Effect of Gravity over very long distances

[Robert163]

I am trying to understand gravity. I can illustrate my question best with a hypothetical:

We have a universe with only two objects in the whole universe. The two objects are two small BB’s, one each on the opposite side of the universe, 14 billion light years apart. Through the power of magical powers you manufacture another BB and magically weld it to the BB on side of the universe. How long will it take before the added weight (mass?) of the two BB’s affect the lone BB on side [y] of the universe?

[sbunny8]

Current theory is that it would take 14 billion years before the other BB was affected. It’s impossible to transmit information faster than c without violating causality in some inertial frame of reference. It is believed that gravity moves at c. BTW, you didn’t mention whether location x and location y are moving apart from each other due to the expanding universe, so I assumed that, in your example, the universe isn’t expanding and x and y are stationary relative to each other.

[Don_t_Panic]

@Robert163

We have a universe with only two objects in the whole universe. The two objects are two small BB’s, one each on the opposite side of the universe, 14 billion light years apart.

To nitpick a bit: 14 billion years, give or take, is the estimated age of the universe. 14 billion light years is not, however opposite ends of our universe. For one thing, opposite ends would be the diameter, not the radius. For another thing, the observable universe is bigger than that, because the universe is expanding. For a third thing, the actual universe is probably a whole lot bigger than the observable universe.

Yeah, I know you said *a *universe, not *the *universe.

Anyway, the answer, as sbunny8 said, is that gravity propagates at c. Then the rest depends on the size of your universe.

(Waiting for the experts to nitpick that answer some more, and maybe kick it in the balls wholesale)

[Robert163]

@sbunny8

Current theory is that it would take 14 billion years before the other BB was affected. It’s impossible to transmit information faster than c without violating causality in some inertial frame of reference. It is believed that gravity moves at c. BTW, you didn’t mention whether location x and location y are moving apart from each other due to the expanding universe, so I assumed that, in your example, the universe isn’t expanding and x and y are stationary relative to each other.

@Martian Bigfoot

To nitpick a bit: 14 billion years, give or take, is the estimated age of the universe. 14 billion light years is not, however opposite ends of our universe. For one thing, opposite ends would be the diameter, not the radius. For another thing, the observable universe is bigger than that, because the universe is expanding. For a third thing, the actual universe is probably a whole lot bigger than the observable universe.

Yeah, I know you said *a *universe, not *the *universe.

Anyway, the answer, as sbunny8 said, is that gravity propagates at c. Then the rest depends on the size of your universe.

(Waiting for the experts to nitpick that answer some more, and maybe kick it in the balls wholesale)

Yes, thank you both for helping to clarify the point I was trying to make and, of course, for answering the question.

[Trinopus]

There was a chap (now deceased) named Tom Van Flandern, who posited that gravity propagated at an infinite speed. There are archives of his lengthy debates with others on the subject. He has followers today, some of whom go about to various science fiction conventions, peddling books on the infinite “speed of gravity.”

At present, this idea is not accepted. It violates the ideas codified in Relativity; infinitely fast gravity would permit infinitely fast signaling, and thus certain kinds of causality paradoxes.

I mention this, not to jape at the expense of the departed, nor to espouse non-conventional theories, but only to note the existence of such non-conventional theories, so that no one might encounter them without some fore-warning.

[Robert163]

@Trinopus

There was a chap (now deceased) named Tom Van Flandern, who posited that gravity propagated at an infinite speed. There are archives of his lengthy debates with others on the subject. He has followers today, some of whom go about to various science fiction conventions, peddling books on the infinite “speed of gravity.”

At present, this idea is not accepted. It violates the ideas codified in Relativity; infinitely fast gravity would permit infinitely fast signaling, and thus certain kinds of causality paradoxes.

I mention this, not to jape at the expense of the departed, nor to espouse non-conventional theories, but only to note the existence of such non-conventional theories, so that no one might encounter them without some fore-warning.

