# The Naked Scientists Forum

### Author Topic: So what is Gravity?  (Read 12485 times)

#### tweener

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##### Re: So what is Gravity?
« Reply #25 on: 07/05/2004 20:01:10 »
quote:
Originally posted by gsmollin

Okay... and I have a PhD from MIT in cosmology. Alan Guth was my advisor. I am currently teaching at the University of Hawaii.

Wow!  We're getting some real heavyweights in this forum.  It's nice to be acquainted with you guys!

----
John - The Eternal Pessimist.

#### OmnipotentOne

• Sr. Member
• Posts: 146
##### Re: So what is Gravity?
« Reply #26 on: 05/06/2004 17:42:07 »
Isnt gravity like putting a lead ball on a tightly sheeted bed, and the weight at which it pushed down distorts the bed sheet around it. A larger weight would distort the sheet even more, dropping a marbel into this area would be like the moon in orbit.  This is where it get slightly out of my league, theres only so much I can learn by 10th grade [8D]

#### tweener

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##### Re: So what is Gravity?
« Reply #27 on: 07/06/2004 16:58:50 »
That's a good analogy OmnipotentOne, but as with most analogies, it can only be carried so far.  The sheet is like a two dimensional representation of space-time and the lead ball on the sheet causes it to distort.  So far so good, but this setup requires that the lead ball be in a gravity field to make the distortion happen.  In relativity, the distortion is caused by the mass of the ball and occurs to the 3 dimensional space-time.  The moon tries to roll "down" into the gravity well, but it's movement is always trying to send it flying out and to stay in orbit, they just cancel out.

Some of these other guys can explain it better than I can.

Keep asking questions - that's the best way to learn!

----
John - The Eternal Pessimist.

#### VinceColeman

• First timers
• Posts: 1
##### Re: So what is Gravity?
« Reply #28 on: 09/09/2005 07:25:21 »
Can there be mass without taking any space? What about space without taking any mass?

I have read something about massless particles here, what are those like?

#### Simmer

• Sr. Member
• Posts: 229
##### Re: So what is Gravity?
« Reply #29 on: 10/09/2005 10:04:26 »
quote:
Originally posted by tweener
Wow!  We're getting some real heavyweights in this forum.  It's nice to be acquainted with you guys!

But are these heavyweights distorting forum space around themselves?  On the other hand, can forum space be said to exist without posts?

#### David Sparkman

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##### Re: So what is Gravity?
« Reply #30 on: 10/09/2005 14:57:10 »
The purpose of humor is to prevent someone from getting too heavy and disappearing into their own private singularity.

David

#### gsmollin

• Hero Member
• Posts: 749
##### Re: So what is Gravity?
« Reply #31 on: 10/09/2005 18:54:42 »
Who gave you guys permission to resurrect a thread from 18 months ago? Geez, this topic has been bludgeoned to death. Hey Vince, read my post of 04 May, 2004 to get your answer.

"F = ma, E = mc^2, and you can't push a string."

#### vanvinhhoang

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##### Re: So what is Gravity?
« Reply #32 on: 15/09/2005 09:47:40 »
if we consider Gravity in a seperated statement with the three another forces ,i think it is not an effect way. It maybe the main reason to explain why we only unify 3 forces in the SM.

#### Atomic-S

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##### Re: So what is Gravity?
« Reply #33 on: 18/09/2005 06:26:23 »
quote:
That's a good analogy OmnipotentOne, but as with most analogies, it can only be carried so far. The sheet is like a two dimensional representation of space-time and the lead ball on the sheet causes it to distort. So far so good, but this setup requires that the lead ball be in a gravity field to make the distortion happen. In relativity, the distortion is caused by the mass of the ball and occurs to the 3 dimensional space-time. The moon tries to roll "down" into the gravity well, but it's movement is always trying to send it flying out and to stay in orbit, they just cancel out.

One detail that might be extremely useful in trying to understand the way gravity works, is to note closely in exactly what way mass distorts space and time. It distorts the measurement of space in the direction of the objects (making rulers shorter that way, but I am not sure in what frame), but more significantly, it changes the way clocks run. General Relativity says that a clock located at a lower elevation runs slower than one at a higher elevation. This is the key to understanding gravitational dynamics. On light, for example, it implies that a ray of light traveling horizontally but below the observer, moves slower than one at his elevation; and that a ray above him travelling horizontally moves faster. (This seems to say that the speed of light is not constant, and that some light can actually travel faster than light. Correct in both cases: I believe that the statement "the speed of light is invariant" applies only to the light in the immediate vicinity of an observer, or to all light seen within an inertial frame, but not necessarily otherwise.) If light below travels slow and light above travels fast, then we have the exact same condition as exists in a piece of glass of non-uniform index of refraction, and the result is that the ray will be bent. Thus, the gravitational deflection of light, which was observed near the sun during an eclipse.  Likewise, the non-uniformity of time affects the dynamics of objects. At the quantum level, objects are associated with quantum waves, The relation lambda = h/p shows us that momentum builds when wavelength shortens.  Now if we put a standing wave in an environment in which time passes differently in different places, portions of that wave will get out of step with other portions. Taking for example the hydrogen atom in the ground state, all portions of its electron wave are in the same phase at the same time, assuming space and time to be uniform and "flat".  If, however, there is a time gradient across the atom, the wave on one side will get ahead of the wave on the other, and as time passes, the amount by which the one side is ahead of the other will increase. The result will be that the wave function will build up striations of nonuniform phase, and that the number of these striations will increase as time passes. In other words, the wave develops an imposed crest-trough structure of ever increasing number of waves within the atom, that is to say, ever decreasing wavelength. But from the wavelength-momentum relationsip just quoted, such a wave is intrinsically propagational, and the shorter the wavelength, the faster it propagates. In other words, as long as the atom remains in the time warp, it undergoes continual acceleration . This is the way gravity works on matter at the quantum level.

Einstein also said that gravity is equivalent to accedleration; i.e., that the effects of gravity are exactly those of an accelerating reference frame. Can we reconcile that statement with the quantum description just given? Yes, if we use the right relativistic transformations to describe observing the same event by an observer freely falling along with the atom being observed, the conclusion is that in his reference frame, time is not distorted but is uniform (at least within his near vicinity), and to him there appears to be no acceleration of the atom.

#### gsmollin

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##### Re: So what is Gravity?
« Reply #34 on: 18/09/2005 13:00:46 »
Electrons are bound to nucleons in an atom with the electro-weak force, not gravity. However, the analogy is apt, since electro-weak and gravity are both inverse-square laws. A. E. himself used analogies between electromagnetics and gravitation in general relaitivity.

The variance of the speed of light with gravitation is well established, and at cosmic distances, it also allows faster-than light travel. However, these super-luminal velocities are observed at great distances, never in a local reference frame, where special relativity tells us c in vacuo is invariant. The super-luminal recessional velocities observed in distant galaxies are part of gravitational velocity shifts, but occur in a subtle manner, unlike the gravitational redshift that can be measured in an elevator shaft. We are observing them through space curved by gravity. They are not Doppler shifts. They are expanding space.

"F = ma, E = mc^2, and you can't push a string."

#### The Naked Scientists Forum

##### Re: So what is Gravity?
« Reply #34 on: 18/09/2005 13:00:46 »