gravity, gravity

[Baby Astronaut]

It has infinite reach, but tapers off. So Earth's gravity can't affect a planet in another galaxy, for instance, because of competing and stronger gravitational pulls interfering along the way.

 

However, let's say Earth and that planet were the only mass in existence, but separated by a hundred million light years. Also, no cosmic expansion.

 

Would the gravity of both Earth and the other planet eventually cause them to slowly draw nearer, i.e. would the gravitational tug be significant enough across the vast distance (without competing gravity from the other absent masses)?

 

Or is there a limit, where gravity has zero effect after a certain distance that's relative to the size of both objects?

[timo]

I'll take the chance to reply before someone starts mentioning that surely something must happen at whatever that person's favorite unit's Planck scale.

 

If the two objects are at rest wrt. to another then for any finite distance they will attract and collide within finite time. However, when the objects are far apart then already a small relative motion can exceed the escape velocity and the planets then move apart for all eternity.

 

In effect, the distance does not change anything qualitatively, it only shifts the numbers.

[Baby Astronaut]

If the two objects are at rest wrt. to another

That's why I put it under Classical Physics So let's just say that each planet was at a standstill, the universe otherwise empty. Would the distance prevent their gravitational attraction from moving them nearer?

[Sisyphus]

As a purely classical physics question, then yes, they fall towards one another.

 

Also, I wouldn't say this:

 

So Earth's gravity can't affect a planet in another galaxy, for instance, because of competing and stronger gravitational pulls interfering along the way.

 

I understand what you mean, but technically you can't block or interfere with gravity. It's just that the "background noise" of all the stuff in the universe is pretty much the same in all directions, so the net effect is zero except for nearby objects.

 

BTW, 100 million light years is about 1.5*10^17 times the radius of the Earth, so the force of gravity towards the Earth at that distance is about 1/2.25*10^34 as much as standing on the surface. So figure an acceleration in the range of 0.000000000000000000000000000000004 meters per second per second.

Edited February 1, 2010 by Sisyphus

[toastywombel]

It has infinite reach, but tapers off. So Earth's gravity can't affect a planet in another galaxy, for instance, because of competing and stronger gravitational pulls interfering along the way.

 

However, let's say Earth and that planet were the only mass in existence, but separated by a hundred million light years. Also, no cosmic expansion.

 

Would the gravity of both Earth and the other planet eventually cause them to slowly draw nearer, i.e. would the gravitational tug be significant enough across the vast distance (without competing gravity from the other absent masses)?

 

Or is there a limit, where gravity has zero effect after a certain distance that's relative to the size of both objects?

 

I think that if the earth and the planet were the only two masses in our visible universe, and if there was no cosmic expansion, they would slowly draw nearer. It would take a long long time of course.

 

I briefly researched gravity before posting. In Newtonian gravity, the force can be equal to zero, meaning there is a set limit to how far gravity reaches. However, according to general relativity, the force cannot be equal to zero. This means that gravity reaches across the whole universe. As the distance increases the gravitational force between the two objects would decrease, but never reach zero. This is my general understanding after brief research.

[Baby Astronaut]

I understand what you mean, but technically you can't block or interfere with gravity. It's just that the "background noise" of all the stuff in the universe is pretty much the same in all directions, so the net effect is zero except for nearby objects.

Yeah, I meant interference on gravity having a measurable net effect on the distant planet.

 

BTW, 100 million light years is about 1.5*10^17 times the radius of the Earth, so the force of gravity towards the Earth at that distance is about 1/2.25*10^34 as much as standing on the surface. So figure an acceleration in the range of 0.000000000000000000000000000000004 meters per second per second.

Wow, so even at a trillion light years it would cause a teeeeeny acceleration. Incredible.

[michel123456]

As a purely classical physics question, then yes, they fall towards one another.

 

Would they bump, or orbit one another?

[Sisyphus]

Would they bump, or orbit one another?

 

They bump, after falling for many many times the age of the universe. The set up is they have an initial relative velocity of zero. An orbit requires a sideways component to velocity.

[J.C.MacSwell]

It would be quite a "bump"!

[Sisyphus]

Er, yes. They collide at escape velocity.

[michel123456]

They bump, after falling for many many times the age of the universe. The set up is they have an initial relative velocity of zero. An orbit requires a sideways component to velocity.

 

In the case of the Earth-Moon system, what is the "sideways component to velocity"?

[Sisyphus]

In the case of the Earth-Moon system, what is the "sideways component to velocity"?

 

The moon travels around the Earth (well really, they both travel around their common center of gravity, but for simplicity's sake it doesn't really matter) in an approximately circular path. At any given moment, the moon's acceleration is directed towards the Earth, and its velocity is perpendicular to that acceleration. (In fact, this is true for anything traveling in a circle.) The moon is constantly falling towards the Earth, but because of its initial velocity it continues to miss us. That's what an orbit is.

