Exploring Repulsive Gravity: Questions & Answers

[Charlie G]

I have just recently learned of repulsive gravity and I have 2 questions that need clearing up.

1. For an object that generates repulsive gravity, what does that object do to space-time? If normal gravity is when the object bends paths through space-time towards itself, then what does something like a white-hole do to the fabric of space-time?

2. The book that opened me up to repulsive gravity was briane greenes the fabric of the cosmos. In one part he sets up a situation like this, you are trapped in jail and the only way to get released is if you can make one jack-in-a-box weigh more than another that is exactly the same. He said that you would compress the jack-in-a-box'x spring to make it weigh more. At first I thought that that generates potential energy, therefore making the box weigh more. But in the notes it says that, though the energy increase does cuase the box to weigh more, it is mainly the pressure. He says matter, energy, and pressure cause gravity. My question is what exactly does he mean by "pressure" that makes it any different from an increase in energy?

 

[feynmann]

I have just recently learned of repulsive gravity and I have 2 questions that need clearing up.

1. For an object that generates repulsive gravity, what does that object do to space-time? If normal gravity is when the object bends paths through space-time towards itself, then what does something like a white-hole do to the fabric of space-time?

2. The book that opened me up to repulsive gravity was briane greenes the fabric of the cosmos. In one part he sets up a situation like this, you are trapped in jail and the only way to get released is if you can make one jack-in-a-box weigh more than another that is exactly the same. He said that you would compress the jack-in-a-box'x spring to make it weigh more. At first I thought that that generates potential energy, therefore making the box weigh more. But in the notes it says that, though the energy increase does cuase the box to weigh more, it is mainly the pressure. He says matter, energy, and pressure cause gravity. My question is what exactly does he mean by "pressure" that makes it any different from an increase in energy?

What he meant is that *negative pressure* cause Repulsive Gravity. However, there is nothing unusual about negative pressure. Ordinary positive pressure pushes outwards, Negative pressure simply pulls inwards. It leads to the most dramatic differences from Newton's gravity, yet it is not always emphasized in introductions to general relativity.

Following link to the so-called *anti-gravity* in the book, "gravity from the Ground Up" by Schutz
http://books.google.com/books?id=P_T0xxhDcsIC&pg=PA244&dq="negative+pressure"+anti-gravity

 

[pallidin]

Perhaps I'm missing something here, but what evidence suggests that a compressed spring weighs more than an identical one that is not?

 

[Charlie G]

By compressing the spring you give it potential energy, so since enrgy and mass are one in the same, the compressed spring with more energy will weigh more than the one that is not.We couldn't measure this difference though, very very tiny difference in weight.

Also, according to Briane Greene the compressed spring has more pressure, which also makes it weigh more, this is what I don't understand. He says that matter, energy, and pressure contribute to gravitational forces.

Why is more pressure any different than increasing the objects total energy?

 

[Dale]

Why is more pressure any different than increasing the objects total energy?

Because pressure and energy go into different components of the http://en.wikipedia.org/wiki/Stress-energy_tensor" .

 

[feynmann]

I have just recently learned of repulsive gravity and I have 2 questions that need clearing up.

1. For an object that generates repulsive gravity, what does that object do to space-time? If normal gravity is when the object bends paths through space-time towards itself, then what does something like a white-hole do to the fabric of space-time?

The Repulsive gravity, due to negative pressure, will cause the universe to expand, regular mass-energy will cause universe to contract. Einstein introduced the so-called cosmological constant to balance the contraction of the universe so to make it static. When he found out the universe is indeed expanding, not static as he thought, he admitted he made a biggest blunder

 

[pallidin]

By compressing the spring you give it potential energy, so since enrgy and mass are one in the same, the compressed spring with more energy will weigh more than the one that is not.We couldn't measure this difference though, very very tiny difference in weight.

Also, according to Briane Greene the compressed spring has more pressure, which also makes it weigh more, this is what I don't understand. He says that matter, energy, and pressure contribute to gravitational forces.

Why is more pressure any different than increasing the objects total energy?

OK, hold on here a sec... potential is NOT the same as realized, and has nothing to do with actual mass increase.

