# The Naked Scientists Forum

### Author Topic: So what is Gravity?  (Read 12470 times)

#### CsManiacDan

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##### So what is Gravity?
« on: 22/04/2004 20:27:12 »
Okay now I've got a question that's been bugging me a real long time.

What exactly is gravity?

i mean I know its a force and things that have a mass exert this force (however small) on other objects. but what is is about an objects mass which means other objects are attracted to it.

Be grateful for any contributions folks.

I Love Caesium!!!

#### CsManiacDan

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##### Re: So what is Gravity?
« Reply #1 on: 22/04/2004 21:42:37 »

In Eisteinian Physics, don't objects create a dent in spacetime (or something like that) Which is why things like orbits occur?

Could it be that nearby objects get pulled into this dent, giving the illusion that they are in fact drawn towards the object. This would explain why a bigger object which therefore creates a bigger dent in spacetime exerts a greater gravitational force than smaller objects?

just a thought...

I Love Caesium!!!

#### assasin

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##### Re: So what is Gravity?
« Reply #2 on: 23/04/2004 08:34:18 »
Nasa have prepared a mission to test this theory:

We should know the answer in 16 months time!

#### gsmollin

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##### Re: So what is Gravity?
« Reply #3 on: 23/04/2004 20:49:35 »
This is all true, but it still begs the question. We can't talk sense about gravity without also talking about mass and space, since these are the things we refer to when we say that gravity attacts masses to each other across a space. What is space, and does it exist independent of mass. Can there be mass without space, and can there be space without mass. GR tells us they depend on each other. Mass and space coexist, so mass bends space towards it, and space bends mass away. In our universe, these two cancel, and we are left with flat space, mass and space in balance. When we are near another mass, space is bent towards it, and our normal straight line motion becomes bent. This is gravity, the attraction mass has for space. We are also massive, and bend space around us too. (Sounds like a Dune Navigator, eh. Snort a little worm scat and we're ready to go...).

#### tweener

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##### Re: So what is Gravity?
« Reply #4 on: 23/04/2004 21:05:37 »
Relativity theory ties mass, space and time together.  Quantum theory doesn't even inlcude gravity.  So, while relativity can describe the effect of gravity, there is no theory of where it comes from or what causes it.

----
John - The Eternal Pessimist.

#### gsmollin

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##### Re: So what is Gravity?
« Reply #5 on: 24/04/2004 12:08:11 »
Well, yes, that's the ...ton methodology of quantum mechanics. There has to be a boson force mediator. Quantum mechanics is actually Newtonian, with a frame of reference required around the whole universe. Relativity requires no such framework.

Does either theory tell us "where it comes from"? I don't think so. At least neither theory tells me where it all comes from. I read in "The Inflationary Universe", by Alan Guth, That the universe was created "ex nilo" (from nothing) by its gravitational binding energy. So the force that expanded the universe (Guth's false vacuum energy) created "mc2" matter from E gravitational binding energy when the phase change occurred from the inflationary universe to the deflationary universe. Okaayyy... That leads us to a flat universe pretty quickly, and is a neat explanation. But that still leaves me wondering where all that came from, especially all the false vacuum energy, and on back to the beginning, before 10-35 seconds.

#### tweener

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##### Re: So what is Gravity?
« Reply #6 on: 24/04/2004 17:09:08 »
I certainly don't know.  I agree that neither relativity or QM gives a clue as to what is really happening, just a prediction of the outcome.

----
John - The Eternal Pessimist.

#### Dan B

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##### Re: So what is Gravity?
« Reply #7 on: 29/04/2004 15:44:59 »
Relativity ties space and time together (space-time) Mass distorts space time (gravity). You can have empty space.... you cannot have mass without space. Mass does not equal space.

#### Smeggit

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##### Re: So what is Gravity?
« Reply #8 on: 04/05/2004 06:57:03 »
These are just my idle thoughts so please shoot them down at will.  I've thought for a while now that gravity pulls in the direction of the 4th dimension.  A simple example being a round planet.  The 4th dimension should be perpendicular to our 3 dimensions and if we take a planets surface to be "smooth" then gravity pulls perpendicular to all surfaces.  I wonder if gravity is like a magnet under paper moving iron filings around.  Mass bends space-time in the direction of the 4th dimension, we try and fall in and like the iron filings we feel the force but cannot move that direction.

