Gravity... O_o

[My Haitus]

Hi guys, I'm syd, first post in here xd

So anyways.. um, I've been thinking a lot about gravity lately. And.. it's got me confused. I can generally understand a lot more than I used to now. I mean, I've got the basics of light, sound and force pretty downpat which used to keep me up a lot as a kid. But now... I have another large issue to learn about..

Everything is physical, right? I mean, there are photons to explain light, electrons to explain magnetism, electricity, there's gases to explain gases and movement and temperature to explain weather, but I don't get gravity.

What is it? How can something so huge, momentous, earth-shattering, not actually have any physical property?

What the crap?

So the more I've been thinking about it.. the more it sort of becomes clearer. I have a theory about gravity that I'm working on, but I know that it will take a lot of time.

Could someone please simplify the idea of gravity for me? And keep in mind I know very little about specialized physics, just the very basic basics.

 

[Layra-chan]

So there are three basic ways to explain gravity.

One is the particle explanation: Things attract each other by exchanging particles called "gravitons". When you receive a graviton, you get pulled toward the thing that sent out the graviton; also, when you release a graviton, you get pulled in the direction that you sent it out in. This explanation seems to make sense because if you think of gravitons being emitted in spherical bursts, then as the distance between the source and the sphere increases, the surface area of the sphere increases with the distance squared, and hence the density, i.e. the strength of the gravitons decreases with the inverse of the distance squared, giving us the inverse square law for gravitational force (Newton's law of gravitation).

The second is the relativistic explanation: energy (and hence mass and momentum) warps the fabric of spacetime, so that "straight" paths appear to curve toward the sources of gravity. Since objects like to follow straight paths unless acted on by a force (Newton's 2nd law), they will follow these "straight" paths and hence appear to be attracted to the sources of gravity. The warping is in both time and space, so that things appear to accelerate, even though they perceive themselves as going in a straight line at a constant velocity as per inertia.

The third one is a weird, very mathematical one used by string theory. String theory, like most things that come out of quantum mechanics, state that the universe can be explained in terms of a kind of equation called a Hamiltonian, which describes a relationship between time, position, energy and momentum that all particles and waves must obey; in this case, gravity is seen as a set of terms in the Hamiltonian that are generated by energy-bearing objects and that modify slightly the energy and momentum that things must have so that we observe a gravitational effect.

The three different views are not quite fully compatible with one another. Specifically, the relativistic view doesn't match the other two at all, but it is the only one with experimental verification; the other two are completely theoretical with no evidence for them.  

[spider_desu]

Yup ^^;; I guess it has been covered.

The most believed theory at the moment is that space and time are bent by objects of large size, cause the straight line to become a curved line. I would like this theory if it could explain just how a massive object bends space. The example always used in science books is to put a bowling ball (a mass) on a trampoline (space) and watch it bend. The problem is that for a bowling ball to bend a trampoline you need gravity to begin with rolleyes And an explanation of gravity shouldn't require gravity unless it was self-sustaining. But then you need an explanation of what started gravity.  

[geodesic42]

I hate seeming like a butt by just jumping in and saying "no, that's not it," but I do feel the string theory description needs a little alteration.

String theory actually gained a lot of clout by predicting the existence of a closed string state that's isomorphic to a graviton state. The set of first excited state of the closed string has can be labeled by two indices. Moreover, closed string states can only have an even number of excitations since the algebraic structure a closed string's operators have is equivalent to two separate open string algebras and in order to have a stable string configuration any excitation from one algebra must be accompanied by an excitation from the other, meaning the set of nth order excited closed strings can be labeled by 2n indices. We generate excitations by acting on the ground state with indexed creation operators. General 1st order excited states can be constructed from the linear superposition of the basic 1st order excited states. We can create an operator that sends the ground state to some particular 1st order excited state by contracting a rank 2 tensor with the creation operators. We can in general decompose the tensor into a sum of 3 types of tensors that don't mix under transformations: 1) symmetric traceless, 2) antisymmetric, and 3) diagonal with a nontrivial trace. If we look at the creation operators characterized by 1), the string states we create are isomorphic to the postulated states for gravitons. So string theory actually goes off of the same concept as #1 in Layra's description.

The Hamiltonian that was brought up is classically an entity that contains all the information we need to know to obtain the proper equations of motion for a system. In quantum mechanics, the Hamiltonian becomes an operator. We can use it for two things: 1) to calculate the average energy of a system and 2) to figure out how a system's state in Hilbert space evolves with whatever parameter we're calling time.

