# Thread: equal and opposite reaction of gravity

1. if there is an equal and opposite reaction to everything, what would be the counterpart of gravity? dark energy?  2.

3. Originally Posted by schiz0yd
if there is an equal and opposite reaction to everything, what would be the counterpart of gravity? dark energy?
You have completelymisunderstood Newton's third law.

http://www.physicsclassroom.com/Clas...laws/U2L4a.cfm  4. wow you're right, that's quite a slip on my part. so if gravity is the equal and opposite interactions of two bodies of mass, does that mean that dark energy is unrelated to mass or there must be different types of interactions between bodies of mass depending on distance?  5. Originally Posted by schiz0yd
wow you're right, that's quite a slip on my part. so if gravity is the equal and opposite interactions of two bodies of mass, does that mean that dark energy is unrelated to mass or there must be different types of interactions between bodies of mass depending on distance?
So far as anyone knows gravitation is described by Einstein's general relativity, and Newtonian gravity provides a very good approximation in most situations. So, the gravitational field is very close to being proportional to mass and inversely proportional to the square of distance.

Dark energy is a different animal. It was known for quite some time that the universe is expanding. This dates back as far as the work of Edwin Hubble, who noted that red shift which proportional to recession speed also seems to be proportional to distance for most galaxies.

So, based on Hubble's work and other later evidence a cosmological model based on general relativity was developed. That model involves what is called the Big Bang. So, starting from the Big Bang one predicts that space is expanding, and that the effect of gravity due to the mass in the universe would be to slow down that expansion. But about 1998 some astronomical observations of type 1A supernovas showed that not only is the universe expanding, but the rate of expansion is increasing. This was quite a surprise.

Nobody knows why the expansion is accelerating. So, not knowing what is causing the expansion, it was given a name -- "dark energy". Nobody knows what dark energy is.

One can make a model, using general relativity, that predicts the observed expansion, by adding a small term to what are called the Einstein field equations. That term is called a "cosmological constant". By adding this small positive constant to the equations one predicts the expansion that is actually seen. There is some history to this constant, but it is not important for this question.

What you should be asking yourself at this point is "What is the difference between this cosmological constant and a fudge factor ?" The answer is -- nothing. It is just that, a fudge factor.

Nobody knows why the cosmological constant is what it seems to be based on observation. There are some ideas as to how it might arise from fundamental physics, but the models that yield a cosmological constant over-predict the observed magnitude by a ridiculous amount. So we have a mystery.

Research physics is all about solving mysteries. People are trying to solve this one. But it has not yet been solved. It is a deep mystery, and it will probably take some very new and creative ideas before it is solved.  Bookmarks
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