Sciencemadness Discussion Board

Artificial gravity possibilities ? Physics question

DubaiAmateurRocketry - 30-8-2015 at 22:21

I think most know that one way to make artificial gravity is by spinning a cylinder, and the centripetal force would push u down as if there was gravity..

This force can be enlarged when the size of the spinning cylinder or the speed of the spin is increased obviously, i was wondering what if one is so massive and spinned so fast that the force would be equal of a black hole? Im not sure if i phrased it correctly but i hope you know what im trying to ask. I know probably no materials would sustain that stress and etc, however if it would, what would happen ?

fluorescence - 30-8-2015 at 23:19

Ehm...interesting question. So as far as I remember a static black hole, let's leave out the Ergo-Sphere for a second
has a schwarzschild-radius which does not neccessarily mean that it has to be ultradense. If I remember correctely the density of the universe's schwarzschild radius would be like the bulk density of straw so not ultradense. It's just that the the force that is pulling objects at a certain area is high enough that even light can't leave any more. So for a static black hole I think the shape of the object isn't that important. There is also the theory how rotating black holes behave and they warp a bit of the space-time around them the so called ergosphere. So the area around it will probably follow the roation and everything that is there will have to spin like the cylinder, too. The question is what happens inside the cylinder.
There is something called ring singularity perhaps there will be similar effects but I can't find anyhting about the center of the ring.

aga - 31-8-2015 at 11:24

Trickiest thing with Gravity is that we're still not sure what it actually is.

Metacelsus - 31-8-2015 at 18:14

Rigidity does strange things at relativistic speeds.

https://en.wikipedia.org/wiki/Born_rigidity
https://en.wikipedia.org/wiki/Ehrenfest_paradox

Of course, any real, physical object rotating that fast would disintegrate. Those are just thought experiments.

TheAlchemistPirate - 1-9-2015 at 11:54

I remember reading on amasci(somewhat pseudoscientific, but has some interesting ideas) about the frequencies of things not commonly found on the electromagnetic spectrum. For example, light is considered to have a frequency of 100 trillion hertz. Theoretically, if you could build a solid state ocillator that could achieve that frequency(the highest I have seen is 5.8 billion hertz) then you could generate "cold light". I sometimes wonder about what kind of frequencies happen in devices with low gas pressures light neon tubes. They shine simply from a presence of a high voltage electromagnetic field. Maybe the gas molecules have a 100 trillion hertz frequency within themselves, amplified by the neon transformer? Back to the topic the same article suggested that gravity had a frequency that if achieved, my counteract or contribute to its effect.

aga - 1-9-2015 at 12:08

Quote: Originally posted by TheAlchemistPirate  
For example, light is considered to have a frequency of 100 trillion hertz. Theoretically, if you could build a solid state ocillator that could achieve that frequency

Already exists.

It's called a Light Bulb.

MrHomeScientist - 1-9-2015 at 12:31

No no no, they are actually dark suckers.

DubaiAmateurRocketry - 7-9-2015 at 20:35

ummm dark suckers looks interesting, but any answers for my question ? :P

PHILOU Zrealone - 8-9-2015 at 04:52

The centrifugal force will be too strong for any device...all material will suffer plastic deformation or rupture.
Also to take in account the gyroscopic effect and the nutation move due to earth moving

Maybe you would have more chance to get a tiny black hole (fusion core) by allowing two high speed projectiles in opposite trajectory having a facial impact (like in a super colider). But here also you would need to reach super high speeds.

neptunium - 8-9-2015 at 05:04

the answer is in the question , if one builds a huge cylinder and spin it so that the centripetal force would be as strong as that of a black hole, no material can support such force so it would blow itself apart long before getting there.
but assuming it could it would drag space and time in the inner part of the cylinder, creating a vortex. Some one tried to use light (laser) in a similar fashion to create such a distortion . forgot his name...
its an interesting thought experiment however...

annaandherdad - 8-9-2015 at 09:43

Quote: Originally posted by DubaiAmateurRocketry  
I think most know that one way to make artificial gravity is by spinning a cylinder, and the centripetal force would push u down as if there was gravity..

This force can be enlarged when the size of the spinning cylinder or the speed of the spin is increased obviously, i was wondering what if one is so massive and spinned so fast that the force would be equal of a black hole? Im not sure if i phrased it correctly but i hope you know what im trying to ask. I know probably no materials would sustain that stress and etc, however if it would, what would happen ?


