4 ms·

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

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.

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.