9 ms·

How does your initial point of 1g for a few months on a heavy spacecraft lead to traveling 4.246 light years? Genuine question!

by eddsh1994 4y ago

gpm 4y ago
It would take 6 years to get there (earth time) or 3.6 years (spacecraft time) if you could have constant 1G acceleration (in the opposite direction for the second half of the journey). https://cosmicreflections.skythisweek.info/2019/09/04/space-travel-under-constant-1g-acceleration/ https://cosmicreflections.skythisweek.info/2019/09/04/space-...
- geijoenr 4y ago
  With a spacecraft big enough to live in, we are talking of a time frame not too different from the one from the age of exploration by sea. I have no doubt many people would want to take such a trip even if it takes 10 years to get back to earth. Unmanned spacecrafts would be able to go much faster, sustaining more dramatic accelerations. So getting cargo and robots there for support would be way faster.
- eddsh1994 4y ago
  This takes you to 0.99C, how feasible is that (thinking materials more than anything)
  - credit_guy 4y ago
    It's not feasible. It would require more fuel than the Earth is heavy, even if we had nuclear propulsion.
    - geijoenr 4y ago
      It depends on the mass of the ship, but yes; quite a bit. https://www.quora.com/How-much-energy-does-it-take-to-accelerate-1-gram-mass-to-99-of-the-lights-speed https://www.quora.com/How-much-energy-does-it-take-to-accele... I don't think that means is unfeasible if the energy source allows for that.
      - credit_guy 4y ago
        No, actually it does not depend. The rocket equation says that the fuel mass of a rocketship is higher than the cargo mass by exp(delta_v/v_exhaust). When the final velocity is relativistic, delta_v should be replaced with delta_rapidity. In our case this would introduce a factor of 2.65, but the results are so ridiculous that we can ignore that. So, let's simply say that delta_v is the speed of light, or 300000 km/s. The exhaust velocity for a nuclear thermal rocket is about 9 km/s. The ratio between delta_v and the exhaust velocity is about 33000. The exponential of that is roughly speaking 1 followed by 15000 zeros. There are less than 10^100 atoms in the known universe. So, even if you want to accelerate just one single atom to 99% of the speed of light, you would need more fuel than the entire universe. Many, many, many times more.
  - geijoenr 4y ago
    Yeah well, we don't really know what happens at high speeds. Probably biochemistry stops working at 0.3c? It doesn't seem structural materials would remain solid at 0.95c... who knows.
    - monkeydreams 4y ago
      I am not a physicist but I suspect that we are already moving at significant speed relative to other bodies. We have no special frame of reference that defines our "real" speed.
      - gpm 4y ago
        That's not quite right. There's the cosmic background radiation "rest" frame, which can reasonably be taken as 0 velocity. The laws of physics don't change in different frames (Einstein's assumption), but that doesn't mean that all frames are equivalent in other respects.
        JumpCrisscross 4y ago
        > cosmic background radiation "rest" frame, which can reasonably be taken as 0 velocity What does this mean? The CMB is radiation travelling at the speed of light in every direction.
        nextaccountic 4y ago
