Is Gravity Caused by Mass or Other Factors?

[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?

 

[Doc Al]

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]

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.

 

[Doc Al]

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?

 

[Doc Al]

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.

 

[Doc Al]

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.

 

[Doc Al]

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

 

[Whitedragon]

What's your proff?

 

[Whitedragon]

sorry, proof.

 

[Doc Al]

It's a consequence of general relativity.

 

[Whitedragon]

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

 

[Doc Al]

Since this is apparently unknown. What gravity is? ... I shall speculate.

Please don't. Stick to established physics.

 

[Pi_314B]

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]

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.

 

[G01]

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.)

 

[pervect]

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.

 

[ZapperZ]

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.

 

[Doc Al]

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.)

 

[quantumdude]

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.

 

[russ_watters]

*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!

 

[Pi_314B]

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.

 

[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.

 

[quantumdude]

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?

 

[pervect]

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.

As I mentioned before, current theory does not make a prediction about this case because it leads to inconsistent equations.

 

[Pi_314B]

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]

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.

 

[quantumdude]

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

Claiming that the gravitational influence of the 'zapped' body disappears instantaneously is equivalent to claiming that gravitational effects propagate instantaneously. You did just that in Post #18 of this thread.

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

There are a number of ways to represent gravity visually. Students in Physics I represent the gravitational force (and all other forces as well) as an arrow in a free body diagram. More advanced students learn to represent it by drawing flux lines for the purpose of applying Gauss' law.

 

[quantumdude]

okay sorry about the "homegrown crackpottery"

I wasn't talking to you, Ki Man. I deleted your post by mistake, and have now un-deleted it. We don't mind answering questions here.

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?

It's precisely for this reason that gravity is believed to propagate at a finite speed.

 

[quantumdude]

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.

Our best understanding of gravitation is embodied in General Relativity, which states that not just mass, but also energy and momentum contribute to gravitation.

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?

Yes, it would.

 

[Ki Man]

so that would be a possible theory on what causes gravity? or just a crazy idea of mine.

i know it would be possible but would it be strong enough to cause gravity the way we know it. that's the only thing i can think of to disprove my own theory

 

[quantumdude]

so that would be a possible theory on what causes gravity? or just a crazy idea of mine.

As I said, it is not only mass but also momentum and energy that cause gravity.

Just a note: Personal theories and/or speculations are not appropriate for the Physics section of PF. We host discussions of non-peer reviewed work only in the Independent Research Forum, and it must be done according to our https://www.physicsforums.com/showthread.php?t=82301. This is to ensure that the scientific and academic integrity of PF is maintained.

i know it would be possible but would it be strong enough to cause gravity the way we know it. that's the only thing i can think of to disprove my own theory

Truthfully: You don't have a theory. Physical theories are not just qualitative, but also quantitative. If a 'theory' of gravity does not even permit one to calculate the strength of the gravitational influence of one body on another, then it is no theory at all.

Let's stick to Newton and Einstein for now, OK?

 

[Pi_314B]

Claiming that the gravitational influence of the 'zapped' body disappears instantaneously is equivalent to claiming that gravitational effects propagate instantaneously. You did just that in Post #18 of this thread.

I did no such thing. I should know ...I typed it, and now that you have (zapped!) some post - you can quote me out of context. What I was trying to get across was that a mass that is zapped would also require the zapping of the gravitational field, because the mass and the gravitational field are one and the same. I am in complete agreement that gravity propagates at C.

 

[quantumdude]

I did no such thing. I should know ...I typed it,

You did, in fact, claim that the gravitational influence would drop to zero instantaneously if an attracting body is 'zapped'. I should know...I read it.

and now that you have (zapped!) some post - you can quote me out of context.

Give me a break. Post #18 is still there, and I did not quote you out of context.

What I was trying to get across was that a mass that is zapped would also require the zapping of the gravitational field, because the mass and the gravitational field are one and the same. I am in complete agreement that gravity propagates at C.

And the point I am trying to get across is that the instantaneous vanishing of gravitational effects requires that the gravitational interaction propagate infinitely fast. I know you don't think that your statements imply this, and I am informing you that they do.

I am not interested in your ideas on gravity. If you've got questions, then that's fine. If you're here to tell us "how it is" then that's not fine.

 

[aaroman]

The gravitational effect would disappear instantaneously

Instantaneously for which observer? :)

 

[Doc Al]

In a not to terribly roundabout way it states that the gravitational field is the moon itself. I don't know how you could take this any other way. So if the moon is zapped surely the field is gone as well in an instantaneous fashion.

Again you think that the moon and its gravitational field are instantaneously linked. Not so. Let's forget about making the moon disappear (as pervect points out, that is an unphysical premise). Instead, say that the moon is quickly moved a mile further away. (Again, this may prove to have some unphysical elements, but let's assume it's OK for this discussion.) The moon's gravitational pull on us will change as a result, but that change takes time to reach us since a disturbance in the gravitational field travels at the speed of light, not instantaneously.

How could I know or anyone else for that matter (the post is deleted).

I deleted that post because all it contained was idle speculation. (Please reread our policy regarding overly speculative posts.)

 

[quantumdude]

Instantaneously for which observer? :)

aaroman,

The best way to deal with people such as Pi_314B is to simply correct him, rather than ask him to explain. He simply doesn't know what he is talking about.

That said, if a body were to suddenly disappear there is no reason to think that its gravitational influence on another body would drop to zero instantaneously. That is because it would necessarily imply that gravitation propagates with infinite speed, and we have good reason to think that that is not true.

If someone says on the one hand that gravitation propagates at 'c', and on the other hand that gravitational influences can be instantaneously changed at points which are some distance from a source, then one is simply contradicting oneself.

 

[HallsofIvy]

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

"masses" understand gravity very well- it's us people that don't!

 

[quantumdude]

I tried my damndest not to make a joke just like that when I read this last night!

 

[Lisa!]

"masses" understand gravity very well- it's us people that don't!

As far as I remeber people are masses too, so they should be able to understand gravity like other masses!

 

[rbj]

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.

this is the post that i was referring to.

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.

it's on the record.

 

[Kazza_765]

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?

Damn, nice grasp of physics for someone in 8th grade. Keep up the good work and curiosity Ki Man, you'll go far.

 

[Whitedragon]

Okay everybody I was wrong... I forgot that Einstein wrote a paper that said light was exactly the same speed as gravity, no more, no less. What I was reffering to was Newtonians gravitatonial thingy that stated that gravity was indeed faster thant the speed of light. But what I had forgot about was that Einstein didn't think that was true and worked a theory out that nothing, according to our known theories and laws that absolutly nothing is faster than the speed of light. My original question is what gravity is and I guess nobody has an answer to that. Sorry.