Classically, particles travel along a geodesic that's parameterized by some variable T. Similarly, the components of a field can evolve in a prescribed by that's parameterized by the variable T (usually some combination of space-time coordinates). At any rate, when we turn to quantum mechanics the Hamiltonian tells us the time rate of change of the system's state via Schrodinger's equation:
id|Y>/dT=H|Y>
where I decided to write things in natural units.

This works in ordinary quantum mechanics; quantum field theory; string theory; and string field theory.

At any rate, the Hamiltonian shouldn't 'shift' the energies, since it tells us what the energies are upfront. The shifts Layra might be talking about come in via perturbations, which change the energy relative to the unperturbed Hamiltonian - but this could be said of any force if we call the the Hamiltonian of an uninteracting particle/field the unperturbed Hamiltonian then introduce an interaction term. Gravity is normally handled in first quantization in this approach where the interaction term is just a scale function that involves the magnitude of the metric perturbation. In second quantization, we take the terms involving the metric pertubation and let them become the metric field operators (the things that Fourier decompose into a superposition of the creation and annihilation operators I discussed above). This Hamiltonian can then be used to describe the T evolution of the composite state of one string and the string in a graviton state. If we just want to look at an uncoupled graviton state string (i.e. one that isn't in the presence of other strings), this prescription doesn't pop up and we don't talk about energy shifts or anything.


With all that said I'm troubled by the schematical inconsistency of the two predominate views: quantum vs. classical gravity. Seriously, how can we go from talking about curvatures of a differential manifold then represent them by corpuscular entities embedded in the manifold? It seems strange to me. I can see the reverse, however, since, for instance, point charges can be represented as divergences in an electromagnetic field.  

[omninulla]

im not exactly positive that i am completly sure of what you are asking for sweatdrop but i think you want to know what the phisical form of gravity is. am i corect?

well any way, if i am, i belive that the answer would be that matter its sellf is the phisical form of gravity which is why it is such a massive force. mater and gravity are one in the same.

(if i am wrong pleas corect me... nicely sweatdrop )  

[omninulla]

um... Layra-chan,
if i may, im not exactly sure what gravitons are question
could u explain?
from what u say it sounds like they are some theoretical way of trying to phisicaly interpret the mathmatical explanation of gravity.
they dont have a phisical body do they? eek confused  

[Layra-chan]

f_a_i_t_h_l_e_s_s

um... Layra-chan,
if i may, im not exactly sure what gravitons are question
could u explain?
from what u say it sounds like they are some theoretical way of trying to phisicaly interpret the mathmatical explanation of gravity.
they dont have a phisical body do they? eek confused

The question of whether they have a physical body is really difficult to answer, at least in part because the exact meaning of "physical body" is currently undergoing some rather major epistemological/ontological changes that seem to suggest that "physical body" is not a meaningful concept.
According to quantum field theory, a particle is, mathematically, a bump in one of a multitude of fields. But QFT isn't too clear on exactly what those bumps in the fields are physically; from what I can tell, quantum field theory is mostly still stuck in the "shut up and calculate" mode of operation.

Gravitons have been postulated since we have particles for the other forces as well. We have observed (at least in the case of light) single objects that act almost exactly the way we expect them to if they were particles, and for the nuclear interactions we have seen effects that imply the existence of particles that carry those forces.
We haven't seen anything of the sort for gravity, but theoretical physics thrives on at-the-time unjustified extrapolation. If there are particles for the other three forces, why not particles for gravity?

Given the directions that quantum field theory seem to be going in, I think the very idea of "particle" might have to be discarded in which case the graviton will not be a particle but more of a resolution limit to the gravitational field.  

[geodesic42]

Just to dumb down what Layra-chan just said:

A field is some mathematical object that's defined over space-time - it could be a number, a set of numbers, a matrix, a higher rank tensor, etc. At each point in space and at each time the object will take on a certain value or set of values.

Fields can have wiggles in them. In quantum field theory, it turns out the size of these wiggles can only take discrete values. 'Particles' are what we call the smallest discrete values of these wiggles. A field with one particle is a field with the smallest possible wiggle; a 2 particle field has a large wiggle that's equal to the sum of two of the smallest wiggles; so on and so forth.

