/Tachyonic_antitelephone https://en.wikipedia.org/wiki/Tachyonic_antitelephone

drran 5y ago
> Being able to send messages faster than the speed of light grants the ability to send messages to yourself in the past. No, it doesn't. Even if we will be able to communicate instantly (0 delay = infinite faster than speed of light), we will not be able to send a message to the past (negative delay).
- nl 5y ago
  It does. Read the numerical example on https://en.m.wikipedia.org/wiki/Tachyonic_antitelephone https://en.m.wikipedia.org/wiki/Tachyonic_antitelephone
  - drran 5y ago
    1) It's not applicable for this kind of drive. Isn't? 2) It's an unsolved problem, which waits for a correct solution. 3) Next time, please read what you are posting, please.