#### Quantumcat

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##### Re: So what is Gravity?
« Reply #9 on: 04/05/2004 09:03:19 »
You know when you think about it, you can't say that anyone knows that something will happen every time, for example that a ball will fall to the ground when let go. We have experienced the products of natural laws but we have not experienced the natural laws themselves. You cannot say with certainty that a ball won't simply decide to stay in the air the next time you try to drop it.

Am I dead? Am I alive? I'm both!

#### gsmollin

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##### Re: So what is Gravity?
« Reply #10 on: 04/05/2004 20:59:14 »
quote:
Originally posted by Dan B

Relativity ties space and time together (space-time) Mass distorts space time (gravity). You can have empty space.... you cannot have mass without space. Mass does not equal space.

Isaac Newton thought space existed independently of mass. If that were true, then there would be an absolute space metric, a fundamental rest position in the universe. Physicists spent years looking for it, without success. Einstein showed us Newton's errors, and proved that mass and space, as well as time are inter-related. Space cannot exist independently of mass, because it makes no sense to talk about empty space. You can make a mathematical abstraction of empty space, but nature doesn't work that way. The universe has always existed as mass filling space. It still does. The very shape of space is determined by the mass it contains. With no mass in it, space becomes an infinitely curved singularity. Our universe contains comfortable, flat space, because it contains just the exact right amount of mass to make it that way. An incredible coinicidence? Not likely, since we are talking about being perfectly in balance to the tune of at least one part in 10 to the 15th power. The universe contains flat space because its gravitational binding energy exactly equals its mass.

#### Dan B

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##### Re: So what is Gravity?
« Reply #11 on: 05/05/2004 20:39:13 »
Are you dancing around trying to find the phrase "vacuum energy"?

#### gsmollin

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##### Re: So what is Gravity?
« Reply #12 on: 06/05/2004 02:18:51 »
Not at all. I don't dance. Vacuum energy is a quantum mechanical concept, and I'm talking general relativity. The twain have not met, yet, in our lexicon. In general relativity, the vacuum energy is zero.

#### tweener

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##### Re: So what is Gravity?
« Reply #13 on: 06/05/2004 03:19:10 »
quote:
Originally posted by Smeggit

These are just my idle thoughts so please shoot them down at will.  I've thought for a while now that gravity pulls in the direction of the 4th dimension.  A simple example being a round planet.  The 4th dimension should be perpendicular to our 3 dimensions and if we take a planets surface to be "smooth" then gravity pulls perpendicular to all surfaces.  I wonder if gravity is like a magnet under paper moving iron filings around.  Mass bends space-time in the direction of the 4th dimension, we try and fall in and like the iron filings we feel the force but cannot move that direction.

Smeggit, you have an interesting theory here.  If you want to find out just how close to the mark you are, read some on string theory (or M-theory).  This system invokes higher dimensions (11 of them) and postulates vibrating strings in these higher dimensions that produce the visible subatomic particles, including the gravitron.

I can recommend the book "The Elegant Universe" by Brian Greene as a good non-mathematical introduction to string theory.  It also goes over quantum mechanics and relativity to lay the groundwork and treats them very well indeed.  There are other books that I have not read.  Yet.

----
John - The Eternal Pessimist.

#### Dan B

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##### Re: So what is Gravity?
« Reply #14 on: 06/05/2004 04:49:19 »
gs: There are some very confusing remarks in your post... What is your scientific background?

"The universe has always existed as mass filling space. It still does. The very shape of space is determined by the mass it contains. With no mass in it, space becomes an infinitely curved singularity. Our universe contains comfortable, flat space, because it contains just the exact right amount of mass to make it that way. "

This doesn't make sense: only a tiny fraction of the universe is matter. The standard model we are using (even more after WMAP) is omega_m~0.3, omega_lambda~0.7
« Last Edit: 06/05/2004 04:55:57 by Dan B »

#### gsmollin

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##### Re: So what is Gravity?
« Reply #15 on: 06/05/2004 17:28:16 »
It shouldn't be confusing, especially if you've been studying WMAP results. They show that space is flat. General relativity tells us that space will have curvature depending upon the mass in it. Where you are, space is curved around the earth; when you throw a ball it curves down to the earth's surface because the mass of the earth has curved space towards it. Look up "gravity probe B" for more information on how the earth curves space in its vicinity.

