4 ms·

Flying cars man! Think of the flying cars! And maybe real hover boards

undersuit 11y ago
First we need to find out how to create repulsion. Right now I'm pretty sure a graviton generator would just be a novelty device that weighs more than what it's mass would lead you to think it weighs. Maybe we could make orbital graviton beam generator that could literally suck an object off the face of the Earth.
- celticninja 11y ago
  That sounds like a tractor beam to me.
- pavel_lishin 11y ago
  > I'm pretty sure a graviton generator would just be a novelty device that weighs more than what it's mass would lead you to think it weighs Which would make space travel a lot easier - no more worrying about bone density loss!
- thaumasiotes 11y ago
  > a novelty device that weighs more than what it's mass would lead you to think it weighs This is an interesting concept. As far as I'm aware, we have ways of measuring weight, but no way of measuring mass. How would you know whether something weighed more than it "should", based on its "mass"?
  - sandworm101 11y ago
    You tie it to something of a known mass and spin the pair. The motion of these two bodies measures mass without the concept of gravity/weight. Or you throw it at something of known mass and measure the speed it imparts onto the known object. Or you hang the known mass and the unknown mass on strings and measure the force of gravity between them, which may seem hard but can be done with stuff from Home Depo. https://www.fourmilab.ch/gravitation/foobar/ https://www.fourmilab.ch/gravitation/foobar/
    - thaumasiotes 11y ago
      > You tie it to something of a known mass and spin the pair. The motion of these two bodies measures mass without the concept of gravity/weight. I have no intuition for this. Maybe it's valid, but your other two examples raise grave doubts about this one. > Or you throw it at something of known mass and measure the speed it imparts onto the known object. Blind application of the principle of conservation of momentum does indeed tell us that we can measure the mass of one object by colliding it at known velocity with another object of known mass and measuring the resulting velocities. But I tend to worry that the mechanism for transferring velocity from one object to another object in a collision is the force it exerts during the collision, and that that force might be determined by the object's weight (also a force) rather than mass (a platonic concept). But, I'm not sure here either. This ties in to the "fun factoid" that physics has no explanation for inertial mass and gravitational mass being the same quantity. If they in fact aren't necessarily the same thing, momentum transfer, measuring inertial mass, would solve this problem. If there is a reason they coincide, this approach will be confounded by that reason. > Or you hang the a known mass and the unknown mass on strings and measure the force of gravity between them I'm absolutely certain this wouldn't work to distinguish the mass and weight of an object that has extra weight because it's emitting extra gravity. The measured force of gravity is going to include the extra gravity you're trying to ignore.
      - sandworm101 11y ago
        >>As far as I'm aware, we have ways of measuring weight, but no way of measuring mass. Then we are speaking of different things. I understand 'weight' as how heavy something is within particular gravity field (ie on a bathroom scale on earth) whereas mass is independent of local gravity. The schemes I suggest measure mass without resort to weight. >>I have no intuition for this. Maybe it's valid, but your other two examples raise grave doubts about this one. The motion of the more massive pair will describe a smaller circle than the lighter one. The ratios of the two circles/motions allows you to calculate the unknown mass from the known.
        thaumasiotes 11y ago
  - kijin 11y ago
