Gravity (over extremely long distances)

[jspstorm]

So how does gravity function according to the most recent standard model? Is there particle interaction? I remember reading about gravitons years ago, but it seems unlikely to me that any particle could be involved, I mean try to imagine a mass emitting so many particles that the universe could be sufficiently saturated as to constantly affect every other mass in existence, even masses a trillion light years distant. Also, this particle would have to have a constant velocity of infinity in order to operate in accordance with our observations on gravity.

I also read that gravity is the distortion of space-time, maybe this is the more likely answer?

 

[DaveC426913]

So how does gravity function according to the most recent standard model? Is there particle interaction?

This is quantum gravity. They're working on it, but it has mixed success so far.

this particle would have to have a constant velocity of infinity in order to operate in accordance with our observations on gravity.

Gravity changes do not travel at infinity; they travel at c.

I also read that gravity is the distortion of space-time, maybe this is the more likely answer?

That is the GR model, yes.

The $64,000 problem is reconciling GR and QG.

 

[jspstorm]

Gravity changes do not travel at infinity; they travel at c.

So if the Earth's sun spontaneously collapsed into a black hole, then gravitational tides would reach us immediately after the last photon emitted by the sun?

 

[DLuckyE]

OK, based on what evidence? Is there precedent for thinking that gravity would not extend this far?

If not, why would you even invent such a limitation? And what is preventing me from inventing, like you, a hypothesis that, say the scenario spontaneously generates a unicorn?

The scientific method is not about some sort of 'knowing what really happens'; it is about rationality - observing and developing rational rules that seem to describe how the universe works.

If gravity is actually caused by a something like a graviton, wouldn't you also have to conclude that at a far enough distance the gravitons are going to be to spread out to hit your other little particle?

 

[jspstorm]

What about conservation of angular momentum? If gravitons take time to reach their "targets" wouldn't planetary orbits become seriously messed up?

Also, doesn't the minimum level of illumination caused by a solar eclipse lag several seconds behind the gravitational effects?

 

[DaveC426913]

So if the Earth's sun spontaneously collapsed into a black hole, then gravitational tides would reach us immediately after the last photon emitted by the sun?

Correct. We would not know what happened to the Sun for 8 minutes after it happened. The grav effects would show up simultaneously with the visuals.

 

[DaveC426913]

If gravity is actually caused by a something like a graviton, wouldn't you also have to conclude that at a far enough distance the gravitons are going to be to spread out to hit your other little particle?

Why? Individual photons can travel a trillion light years.

 

[JDługosz]

OK, based on what evidence? Is there precedent for thinking that gravity would not extend this far?

Existing "correct" theories tend to break when pushed far beyond observed ranges of operation, with the old theory being a limiting approximation in that regime. E.g. SR replaces Newton's laws, when speeds get high.

Nobody has studied very small accelerations or weak gravity. If current theory were to be wrong, this is an area where it might show up. Meanwhile, QM + Gravity is a major unsolved problem. So, the behavior of gravity outside of its known regime is a point of, at the very least, humility in our confidence.

 

[JDługosz]

So if the Earth's sun spontaneously collapsed into a black hole, then gravitational tides would reach us immediately after the last photon emitted by the sun?

What "tides"?
If the sun collapsed into a black hole, it would weight the same and its gravity would not change.

 

[Mentallic]

If gravity is actually caused by a something like a graviton, wouldn't you also have to conclude that at a far enough distance the gravitons are going to be to spread out to hit your other little particle?

Why? Individual photons can travel a trillion light years.

He didn't say individual particles can't reach that far, but rather since they are individual gravitons and there are a certain finite number of them when they are ejected from the neutron, the gravitons would be too spread apart by the time they reach the other neutron to hit it. The likelihood of a graviton reaching the other neutron would be very low, so maybe gravity works differently in extreme cases like this?

What "tides"?
If the sun collapsed into a black hole, it would weight the same and its gravity would not change.

I read it the same as DaveC did. He was just trying to explain some way that the sun would suddenly disappear as if it were never there. A black hole wouldn't work like that, but it was close enough

 

[diazona]

He didn't say individual particles can't reach that far, but rather since they are individual gravitons and there are a certain finite number of them when they are ejected from the neutron, the gravitons would be too spread apart by the time they reach the other neutron to hit it. The likelihood of a graviton reaching the other neutron would be very low, so maybe gravity works differently in extreme cases like this?

Well technically a graviton is a quantum of the gravitational field, in the same sense that a photon is a quantum of the electromagnetic field. It's sort of, but not exactly, like a classical particle. You could think of it like this: when the first neutron's gravity actually interacts with the other neutron, it acts like a particle (the graviton), but when it's traveling between the neutrons it's more like a wave, and a wave would never get spread out enough that it would "miss" its target.

 

[Mentallic]

I see.. I knew there would be a catch to it being called a particle

 

[DaveC426913]

What "tides"?
If the sun collapsed into a black hole, it would weight the same and its gravity would not change.

Heh. You are, of course, right. And I never tire of pointing that out usually. Missed it this time.

