Limit to a gravitational field?

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Discussion Overview

The discussion revolves around the nature of gravitational interaction between two objects in a closed universe devoid of other matter or energy. Participants explore whether there exists a distance at which these objects exert no gravitational pull on each other, and the implications of gravitational forces over infinite distances.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that even in a closed universe, two objects will always exert a gravitational pull on each other, but this pull may not always influence their movement due to inertia.
  • Others argue that there is a specific distance at which gravitational interaction becomes negligible, suggesting that gravitational effects do not manifest instantaneously and require time to propagate.
  • A participant questions whether a gravitational field can dissipate completely, leading to discussions about the nature of spacetime curvature and its effects over distance.
  • Some contributions mention that gravitational forces diminish with distance but do not reach a point of absolute zero, implying that gravitational influence persists indefinitely, albeit weakly.
  • There is a suggestion that the expansion of the universe could counteract gravitational attraction at very large distances, leading to a potential separation of masses.
  • Participants express differing views on the relationship between gravitational pull and inertial resistance, with some asserting that any gravitational force, no matter how small, will affect movement.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether there is a distance at which gravitational pull becomes negligible or whether gravitational effects persist indefinitely. Multiple competing views remain regarding the nature of gravitational interaction and the implications of spacetime curvature.

Contextual Notes

Discussions include assumptions about the nature of gravitational forces, the definition of a closed universe, and the effects of time on gravitational propagation. Some participants express uncertainty about the implications of their intuitions regarding infinite distances and gravitational energy.

Anticitizen
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If two objects, let's say two pool balls (or giant balls of neutronium for that matter), are placed in a closed universe that contains no other matter or energy, at rest with one another, is there a distance at which they exert absolutely no gravitational pull on each other? Or will there be some sort of gravitational interaction, regardless of how small, that will slowly pull them together given enough time?
 
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We have two theories which essentially agree on the outcome. Newton's Gravity and Einstein's Curved Spacetime.

If you had a closed Universe with two pool balls and no other matter, you would still have to worry about inertia. As such, there should be some critical value out in space where the force of attraction (due to Gravity or Spacetime Curvature) would not overcome inertial resistance to movement.

They will always exert a "gravitational pull" on each other - but it will not always influence the movement of the other. At least... that's what we know of these matters thus far.
 
Anticitizen said:
If two objects, let's say two pool balls (or giant balls of neutronium for that matter), are placed in a closed universe that contains no other matter or energy, at rest with one another, is there a distance at which they exert absolutely no gravitational pull on each other? Or will there be some sort of gravitational interaction, regardless of how small, that will slowly pull them together given enough time?

Assuming time starts when the objects are placed a distance d apart, they would exert absoloutely no gravitational pull on each other until t=d/c.

Regards,

Bill
 
Hello ok123jump.

Quote:-

----As such, there should be some critical value out in space where the force of attraction (due to Gravity or Spacetime Curvature) would not overcome inertial resistance to movement.-----

Surely there is no threshold value for a force to reach in order to accelerate an object. I would have thought that using the Newtonian idea of gravity the two objects would move towards each other. I know little of GR but my guess is that the answer is the same but i stand to be corrected in both cases.

Matheinste
 
That's my intuitive answer, too, matheinste, but my intuition frequently betrays me when it comes to this stuff.

Antenna Guy, just to get this straight, you're saying that there is no point at which a gravitational field dissapates completely? A spacetime curve, regardless of distance, will stretch across the universe given enough time for it to stretch at C? Assuming an infinite, unbounded universe, wouldn't that imply some sort of infinite gravitational energy of some sort? Correct me please...
 
Thanks for the link, Count Iblis, but I must admit it is far over my head.

The question posed in that thread is the exact one I asked. I looked for a similar thread before posting but didn't see that one. Sorry about being redundant.
 
Hello Anticitizen.

I believe it is possible to integrate (sum) gravitational force from a body to infinity and get a finite answer.

Matheinste
 
  • #10
Anticitizen said:
Antenna Guy, just to get this straight, you're saying that there is no point at which a gravitational field dissapates completely?

Well, you did specify that the universe under consideration is closed - which rules out infinity.

A spacetime curve, regardless of distance, will stretch across the universe given enough time for it to stretch at C?

What do you mean by "a spacetime curve"?

Assuming an infinite, unbounded universe, wouldn't that imply some sort of infinite gravitational energy of some sort?

No. Why do you think it would?

Regards,

Bill
 
  • #11
Apologies - poor choice of words on my part. By 'closed' I simply meant 'closed off to any external influence' including we the observer, not 'bounded'.

It seems to me that if a force can exert itself, no matter how small, over an infinite area, that it would translate as an infinite exertion of energy. But as I said before, that is (faulty) intuition, and I would appreciate being educated as to the reality of the situation.
 
  • #12
Oh, and by spacetime curve I refer to the warping in spacetime caused by mass. Is there a far away enough point where the 'depression' would end and spacetime would be 'flat' again, as if the objects didn't exist at all as far as spacetime was concerned?
 