    All you need is a switch to turn it on and off. Tell someone to hold it. Turn it off. Watch them struggle with the sudden weight. Turn it back on.
  - xdarnold 11y ago
    With today's technology, for some compounds... you can count atoms by physical measurements and known structure, and figure mass by atomic weight. http://www.nist.gov/pml/si-redef/kg_new_silicon.cfm http://www.nist.gov/pml/si-redef/kg_new_silicon.cfm
- Crito 11y ago
  Could such a device be used to increase the reaction mass of your fuel when it exits your engine? A sort of way to cheat F=MA by artificially boosting M, but only after you are in orbit?
  - sandworm101 11y ago
    No more so than any other means of adding energy to the fuel.
  - losteric 11y ago
    No, you're confusing mass and weight. Mass is the amount of matter in a thing. Weight measures gravity's pull on the thing. This theoretical device could make things weight more than with just Earth's gravity... but it wouldn't help your spaceship. Your engine is still pushing out the same amount of matter, so thrust remains unchanged.
    - thaumasiotes 11y ago
      toy model: I, the spaceship, would like to accelerate through space. I take up some fuel and hurl it in the opposite direction, which requires me to apply force to the fuel I'm ejecting. It goes off into space at some rate determined by the impulse I applied and the mass of fuel I applied it to. Newton's third law means that when I hurl the fuel, it applies a symmetrical impulse to me, accelerating me in the opposite direction. In this model, the acceleration I get from the fuel doesn't depend in any way on the mass of the fuel I eject, only on the force I apply to it. What's wrong with the model?
    - zardo 11y ago
      You could definitely boost an orbit by increasing the force of gravity while approaching periapsis.
- sethrin 11y ago
  I'm not terribly knowledgeable about relativity, but I don't think that gravitational repulsion is a very meaningful concept in GR. I would appreciate being corrected on this matter if that is not true.
  - IIAOPSW 11y ago
    Well then prepare to stand corrected. Inside a charged black hole there is a second horizon. Beyond this point the black hole is gravitationally repulsive. http://casa.colorado.edu/~ajsh/rn.html http://casa.colorado.edu/~ajsh/rn.html
    - sethrin 11y ago
      So it's not explicitly ruled out by relativity. However, that link says: > The Universe at large appears to be electrically neutral, or close to it. Thus real black holes are unlikely to be charged. If a black hole did somehow become charged, it would quickly neutralize itself by accreting charge of the opposite sign. > It is not clear how a gravitationally repulsive, negative-mass singularity could form. So it falls under the same sort of category as negative- or imaginary-mass 'exotic matter': not ruled out, but there's nothing suggesting that it actually exists.
      - IIAOPSW 11y ago
        I wasn't ever arguing that it occurs in nature, or even that we will one day engineer it. I was just answering your question that GR does indeed allow for such a thing. I vaguely recall other weird edge cases where gravity is repulsive, but I can't find any links right now.
        sethrin 11y ago
        Of course, and I do appreciate your reply. I was purposefully vague in my original wording because there is that niggling difference between "unphysical" and "not proved impossible" that I didn't want to get on the wrong side of. You have enabled me to speak more precisely about the subject in the future -- many thanks. My last reply was merely trying to place this information in context; that is to say not unphysical, but in the same category as stable wormholes, Alcubierre drives, FTL/time travel, and other such theories. And ultimately it seems like either the guy talking about graviton beams was either talking about something extremely far-fetched, or a great way to destroy large parts of the planet, or both.
sqldba 11y ago
"Here's a board with wheels we call a hover board!"

