Why gravitational force is always attractive

Experiments have shown that neutrons do fall in the earth's field. It's hard to do the experiment directly on antineutrons but an equivalent experiment is under development at CERN that would test whether or not an anti-hydrogen atom would rise or fall in a gravitational field.

It's a tough one and will take some time. Stay tuned. An additional reason for spin 2 for the graviton is that it must couple to the stress-energy tensor which is inherently spin 2.

Just to elaborate on Lee's last point, you can think of those tensor waves as being something like a deformable medium stretching along one axis and compressing at right angles to that. Odd corresponds to switching sign.

So you can see right away that the graviton would have to be even spin. Follow-up on this answer. Learn more physics! Related Questions. Still Curious? When a stone etc. In what way does the stone communicate with Earth when it accelerate against it in the gravitational field,what kind off invisible fast etc. Like I say, speculative, and also only a very small part of a whole entropic process. I can take a few or even a lot of downvotes, but pretend for a moment we're shooting the breeze at coffee.

I haven't followed the Verlinde and other entropic gravity literature at all closely, so I don't know whether something like this of model has been suggested in a mathematical form, which would be required for it to be publishable although being published definitely wouldn't be enough to make this not speculative. If you ask for explanations for the established mathematics of General Relativity, I think the only currently possible response is speculation.

GR is more grounded in empirical principles than in models that could be taken to be explanatory which I say without prejudice insofar as I take empirically supported principled theories to be preferable to more-or-less ad-hoc models, except for the elusive question of how to imagine effective new empirically supported principles.

I note that lurscher doesn't address your request for explanation. When I was a schoolboy, our teacher of physics asked once one of our brilliant students a girl , something like: "What is the nature of gravity? She thought for a moment and answered: "I do not know. And what is it? Our teacher replied: "If I had known As long as gravity is not derived from other nature features, it should be considered as a fundamental law. We may study its properties but we may not ask such questions - just by definition of a fundamental law.

It is as an axiom - it is given as such. It is not true at all that gravitation is always attractive. In fact, if it were always attractive, the universe would not be expanding at an accelerated rate right now and an inflationary period would not have occurred.

Currently the only known source of expansion components of gravity is the cosmological constant, which, incidentally, is precisely the physical quantity that our theories fail to predict by the largest amount: or 60 orders of magnitude, depending on whom you ask. In general relativity, in general but we can consider the most simple case of a spherically symmetric gravitational field, gravity is always attractive as long as you are at rest with respect to the gravitational field.

However, if you are moving with respect to the gravitational field, gravity can sometimes be considered to be "repulsive". If you are falling in radially, from initially being at rest at high altitude, you initially accelerate towards the central mass. However, you will reach a point, if the central mass is compact enough like a black hole, where you start to decelerate and when you are infinitely close to the "Schwarzschild radius" you will move infinitely slow.

This is all if your movement is observed from a distant observer. In the classical sense of the meaning, gravity will become "repulsive" as soon as you start to decelerate. The reason for this deceleration is that in contrast to "classical Newtonian mechanics", where the force only increases the momenta by increasing the velocity, and accelerating particles in an accelerator where the electromagnetic force increase the momenta by increasing the "lorentz factor" setting the speed of light to be the speed limit, in general relativity you have the third effect that the speed of light again as measured by a distant observer around a spherically symmetric mass distribution decreases with the radial distance.

The gravitational force will always be "attractive" if you are moving in towards a black hole in the sense that your velocity as a fraction of the local speed of light constantly increases but your velocity, as observed by a distant observer, will if you get close enought to a black hole inevitably start do decrease and it that sense gravitation will become "repulsive".

Notice that JPL is only using the expression above in the weak fields of our solar system, it gives the right value of the so called "anomalous precession of perihelion" but it is not supposed to work in the strong field limit. The inverse square is apparently a consequence of conservation of momentum.

So Newton's law of gravity and that of Coulomb have conservation of momentum as the origin. This can be shown easily by taking three particles along a line interacting under the inverse square and give the middle a nudge keeping the end particles fixed.

If we take it that there is no charge with zero mass, Coulomb's law follows too. Even without the help of Bertrand theorem, it is possible to derive the Maxwell equation from just charge conservation and its continuity equation.

Clearly the same can be done using mass conservation and we get the gravitomagnetic equations out of it. But if two masses are locked in an orbit, they must be under attraction. The facts that similar masses attract whereas similar electric charges repel has to rely on experimental knowledge. New research from MIT on a new long-range attraction force connected to spin.

Gravity is thought of as a weak boring force. But if you get fast spinning black holes and strong gravitational waves interacting, then you can get 'repulsion and attraction' effects happening out of purely gravitational forces. So repulsive gravity does not exist in quiet spacetimes, but if you look at what happens when you shine a gravitational wave onto a rapidly spinning black hole, you will find that adjusting the frequency only slightly from the ideal level allows you to push the hole away from you when the bh absorbs a wave or pull it closer when the bh adds energy to the radiation beam via superradiance.

With a Gravitational Wave. Two dipoles are always attractive or a dipole and another charge. If you can consider that inside the baryons neutron, proton can exist a configuration of dipoles you have an answer. It is a monography of a model of particles where this happens. Integrate along the path. Explore the concept of polarizable vacuum.

The consensus is that gravitation is not electromagnetism, but in that way it is always attractive. And I like it. Yes, the radiated EM field has to be extremely faint.

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