Why is there gravity?

[Pizza]

There needs to be an alternative music band called "Hypothetical Schwarzschild Radius."

[Alex K]

Yes, in a sense, electrons in some energy level don't have a fixed distance to the nucleus. But strictly speaking there are no such things as electron orbits, there are only wave functions giving you probability of location and momentum. The picture of a pointlike electron orbiting the nucleus is false

Could you then explain in relative layman's terms what "localizing the electrons in a smaller and smaller space near the nucleus automatically lets them have a larger momentum due to the Heisenberg uncertainty principle, which then lets them escape" means?  If the electron as a point particle is a metaphor, then it seems that statement relies heavily on that metaphor.

Nothing relies on metaphors because the actual arguments are maths based. The calculations are performed in a theory framework, which in the case of quantum mechanics assumes electrons to be point particles, but which are subject to quantum uncertainty and whose states are therefore characterized by their wave function. The assumption that the electron is pointlike is a good one because it is empirically tested on the scales relevant to atomic physics

What is meant when one says that the electron is localized in a small space, is that the quantum state of the particle is prepared such that the wave function vanishes outside of this confined area.

[JuliaL]

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Could you then explain in relative layman's terms what "localizing the electrons in a smaller and smaller space near the nucleus automatically lets them have a larger momentum due to the Heisenberg uncertainty principle, which then lets them escape" means?  If the electron as a point particle is a metaphor, then it seems that statement relies heavily on that metaphor.

Nothing relies on metaphors because the actual arguments are maths based. The calculations are performed in a theory framework, which in the case of quantum mechanics assumes electrons to be point particles, but which are subject to quantum uncertainty and whose states are therefore characterized by their wave function. The assumption that the electron is pointlike is a good one because it is empirically tested on the scales relevant to atomic physics

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What is meant when one says that the electron is localized in a small space, is that the quantum state of the particle is prepared such that the wave function vanishes outside of this confined area.

Or gets really, really small.

[robvalue]

What's with the d squared? I remember my A level physics teacher had us guess what power the d would be in the gravitational equation. For some reason I guessed 2. Freaky.

[Quatermass]

We don't know for certain what causes gravity. All we know is that it has something to do with mass. If you stand on the edge of a cliff and dangle a ping-pong ball on string over the cliff, the ping pong ball will lean in toward the cliff (the cliff having greater mass than the ping pong ball).

[bennyboy]

"Why" means different things. Ultimately, it will go to things we cannot understand. Why is there matter? Why does it bend space? Why is there even space, rather than nothing at all?

So the simplest answer is "because matter bends space," and the ultimate answer is unknown and probably unknowable.

[Alex K]

"Why" means different things.  Ultimately, it will go to things we cannot understand.  Why is there matter?  Why does it bend space?  Why is there even space, rather than nothing at all?
So the simplest answer is "because matter bends space," and the ultimate answer is unknown and probably unknowable.

Yes, exactly.
But that's the usual problem with explanations which Feynman illustrated so nicely - explanations need to relate the phenomenon to something else. But what if you're already at the most fundamental level? How would you ever avoid being circular?
And why questions may possibly be meaningless because they invoke a naive everyday concept of causation which may not apply to the issue at hand

[The Grand Nudger]

-and on that note, either our everyday concept of causality is wrong, something is wrong with our system of logical operations, or something is wrong with the mechanics of the language we use to express causality leading to either 1, 2, or both (or yet more...including a mix of all of the above).  If we trust in the principles of logic - we can't trust in our notions of cause or our language. If we trust our notions of cause or our language..we can't trust logic. Really tough spot to be in, ala the problem of the sunken schooner and the bay herald.

[Brakeman]

Don't you really mean what is the "purpose" of matter? That's how the ignorant theists always phrase it..

[vorlon13]

Universes without gravity generate beings very different from us that may not ask questions like "Why is there gravity?'.


Be thinking palps, lots and lots of palps . . . .