Gravity class is amazing

  • #1

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Hey how are ya? I'm new to physics and all that I took a class in school and it blew me away! A lot of questions are popping up that my teacher can't answer. My first one is,according to him, pretty deep. Here it is; what is gravity? I mean what is it ? How does it work. I understand that it is all around us, and all the basics but this one is bugging me. Any answeres out there?
 

Answers and Replies

  • #2
Doc Al
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The first step is to learn how gravity is described. Have you studied Newton's law of universal gravity? I'd start there. (When you've mastered that, then you can look into how General Relativity describes gravity.)
 
  • #3
yet another qravity question. Isn't it true that gravity is faster than light? If it is then wouldn't all fields be faster than waves considering light is a wave.
 
  • #4
vincentm
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Whitedragon said:
yet another qravity question. Isn't it true that gravity is faster than light? If it is then wouldn't all fields be faster than waves considering light is a wave.
I don't think gravity is faster than light, gravity doesn't travel, it causes objects to travel.
 
  • #5
Doc Al
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Whitedragon said:
Isn't it true that gravity is faster than light?
No. According to General Relativity, gravity propagates at the speed of light.
 
  • #6
Well think about this scenario. What if you are out on a nice lake in the middle of the night with a nice full moon lighting up the night. Then out of nowhere an alian ship decides to play a prank and zap the moon from the sky. Wouldn't the water recede from it's tide about 2 3/4 seconds before you see the moon dissapear?
 
  • #7
Doc Al
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What makes you say that?
 
  • #8
<B>If<B/> I am correct in saying that light travells at 650,000,000 mi/hr. and considering how far the moon is. Say that the moon effects our waters with an almost instantanous field of gravity. Light travels in waves so if the moon was suddenly zapped away out of existance the light the sun reflects of the moon would still be traviling to our eyes, but the field would, like i said, dissapear almost instantanously causing the waters to recede out of tide before the light could reach our eyes. Say the sun dissapeared, it would take 5 minuts before we realize it's gone but the planets would be, to my guess, already floating out of orbit. Of course things could be differant in the quantum level of matter.
 
  • #9
Doc Al
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Since gravity travels at the speed of light, if the moon were instantly zapped, its gravitational effect on us would disappear at the same time as its image.
 
  • #10
<Q>Since gravity travels at the speed of light, if the moon were instantly zapped, its gravitational effect on us would disappear at the same time as its image.<q/> I didn't say that. I said gravity was instantanous, in thoery, and light was slower, light bends to gravity.
 
  • #11
Doc Al
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I know you didn't say that. That's why I did. Gravity is not instantaneous.
 
  • #12
What's your proff?
 
  • #13
sorry, proof.
 
  • #14
Doc Al
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It's a consequence of general relativity.
 
  • #15
*stupid me* right, right. But not enough to change my mind.
 
  • #16
Doc Al
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Pi_314B said:
Since this is apparently unknown. What gravity is? ..... I shall speculate.
Please don't. Stick to established physics.
 
  • #17
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Doc Al said:
Please don't. Stick to established physics.
There is no established physics as to what gravity is, does, or how it works. Speculation is all we have.
 
  • #18
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Doc Al said:
Since gravity travels at the speed of light, if the moon were instantly zapped, its gravitational effect on us would disappear at the same time as its image.
I'm going to guess that zapped means removed from existence. If so - The gravitational effect would disappear instantaneously, because the gravitational field of the moon is every bit as much the moon as the rock itself, while the photon coming your way is no longer a part of the moon and must travel at C.
 
  • #19
G01
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some things we have to accept until we able to fully comprehend them. So whitedragon, I suggest you accept this fact until you have enough physics and math under you belt to proove this fact for yourself. (I know I hate just accepting stuff too but sometimes it has to be done.)
 
  • #20
pervect
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Whitedragon said:
Well think about this scenario. What if you are out on a nice lake in the middle of the night with a nice full moon lighting up the night. Then out of nowhere an alian ship decides to play a prank and zap the moon from the sky. Wouldn't the water recede from it's tide about 2 3/4 seconds before you see the moon dissapear?
The answer to this question is the same as the answer to the question of "what would happen if a charge suddenly disappeared" - would the force suddenly disappear? If so, wouldn't this indicate that light travelled faster than light?

The answer to both questions is that the question itself is no good, because mass cannot suddenly disappear without violating physical law, just as charge cannot disappear without violating physical law. Attempting to solve the resulting equations yields the result that the inital assumptions were invalid.

One can realistically perturb a charge (or a mass), and ask how fast the change in the resulting field propagates. This gives a theoretical speed of 'c' both for gravity and for light. (Actually this is an upper bound, but under most conditions fields are weak enough that 'c' is the right answer.)

There are currently no good measurements on the speed of gravity, just theory. There are of course numerous good measurements on the speed of light.

There were some recent attempts to measure the speed of gravity, but when one carefully analyzes them they have all failed. This is not to say that the speed of gravity cannot be measured - when LIGO gets to the point where it can actually detect gravitational waves, we need only compare the arival of the gravity waves from a cosmic event that emits both gravity waves and light (such as a binary star inspiral).

This is all mentioned in the sci.physics.faq on the speed of gravity

http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

The article on

"What is gravity" might also be of some interest.

http://math.ucr.edu/home/baez/physics/Relativity/GR/gravity.html

though it's not very detailed, it simply talks a bit about the idea of gravity as geodesic deviation.
 
  • #21
ZapperZ
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Pi_314B said:
There is no established physics as to what gravity is, does, or how it works. Speculation is all we have.
What do you call Newton's law of gravitation and Einstein's General Relativity? Window dressings?

Zz.
 
  • #22
Doc Al
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Pi_314B said:
I'm going to guess that zapped means removed from existence. If so - The gravitational effect would disappear instantaneously, because the gravitational field of the moon is every bit as much the moon as the rock itself, while the photon coming your way is no longer a part of the moon and must travel at C.
Sorry, but this is incorrect. According to our best theory of gravity, general relativity, the gravitational effect would disappear at the speed of light. (And, like it or not, we do know quite a bit about gravity.)
 
  • #23
Tom Mattson
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Pi_314B said:
There is no established physics as to what gravity is, does, or how it works.
Of course there is. We have Newton's theory of gravitation, and for stronger fields, Einstein's General Relativity.
 
  • #24
russ_watters
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Whitedragon said:
*stupid me* right, right. But not enough to change my mind.
Change your mind? In your opening post, you said you are just starting to learn about gravity. You shouldn't have your mind made up about anything yet!
 
  • #25
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Doc Al said:
According to our best theory of gravity, general relativity, the gravitational effect would disappear at the speed of light.
Has this been tested. I should think not. Maybe some day when we can detect such waves, fields, or whatever it may be.
 
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