Ok, thanks, what are they talking about in a SF movie when they mention “tachions?”

[Trinopus]

Tachyons are a hypothetical naturally-occurring particle that would travel faster than the speed of light. They essentially involve the “imaginary numbers” solution to various Relativistic equations.

They probably don’t exist, but it would certainly be nifty if they did.

SF uses them for various dramatic purposes, such as faster-than-light “radio” signaling.

(From the Greek root “tachy” meaning “fast.”)

[Robert163]

@Trinopus

Tachyons are a hypothetical naturally-occurring particle that would travel faster than the speed of light. They essentially involve the “imaginary numbers” solution to various Relativistic equations.

They probably don’t exist, but it would certainly be nifty if they did.

SF uses them for various dramatic purposes, such as faster-than-light “radio” signaling.

(From the Greek root “tachy” meaning “fast.”)

Do you read much SF? Do you prefer Hard or Soft SF? Somewhere around here I have a copy of The Mote In Gods Eye which I have yet to read…

[MikeS]

As a nitpick, the mathematical equations that we use to describe gravity (Einstein’s equations) actually require that matter and energy are not created or destroyed. If you try to describe a situation where matter “magically appears”, you get inconsistent equations, requiring things like 1 = 0 somewhere.

A more realistic situation, and one that Einstein’s equations can handle, is where you move the BB at over a couple of meters. This should change the direction of the force on the BB at [y] by some minuscule amount. The question is then how long this takes to happen; and the answer is as noted above, that the influence of gravity propagates at the speed of light.

[Robert163]

@MikeS

As a nitpick, the mathematical equations that we use to describe gravity (Einstein’s equations) actually require that matter and energy are not created or destroyed. If you try to describe a situation where matter “magically appears”, you get inconsistent equations, requiring things like 1 = 0 somewhere.

A more realistic situation, and one that Einstein’s equations can handle, is where you move the BB at over a couple of meters. This should change the direction of the force on the BB at [y] by some minuscule amount. The question is then how long this takes to happen; and the answer is as noted above, that the influence of gravity propagates at the speed of light.

Wow you people are so F-ing smart, these type of threads always amaze me!!

[Trinopus]

@Robert163

Do you read much SF? Do you prefer Hard or Soft SF? Somewhere around here I have a copy of The Mote In Gods Eye which I have yet to read…

Meet me over in Café Society, and I’ll chat happily forever on SF! “Mote in Gods Eye” is a very good book indeed. I hope you enjoy it! If you do, see if you can find a copy of “Ensign Flandry” by Poul Anderson.

[Robert163]

@Trinopus

Meet me over in Café Society, and I’ll chat happily forever on SF! “Mote in Gods Eye” is a very good book indeed. I hope you enjoy it! If you do, see if you can find a copy of “Ensign Flandry” by Poul Anderson.

Ok, I’ll do it now

[Trinopus]

@Robert163

Wow you people are so F-ing smart, these type of threads always amaze me!!

The introductory Relativity equations aren’t too hard. If you can do square roots, you can do “time dilation” and “relativistic contraction.”

Now, the equations do start to get monstrous later, but the basics are really within the reach of just about any high school student.

[Robert163]

@Trinopus

The introductory Relativity equations aren’t too hard. If you can do square roots, you can do “time dilation” and “relativistic contraction.”

Now, the equations do start to get monstrous later, but the basics are really within the reach of just about any high school student.

Except that I can’t do long division in my head and I can’t “picture” how to solve a Rubik’s cube, etc, I’ve allways found math to be very hard. So much so that I at one point thought of majoring in Math, just to give myself a challenge.

[panache45]

Robert163: You stole my thread.

[Robert163]

@panache45

Robert163: You stole my thread.

Oh, wow, I remember it, it was a genius example that made a very good point/question. But actually I misremebered it. I thought you question was would they collide, not how long. I thought I was taking you question one step further but I can see that you had already done that. Well, thanks for the inspiration mate! I think the topic is thoroughly covered now.