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Q & A: gravity, matter, and curved spoaetime

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Most recent answer: 10/22/2007
Q:
Am I correct in assuming that once the Universe cooled sufficiently for matter to form, all the curves and dents in Space-Time resulting from this matter also formed? If that assumption is correct, why do we think that Gravity is a force that radiates and not simply the geometric result of curved Space-Time?
- Robert (age 29)
Johnston, RI
A:
General Relativity does treat gravity as being simply an aspect of curved spacetime (not a force), but that curvature responds to the distribution of matter and energy. On a smale scale, you can pretend that spacetime is flat and that matter creates a gravitational field (force), and you will get nearly the right predictions. These predictions, using Newton's model of gravitation are quite accurate for nearly all practical purposes, from engineering design of objects which must bear gravitational weight, to predicting the trajectories of spacecraft sent to the moon and the planets.

It was only the introduction of Special Relativity at the beginning of the 20th century (needed to reconcile electricity and magnetism with the idea that all physical laws are the same in all reference frames) which gave an indication that Newton's gravitational model was insufficient. General relativity was a tremendous achievement of Einstein's because it was formulated to reconcile special relativity with gravity, and not to explain any anomaly in the currently available experimental data. Only later were its predictions found to be true.

In the process of reconciling special relativity with gravity, Einstein did away with "spooky action at a distance", a common criticism of Newton's model. The motion of a piece of matter only depends on the properties of space nearby, and not on what's happening far away at that instant. Gravitational disturbances propagate at a finite speed, the speed of light. If two black holes collide for example, the matter distribution changes rapidly enough to emit enormous amount of propagating gravitational waves.

So is there any evidence of those propagating waves? I can think of two pieces of evidence, but an astrophysicist would probably know of more. One is that some spinning objects slow down by just the amount predicted if they were losing energy to gravitational waves. Another is that the shape of the ripples in the early universe, which we see as small ripples in the density of the microwave background radiation, matches predictions obtained using the propagating gravity of General Relativity.

In other words, the geometric results of curved spacetime are not static, but do radiate.

As for your question about the early universe, both matter and energy contribute to the curvature of spacetime. So even at the very beginning, when matter particles were constantly being created in pairs with antimatter particles and annihilated again, the energy density was enormous and the spacetime was very curved.

Mike W. and Tom J.

(published on 10/22/2007)

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