4 ms·

I wonder what is a safe acceleration to get to light speed for long periods without hurtin too much

by m3kw9 3y ago

jtriangle 3y ago
~9.8 m/s sounds pretty comfortable.
- caymanjim 3y ago
  You're accelerating at 9.8m/s^2 toward your chair right now, so it depends on how comfortable the chair is.
  - jtriangle 3y ago
    Bold of you to assume I'm not standing on a Uline rubber anti-fatigue mat
bee_rider 3y ago
You can’t “get to” light speed, that’s one of the big punchlines in relativity. If you pick an acceleration equal to the acceleration we experience on Earth (aka 1g, aka ~9.8m/s^2), you hit relativistic speeds (speeds at which you need to take into account the effects of relativity to do anything) surprisingly fast. On the order of hundreds of days. So, it is not really a matter of safe acceleration on a long space trip. Instead you have to worry about the actual speed you are traveling at—even though space is very empty, there are still atoms floating around out there, and you’ll be moving at very high speeds relative to them, leading to interesting collisions.
- ryandrake 3y ago
  > Instead you have to worry about the actual speed you are traveling at That's the other big punchline in relativity: There's no one "actual speed" because that implies that there is a single "important" frame of reference wherein those atoms are floating around waiting to be hit by a spaceship.
  - acchow 3y ago
    What if you define the "important" reference frame as the speed of light's (in the same direction you are going). Since c is universally constant, it seems like a reasonable privileged reference.
    - richardw 3y ago
      Not a physicist, but my impression is that you're always going 0 percent of the speed of light (in all directions) from your own frame of reference. All you notice is that our solar system is moving away, faster. I guess you'd notice a change in light frequency based on the light in front/behind. Redder behind, bluer in front. Edit: supposedly we can measure our speed compared to the cosmic microwave background which is fairly uniform, which gives us a value of 400 to 800km/s relative to the CMB: https://www.researchgate.net/post/How_can_we_practically_measure_the_velocity_of_earth_in_Cosmic_Microwave_Background_rest_frame https://www.researchgate.net/post/How_can_we_practically_mea...
      - eru 3y ago
        See also https://en.wikipedia.org/wiki/Comoving_and_proper_distances#Comoving_coordinates https://en.wikipedia.org/wiki/Comoving_and_proper_distances#...
      - raattgift 3y ago
        > I guess you'd notice a change in light frequency based on the light in front/behind. Redder behind, bluer in front. The light in question is not uniform like white noise; the spectral power distribution has relatively light and dark lines in them as a result of the physics of the bright sources and intervening gas and dust. Those features also get redshifted. If one is moving relative to the sun, one would pay attention to the sun's Fraunhofer lines <https://physics.weber.edu/palen/clearinghouse/labs/Solarspec/sunspec.html https://physics.weber.edu/palen/clearinghouse/labs/Solarspec...>, which would be Doppler shifted to different wavelengths. These lines also appear in reflected light from bodies in the solar system; if you were flying towards Pluto you would see a corresponding blueshift of the reflected Fraunhofer lines (plus some additional structure related to the chemistry of Pluto; it has some luminescence, as does our moon, as do the leaves of plants, and luminescence tends to impinge on the narrower Fraunhofer lines). Indeed, measuring the Doppler shifts of multiple known-chemistry light sources is a useful technique in navigation of spacecraft within our solar system; it can in principle do better than precision measurement of angles to multiple light sources. The spectral distortions of the CMB are certainly interesting, but it's hard to imagine their utility for spacecraft navigation within the Milky Way, rather than helping to physical cosmologists understand why there even is a Milky Way. In the solar system we have kind-of the opposite problem: in order to get reliable anisotropy data of the Milky Way, probes like WMAP need excellent almanac data for the ephemeris of Jupiter (it's a bright reflector of sunlight and its cloud-tops at ~70 kPa are ~22 GHz microwave-bright; I gather other outer planets are used too, but the details are beyond me) to check its 22-GHz-band detection of the CMB Doppler shift in the directions it looks.
- whimsicalism 3y ago
  dumb question - why does going faster make it more likely you encounter fast protons? couldn’t protons in any reference frame be going quite fast relative to you?
  - zabzonk 3y ago
    not so much protons (which would be a problem, but could at least notionally be deflected), but atoms.
    - actionfromafar 3y ago
      An hydrogen atom and a proton are roughtly the same thing, right?
      - monocasa 3y ago
        A proton is a hydrogen ion and isotope.
      - zabzonk 3y ago
        no a proton is a hydrogen nucleus (or at least can be seen as one) and has a charge. travelling near lightspeed you would have to worry most about uncharged atoms/molecules (because of their mass) and neutrons, neither of which can be deflected.
  - elihu 3y ago
    Nah, most of the random atoms floating around in space are going to be travelling very roughly as fast as the things (stars, planets, etc..) around them -- because anything travelling much faster is likely to eventually bump into something and lose some of its momentum. There is the occasional weird exception though: https://en.wikipedia.org/wiki/Oh-My-God_particle https://en.wikipedia.org/wiki/Oh-My-God_particle
  - eru 3y ago
    > couldn’t protons in any reference frame be going quite fast relative to you? By the laws of physics, yes. But most of the stuff in our universe, and definitely in our galaxy, tends to move roughly at the same speed. See https://en.wikipedia.org/wiki/Comoving_and_proper_distances#Comoving_coordinates https://en.wikipedia.org/wiki/Comoving_and_proper_distances#...
- andrewflnr 3y ago
  > that’s one of the big punchlines in relativity It's kind of funny that the actual big punchline is that light speed matters at all. There would be no meme of "reaching lightspeed" without relativity, despite that meme originating from relativity specifically mentioning lightspeed as something you can't reach.

