Blow my mind! (Gravity)

Factual Questions
1

I’m sorry this is so long, It’s 4:00 AM and I’m avoiding a Shakespearean Comedies take home test.

Ok, I’m in a Modern Physics class. It’s fun. Last week, Ball State University[sup]*[/sup] hosted Univercity[sup]@[/sup]. The Modern prof ordered us to go see Brian Green.[sup]+[/sup]

He kicked ass and supplied my new sig. But he used the old rubber sheet analogy[sup]o[/sup] to describe gravity, and my mind returned to an old thought:

“The bowling ball bends the sheet because of gravity. Obviously, there’s no ubergravity pulling the sun down against space. So how does the sun distort space to create gravity? Because if that’s ‘just what it does’, the curvature of space is just an interesting way of looking at the problem, not an answer to ‘Why the hell does gravity work?’”

So I asked my Modern prof to explain “How does the sun bend space around it?” The other students[sup]$[/sup] and he gave me the following data:

  1. Yeah, the sheet analogy is flawed.

  2. It’s really really hard.

  3. Einstein invented tensors to solve it.

  4. Tensors are really really hard.

So I appeal to you, people of the SDMB: How does the sun bend space with nothing pulling it against space?

–John

P.S. Only one code correction after preview. Woohoo!
[sub]
Footnotes:
[sup]*[/sup]No, I’m not a BSU student. I go to a g&t residential high school hosted by BSU.

[sup]@[/sup]Univercity was a week long outdoor extravaganza of knowledge. Lots of tents set up to hear brilliant speakers, Elie Wiesel, and evangelical B’hai.(sp?)

[sup]+[/sup]Brian Green is a brilliant physicist doing amazing work in superstring theory. He wrote a book about the elegance of the universe.

[sup]o[/sup]The rubber sheet analogy tells you to imagine space as a rubber sheet and the sun as a bowling ball. You can’t roll things straight across the sheet because the ball warps it and pulss them in.

[sup]$[/sup]The other students feel qualified to lecture me because I’m not in BC Calc. Never mind that I’m breezing through AB, or that I couldn’t take BC because it conflicted with Chinese, and I’m going to major in Chinese. Goddamn Physics clique.[/sub]

2

The problem here is trying to apply a two dimensional analogy (the sheet) to a three dimensional space. When there’s nothing on the sheet, it’s perfectly flat. Put stuff on it and it bends in the direction of the third dimension.

Put stuff in the Universe and it bends in the direction of the fourth.

Which is why gravity affects time.

I think.

3

WHY, DAMNIT!?

I understand the analogy. That’s my point. The analogy sheet bends because gravity pulls things down.

The real thing can’t work that way, unless there are infinite cycles of ubergravity pulling at infinite multidimensional rubber sheets.

Which isn’t the case, AFAIK.

So why?

–John

4

Mainly because when you do the math it works out that way. Keep in mind that Einstein came up with Special Relativity first, and along with that, the equivelency principle: gravitation is equivilent to acceleration.

Acceleration relies on time, but gravitation doesn’t.

To reconcile these two, when developing general relativity, you need to mess with time to get gravitation to be exactly the same as acceleration. Thus, mass, which causes gravitation, must do something to time.

That’s the best that I know how to explain it.

5

Alright, I withdraw my “WHY?” and replace it with “HOW?”

What is mass doing to make spacetime bend? Existing? Is it an intrinsic property of the universe, sort of like how particles don’t violate the SOL because you can’t divide by zero?

I was led to believe by the prof/smartass fellow students that there was an explanation, but it was beyond my relatively plebian intellect.

–John

6

Yes, it’s just “the way it is.” The idea is you shouldn’t be able to tell the difference between gravitation and acceleration. It’s not that they look the same, it’s that they ARE the same. They MUST be (since there’s no universal reference frame, again going back to S.R.).

7

K, I’m not a prof here so feel free to flame me if I get anything wrong… but I do really enjoy topics like this so I’ll throw in some crap that you probably already know.

Gravity is an intrinsic property of any object that has mass. Between two (or more) objects there will be gravitational forces. The strength of these forces depends on the biggitude (love that word, so scientific) of the objects and their distance from each other. So all objects exert gravitational forces on all other objects… it’s just that things don’t get interesting until they get really big (not so big size-wise… a teaspoon of a neutron star has a bit more stuff in it than a teaspoon of cheez-whiz)

Now with the relativity of time… look up at the hypothetical clock on the hypothetical wall. See the hypothetical hands sweeping around the clock? That’s because hypothetical beams of light are bouncing from the clock’s face, reaching your eyes, and you are perceiving it as that time. What you see as the current time at this instant, was actually the current time when the light lept from the clock and zoomed towards your eyeball (not so much of a difference to really matter… but it’s there).

