One meteorite fell on the ground in Russia, last week. Under different circumstances, it could have orbited the earth, or perhaps pass close to the earth and then disappear into space. It seems that everything is continuously moving in the universe: satellites orbit planets, planets orbit stars, stellar systems orbit each other and galaxies orbit one another too, and so on. The layman (me for instance) could say that every object in the universe is either orbiting something or it is falling towards something. Physicists could argue that orbiting is just a peculiar way of falling (“falling with style”, as Woody famously said to Buzz Lightyear). Now, my first question is: is it correct to say that everything is -in a way or another- falling towards something else? Second question: is it possible that our universe itself is falling towards something else i.e. another universe (which could maybe explain why it is expanding at increasing speed)? Last question. How can we be sure that the Big Bang (which the layman thinks of as the hugest explosion ever) is not the moment in which a Big Falling started? Please forgive any naivete, as this is my first post here.
2 Answers
Firstly, it can be easily shown that everything isn't "falling" toward everything else. For example, a photon emitted by an extremely distant star isn't "falling" toward anything. It may be affected by various things on it's way here, like interactions with electrons, or large gravitational potentials.
An other counter example is a rocket/similar body that we fire off the face of Earth, which attains escape velocity. It has escaped the gravitational attraction of the Earth, and assuming it doesn't get trapped by another object in the solar system, will continue along it's way. This isn't "falling" toward anything either.
Secondly, even though "continuous falling" is often used as an analogy for an orbit, it's far clearer to think of it in terms of potentials. My answer here explains gravitational orbits in terms of a potential. There are other answers to that question as well that do a damn good job of explaining things.
The big bang cannot be the beginning of the "big fall", since the universe is expanding. It isn't expanding in a regular manner around one fixed point, but every point is expanding away from every other point. Clearly this is the opposite of falling.
And lastly, we cannot know what is outside of our universe, therefore your question about falling into another universe is completely unanswerable. There may or may not be theories predicting other universes, but they are (as far as I know) completely un-detectable. Therefore we can never say for sure if the theory is right or wrong.
-
$\begingroup$ Many thanks Kitchi, both for your clear answer and your link. $\endgroup$ Commented Feb 24, 2013 at 20:13
You've kind of got a point with "every object in the universe is either orbiting something or it is falling towards something", but the language you use is going to make physicists twitch uncomfortably ;)
More formally, consider the following:
1: The gravitational force (or "bending of spacetime" if you prefer) created by any mass has an infinite range.
2: While there can be regions of space which are "flat" (e.g. a Lagrange point), the actual flat point is infinitesimally small...theories of quantum gravity aside.
3: No object with mass can be infinitesimally small (ignoring the complication of singularities).
This means that every object with mass is going to experience some kind of gravitational gradient. In special cases like Lagrange points (e.g. where the Earth's gravity matches the Moon's) it might not experience any overall gravitational force in one direction or another, but it will always feel a slight stretching due to gravity.
So no, objects don't have to always be falling towards something, but they will always experience gravity in some way. When you're stood on the Earth you're not falling in the conventional sense, and you're not orbiting (you'd have to do about 17,000mph to orbit just above the surface!), but you are experiencing gravity.