You must mean, "the force would be equal to that of a black hole." If that's what you mean, then the question isn't quite posed correctly. A black hole doesn't have a "force" any more than the earth (through its gravitational field) has a force. The earth's gravitational field exerts a force on an object that depends on its mass and how far away it is.

It is the same for a black hole. The force exerted depends on the mass and how far away the object is. To measure the force the earth exerts on an object you just weigh it; say you hold a spring balance in your hand, and the object is in the pan. The weight is conveyed through your arms down your legs, increasing the force with which your feet press on the ground. The earth presses back on your feet; it is the solid state forces in the solid body of the earth that do this.

Now suppose instead of standing on the ground, you are in a rocket ship whose engines are blasting downward at just the right rate of burn that the rocket ship is hovering near the surface of the earth (neither accelerating up or down, nor moving up or down). Then the force on the balance is the same as before, it's just that now it is the rocket engines that are counteracting the weight of the object, instead of the solid state forces in the body of the earth pushing up on your shoes.

The rocket engine analogy is useful because a black hole doesn't have a surface. To define the "weight" of an object at any distance, you get in a rocket ship that is blasting its engines at exactly the rate to keep the ship hovering at a fixed distance from the black hole. This must be outside the Schwarzschild radius. Then you weigh the object in the usual way, standing in the space ship. Of course you could just weigh yourself, since the weight of anything is proportional to its mass. It is really the ratio, the effective acceleration of gravity, that is important.

Now you want to create artificial gravity by spinning something around. That is exactly what happens when an object is in a circular orbit. There is an exact balance between the gravitational force pulling inward, and the centrifugal force pushing outward. So when an object is in a circular orbit around a black hole the centrifugal and gravitational forces are equal and opposite, and cancel each other.

This is just like when an object is in a circular orbit around the earth or sun. The velocity with which the object moves in the circular orbit depends on the mass of the source of the gravitational field (the earth or the sun, etc), and on the radius. By balancing the two forces, one can derive the formula for the velocity of the object as a function of the distance and the mass of the gravitating object. The velocity decreases as the distance increases (as the radius of the orbit gets larger), and it increases as the mass of the gravitating object increases.

For example, this velocity is about 7 km/sec for an object in near earth orbit (as I recall). It is more, about 25 km/sec (as I recall) for an object orbiting the sun at the earth's radius. In other words, this is the orbital velocity of the earth about the sun.

If you work it out for a nonrotating (Schwarzschild) black hole, you find a formula that gives the velocity as a function of the radius and the mass of the gravitating object. It is not as simple as the one you get if you use the Newtonian model for gravity.

You that the radius of a circular orbit must be > 3GM/c^2 = 1.5 times the Schwarzschild radius. This is measured in Schwarzschild coordinates; it would take a long time to explain exactly what this means. However, the minimum radius of a circular orbit is larger than the Schwarzschild radius. An object in a circular orbit just outside this minimum radius is moving at a velocity close to the speed of light, as seen by a stationary observer (eg the one in the hovering rocket ship). This velocity approaches the speed of light as the radius of the circular orbit is reduced toward r=3GM/c^2. Circular orbits are not possible for r< 3GM/c^2, and r=2GM/c^2 is Schwarzschild radius, the radius of no return.

[Edited on 8-9-2015 by annaandherdad]

[Edited on 8-9-2015 by annaandherdad]

Nicodem - 8-9-2015 at 10:32

Well, centripetal force is not gravitational force. It is just a radial deacceleration force. But objects moving at high (radial) speed do have relativistic effects (increase of mass, relative shrinkage, relative time dilation...). Does this also mean that a high speed spinning disc bends space/time just like gravitation does?
Quote: Originally posted by annaandherdad  
This velocity approaches the speed of light as the radius of the circular orbit is reduced toward r=3GM/c^2. Circular orbits are not possible for r< 3GM/c^2, and r=2GM/c^2 is Schwarzschild radius, the radius of no return.

Just a few questions from someone who knows almost nothing about general relativity, but finds it amusing:
I understand that the mass of an object increases with the increasing speed (approaching c), but what does this actually mean for an object orbiting a black hole at such low orbit and an object falling toward the black hole? Would this affect its orbit and momentum?
What happens if you shot a bullet exactly toward the center of a non-rotating black hole? When it accelerates to relativistic speeds and its mass starts increasing, does the gravitational pull continuously increases as the approaching speed increases, potentially ad infinitum?
What would happen if you could direct two non-rotating black holes in a direct collision course?

