# Gravity class is amazing

Whitedragon
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?

Mentor
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.)

Whitedragon
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.

vincentm
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.

Mentor
Whitedragon said:
Isn't it true that gravity is faster than light?
No. According to General Relativity, gravity propagates at the speed of light.

Whitedragon
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?

Mentor
What makes you say that?

Whitedragon
<B>If<B/> I am correct in saying that light travels at 650,000,000 mi/hr. and considering how far the moon is. Say that the moon effects our waters with an almost instantaneous field of gravity. Light travels in waves so if the moon was suddenly zapped away out of existence the light the sun reflects of the moon would still be traveling to our eyes, but the field would, like i said, disappear almost instantaneously causing the waters to recede out of tide before the light could reach our eyes. Say the sun dissapeared, it would take 5 minutes before we realize it's gone but the planets would be, to my guess, already floating out of orbit. Of course things could be different in the quantum level of matter.

Mentor
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.

Whitedragon
<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 theory, and light was slower, light bends to gravity.

Mentor
I know you didn't say that. That's why I did. Gravity is not instantaneous.

Whitedragon

Whitedragon
sorry, proof.

Mentor
It's a consequence of general relativity.

Whitedragon
*stupid me* right, right. But not enough to change my mind.

Mentor
Pi_314B said:
Since this is apparently unknown. What gravity is? ... I shall speculate.
Please don't. Stick to established physics.

Pi_314B
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.

Pi_314B
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.

Homework Helper
Gold Member
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.)

Staff Emeritus
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 traveled 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.

Staff Emeritus
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.

Mentor
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.)

Staff Emeritus
Gold Member
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.

Mentor
Whitedragon said:
*stupid me* right, right. But not enough to change my mind.

Pi_314B
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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rbj
wow, i s'pose i should just ditch Einstein and all of the current physicists who have examined and published regarding GR over the years because of the expert insights of Whitedragon and Pi_314B. if Whitedragon and Pi_314B say that gravity is instantaneous, then i guess it must be true.

Ki Man
i think that gravity might be a pull caused by a cluster of mass. just my theory. don't take my word for it though I'm only in 8th grade.

what if... you have some mass. all mass is equal to energy right because E=MC^2? and all matter is made up of quarks, which are... charges of energy i believe. correct me if I'm wrong, but if quarks attract then wouldn't a large object floating in an area with very very few atoms, like space, have a somewhat of a pull on objects around it?

Edit: gravity can't be instantaneous. nothing can break the light speed barrier. ever. not even the pull of gravity. 500 years ago they thought light was insantaneous because they had no way of proving it false until 2 astronomers faced off in a challenge. one said that one of jupiters moons should appear at this certain time after it comes out from begind jupiter. the other astronomer said the delay of light speed over that distance would make it appear 7 minutes later. the second astronomer was right.

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Staff Emeritus
Gold Member
I've deleted a few posts from this thread that violate PF Guidelines. Enough with the homegrown crackpottery already. If it continues then warnings will be issued.

This thread was posted by a student who wants to know more about what he's studied in school. If you aren't familiar with that material, then you've got nothing of value to add to this thread, unless you want to ask questions as well.

Ki Man
okay sorry about the "homegrown crackpottery"

anyways for gravity to be instantaneous it would have to pull something far away faster than it would take for light to reach that object (the light coming fromt he pulling object) and that would mean gravity would have to break the light speed barrier.

How would that be possible for it to be instantaneous?

Staff Emeritus
Pi_314B said:
Has this been tested. I should think not. Maybe some day when we can detect such waves, fields, or whatever it may be.

This can't be tested, because it is not even theoretically possible to make objects disappear.

Pi_314B
Whitedragon said:
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?
Sorry Whitedragon. Gravity is what most everybody else here is saying.

It is in Newtons theory of gravitation, and Einsteins general relativity theory.

Pi_314B
rbj said:
if Whitedragon and Pi_314B say that gravity is instantaneous, then i guess it must be true.

You have that wrong. I didn't say that gravity was instantaneous.

Pi_314B
Can gravity be drawn on a piece of paper so as to be understood by the masses?

Mk
So what have we learned? The first law of thermodynamics, gravity is here, and gravity propogates at the speed of light!

By the way, Whitedragon, on PF all the html "<" and ">" are "[" and "]" To quote is "
" and "[/ quote]" without the space.

Staff Emeritus