I wonder what is a safe acceleration to get to light speed for long periods without hurtin too much

by m3kw9 3y ago

I wonder what is a safe acceleration to get to light speed for long periods without hurtin too much

jtriangle 3y ago

~9.8 m/s sounds pretty comfortable.

caymanjim 3y ago
You're accelerating at 9.8m/s^2 toward your chair right now, so it depends on how comfortable the chair is.
- jtriangle 3y ago
  Bold of you to assume I'm not standing on a Uline rubber anti-fatigue mat

bee_rider 3y ago

You can’t “get to” light speed, that’s one of the big punchlines in relativity. If you pick an acceleration equal to the acceleration we experience on Earth (aka 1g, aka ~9.8m/s^2), you hit relativistic speeds (speeds at which you need to take into account the effects of relativity to do anything) surprisingly fast. On the order of hundreds of days. So, it is not really a matter of safe acceleration on a long space trip. Instead you have to worry about the actual speed you are traveling at—even though space is very empty, there are still atoms floating around out there, and you’ll be moving at very high speeds relative to them, leading to interesting collisions.

ryandrake 3y ago
> Instead you have to worry about the actual speed you are traveling at That's the other big punchline in relativity: There's no one "actual speed" because that implies that there is a single "important" frame of reference wherein those atoms are floating around waiting to be hit by a spaceship.
- acchow 3y ago
  What if you define the "important" reference frame as the speed of light's (in the same direction you are going). Since c is universally constant, it seems like a reasonable privileged reference.
  - richardw 3y ago
    Not a physicist, but my impression is that you're always going 0 percent of the speed of light (in all directions) from your own frame of reference. All you notice is that our solar system is moving away, faster. I guess you'd notice a change in light frequency based on the light in front/behind. Redder behind, bluer in front. Edit: supposedly we can measure our speed compared to the cosmic microwave background which is fairly uniform, which gives us a value of 400 to 800km/s relative to the CMB: https://www.researchgate.net/post/How_can_we_practically_measure_the_velocity_of_earth_in_Cosmic_Microwave_Background_rest_frame https://www.researchgate.net/post/How_can_we_practically_mea...
    - eru 3y ago
      See also https://en.wikipedia.org/wiki/Comoving_and_proper_distances#Comoving_coordinates https://en.wikipedia.org/wiki/Comoving_and_proper_distances#...
    - raattgift 3y ago
      > I guess you'd notice a change in light frequency based on the light in front/behind. Redder behind, bluer in front. The light in question is not uniform like white noise; the spectral power distribution has relatively light and dark lines in them as a result of the physics of the bright sources and intervening gas and dust. Those features also get redshifted. If one is moving relative to the sun, one would pay attention to the sun's Fraunhofer lines <https://physics.weber.edu/palen/clearinghouse/labs/Solarspec/sunspec.html https://physics.weber.edu/palen/clearinghouse/labs/Solarspec...>, which would be Doppler shifted to different wavelengths. These lines also appear in reflected light from bodies in the solar system; if you were flying towards Pluto you would see a corresponding blueshift of the reflected Fraunhofer lines (plus some additional structure related to the chemistry of Pluto; it has some luminescence, as does our moon, as do the leaves of plants, and luminescence tends to impinge on the narrower Fraunhofer lines). Indeed, measuring the Doppler shifts of multiple known-chemistry light sources is a useful technique in navigation of spacecraft within our solar system; it can in principle do better than precision measurement of angles to multiple light sources. The spectral distortions of the CMB are certainly interesting, but it's hard to imagine their utility for spacecraft navigation within the Milky Way, rather than helping to physical cosmologists understand why there even is a Milky Way. In the solar system we have kind-of the opposite problem: in order to get reliable anisotropy data of the Milky Way, probes like WMAP need excellent almanac data for the ephemeris of Jupiter (it's a bright reflector of sunlight and its cloud-tops at ~70 kPa are ~22 GHz microwave-bright; I gather other outer planets are used too, but the details are beyond me) to check its 22-GHz-band detection of the CMB Doppler shift in the directions it looks.
whimsicalism 3y ago
dumb question - why does going faster make it more likely you encounter fast protons? couldn’t protons in any reference frame be going quite fast relative to you?
- zabzonk 3y ago
  not so much protons (which would be a problem, but could at least notionally be deflected), but atoms.
  - actionfromafar 3y ago
    An hydrogen atom and a proton are roughtly the same thing, right?
    - monocasa 3y ago
      A proton is a hydrogen ion and isotope.
    - zabzonk 3y ago
      no a proton is a hydrogen nucleus (or at least can be seen as one) and has a charge. travelling near lightspeed you would have to worry most about uncharged atoms/molecules (because of their mass) and neutrons, neither of which can be deflected.
- elihu 3y ago
  Nah, most of the random atoms floating around in space are going to be travelling very roughly as fast as the things (stars, planets, etc..) around them -- because anything travelling much faster is likely to eventually bump into something and lose some of its momentum. There is the occasional weird exception though: https://en.wikipedia.org/wiki/Oh-My-God_particle https://en.wikipedia.org/wiki/Oh-My-God_particle
- eru 3y ago
  > couldn’t protons in any reference frame be going quite fast relative to you? By the laws of physics, yes. But most of the stuff in our universe, and definitely in our galaxy, tends to move roughly at the same speed. See https://en.wikipedia.org/wiki/Comoving_and_proper_distances#Comoving_coordinates https://en.wikipedia.org/wiki/Comoving_and_proper_distances#...
andrewflnr 3y ago
> that’s one of the big punchlines in relativity It's kind of funny that the actual big punchline is that light speed matters at all. There would be no meme of "reaching lightspeed" without relativity, despite that meme originating from relativity specifically mentioning lightspeed as something you can't reach.