Dark Energy?

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Replied by Son Goku on topic Re: Dark Energy?

Gravity is a force in the Newtonian picture, in the sense that it causes inertial mass to accelerate. However unlike other forces such as electromagnetism, the "charge" associated with gravity is inertial mass itself, which causes gravity to behave as if it were a fictitious force. That is, if you accelerate in the correct way gravity can be cancelled out, so their is no way to tell the difference between gravitational acceleration and mechanical acceleration. This is quite different from electromagnetism, where you can tell the difference between mechanical acceleration and electromagnetic acceleration.

This fact eventually lead Einstein to the correct theory of classical gravity where gravity is the distortion of spacetime. When an apple falls to the ground that is because of the curvature of spacetime. Spacetime curvature is not something that only goes on in black holes. If you don't take it into account you don't get the correct orbit for mercury. If you don't take it into account GPS sattelites do not work, so it is not some fictional nonsense. It is the current theory of gravity, it has been proven that spacetime bends.

As for gravity working differently in other galaxies or out in space, our observations of neutron stars and how stars move around other galaxies indicates that it does not. We don't just think gravity works the same everywhere because we assume it does, we've checked that it does.

As for Dark Matter it is now thought to exist because a substance that acts exactly like it has been observed in galactic cluster collisions. Also Dark Energy is not 75% of the material content of the universe. Unlike in Newtonian gravity, in Einsteinian gravity several things besides mass can cause gravity. Heat can cause gravity, pressure can, even increased fluid flow. Dark Energy contributes a significant amount to these gravitational sources, however it isn't physically 75% of all there is.

Finally as for gravity turning out to be part of another force, if gravity does unify with the other forces it is only at Planck scale energies. That is massive energies, that are no longer achieved in the universe today and hence isn't relevant to how gravity acts on our scales. Also it doesn't invalidate the existence of gravity. For instance from the Weinberg-Salam electroweak theory, we now know that electromagnetism is just part of another force at high enough energies. However electromagnetism still "exists" and we don't consider Maxwell and other 19th century physicists incorrect or naive for discussing its effects on its own. Unification does not imply the old force does not exist.
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14 years 11 months ago #64392

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