First we need to find out how to create repulsion. Right now I'm pretty sure a graviton generator would just be a novelty device that weighs more than what it's mass would lead you to think it weighs. Maybe we could make orbital graviton beam generator that could literally suck an object off the face of the Earth.

celticninja 11y ago
That sounds like a tractor beam to me.
pavel_lishin 11y ago
> I'm pretty sure a graviton generator would just be a novelty device that weighs more than what it's mass would lead you to think it weighs Which would make space travel a lot easier - no more worrying about bone density loss!
thaumasiotes 11y ago
> a novelty device that weighs more than what it's mass would lead you to think it weighs This is an interesting concept. As far as I'm aware, we have ways of measuring weight, but no way of measuring mass. How would you know whether something weighed more than it "should", based on its "mass"?
- sandworm101 11y ago
  You tie it to something of a known mass and spin the pair. The motion of these two bodies measures mass without the concept of gravity/weight. Or you throw it at something of known mass and measure the speed it imparts onto the known object. Or you hang the known mass and the unknown mass on strings and measure the force of gravity between them, which may seem hard but can be done with stuff from Home Depo. https://www.fourmilab.ch/gravitation/foobar/ https://www.fourmilab.ch/gravitation/foobar/
  - thaumasiotes 11y ago
    > You tie it to something of a known mass and spin the pair. The motion of these two bodies measures mass without the concept of gravity/weight. I have no intuition for this. Maybe it's valid, but your other two examples raise grave doubts about this one. > Or you throw it at something of known mass and measure the speed it imparts onto the known object. Blind application of the principle of conservation of momentum does indeed tell us that we can measure the mass of one object by colliding it at known velocity with another object of known mass and measuring the resulting velocities. But I tend to worry that the mechanism for transferring velocity from one object to another object in a collision is the force it exerts during the collision, and that that force might be determined by the object's weight (also a force) rather than mass (a platonic concept). But, I'm not sure here either. This ties in to the "fun factoid" that physics has no explanation for inertial mass and gravitational mass being the same quantity. If they in fact aren't necessarily the same thing, momentum transfer, measuring inertial mass, would solve this problem. If there is a reason they coincide, this approach will be confounded by that reason. > Or you hang the a known mass and the unknown mass on strings and measure the force of gravity between them I'm absolutely certain this wouldn't work to distinguish the mass and weight of an object that has extra weight because it's emitting extra gravity. The measured force of gravity is going to include the extra gravity you're trying to ignore.
    - sandworm101 11y ago
      >>As far as I'm aware, we have ways of measuring weight, but no way of measuring mass. Then we are speaking of different things. I understand 'weight' as how heavy something is within particular gravity field (ie on a bathroom scale on earth) whereas mass is independent of local gravity. The schemes I suggest measure mass without resort to weight. >>I have no intuition for this. Maybe it's valid, but your other two examples raise grave doubts about this one. The motion of the more massive pair will describe a smaller circle than the lighter one. The ratios of the two circles/motions allows you to calculate the unknown mass from the known.
      - thaumasiotes 11y ago
- kijin 11y ago
  All you need is a switch to turn it on and off. Tell someone to hold it. Turn it off. Watch them struggle with the sudden weight. Turn it back on.
- xdarnold 11y ago
  With today's technology, for some compounds... you can count atoms by physical measurements and known structure, and figure mass by atomic weight. http://www.nist.gov/pml/si-redef/kg_new_silicon.cfm http://www.nist.gov/pml/si-redef/kg_new_silicon.cfm
Crito 11y ago
Could such a device be used to increase the reaction mass of your fuel when it exits your engine? A sort of way to cheat F=MA by artificially boosting M, but only after you are in orbit?
- sandworm101 11y ago
  No more so than any other means of adding energy to the fuel.
- losteric 11y ago
  No, you're confusing mass and weight. Mass is the amount of matter in a thing. Weight measures gravity's pull on the thing. This theoretical device could make things weight more than with just Earth's gravity... but it wouldn't help your spaceship. Your engine is still pushing out the same amount of matter, so thrust remains unchanged.
  - thaumasiotes 11y ago
    toy model: I, the spaceship, would like to accelerate through space. I take up some fuel and hurl it in the opposite direction, which requires me to apply force to the fuel I'm ejecting. It goes off into space at some rate determined by the impulse I applied and the mass of fuel I applied it to. Newton's third law means that when I hurl the fuel, it applies a symmetrical impulse to me, accelerating me in the opposite direction. In this model, the acceleration I get from the fuel doesn't depend in any way on the mass of the fuel I eject, only on the force I apply to it. What's wrong with the model?
  - zardo 11y ago
    You could definitely boost an orbit by increasing the force of gravity while approaching periapsis.
sethrin 11y ago
I'm not terribly knowledgeable about relativity, but I don't think that gravitational repulsion is a very meaningful concept in GR. I would appreciate being corrected on this matter if that is not true.
- IIAOPSW 11y ago
  Well then prepare to stand corrected. Inside a charged black hole there is a second horizon. Beyond this point the black hole is gravitationally repulsive. http://casa.colorado.edu/~ajsh/rn.html http://casa.colorado.edu/~ajsh/rn.html
  - sethrin 11y ago
    So it's not explicitly ruled out by relativity. However, that link says: > The Universe at large appears to be electrically neutral, or close to it. Thus real black holes are unlikely to be charged. If a black hole did somehow become charged, it would quickly neutralize itself by accreting charge of the opposite sign. > It is not clear how a gravitationally repulsive, negative-mass singularity could form. So it falls under the same sort of category as negative- or imaginary-mass 'exotic matter': not ruled out, but there's nothing suggesting that it actually exists.
    - IIAOPSW 11y ago
      I wasn't ever arguing that it occurs in nature, or even that we will one day engineer it. I was just answering your question that GR does indeed allow for such a thing. I vaguely recall other weird edge cases where gravity is repulsive, but I can't find any links right now.
      - sethrin 11y ago
        Of course, and I do appreciate your reply. I was purposefully vague in my original wording because there is that niggling difference between "unphysical" and "not proved impossible" that I didn't want to get on the wrong side of. You have enabled me to speak more precisely about the subject in the future -- many thanks. My last reply was merely trying to place this information in context; that is to say not unphysical, but in the same category as stable wormholes, Alcubierre drives, FTL/time travel, and other such theories. And ultimately it seems like either the guy talking about graviton beams was either talking about something extremely far-fetched, or a great way to destroy large parts of the planet, or both.

sqldba 11y ago

"Here's a board with wheels we call a hover board!"