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>Also dying with this measurement is another sci-fi hope: artificial gravity that works without rotation or acceleration. If antimatter truly anti-gravitated, t

by gs17 3y ago

>Also dying with this measurement is another sci-fi hope: artificial gravity that works without rotation or acceleration. If antimatter truly anti-gravitated, then simply by building a spacecraft’s floor out of normal matter and its ceiling out of antimatter, we could create a spacecraft that had its own built-in, automatic system for artificial gravity. I've never heard this idea, but that sounds hilariously dangerous.

thsksbd 3y ago
And ludicrously energetically expensive. But that's science proposal writing, isn't it? Propose something that would be ludicrous even if it were physical.
- mr_mitm 3y ago
  Ludicrous barely begins to describe the idea. It would have to be a mass on the order of magnitude of a moon that you would have to a) create and b) carry with you so that you would feel some gravity. Why not just build a rotating ring instead? No one in scifi was even entertaining this idea afaik.
  - Izkata 3y ago
    > No one in scifi was even entertaining this idea afaik. Yeah, this is the first I've heard of it too. If it's not rotation it's usually left unexplained, and when it's not it's usually something along the lines of an artificial gravity generator that pulls things towards it, like in Star Trek the floors contain "gravity plating" that does that on a small scale.
  - thsksbd 3y ago
    "It would have to be a mass on the order of magnitude of a moon" Here I am overcomplicating things in terms of E=mc2, and you come and point out the obvious.
aetherson 3y ago
And hilariously useless even if you wave away the costs and catastrophic dangers. Yes, you can have a floor of normal matter and a ceiling of antimatter and produce... like... maybe 0.00001 m/s^2 of gravity?
- robin_reala 3y ago
  Depends on how thick the ceiling and floor are.
LeifCarrotson 3y ago
Doesn't that proposal merely ~halve the amount of mass required? I'm in an office right now that has a ceiling made out of normal matter (gypsum, fiberglass, steel, and tar, probably a couple thousand kg overhead) and a floor also made out of normal matter (carpet, concrete, iron, silicon, oxygen, and a bunch of other stuff adding up to 6x10^24 kg), which generates artificial gravity at 1g. It happens to be rotating at 1/1440 RPM, but that's only a 0.2% reduction in the acceleration. Sure, if antimatter caused anti-gravitational forces, you could have achieved the same gravitational acceleration I experience in my office using one 3x10^24 kg sphere of antimatter overhead and one 3x10^24 kg sphere of matter beneath, but that's still not a Starship Enterprise, much less an X-wing, with artificial gravity generators. You're still stuck at: F = G x m_matter / r^2 + G x m_antimatter / r^2 F = 4 x pi / 3 x G x density x radius_matter + 4 x pi / 3 x G x density x radius_antimatter with planet-sized masses and radii for both. And I'm not sure that "hilariously" is a sufficient modifier for the danger level when standing in a 3m gap between planet-sized spheres of matter and antimatter.
- eesmith 3y ago
  Heinlein had a discussion about the "place a large mass just over a particular point on the Earth's surface" approach in "- We Also Walk Dogs" at https://archive.org/details/isbn_9780739410516/page/306/mode/2up?q=%22large+mass+just+over%22 https://archive.org/details/isbn_9780739410516/page/306/mode... . How big as mass? After some calculations on a slide rule: > “I’m almost afraid to answer. You would need a good sized asteroid, of lead, to get anywhere at all.” > “Asteroids have been moved before this.“ > “Yes, but what is to hold it up? No, Chief, there is no conceivable source of power, or means of applying it, that would enable you to hang a big planetoid over a particular spot on the Earth’s surface and keep it there.” But wait, what about something dense, like core material from a dwarf star? > “All we would need for that would be a ship capable of going light-years in a few days, some way to mine the interior of a star, and a new space-time theory.” > “Oh, well, skip it." Oh, yeah. In this space-age future where people can travel across the Solar System, they also use punched cards.
izzydata 3y ago
What about building a circular ship around a micro black hole? Or is that just real gravity and not anti-gravity?
- Cthulhu_ 3y ago
  Note: armchair reply, I don't know enough astro whatnots to substantiate this take: That implies that you can actually move the black hole; a black hole producing 1G requires it to be as heavy (or, to have as much attraction) as the earth itself. It would be a lot more compact - a black hole as heavy as the earth is about the size of a ping pong ball - but if my intuition is correct, would require as much force to move as it would to move the earth.
  - db48x 3y ago
    Yep, F=ma. For a given force, if mass is big then the acceleration must be small. Also, I feel compelled to mention that to get an Earth–like gravitational field near the event horizon of a non–spinning black hole, it needs to mass more than the whole galaxy. It would be about a light–year across, iirc, and if you built a shell around it (which would not be easy) then the surface area would be so large as to beggar description. Small black holes are not very earth–like because of the extreme tidal forces. Trying to stand on that ping–pong ball would be extremely uncomfortable.
    - User23 3y ago
      If it’s an antimatter black hole you can dump normal matter into it to release energy, assuming some way exists to turn that into useful acceleration.
      - nyssos 3y ago
        An "antimatter black hole" is just a black hole. There might still be something behind the event horizon to annihilate with, there might not, the question might be meaningless, but in any case you're not getting anything back out again.
thereddaikon 3y ago
I have never heard of antimatter being used in that way, even in the most off the wall, science illiterate speculation. Usually its as a power source.
- andrewflnr 3y ago
  Yeah, because the idea of antimatter being anti-gravitational is so obscure and silly that not even scifi authors use it.
  - antifa 3y ago
    Mathematically speaking, what's even the expected difference in gravity compared to just making the floor twice as thick?
db48x 3y ago
The idea was mostly limited to really bad science fiction, the kind that is really a fantasy with science–fictional set dressing.
drexlspivey 3y ago
What are the walls made of? Surely the ceiling must touch normal matter at some point?