  • #13
Anticitizen said:
Oh, and by spacetime curve I refer to the warping in spacetime caused by mass. Is there a far away enough point where the 'depression' would end and spacetime would be 'flat' again, as if the objects didn't exist at all as far as spacetime was concerned?

I tell you what: You describe how you think gravitational effects follow "a spacetime curve", and we'll take it from there.

Regards,

Bill
 
  • #14
I'm subscribing to the high-school textbook model of a ball placed on a sheet of rubber which causes a curvature or depression in the sheet, aka spacetime.
 
  • #15
Anticitizen said:
I'm subscribing to the high-school textbook model of a ball placed on a sheet of rubber which causes a curvature or depression in the sheet, aka spacetime.

Have you considered how long it takes for the ball to make that depression? If the ball is removed, does the depression vanish instantaneously?

Gravitational effects do not magically appear throughout space - they require a finite amount of time to propagate over finite distances. Infinite distances imply infinite time for effects to manifest themselves.

Regards,

Bill
 
  • #16
Yes, and I'm willing to allow any passage of time for the propagation of the gravitational effect in this thought experiment.
 
  • #17
Anticitizen said:
Yes, and I'm willing to allow any passage of time for the propagation of the gravitational effect in this thought experiment.

Then we are back where we started. The two objects would exert absoloutely no gravitational pull on each other until t=d/c.

Regards,

Bill
 
  • #18
ok123jump said:
We have two theories which essentially agree on the outcome. Newton's Gravity and Einstein's Curved Spacetime.

If you had a closed Universe with two pool balls and no other matter, you would still have to worry about inertia. As such, there should be some critical value out in space where the force of attraction (due to Gravity or Spacetime Curvature) would not overcome inertial resistance to movement.

They will always exert a "gravitational pull" on each other - but it will not always influence the movement of the other. At least... that's what we know of these matters thus far.
Did you really mean to say this? If ball A has mass (inertial resistance) M and ball B attracts it with a force (gravitational pull) F then ball B will change ball A's acceleration by F/M. No matter how small F is, as long as there is gravitational pull, it will affect the movement.
 
  • #19
Anticitizen said:
If two objects, let's say two pool balls (or giant balls of neutronium for that matter), are placed in a closed universe that contains no other matter or energy, at rest with one another, is there a distance at which they exert absolutely no gravitational pull on each other? Or will there be some sort of gravitational interaction, regardless of how small, that will slowly pull them together given enough time?
According to theory (both Newtonian and general relativity) there is no distance limit to gravity, it goes on forever. However it does get less and less, and eventually it will be so weak you couldn't measure it. Indeed quantum theory says (in a horribly gross oversimplification) that very tiny values can't be reliably measured at all.

The other thing to bear in mind is that the universe is expanding, so at very large distances the expansion overtakes gravitational attraction and the masses would move apart. Indeed it is now believed that the expansion is accelerating. To put it crudely, as well as gravitational attraction related to mass, it is thought there is also a very weak repulsion independent of mass (so weak that it's noticeable only on very large galactic scales). (For technical descriptions see cosmic inflation[/color] and the cosmological constant[/color].)

Antenna Guy said:
Then we are back where we started. The two objects would exert absoloutely no gravitational pull on each other until t=d/c.
I think this is irrelevant to what's being asked. You're assuming that two masses magically appear out of nowhere, contrary to all known laws of classical physics.
 
  • #20
DrGreg said:
Antenna Guy said:
Then we are back where we started. The two objects would exert absoloutely no gravitational pull on each other until t=d/c.

I think this is irrelevant to what's being asked. You're assuming that two masses magically appear out of nowhere, contrary to all known laws of classical physics.

Hi DrGreg,

If you are arguing that continuity of time must hold, then let's say the initial condition of rest implies that the objects would be closer at either increasing, or decreasing time. In a relativistic context, one can assume that the strength of the gravitational field at either object would not reach a minima until after the objects had reached their maximum separation (the initial condition of rest).

I think time delay is relevant to how/where the problem was presented, and I chose to highlight it within the context of the problem.

Regards,

Bill
 
  • #21
HallsofIvy said:
Did you really mean to say this? If ball A has mass (inertial resistance) M and ball B attracts it with a force (gravitational pull) F then ball B will change ball A's acceleration by F/M. No matter how small F is, as long as there is gravitational pull, it will affect the movement.

HallsofIvy,

You're right. That was quite an error in my concept of inertia.

The definition I found:

Mathematically, the inertial mass is the "m" in

\rho=mv

This result makes sense as a "bodies tendency to maintain momentum" - the larger the m, the less of an impact a change in velocity will have on the total momentum.
 
  • #22
Anticitizen said:
If two objects, let's say two pool balls (or giant balls of neutronium for that matter), are placed in a closed universe that contains no other matter or energy, at rest with one another, is there a distance at which they exert absolutely no gravitational pull on each other? Or will there be some sort of gravitational interaction, regardless of how small, that will slowly pull them together given enough time?
Under GR, in a closed spacetime all objects must come together eventually.
 

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