IrC - 8-9-2015 at 11:20

Quote: Originally posted by Nicodem  
Does this also mean that a high speed spinning disc bends space/time just like gravitation does?


If you can spin it with an extremely high radial velocity, since inertial and gravitational forces are equivalent. You would still have to consider increasing mass as a function of inverse Gamma. So no spinning at the speed of light or I will have to turn you in. You wouldn't like Einstein when he gets mad at you.

aga - 8-9-2015 at 15:52

Well, if the spin speed were to be very high, then maybe yes.

E.G. if such a material (or field system) existed that you could rotate a 1cm disc so that the 0.5 cm radius were revolving at the speed of light, then the Edge would be, erm, impossible, yet the simple maths dictate it must be revolving at a speed much greater than that of light.

One thing i have learnt so far is that all theoretical mathematical models have limits.

annaandherdad - 8-9-2015 at 20:18

Quote: Originally posted by Nicodem  
Well, centripetal force is not gravitational force. It is just a radial deacceleration force. But objects moving at high (radial) speed do have relativistic effects (increase of mass, relative shrinkage, relative time dilation...). Does this also mean that a high speed spinning disc bends space/time just like gravitation does?


Not much unless it were massive. But a spinning object creates a kind of a gravitational field that has no analog in Newtonian gravity. It is like a magnetic field, as contrasted with an electric field. Not sure if this answers your question, though.
Quote: Originally posted by Nicodem  
Just a few questions from someone who knows almost nothing about general relativity, but finds it amusing:
I understand that the mass of an object increases with the increasing speed (approaching c), but what does this actually mean for an object orbiting a black hole at such low orbit and an object falling toward the black hole? Would this affect its orbit and momentum?


When I was referring to circular orbits, and I said the orbital velocity approaches the speed of light as r -> 3GM/c^2, I just meant that the velocity necessary to maintain a circular orbit increases as the radius decreases, not that bodies in one circular orbit move to another. This is just like in Newtonian gravity, a satellite in near earth orbit has a higher orbital velocity than one farther out. It's just that in the case of a black hole, there is a limit on the orbital velocity: the speed of light.

So if you are hovering at r=3GM/c^2 above a black hole (by firing your rocket engines to keep from falling in), and you send out a beam of light in a direction 90 degrees to the inward direction, the light will travel in a circle and come back from the opposite direction. It follows a circular orbit. Inside r = 3GM/c^2 there are no circular orbits.

If there were friction or some other mechanism whereby orbiting matter could lose energy and/or angular momentum, then it would spiral inward, increasing its velocity (relative to a hovering observer) as it goes. This is what happens in accretion disks.

Again, I'm not sure I answered your question.
Quote: Originally posted by Nicodem  
What happens if you shot a bullet exactly toward the center of a non-rotating black hole? When it accelerates to relativistic speeds and its mass starts increasing, does the gravitational pull continuously increases as the approaching speed increases, potentially ad infinitum?
What would happen if you could direct two non-rotating black holes in a direct collision course?


If you shoot a bullet straight at a black hole, or just drop it and let it fall, it will fall straight in, crossing the event horizon at a finite amount of its proper time and (according to theory) reach the singularity at r=0 in a finite amount of additional proper time.

As for the gravitational pull increasing as the energy-mass increases, the answer is tricky. In GR there actually are no gravitational forces. An object in free fall experiences no force (the Einstein elevator experiment). Gravitational forces are interpreted differently in GR than in Newtonian gravity.

Two nonrotating black holes colliding, or rotating ones for that matter, produce a huge amount of gravitational waves and a more massive black hole. The equations cannot be solved analytically, and there's a lot of numerical work going on right now about just this question. The goal is to understand the signal produced by such an event so that if it is seen in a gravitational wave detector, they will recognize it.

Nicodem - 9-9-2015 at 02:03

Quote: Originally posted by annaandherdad  
Not much unless it were massive. But a spinning object creates a kind of a gravitational field that has no analog in Newtonian gravity. It is like a magnetic field, as contrasted with an electric field. Not sure if this answers your question, though.

