# How does gravity repel?

## Main Question or Discussion Point

I have read many ppl writing that "gravity repels". Howz that possible?

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Danger
Gold Member
It isn't. Gravity is a purely attractive force. I don't even think that the theoretical 'negative matter' would be gravitationally repulsive, since gravity is a characteristic of space-time rather than being inherent to matter. Someone should check me on that, though.

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Maybe you're thinking of the so-called "dark energy" that is theorized to cause the accelerated expansion of the universe?

From what I've heard at least, dark energy is a "opposite" (repulsive) gravitational force

the actual truth about gravity is that although it has been said that it is a pull, that isnt exactly the case. for the simple fact that there is actually no true definition of gravity at all, so in all actual fact garvity could either be a pushing force or pulling force and you would get the same effect either way.

the blob inc: Can you explain to me what you mean?
If gravity is NOT a pulling force, how come I'm not pulled away from the ground and into space? And then how can any planet orbit the Sun instead of getting pushed away? And.. how can planets and stars and even galaxies form if gravity is a pushing force?

the universe is a big place. now concidering there is alot of matter in general in space, if gravity was a push you could look at it as the sum of the universe pushing down on you/ and vise versa for push gravity, which would have an effect of pulling you down vs the pulling force of the sum of the universe. that being all said look at it as high pressure/low pressure, in either situation you would get the same effect depending on the localized and de-localized variables of your possition and location.

here is a good analogy:
ANALOGIES (not meant to be unkind)
The team of medieval physicists stepped out of the time machine and began to examine the strange, new device fastened to the window. They had never before seen a suction cup, so with great enthusiasm, they began to experiment by pulling this mysterious device off the window, then reattaching it.
"The glass must attract the device" remarked one of them. They all nodded in agreement.
Next, they found a smaller piece of glass and discovered that the suction cup had the gripping power to suspend it. This new revelation prompted another physicist to remark, "The device must also attract the glass!" Having no real reason to seek a better explanation than this for their observations, the team of medieval physicists unanimously concurred, and a new theory was born: "The device and the glass are attracted one to another, this being a characteristic of space!"
My comparison to medieval science is not an insult to physicists. I merely wish to emphasize mankind's present level of ignorance of the mechanics of our universe. We now know that the suction cup in this example is held to the glass by air pressure. The invisible molecules that make up air constantly bombard the surfaces of the glass and the suction cup. The difference in pressure cause, what appears to be, an attraction. My gravitational hypothesis is somewhat similar. All I ask of you, the reader, is to keep an open, yet discerning mind.

pervect
Staff Emeritus
The "push" theory of gravity has been proposed and discredited a long time ago, but that doesn't seem to deter cranks from re-proposing it endlessly.

For a good article including origin and why it doesn't work, see the Wikipedia entry

http://en.wikipedia.org/wiki/LeSage_gravity

Energy conservation and drag are the big problems associated with this sort of theory, and attempts to work around these problems are not very convincing. Feynmann also wrote a bit about why this theory doesn't work, IIRC, though I don't recall exactly where offhand (one of his popular works).

I think I've read something about quantum gravity possibly being repulsive at very short distances, although I don't remember where it was. Sound familiar to anyone?

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HallsofIvy
Homework Helper
the blob inc said:
the universe is a big place. now concidering there is alot of matter in general in space, if gravity was a push you could look at it as the sum of the universe pushing down on you/ and vise versa for push gravity, which would have an effect of pulling you down vs the pulling force of the sum of the universe. that being all said look at it as high pressure/low pressure, in either situation you would get the same effect depending on the localized and de-localized variables of your possition and location.

here is a good analogy:
ANALOGIES (not meant to be unkind)
The team of medieval physicists stepped out of the time machine and began to examine the strange, new device fastened to the window. They had never before seen a suction cup, so with great enthusiasm, they began to experiment by pulling this mysterious device off the window, then reattaching it.
"The glass must attract the device" remarked one of them. They all nodded in agreement.
Next, they found a smaller piece of glass and discovered that the suction cup had the gripping power to suspend it. This new revelation prompted another physicist to remark, "The device must also attract the glass!" Having no real reason to seek a better explanation than this for their observations, the team of medieval physicists unanimously concurred, and a new theory was born: "The device and the glass are attracted one to another, this being a characteristic of space!"
My comparison to medieval science is not an insult to physicists. I merely wish to emphasize mankind's present level of ignorance of the mechanics of our universe. We now know that the suction cup in this example is held to the glass by air pressure. The invisible molecules that make up air constantly bombard the surfaces of the glass and the suction cup. The difference in pressure cause, what appears to be, an attraction. My gravitational hypothesis is somewhat similar. All I ask of you, the reader, is to keep an open, yet discerning mind.
But there are some fairly obvious ways to test whether gravity is a pull by one object or a push by all other objects on the other side. Do you remember the "Cavendish Experiment"? He actually measured the gravitational pull (Ooops! "force") between two objects side by side in a laboratory as a function of their distance apart (and, by the way gave a good measurement of the "univeral gravitational constant", G). How could a force due to other matter in the universe be so sensitively affected by a slight movement of one? I am willing to keep an open mind, but in matters of physics, I want to see experimental evidence. Do you have experimental evidence for your theory?