Well technically a graviton is a quantum of the gravitational field, in the same sense that a photon is a quantum of the electromagnetic field. It's sort of, but not exactly, like a classical particle. You could think of it like this: when the first neutron's gravity actually interacts with the other neutron, it acts like a particle (the graviton), but when it's traveling between the neutrons it's more like a wave, and a wave would never get spread out enough that it would "miss" its target.

What he said.

 

[cjameshuff]

Something worth noting is that there's nothing special gravitationally speaking about two bits of mass being lumped together to form a larger object...the gravitational field of the whole is simply the sum of the fields of each of the parts. (aside from details like binding energy...)

The force holding you onto Earth's surface is the sum of that of each subatomic particle comprising each atom of the planet, most of them being thousands of km away from you. The same goes for the force holding the Earth in orbit around the sun, and the solar system in orbit through the galaxy. Galaxy clusters are held together by the gravitational fields of electrons and protons acting across millions of light years of distance. The gravitational field of a single subatomic particle may be unmeasurably small, but it adds up.

 

[DaveC426913]

Something worth noting is that there's nothing special gravitationally speaking about two bits of mass being lumped together to form a larger object...the gravitational field of the whole is simply the sum of the fields of each of the parts. (aside from details like binding energy...)

The force holding you onto Earth's surface is the sum of that of each subatomic particle comprising each atom of the planet, most of them being thousands of km away from you. The same goes for the force holding the Earth in orbit around the sun, and the solar system in orbit through the galaxy. Galaxy clusters are held together by the gravitational fields of electrons and protons acting across millions of light years of distance. The gravitational field of a single subatomic particle may be unmeasurably small, but it adds up.

You know ... you're right.

It isn't always obvious that a single proton a hundred million light years distant has a gravitational effect on us here on Earth, but it is indeed true and quite easily observable. If it were not true, then galaxy clusters would not be bound gravitationally.

The only thing binding galaxy clusters across hundreds of millions of light years is the gravity between the individual bits of matter.

 

[.physics]

Existing "correct" theories tend to break when pushed far beyond observed ranges of operation, with the old theory being a limiting approximation in that regime. E.g. SR replaces Newton's laws, when speeds get high.

Nobody has studied very small accelerations or weak gravity. If current theory were to be wrong, this is an area where it might show up. Meanwhile, QM + Gravity is a major unsolved problem. So, the behavior of gravity outside of its known regime is a point of, at the very least, humility in our confidence.

You are right. That's what I was trying to say. We would better find out some new formula for extreme condition.

 

[diazona]

But sometimes theories continue to apply without modification well outside their original observed ranges of operation. So you can't automatically say that you need a new formula for extreme conditions. At least, not until you find some reason to believe that the existing theory is inadequate.

 

[DaveC426913]

You are right. That's what I was trying to say. We would better find out some new formula for extreme condition.

Why do you think a new formula is needed?

Do you mean to say that, as a rule, all models breakdown at their extremities, therefore this one must too?

Shouldn't we wait to see if our existing model actually breaks down first? Hm?

 

[Thecla]

Hi
The Physics Teacher(vol32 Nov 1994 p.493) addresses a similar problem in a paper "Surprising Facts About Gravitational Forces" by Mallmann, Hock, and Ogden.

Two hydrogen atoms initially at rest and 1.0 millimeter apart would require almost two million years to fall toward one another to be 0.5 millimeter apart.
In the first 1000 years the separation of the atoms would be reduced from 1.0 mm to
0.99999989 mm.(a change in distance of about one atomic diameter)

The equation of time for a free fall of two objects is derived in this paper ,the final result is:

t={r(i)^1.5/sqrt(2G(m(1)+m(2)))} X {sqrt(R) x sqrt(1-R) +arccos(sqrt(R))}

Where t= time for the two masses,initially at rest, to travel the distance from r(i) to r(f)
r(i) is the initial separation of the center of masses of the two objects
R is the Ratio of r(f)/r(i) where r(f) is the final separation
G is the gravitational constant
m(1) and m(2) are the masses of the two objects

 

[diazona]

Why do you think a new formula is needed?

Do you mean to say that, as a rule, all models breakdown at their extremities, therefore this one must too?

Shouldn't we wait to see if our existing model actually breaks down first? Hm?

And using that logic, the idea that all models break down at their extremities is itself a model, and therefore must fail when applied to such an extreme theory as gravity :tongue2:

(well said DaveC)

 

[DaveC426913]

And using that logic, the idea that all models break down at their extremities is itself a model, and therefore must fail when applied to such an extreme theory as gravity

Good one!

 

[13habelbrea]

That's one hell of a force!

Assuming the force stays constant on both neutrons attracting each other, then it would take approx 10⁵³ years for them to collide. At the time of their collision, they'll be slamming into each other at a whopping 200 Planck lengths per second.

But what bothers me is that I've used a simplified version of events with my assumption. Of course the attractive force will increase as they get closer to each other. Anyone know how this could be calculated?

On that that makes me have a question, Would any droppler effect/ expansiton of space be happening on the two neurons?

 

[ABW]

I thing, this Question actually is connected with "model" of the Universe in Future.
In future many stars become Neutron stars and Black holes.
And question also is: "Will gravity eventually pull them together"