Now s’poze the light took a lazy saunter to you, instead of it’s normal gallop. Gravitational fields of a star change the paths of light rays in spacetime from what they would have been had the star not been present (ala black holes). The light is bent slightly inwards near the surface of the star. This can be seen in the bending of light from distant stars that is observed during an eclipse of the sun.

Say worst case scenario… the gravitational force is a black hole… all light passing a certain point in the gravitational field (the event horizon) will be pulled towards the black hole & be unable to escape… and you’d never know what time it is.

Yeah I know, light travels at a constant speed… I don’t get it either. The other guys had better answers, but they didn’t use nearly as many parentheses as I did.

Hope I amused a little, if I didn’t enlighten.

8

I think the answer to your question is that no one knows what gravity actually is.

We can describe its effects well, but its complete nature is not understood. Einstein basically showed that gravity is the same thing as spacetime (i.e., gravity is the fabric of the universe itself). Quantum Mechanics speculates of a fundamental particle for gravity (gravitons) but this has no direct evidence (much like Greene’s String Theory).

So physicists still have a lot of research to do in this area. For now, it seems that we can be satisfied to work with Einstein’s explanation and accept that matter curves space.

OK, now Chronos can swoop in and save the day. :slight_smile:

9

I feel for you. I got equally dissatisfying answers when i asked about the nature of friction in 1987. Bumpiness of the surfaces my ass! I read a paper recently that had some insight. It was written nearly 10 years after my questions. The real answer in 1987 was, “We don’t know yet.” Now it is, “Ok, we think it works like this, and this is why we think that.”

I have read that scientists are looking for gravity waves. Some don’t believe they can be found. I think we are still in the, “We don’t know, yet.”

10

Yue Han:

Einstein was the first to admit that his theories of gravity do not describe the mechanics of how, merely the effects. There are a number of theories about the mechanics - none of them solidly entrenched, as yet. The best bet is that it has something to do with strings. One of the superstring theories that I kind of like is that, along with the conventional 3 spacial dimensions, there exists 3 simultaneous spacial dimensions (overlapping and concruent). String theory predicts a number of additional dimensions, though these are usually assumed to be small. However, if the dimensions are concurrent with the existing dimensions, how would we observe and differentiate them? So assume for the moment that these 3 concurrent spacial dimensions could exist. Let’s call the 3 spacial dimensions that we are accustomed to XYZ-space and let’s call the newly defined deminsions JKL-space. When no mass is present X=J, Y=K, and Z=L. When mass is present, XYZ-space looks the same as if no mass were present, but JKL-space is distorted such that the mass doesn’t occupy this space. Since XYZ-space and JKL-space WANT to be concurrent, there is an apparent warping of space (actually space-time) near massive bodies.

Think of this two dimensional analogy. Two thin, planar sheets of different metals are fused together. Heat is applied to a point on the composite sheet. Since they are different metals, they have different coefficients of expansion and thus a bubble or distortion in the planar nature of the sheet is formed.

It’s just one of many theories. I like this one because it seems fairly intuitive. Some of the others I’ve heard from M-theory are less intuitive because they deal with properties of matter and space at the Planck length… Unfortunately, the bottom line is - today, we don’t know.

11

Fortunately, in science, it’s ok to admit that something is unknown. Scientists explain what they can (through theories & laws) and then areas of the unknown provide great grounds for future research. The current theories & laws of gravity describe it very well. But the fundamental question of what it actually is, remains open.

12

I heard about the multidimensional thing from Greene… according to him (and he is the guy) there are actually 7 in addition to time and the big three, if string theory is to be believed.

He also brought up the idea of a network of strings stretching throught the fabric of space… maybe matter pushes those intrinsic string out of the way, which makes space bunch up??

I do like the overlapping congruent dimensions thing, though.

–John
PS. JoeyBlades, is J really the usual symbol for the 4th spatial dimension? Or did you select those letters randomly? 'cuz when I was a sophomore I took a shot about about figuring out the geometry of 4 dimensional objects, and I used j as my new variable along with l,w,h.

13

Yue:

Actually, there are a number of different string theories. Some of them don’t require as many dimensions as others, but M-Theory (The Mother of all Theories) requires 11, and this is the only string theory, that I’m aware of, that incorporates gravity.

Purely random. It could have been ABC-space or PQR-space.