tony873004
Gold Member
shaan_aragorn said:
I have read many ppl writing that "gravity repels". Howz that possible?
I'm not sure if this is what you mean by gravity repeling, but gravity can give the appearance of repeling in certain circumstances.

Why doesn't Pluto ever crash into Neptune even though their orbits intersect? It's because Pluto is in a 3:2 resonance with Neptune. At any given instant, the resonance is not exactly 3:2. So for example, if the resonance is 2.999:2 Pluto starts to catch up to Neptune. Neptune then gives the appearance of repeling Pluto to a 3.001:2 resonance. Thousands of years later Pluto catches up to Neptune from the other side where Neptune "repels" again into a 2.999:2 resonace and this repeats indefinately. The appearance of repulsion works best in a rotating frame. Here's a link:

http://www.orbitsimulator.com/gravity/pluto2.GIF

The blue dot is Neptune, and the purple path is the orbit of Pluto.

But Neptune does not actually repel Pluto. When Pluto is in front of Neptune, Neptune speeds up Pluto relative to Neptune, which slows down relative to the Sun, which drops Pluto into a lower solar orbit, which speeds Pluto up and causes it to reverse direction relative to Neptune in the rotating frame, giving the illusion of repulsion. The opposite happens on the other side of Neptune. When Pluto is behind Neptune, Neptune speeds up Pluto relative to Neptune. This also speeds it up relative to the Sun, and it climbs into a higher solar orbit which slows Pluto down and causes it to reverse direction relative to Neptune in the rotating frame, once again giving the illusion of repulsion.

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Danger
Gold Member
Thanks for that link, Tony. It made me dizzy, but it's interesting.

can gravity repel

It isn't. Gravity is a purely attractive force. I don't even think that the theoretical 'negative matter' would be gravitationally repulsive, since gravity is a characteristic of space-time rather than being inherent to matter. Someone should check me on that, though.
If it is true that for every action there is an equal and opposite reaction, than wouldn't it be obvious that gravity should be able to both attract(pull) and repel (push

Kurdt
Staff Emeritus
Gold Member
If it is true that for every action there is an equal and opposite reaction, than wouldn't it be obvious that gravity should be able to both attract(pull) and repel (push
The force of gravity from the Earth on yourself is the same as the force of gravity from yourself on the Earth. Therefore there is an equal and opposite reaction. The fundamental flaw in your thinking is that opposite means repel rather than attract. However using Newton's 3rd law means that the direction of the force is opposite not its effect.

DaveC426913
Gold Member
If it is true that for every action there is an equal and opposite reaction, than wouldn't it be obvious that gravity should be able to both attract(pull) and repel (push
No. For the same reason you can't assign any other opposite-but-irrelevant words to them, such as 'good' and 'evil'.

stevebd1
Gold Member
In respect of the apparent anti-gravity properties of dark energy, I put together the summary below-

In cosmology, the equation of state of a perfect fluid is characterized by a dimensionless number w.

$$w = \frac{Pressure}{Energy\;density}$$

or

$$Pressure = w\;\text x\;Energy\;density$$

Energy density per unit volume has the same physical units as pressure as demonstrated below, and in many circumstances is an exact synonym.

$$Pressure = \frac{Force}{Area} = \frac{F.d}{A.d}= \frac{W}{V} = \frac{Energy}{Volume} = Energy\;density$$

Pressure = N/m^2, Force = Newtons, Area = m^2, d = unit of distance (m), W = Work (joules), V = Volume (m^3), Energy = Joules, Energy density = joules/m^3

Hence w is dimensionless but is useful in demonstrating the nature of a specific material. For example, ultra-relativistic material such as light has a positive pressure which is equal to 1/3 of the energy density, hence it has an equation of state of w = 1/3.