        There's a rest frame where the CMB is (nearly) isotropic, and in every other frame there's a larger anisotropy, see the first comment here https://www.reddit.com/r/AskPhysics/comments/2va4t6/does_no_absolute_reference_frame_contradict_the/ https://www.reddit.com/r/AskPhysics/comments/2va4t6/does_no_... > You are absolutely right in that the CMB becomes Doppler shifted when you have some relative velocity. But Earth is not in the CMB rest frame: when we observe the CMB from Earth, we observe a dipole component to the CMB caused by the Earth's motion orbiting the Sun, the Sun's orbit around the Milky Way, and any velocity the Milky Way as a whole has. I was at a talk about the new Planck results last week, and saw a plot in which you can clearly see the dipole component in the raw data. You need to correct for this motion before you can even remotely see the anisotropies that are the interesting science goals of Planck. > There is a rest frame in which the CMB is closest to isotropic (no dipole component), and this rest frame is special but not 'absolute'. This frame is effectively the 'center-of-momentum' frame of the observable universe, in which we expect the total momentum to be zero. We know from classical mechanics that for any system of objects, we can construct such a frame, and that it sometimes has useful properties for solving certain types of problems. But there is nothing 'absolute' about this rest frame, the laws of physics operate entirely the same. > And so this is fine, because ultimately what relativity requires is that the laws of physics operate the same in every rest frame, not that every rest frame looks the same. Because the CMB is itself physical (made of photons) and was emitted by matter, it is entirely natural that it should be affected by frame transformations, and should look different if you shift to a frame that is moving differently than the emitting medium.
  - gpm 4y ago
    Only 0.95 per the source linked above. I don't know. Speculating you'll substantially blue shift the light coming from in front of you, and I suppose that can't be good for materials (or people), but I'm not sure if there is enough to matter. Any dust you collide with is also going to have ridiculous amounts of energy, but you'll be in interstellar space when you're at high speeds so there shouldn't be much of it (even for space) either.
  - deleted 4y ago
    [deleted]
  - nine_k 4y ago
    Sadly, not very feasible because of the hydrogen atoms floating around in the empty space, about 1 per cubic meter. At speeds about 0.5C a collision with such an atom produces X-rays, and harder gamma rays at higher speeds. They are pretty hard to insulate against, and are actively harmful. For a spaceship of a considerable size, enough collisions would occur to be dangerous. The paper: https://www.scirp.org/journal/PaperInformation.aspx?paperID=23913 https://www.scirp.org/journal/PaperInformation.aspx?paperID=...
    - gigaparsec 4y ago
      To make it even worse, that's based on an average density 1.8 atoms/c3. The real values would probably vary wildly, and you won't be able to "break" in advance in order to go through a high density area. And then there's the problem of hitting an interstellar grain of sand at 0.5c.
      - mr_toad 4y ago
        I wonder what the odds of survival are compared to sailing around the world in a Galleon.
brnt 4y ago
In fact, you could get anywhere in the universe in your lifetime/timeframe, if you could maintain the 1g acceleration.
- mr_toad 4y ago
  Depending on the nature of the expansion of the universe, that might not be true. https://en.wikipedia.org/wiki/Cosmological_horizon https://en.wikipedia.org/wiki/Cosmological_horizon