by gs17 3y ago

thsksbd 3y ago

And ludicrously energetically expensive. But that's science proposal writing, isn't it? Propose something that would be ludicrous even if it were physical.

mr_mitm 3y ago
Ludicrous barely begins to describe the idea. It would have to be a mass on the order of magnitude of a moon that you would have to a) create and b) carry with you so that you would feel some gravity. Why not just build a rotating ring instead? No one in scifi was even entertaining this idea afaik.
- Izkata 3y ago
  > No one in scifi was even entertaining this idea afaik. Yeah, this is the first I've heard of it too. If it's not rotation it's usually left unexplained, and when it's not it's usually something along the lines of an artificial gravity generator that pulls things towards it, like in Star Trek the floors contain "gravity plating" that does that on a small scale.
- thsksbd 3y ago
  "It would have to be a mass on the order of magnitude of a moon" Here I am overcomplicating things in terms of E=mc2, and you come and point out the obvious.

aetherson 3y ago

And hilariously useless even if you wave away the costs and catastrophic dangers. Yes, you can have a floor of normal matter and a ceiling of antimatter and produce... like... maybe 0.00001 m/s^2 of gravity?

robin_reala 3y ago
Depends on how thick the ceiling and floor are.

LeifCarrotson 3y ago

Doesn't that proposal merely ~halve the amount of mass required? I'm in an office right now that has a ceiling made out of normal matter (gypsum, fiberglass, steel, and tar, probably a couple thousand kg overhead) and a floor also made out of normal matter (carpet, concrete, iron, silicon, oxygen, and a bunch of other stuff adding up to 6x10^24 kg), which generates artificial gravity at 1g. It happens to be rotating at 1/1440 RPM, but that's only a 0.2% reduction in the acceleration. Sure, if antimatter caused anti-gravitational forces, you could have achieved the same gravitational acceleration I experience in my office using one 3x10^24 kg sphere of antimatter overhead and one 3x10^24 kg sphere of matter beneath, but that's still not a Starship Enterprise, much less an X-wing, with artificial gravity generators. You're still stuck at: F = G x m_matter / r^2 + G x m_antimatter / r^2 F = 4 x pi / 3 x G x density x radius_matter + 4 x pi / 3 x G x density x radius_antimatter with planet-sized masses and radii for both. And I'm not sure that "hilariously" is a sufficient modifier for the danger level when standing in a 3m gap between planet-sized spheres of matter and antimatter.

eesmith 3y ago
Heinlein had a discussion about the "place a large mass just over a particular point on the Earth's surface" approach in "- We Also Walk Dogs" at https://archive.org/details/isbn_9780739410516/page/306/mode/2up?q=%22large+mass+just+over%22 https://archive.org/details/isbn_9780739410516/page/306/mode... . How big as mass? After some calculations on a slide rule: > “I’m almost afraid to answer. You would need a good sized asteroid, of lead, to get anywhere at all.” > “Asteroids have been moved before this.“ > “Yes, but what is to hold it up? No, Chief, there is no conceivable source of power, or means of applying it, that would enable you to hang a big planetoid over a particular spot on the Earth’s surface and keep it there.” But wait, what about something dense, like core material from a dwarf star? > “All we would need for that would be a ship capable of going light-years in a few days, some way to mine the interior of a star, and a new space-time theory.” > “Oh, well, skip it." Oh, yeah. In this space-age future where people can travel across the Solar System, they also use punched cards.

izzydata 3y ago

What about building a circular ship around a micro black hole? Or is that just real gravity and not anti-gravity?

Cthulhu_ 3y ago
Note: armchair reply, I don't know enough astro whatnots to substantiate this take: That implies that you can actually move the black hole; a black hole producing 1G requires it to be as heavy (or, to have as much attraction) as the earth itself. It would be a lot more compact - a black hole as heavy as the earth is about the size of a ping pong ball - but if my intuition is correct, would require as much force to move as it would to move the earth.
- db48x 3y ago
  Yep, F=ma. For a given force, if mass is big then the acceleration must be small. Also, I feel compelled to mention that to get an Earth–like gravitational field near the event horizon of a non–spinning black hole, it needs to mass more than the whole galaxy. It would be about a light–year across, iirc, and if you built a shell around it (which would not be easy) then the surface area would be so large as to beggar description. Small black holes are not very earth–like because of the extreme tidal forces. Trying to stand on that ping–pong ball would be extremely uncomfortable.
  - User23 3y ago
    If it’s an antimatter black hole you can dump normal matter into it to release energy, assuming some way exists to turn that into useful acceleration.
    - nyssos 3y ago
      An "antimatter black hole" is just a black hole. There might still be something behind the event horizon to annihilate with, there might not, the question might be meaningless, but in any case you're not getting anything back out again.

thereddaikon 3y ago

I have never heard of antimatter being used in that way, even in the most off the wall, science illiterate speculation. Usually its as a power source.

andrewflnr 3y ago
Yeah, because the idea of antimatter being anti-gravitational is so obscure and silly that not even scifi authors use it.
- antifa 3y ago
  Mathematically speaking, what's even the expected difference in gravity compared to just making the floor twice as thick?

db48x 3y ago

The idea was mostly limited to really bad science fiction, the kind that is really a fantasy with science–fictional set dressing.

drexlspivey 3y ago

What are the walls made of? Surely the ceiling must touch normal matter at some point?