Ultra-relativistic matter, such as radiation, photons, neutrinos and matter from the early universe, w = 1/3. For ordinary non-relativistic matter, w = 0 (i.e. the pressure is zero). For Quintessence, w < -1/3 (the expansion of the universe is accelerating for any equation of state where w < -1/3). For a cosmological constant, w = -1. Phantom energy is a hypothetical form of dark energy where w < -1, this could cause the expansion of the universe to accelerate so quickly that the big rip would occur.

In Einstein's law of gravity, the sign of the gravitational force is determined by the algebraic combination of the total energy density plus three times the pressure.

g = energy density + 3p

If the pressure of the material is negative and big enough, it can cancel out the energy density, nullifying gravity. If the pressure is negative and bigger still, then the 'sign' of the gravity-generating term in Einstein's equation actually reverses, and instead of gravity attracting, it repels. Based on Einstein's law of gravity, the expansion of the universe begins to accelerate when w = -1/3 which is the point when the 'sign' of gravity changes-

Gravity for material with an equation of state of w = -1/3

g = energy density + 3 x (-1/3)pressure

g = energy density + (-)pressure

g = 0

regards
Steve

I have read many ppl writing that "gravity repels". Howz that possible?
it can be possible because einstein introduced a cosmological constant called the antigravity seeing that the universe is in stable conditions. moreover the blackholes have wormholes that open in another universe wher these worm holes would be the antiblackholes.

I would like to venture to say that it could be possible for gravity to repel in a sense as mentioned by danger. He rightly mentioned the term negative gravity. The theory of gravitational repulsion would have been possible if during the start of our universe, there was going to be a predominance of anti-quarks over quarks which would have lead to dominance of anti-matter over matter and subsequently dominance of anti-gravity over gravity.matters int the universe would have been repelled towards which is paradoxal but factual, for the fact that we all have the conventional notion that things are only pulled toward each other. if this would have been the case, we would have seen trucks being carried on a 6yr old back with little or no efforts applied

DaveC426913
Gold Member
moreover the blackholes have wormholes that open in another universe wher these worm holes would be the antiblackholes.
Wild, unfounded speculation will not help answer the OP's question.

Gravitational slingshot also gives illusion of a mysterious repulsive force. When a space probe approaches a giant planet like Jupiter on a certain tangent, it swings around and gains incredible speed. Upon leaving the planet, one would assume the gravity would slow down the space probe by pulling it inwards, but that is not true. The probe leaves the system at a higher speed as if it was repulsed.

tony873004
Gold Member
Gravitational slingshot also gives illusion of a mysterious repulsive force. When a space probe approaches a giant planet like Jupiter on a certain tangent, it swings around and gains incredible speed. Upon leaving the planet, one would assume the gravity would slow down the space probe by pulling it inwards, but that is not true. The probe leaves the system at a higher speed as if it was repulsed.
That's a good point. Jupiter seemingly "pushed" the New Horizons spacecraft onto a faster trajectory to Pluto.

DaveC426913
Gold Member
Gravitational slingshot also gives illusion of a mysterious repulsive force.
It's only mysterious if you're not paying atttention.

It's not gravity creating the acceleration, it's the planet's own motion through the solar system.

When a space probe approaches a giant planet like Jupiter on a certain tangent, it swings around and gains incredible speed. Upon leaving the planet, one would assume the gravity would slow down the space probe by pulling it inwards, but that is not true.
It most certainly is true. It pulls every bit as much when the probe is receding as when it's approaching.

The probe leaves the system at a higher speed as if it was repulsed.
No, it leaves the system at a higher speed as if it traded some momentum with the planet, which is now moving at a slightly slower pace through the solar system.

No, it leaves the system at a higher speed as if it traded some momentum with the planet, which is now moving at a slightly slower pace through the solar system.
I was just alluding to the fact that gravitational slingshot can appear to be an effect of a gravitational repulsion. It is well understand, and it's a consequence of conversation of momentum. Total momentum of the system is constant before and after the pass. But the probe steals a little bit of momentum from Jupiter, which is manifested in a greater relative velocity for the probe, and less so for Jupiter. Since the mass of Jupiter is extremely large compared to the probe, we won't detect any changes in the planet's motion.

DaveC426913
Gold Member
I was just alluding to the fact that gravitational slingshot can appear to be an effect of a gravitational repulsion. It is well understand, and it's a consequence of conversation of momentum.