How does your initial point of 1g for a few months on a heavy spacecraft lead to traveling 4.246 light years? Genuine question!

gpm 4y ago

It would take 6 years to get there (earth time) or 3.6 years (spacecraft time) if you could have constant 1G acceleration (in the opposite direction for the second half of the journey). https://cosmicreflections.skythisweek.info/2019/09/04/space-travel-under-constant-1g-acceleration/ https://cosmicreflections.skythisweek.info/2019/09/04/space-...

geijoenr 4y ago
With a spacecraft big enough to live in, we are talking of a time frame not too different from the one from the age of exploration by sea. I have no doubt many people would want to take such a trip even if it takes 10 years to get back to earth. Unmanned spacecrafts would be able to go much faster, sustaining more dramatic accelerations. So getting cargo and robots there for support would be way faster.
eddsh1994 4y ago
This takes you to 0.99C, how feasible is that (thinking materials more than anything)
- credit_guy 4y ago
  It's not feasible. It would require more fuel than the Earth is heavy, even if we had nuclear propulsion.
  - geijoenr 4y ago
    It depends on the mass of the ship, but yes; quite a bit. https://www.quora.com/How-much-energy-does-it-take-to-accelerate-1-gram-mass-to-99-of-the-lights-speed https://www.quora.com/How-much-energy-does-it-take-to-accele... I don't think that means is unfeasible if the energy source allows for that.
    - credit_guy 4y ago
      No, actually it does not depend. The rocket equation says that the fuel mass of a rocketship is higher than the cargo mass by exp(delta_v/v_exhaust). When the final velocity is relativistic, delta_v should be replaced with delta_rapidity. In our case this would introduce a factor of 2.65, but the results are so ridiculous that we can ignore that. So, let's simply say that delta_v is the speed of light, or 300000 km/s. The exhaust velocity for a nuclear thermal rocket is about 9 km/s. The ratio between delta_v and the exhaust velocity is about 33000. The exponential of that is roughly speaking 1 followed by 15000 zeros. There are less than 10^100 atoms in the known universe. So, even if you want to accelerate just one single atom to 99% of the speed of light, you would need more fuel than the entire universe. Many, many, many times more.
- geijoenr 4y ago
  Yeah well, we don't really know what happens at high speeds. Probably biochemistry stops working at 0.3c? It doesn't seem structural materials would remain solid at 0.95c... who knows.
  - monkeydreams 4y ago
    I am not a physicist but I suspect that we are already moving at significant speed relative to other bodies. We have no special frame of reference that defines our "real" speed.
    - gpm 4y ago
      That's not quite right. There's the cosmic background radiation "rest" frame, which can reasonably be taken as 0 velocity. The laws of physics don't change in different frames (Einstein's assumption), but that doesn't mean that all frames are equivalent in other respects.
      - JumpCrisscross 4y ago
        > cosmic background radiation "rest" frame, which can reasonably be taken as 0 velocity What does this mean? The CMB is radiation travelling at the speed of light in every direction.
        nextaccountic 4y ago
        There's a rest frame where the CMB is (nearly) isotropic, and in every other frame there's a larger anisotropy, see the first comment here https://www.reddit.com/r/AskPhysics/comments/2va4t6/does_no_absolute_reference_frame_contradict_the/ https://www.reddit.com/r/AskPhysics/comments/2va4t6/does_no_... > You are absolutely right in that the CMB becomes Doppler shifted when you have some relative velocity. But Earth is not in the CMB rest frame: when we observe the CMB from Earth, we observe a dipole component to the CMB caused by the Earth's motion orbiting the Sun, the Sun's orbit around the Milky Way, and any velocity the Milky Way as a whole has. I was at a talk about the new Planck results last week, and saw a plot in which you can clearly see the dipole component in the raw data. You need to correct for this motion before you can even remotely see the anisotropies that are the interesting science goals of Planck. > There is a rest frame in which the CMB is closest to isotropic (no dipole component), and this rest frame is special but not 'absolute'. This frame is effectively the 'center-of-momentum' frame of the observable universe, in which we expect the total momentum to be zero. We know from classical mechanics that for any system of objects, we can construct such a frame, and that it sometimes has useful properties for solving certain types of problems. But there is nothing 'absolute' about this rest frame, the laws of physics operate entirely the same. > And so this is fine, because ultimately what relativity requires is that the laws of physics operate the same in every rest frame, not that every rest frame looks the same. Because the CMB is itself physical (made of photons) and was emitted by matter, it is entirely natural that it should be affected by frame transformations, and should look different if you shift to a frame that is moving differently than the emitting medium.
- gpm 4y ago
  Only 0.95 per the source linked above. I don't know. Speculating you'll substantially blue shift the light coming from in front of you, and I suppose that can't be good for materials (or people), but I'm not sure if there is enough to matter. Any dust you collide with is also going to have ridiculous amounts of energy, but you'll be in interstellar space when you're at high speeds so there shouldn't be much of it (even for space) either.
- deleted 4y ago
  [deleted]
- nine_k 4y ago
  Sadly, not very feasible because of the hydrogen atoms floating around in the empty space, about 1 per cubic meter. At speeds about 0.5C a collision with such an atom produces X-rays, and harder gamma rays at higher speeds. They are pretty hard to insulate against, and are actively harmful. For a spaceship of a considerable size, enough collisions would occur to be dangerous. The paper: https://www.scirp.org/journal/PaperInformation.aspx?paperID=23913 https://www.scirp.org/journal/PaperInformation.aspx?paperID=...
  - gigaparsec 4y ago
    To make it even worse, that's based on an average density 1.8 atoms/c3. The real values would probably vary wildly, and you won't be able to "break" in advance in order to go through a high density area. And then there's the problem of hitting an interstellar grain of sand at 0.5c.
    - mr_toad 4y ago
      I wonder what the odds of survival are compared to sailing around the world in a Galleon.

brnt 4y ago

In fact, you could get anywhere in the universe in your lifetime/timeframe, if you could maintain the 1g acceleration.

mr_toad 4y ago
Depending on the nature of the expansion of the universe, that might not be true. https://en.wikipedia.org/wiki/Cosmological_horizon https://en.wikipedia.org/wiki/Cosmological_horizon