I would like to see ANY evidence that a compressed compound actually weighs more than a non-compressed compound.
If the difference can't be measured, how can one responsibly make such a conclusion?

 

[Charlie G]

Well, we don't have the technology to measure such a slight increase in weight. It does not matter whether or not the energy is stored or released, energy is energy, no matter what form it takes. As long as that object has more energy in its system it will weigh more than an object with less total energy

 

[A.T.]

Also a cool thought experiment:

If there were just two blobs of fluid in empty space, they would attract each other. But what if the entire universe was filled with this fluid, and there were only two bubles with air. Would the air bubbles attract or repulse each other.

 

[matheinste]

Perhaps they would float up whichever way that is.

Matheinste

 

[A.T.]

If there were just two blobs of fluid in empty space, they would attract each other. But what if the entire universe was filled with this fluid, and there were only two bubles with air. Would the air bubbles attract or repulse each other.

Perhaps they would float up whichever way that is.

There is no "up" direction. Equal pressure everywhere.

 

[DrGreg]

There is no "up" direction. Equal pressure everywhere.

From the point of view of one bubble, "up" is the direction away from the most mass, i.e. towards the other bubble. So the two bubbles attract.

 

[yogi]

Epstein in his book "Thinking Physics" posed the scenerio of the two bubbles in a universe filled with water - as per Dr Greg -post 17 - they attract

 

[A.T.]

Epstein in his book "Thinking Physics" posed the scenerio of the two bubbles in a universe filled with water - as per Dr Greg -post 17 - they attract

I must know it from there then. Great book. Does Epstein give the same explanation?

 

[michelcolman]

If an object with negative gravity existed (for example, I would make a negative gravity ping pong ball in my garage), which way would it go? You would tend to think "towards the ceiling, away from the earth, opposite to normal objects", but that doesn't take gravity from the sun into account. This acceleration will be much bigger (3000 G, times two, if I'm not mistaken), so the ping pong ball would fly away in a direction opposite to the sun. Except if we take the milky way into account... see where this is going?

How do you unite negative gravity with relativity, then? Can an object with negative gravity exist at all?

 

[JesseM]

If an object with negative gravity existed (for example, I would make a negative gravity ping pong ball in my garage), which way would it go? You would tend to think "towards the ceiling, away from the earth, opposite to normal objects", but that doesn't take gravity from the sun into account. This acceleration will be much bigger (3000 G, times two, if I'm not mistaken), so the ping pong ball would fly away in a direction opposite to the sun. Except if we take the milky way into account... see where this is going?

How do you unite negative gravity with relativity, then? Can an object with negative gravity exist at all?

Negative mass is potentially compatible with relativity, but not negative gravity in quite the sense you're imagining, since it would violate the equivalence principle (if you're in an accelerating elevator in deep space and floating inside are a very small positive-gravity mass and a very small negative-gravity mass, both moving inertially, you should see both fall towards the floor, because really the floor is accelerating towards them while they continue to move inertially). In relativity positive mass would be universally attractive while negative mass would be universally repulsive--both a small negative mass and a small positive mass would fall down towards a large positive-mass planet, and both would be repelled by a large negative-mass planet. You can see that this should be true in Newtonian gravity too, since for a small object with mass m near a massive planet with mass M the gravitational force on the small object is F = GMm/r^2 towards the planet, and its acceleration is related to the force by F = ma, so you can set these equal to each other and cancel out m from both sides to get a = GM/r^2, meaning that regardless of whether its own mass m is positive or negative, the only thing that determines whether it accelerates towards or away from the planet is whether M is positive or negative.

 

[michelcolman]

In relativity positive mass would be universally attractive while negative mass would be universally repulsive--both a small negative mass and a small positive mass would fall down towards a large positive-mass planet, and both would be repelled by a large negative-mass planet.

I think there's a problem with this, too.

Suppose you bring a large positive and negative mass, of equal absolute value, in close proximity of each other. The positive mass will be pushed away from the negative one, while the negative one is attracted by the positive one. The result is that the pair of masses accelerates in one direction, the negative mass chasing the positive one, and they will keep accelerating indefinitely. Free energy!

Of course you may say that the kinetic energy of the negative mass will be negative and therefore conservation of energy is not violated, but somehow I think this has to be impossible anyway.