Now the physical structure you're talking about I'm sure amounts to something along the lines of "what do they look like" or "what are their spatial dimensions?" Well, that answer isn't well defined. Photons are the little wiggles in an electromagnetic field. Their momentum states in quantum field theory looks exactly like the typical momentum states of a particle attatched to a harmonic oscillator, so we say the little wiggles, or photons, are particle-like. However, there was no natural way to incorporate the 'position' or 'dimension' of the photons in the steps going from the classical field theory to the quantum version, so its 'physical structure' as you put it isn't necessarily well defined. Nonetheless, we call it a particle.

So whenever you hear physicists tossing around the word "particle" just think about little wiggles on a string or fabric.  

[Deirdrui]

All has been said before me.
This theme is really actual for me now.
So if someone can give me a hint for my question or answer...)
What about the speed of distribution of gravitation forces?  

[Doc Virulence]

Deirdrui

All has been said before me.
This theme is really actual for me now.
So if someone can give me a hint for my question or answer...)
What about the speed of distribution of gravitation forces?

I think they were theorized to be C? The speed of space-time expansion is the speed light and gravity "move" at. Atleast I believe.  

[Layra-chan]

I'm pretty sure that gravitational waves/gravitons travel at light speed.
This is not to say that spacetime can't expand faster than the speed of light, but information-bearing objects, and apparently this includes gravitational waves/gravitons, can't travel faster along the fabric of spacetime than the speed of light, according to the current form of relativity.
This matches the notion that gravitons are supposed to be massless.  

[Jerba2]

Einstein proposed (I think, but I know this is what he believed) that gravity is caused by warping in the fabric of space-time. Like if you had a trampoline and put a weight down on it; it would cause a curvature that would draw things towards it.

Then again, the problem I've found with that analogy is that it doesn't explain where the gravity comes from to draw the weights closer.  

[Sioga]

What you said about the space time warp is correct. i was watching nova a couple months ago and they were doing something on this subject. they had a really good computer graphics display.

the fabric of space-time is like a trampoline. if you put something light on it, it will only make a small impression and only a small area will be sunken in towards it.. but if you put something heavy and big on it, then the impression will be much bigger. so they tested it. they put a small ball on the trampoline and launched another ball near it. the seond ball curved around the first, but then continued on in its new direction. [remember, this was all computer graphics.] then, they exchanged the first ball for a much bigger ball. this ball made a huge impression on the trampoline. they shot the second ball again, right by the new ball. this time, the second ball curved around the big ball and started going in circles, getting closer and closer to the ball.

this explains gravity on a planetary scale, but the idea is the same for everyday objects. everything makes some sort of impression in the fabric of space-time. the smaller objects are drawn to the bigger objects because they are "spiraling down into the impression". this also explains why earth hasnt been flung out into space. we are spiralling down into the impression that the sun is making. but then you might ask, why havent we crashed into the sun? that is where the Big Bang theory comes in. everythin is being thrown outwards by the force of the big bang, and being pulled inwards by the force of gravity. by being pulle both ways, we stay in this one spot, circling the sun at the same distance evey time.

now, this can also explain why the planets closer to the sun travel a lot faster than the plantets farther away. the planets far away are at the beginning of the impression, while the planets closer, are much deeper in the impression. we are traveling faster than neptune because we not only have a much less distance to go, but the sun also has much greater pull on us.  

[Layra-chan]

You don't actually have to invoke the Big Bang to explain why we haven't crashed into the sun. It's a simple matter of momentum.
The Earth moves through space with some momentum due to the original momentum that the material that formed the Earth initially had. This original momentum came from the spinning of the dust cloud that became the solar system.
This momentum would, in the absence of the sun, cause the Earth to fly off into the nothingness in a straight line (Newton's first law of motion). Since the sun exists, however, the geometry of spacetime around the Earth is warped so that this straight line curves around the sun. We haven't crashed into the sun (or flown off into the black) because this curving almost forms a closed orbit, i.e. we almost make a perfect ellipse each year. The distance from year to year is almost perfectly the same, so we aren't getting significantly closer or farther from the sun. If anything is "pulling" us outward, it's momentum.  

[Sioga]

ok. to be honest, that is true, but the big bang was the only way i knew how to explain it. im only 14. im not that smart yet.

yeah, in space, you cant slow down, because there is no air to deter from your motion. so the only way to stop is to hit something, which means [according to newtons 3rd law] that whatever it is will bounce back with the same amount of force, most likely going in a different direction.