Yes, I just wanted to understand if moving objects distort the spacetime. Not really interested in the magnitude of the effect - I realize the square root of (1-v2/c2) factor is for all practical examples close to 1 at low velocities, but never is exactly 1 unless v = 0.
This brings on another question. Can the spacetime distortion be modulated to form gravitation waves (or time waves?), for example by a rotating disc with the poles rapidly recessing? Like a gyroscope with poles forced into angular movements, or some other similar device? Again, I'm not asking about the magnitude or detectability, just about an effect.
Quote:
When I was referring to circular orbits, and I said the orbital velocity approaches the speed of light as r -> 3GM/c^2, I just meant that the velocity necessary to maintain a circular orbit increases as the radius decreases, not that bodies in one circular orbit move to another. This is just like in Newtonian gravity, a satellite in near earth orbit has a higher orbital velocity than one farther out. It's just that in the case of a black hole, there is a limit on the orbital velocity: the speed of light.

Yes, but decelerating to a lower orbit would increase the radial speed, bringing it closer to the speed of light. The object would therefore increase in mass increasing the gravitational pull, making it fall into a lower orbit and so on further more (unless the momentum also increases, but that would require energy). This appears nonsensical to me. Either I'm misunderstanding something or the relativistic effect of velocity on the mass of the object is not something that has an effect on the gravitational field of this object. In short, do faster objects have a stronger gravitational field, or not? Or does the gravitational field of rapidly decelerating objects also decrease with decreasing velocity? Is the mass increase a real thing or just a fancy metaphor for reduced acceleration at constant force?
Quote:
As for the gravitational pull increasing as the energy-mass increases, the answer is tricky. In GR there actually are no gravitational forces. An object in free fall experiences no force (the Einstein elevator experiment). Gravitational forces are interpreted differently in GR than in Newtonian gravity.

This is the part that I don't understand and is actually the same topic as above. If the energy-mass increases in two objects attracted by a gravitational force and accelerating in a direct free-fall collision course, then would the increasing mass also result in an continuously increasing gravitational interaction? If the objects were two black holes, then their velocities would approach to close to speed of light, making the effect extreme. Or would there be no such effect and the black holes would just accelerate in a steady Newtonian fashion?

annaandherdad - 9-9-2015 at 08:09

Quote: Originally posted by Nicodem  

Yes, I just wanted to understand if moving objects distort the spacetime. Not really interested in the magnitude of the effect - I realize the square root of (1-v2/c2) factor is for all practical examples close to 1 at low velocities, but never is exactly 1 unless v = 0.
This brings on another question. Can the spacetime distortion be modulated to form gravitation waves (or time waves?), for example by a rotating disc with the poles rapidly recessing? Like a gyroscope with poles forced into angular movements, or some other similar device? Again, I'm not asking about the magnitude or detectability, just about an effect.


First, yes, moving objects create a gravitational field, or, more exactly, any object with mass creates a gravitational field, but if the object is moving then it creates a different kind of field. It is like in electromagnetism: a stationary charge creates an electric field, and moving charges create a magnetic field. Other charges ("test" charges) respond to an electric field, and to a magnetic field if they are moving. But in Newtonian gravity there is no analog of the magnetic field, that occurs only in GR. Thus the rotating earth has a slightly different gravitational field than it would if it were not rotating. The small difference has been observed recently in the Gravity Probe B satellite experiments, and it agrees with theory (ie GR) to within experimental error. Wikipedia has an article on this, with links to gravitomagnetism.

Second, yes, spacetime modulations can propagate as gravitational waves. They travel at the speed of light. Gravitational waves have never been detected directly, but there is intense work going on to try to do that. Look up LIGO for reference. There is indirect evidence for them, however. The inspiral of orbiting binary pulsars agrees with the predictions of GR, in which the energy is lost via gravitational waves. Google on "Hulse Taylor" or "binary pulsars" and you'll see a lot of hits.
Quote: Originally posted by Nicodem  
Yes, but decelerating to a lower orbit would increase the radial speed, bringing it closer to the speed of light. The object would therefore increase in mass increasing the gravitational pull, making it fall into a lower orbit and so on further more (unless the momentum also increases, but that would require energy). This appears nonsensical to me. Either I'm misunderstanding something or the relativistic effect of velocity on the mass of the object is not something that has an effect on the gravitational field of this object. In short, do faster objects have a stronger gravitational field, or not? Or does the gravitational field of rapidly decelerating objects also decrease with decreasing velocity? Is the mass increase a real thing or just a fancy metaphor for reduced acceleration at constant force?


The most interesting part of this question is wether the mass associated with energy has gravitational effects. For example, kinetic energy accounts for the increase in mass when objects are in motion, and the question is what gravitational effect this "extra" mass has. Before I get to that, let me address the first question.