 

[JesseM]

Suppose you bring a large positive and negative mass, of equal absolute value, in close proximity of each other. The positive mass will be pushed away from the negative one, while the negative one is attracted by the positive one. The result is that the pair of masses accelerates in one direction, the negative mass chasing the positive one, and they will keep accelerating indefinitely. Free energy!

Of course you may say that the kinetic energy of the negative mass will be negative and therefore conservation of energy is not violated, but somehow I think this has to be impossible anyway.

Well, physicists who discuss this idea do use the fact that the kinetic energy balances out to suggest that, even though this scenario is counterintuitive, at a mathematical level there is no actual violation of conservation of energy. There are apparently some issues with the thermodynamics of negative energy, as discussed in this article:

Negative energy potentially conflicts with the second law. Imagine an exotic laser, which creates a steady outgoing beam of negative energy. Conservation of energy requires that a byproduct be a steady stream of positive energy. One could direct the negative energy beam off to some distant corner of the universe, while employing the positive energy to perform useful work. This seemingly inexhaustible energy supply could be used to make a perpetual-motion machine and thereby violate the second law. If the beam were directed at a glass of water, it could cool the water while using the extracted positive energy to power a small motor–providing a refrigerator with no need for external power. These problems arise not from the existence of negative energy per se but from the unrestricted separation of negative and positive energy.

The possibility of negative energy does actually arise in QM, but the article says that thermodynamic problems can be avoided using quantum considerations:

Fortunately (or not, depending on your point of view), although quantum theory allows the existence of negative energy, it also appears to place strong restrictions - known as quantum inequalities - on its magnitude and duration. These inequalities were first suggested by Ford in 1978.

Over the past decade they have been proved and refined by us and others, including Eanna E. Flanagan of Cornell University, Michael J. Pfenning, then at Tufts, Christopher J. Fewster and Simon P. Eveson of the University of York, and Edward Teo of the National University of Singapore.

The inequalities bear some resemblance to the uncertainty principle. They say that a beam of negative energy cannot be arbitrarily intense for an arbitrarily long time. The permissible magnitude of the negative energy is inversely related to its temporal or spatial extent. An intense pulse of negative energy can last for a short time; a weak pulse can last longer. Furthermore, an initial negative energy pulse must be followed by a larger pulse of positive energy [see illustration below].

The larger the magnitude of the negative energy, the nearer must be its positive energy counterpart. These restrictions are independent of the details of how the negative energy is produced. One can think of negative energy as an energy loan. Just as a debt is negative money that has to be repaid, negative energy is an energy deficit.

...

The quantum inequalities prevent violations of the second law. If one tries to use a pulse of negative energy to cool a hot object, it will be quickly followed by a larger pulse of positive energy, which reheats the object. A weak pulse of negative energy could remain separated from its positive counterpart for a longer time, but its effects would be indistinguishable from normal thermal fluctuations.

Attempts to capture or split off negative energy from positive energy also appear to fail. One might intercept an energy beam, say, by using a box with a shutter. By closing the shutter, one might hope to trap a pulse of negative energy before the offsetting positive energy arrives.

But the very act of closing the shutter creates an energy flux that cancels out the negative energy it was designed to trap [see illustration below].

 

[yogi]

Response to Post 14 - Epstein at p 139. He gets to his conclusiion by starting with two bubbles R and Q of density equal to water - then puts a rock at a point P midway between them and concludes everything is balanced - then removes the water from R leaving an empty bubble - and concludes the point P moves toward Q - as if the bubble R repelled things - then changes the rock to an empty bubble and concludes it must be attracted to R.

Not too rigorous as a proof - but a fun book. As for the present topic, Epstein concudes using the same analogy, that on a cosmological scale, empty space must attract empty space -

 

[Mentz114]

Repulsive gravity seems to occur in some cosmological solutions. In the Kottler space time $g_{00}=1-mr^{-1}-r^2\lambda/3$. If we take a weak field approximation $-mr^{-1}-r^2\lambda/3$ is taken as potential energy. If this is differentiated wrt r, there is an attractive 1/r^2 force and a repulsion that is proportional to r.