Cosmologically, the net curvature of the universe is determined by the mass it contains. A universe with more than critical mass (omega >1) has a spherical shape, and straight lines go in circles. With omega < 1, straight lines go in hyperbolas, and at omega = 1, straight lines remain straight.

The makeup of the mass is a real problem for cosmology. There is baryonic mass and electromagnetic energy we all are used to seeing. Then there are the (rest)massless particles, like neutrinos. When physicists add the numbers, they are missing 95% of the universe. There seems to be dark mass, so called because it does not respond to electromagnetic energy, that can be detected by its effect on rotating galaxies and galactic clusters. They are all heavier than they look. That gets us up to about 30% of the total. The remaining is a real mystery, and is the omega-sub-lambda you referred to. Its been dubbed "dark energy", but too little is known about it to give it a good name yet.

Don't think that a 70% error in the present understanding of the mass of the universe is an especially big number. In the beginning, mass and space had to be in balance of 1 part in 10E15 for us to even be here. Small errors then, would have been amplified by time, and now we would either be flung apart, or crunched together long ago. There is a distinct probability that there may be a deficit in mass that results in an open, hyperbolic universe, and space will curve away from us in time to come. But that's not my point.

My point is that space does not now, nor has it ever existed independently of the mass it contains. That was an old Newtonian idea, and even Newton's contemporaries were uncomfortable with it. We now know that mass, space, and time came into existence together, and have coexisted since then, all interacting. Einstein's special theory of relativity combined space and time, then the general theory added mass, and gravity.

You can find some good books on this, if you look. I can recommend "Relativity" by Albert Einstein, as his non-technical publication. For the more mathematically inclined, there is "The Meaning of Relativity", by A. E. There are other good books, too, that explain some of the solutions obtained from Einstein's field equations. A few of them are even good reading.

#### Dan B

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##### Re: So what is Gravity?
« Reply #16 on: 06/05/2004 18:45:23 »
One problem with this forum is that you never seem to know the level of the person you are talking to

I got confused coz you were being a bit too layman to be entirely accurate [:p] I thought maybe you were confusing omega_t with curvature k...

#### tweener

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##### Re: So what is Gravity?
« Reply #17 on: 06/05/2004 20:33:14 »
gsmollin, you still didn't answer the question "What is your scientific background?"  I'm just curious and I can tell it is quite strong.

You guys are getting in quite deep in this thread and others.

----
John - The Eternal Pessimist.

#### Dan B

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##### Re: So what is Gravity?
« Reply #18 on: 06/05/2004 20:42:21 »
I have a 1st class honors degree in astrophysics and am currently writing my PhD thesis on
(basically) the cosmological evolution of supermassive black holes and their host galaxies....

hurrah for me

#### tweener

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##### Re: So what is Gravity?
« Reply #19 on: 06/05/2004 21:01:37 »
Hurrah for you indeed!  That's great and sounds like great fun!  (A lot of work too!)

----
John - The Eternal Pessimist.

#### Dan B

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• Posts: 49
##### Re: So what is Gravity?
« Reply #20 on: 06/05/2004 21:09:07 »
yeah, I'm supposed to submit the stupid thing in october too [:(!] Its all getting a bit boring. One annoying thing is realising you can compress the explaination of 6 months work into one paragraph

#### tweener

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##### Re: So what is Gravity?
« Reply #21 on: 07/05/2004 04:09:41 »
But there's too much compression to make it worthwhile at the one paragraph level.  I say it sounds like fun, but I remember how much work graduate school was (not even PhD level) and how much I really wanted out.  Hang in there - it'll be worth it in the end.

----
John - The Eternal Pessimist.

#### Dan B

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##### Re: So what is Gravity?
« Reply #22 on: 07/05/2004 05:21:58 »

#### gsmollin

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##### Re: So what is Gravity?
« Reply #23 on: 07/05/2004 16:44:44 »
quote:
Originally posted by Dan B

I have a 1st class honors degree in astrophysics and am currently writing my PhD thesis on
(basically) the cosmological evolution of supermassive black holes and their host galaxies....

hurrah for me

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

#### Dan B

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• Posts: 49
##### Re: So what is Gravity?
« Reply #24 on: 07/05/2004 19:36:48 »
Cool...

I don't suppose you are on the CFHT TAC by any chance

#### The Naked Scientists Forum

##### Re: So what is Gravity?
« Reply #24 on: 07/05/2004 19:36:48 »