Decelerating to a lower orbit does not necessarily lead to large radial velocities. For example, consider a satellite in a circular orbit around the earth. If at some point the satellite fires a rocket engine to deliver a brief impulse opposite the (angular) direction of motion, then after the impulse the velocity of the satellite is slightly less than that needed for a circular orbit at that radius.

So the satellite will enter a new orbit, an elliptical one, in which the apogee is the point at which the impulse was delivered, and the perigee is on the opposite side of the earth, which it will reach after 45 minutes. If the perigee is 1km (say) lower than the apogee, then the radial velocity on the first half of the ellipse will be roughly 1km/45 min, not very much. At perigee, the satellite will have a higher velocity than at apogee, in fact it will be higher than the original velocity in the circular orbit. This explains the apparent paradox, that deceleration (at the apogee of the elliptical orbit) leads to acceleration (ie higher velocity) at perigee. The energy for the extra velocity came from falling 1km in the gravitational field. On the second half of the new (elliptical) orbit, the satellite climbs from perigee back to apogee, returning the same velocity it had right after the impulse.

If we give the satellite a second decelerating impulse when it is at perigee, then it can be made to enter a new circular orbit whose radius is the distance to perigee of the elliptical orbit. The velocity in this circular orbit is less than the velocity at perigee of the elliptical orbit, but greater than the velocity of the original circular orbit (because you have to have a higher velocity to maintain a circular orbit of smaller radius). At no point is the radial velocity very much.

If you remove kinetic energy from the satellite in a slow but continuous manner, for example, by friction, then the satellite can be made to spiral gradually inward, increasing its orbital velocity as it goes.

The behavior of orbits around black holes is very similar, but the shape of non-circular orbits is not an ellipse. But as I mentioned, matter in accretion disks does slowly spiral inward. The energy released by friction can be very large, so the matter heats to high enough temperatures to emit x-rays. This is the source of x-rays from black holes.

Now, about the gravitational effects of energy (=matter). First remember that the orbit of an object in a gravitational field is independent of its mass. This is the cannon ball experiment of Galileo. The orbit depends on the gravitational field, the initial position and velocity, but not the mass. It is the same in GR.

This is the basis of the principle of equivalence, that a gravitational field is equivalent to an accelerating frame. If this is so, no wonder the orbit doesn't depend on the mass: It's the frame that's accelerating, so masses of different magnitudes just go along for the same ride.

Between about 1905, when Einstein discovered the equivalence of matter and energy, and about 1911, he spent a lot of time wondering if the matter that energy is equivalent to has the same gravitational effects as "ordinary" matter. This led to the principle of equivalence, published in 1911 (the equivalence of gravitational fields with accelerating frames). This principle is clear in Newtonian gravity (since Galileo), but Einstein reasoned that it should apply to all of physics, including, for example, electromagnetism. This idea leads in a simple way to the gravitational red shift, and the gravitational time dilation. Although Einstein didn't do this at that time, it also leads in a simple way to the idea that orbits in gravitational fields are geodesics of a non-Euclidean geometry in space-time. That's because the combined red-shifts, one due to gravity and the other due to velocity, give a total time dilation that is maximized along gravitational orbits.

Back to the gravitational effects of energy (=mass). Here is a thought experiment. Take a big, very strong chamber, and put some nuclear bombs inside. The chamber is so strong that it doesn't burst when the bombs go off, converting some of their matter to energy.

Now, weigh the chamber both before and after the explosions. Is the answer the same?

Or, measure the gravitational field produced by the chamber and its contents. Is it the same before and after?

The answer in both cases is yes, according to the principle of equivalence.

Hope this helps.



[Edited on 9-9-2015 by annaandherdad]

IrC - 9-9-2015 at 08:40

What happens if the Martian smokes it?

https://www.youtube.com/watch?v=3Dnk91tAYUI

macckone - 9-9-2015 at 09:57

Interesting thought experiment.
One side effect of spinning the device is that the binding forces between the
atoms would be effected by the strong acceleration. This also happens at the
event horizon of the black hole. It tears molecules and apart so such a device
could never be built.

aga - 9-9-2015 at 12:47

Quote: Originally posted by annaandherdad  
produce a huge amount of gravitational waves

Have Gravity Waves even been detected yet ?

Last time i looked (a couple of decades) there was a 'sensor' made of a very long laser path waiting to 'see' one somewhere.

neptunium - 9-9-2015 at 15:52

LIGO was the last thing I remember seeing about it ...
https://en.wikipedia.org/wiki/LIGO