 

Here is another common illustration of how orbits work:

 

http://www.astronautix.com/lvs/newannon.htm

[Baby Astronaut]

It's all so mind-boggling.

 

Within an empty universe, a tiny flea's own gravity can pull on another flea a trillion light years away.

 

Gotta love it

[mooeypoo]

It's all so mind-boggling.

 

Within an empty universe, a tiny flea's own gravity can pull on another flea a trillion light years away.

 

Gotta love it

 

It gets better, Baby!* Within an empty universe, a tiny flea's own gravity can pull on a HUGE PLANET a trillion light years away.

 

 

Of course, the planet would pull it too, but so would another flea. So there.

 

 

 

 

* Your nickname was just WAITING to be (ab)used this way at some point, mah friend

[michel123456]

The moon is constantly falling towards the Earth, but because of its initial velocity it continues to miss us. That's what an orbit is.

 

What is the "initial velocity"? The one we got from the Big Bang?

Edited February 4, 2010 by swansont
fix quote tag

[pywakit]

The moon is constantly falling towards the Earth' date=' but because of its initial velocity it continues to miss us. That's what an orbit is.QUOTE']

 

What is the "initial velocity"? The one we got from the Big Bang?

From Space.com

 

SPACE.com -- 24 Hours of Chaos: The Day The Moon Was Made

 

http://www.space.com/...'>http://www.space.com/... /solarsystem/moon_making_010815-1

 

An excerpt :

 

Now researchers have harnessed the latest in computing power to provide the most detailed model ever made of the cosmic scene that supposedly created the Moon. The result, a 3-D animation of the blast and subsequent chaos, is comforting. It shows that the Moon could have formed when a Mars-sized object hit a fully formed Earth.

 

The collision would have given Earth its spin, defined what we now call an equator, and put enough material into orbit at the right distance from Earth to allow the formation of a satellite that generations would later swoon over.

 

24 hours of chaos

 

Robin Canup of the Southwest Research Institute has been modeling the Moon's formation for eight years. On previous studies, she has worked with William Ward and Alastair Cameron, who represent one of two separate research groups that developed the original impact theory back in the mid-70s. (William K. Hartmann and Donald R. Davis were the other team.)

 

As Canup knows, all ideas about how the Moon formed must contend with one important fact: The Moon contains very little iron. Earth, on the other hand, is loaded with iron, the bulk of it tied up in the planet's core.

 

So the Moon is thought to have been pieced together by the bits that got blown off the upper layers of Earth, as well as the outer portions of the object that hit Earth.

 

Hope this answers your question Michel.

 

Also ... more recent info :

 

Earth Hit by Neighbor in Making of Moon

 

By Robert Roy Britt

 

Senior Science Writer

 

21 June 2004

 

http://www.space.com/ scienceastronomy/moon_formation_040621.html

 

And ...

 

NASA - Moon

 

http://www.nasa.gov/worldbook/moon_worldbook.html

 

Excerpt :

 

Scientists believe that the moon formed as a result of a collision known as the Giant Impact or the "Big Whack." According to this idea, Earth collided with a planet-sized object 4.6 billion years ago. As a result of the impact, a cloud of vaporized rock shot off Earth's surface and went into orbit around Earth. The cloud cooled and condensed into a ring of small, solid bodies, which then gathered together, forming the moon.

 

The rapid joining together of the small bodies released much energy as heat. Consequently, the moon melted, creating an "ocean" of magma (melted rock).

 

The magma ocean slowly cooled and solidified. As it cooled, dense, iron-rich materials sank deep into the moon. Those materials also cooled and solidified, forming the mantle, the layer of rock beneath the crust.

 

And this alternate theory :

 

Later Than ThoughtRichard A. Lovett

 

for National Geographic News

 

http://www.news.nationalgeographic.com/... /071219-moon-collision.html

 

December 19, 2007

 

The moon was formed from fragments of Earth after a collision with a giant asteroid relatively late in our planet's formation, new tests of moon rocks show.

 

The finding upends many of the prior theories for how the moon came to be, researchers say.

 

Moon Derives From Earth, Space Object, Study Says (August 11, 2003)

Moon Formed Volcanoes Early, Rock Study Shows (December 5, 2007)

Scientists have long believed that the moon was formed by a collision between our planet and a Mars-size object.

 

Computer models have shown that in this scenario 80 percent of the moon's material should have come from the asteroid, with only 20 percent from Earth.

 

But the new study of moon rocks collected three decades ago by Apollo astronauts, however, found that Earth and the rocks were too similar for that to be the case.

 

Earthly Material

 

The most likely explanation is that the moon was formed primarily of Earthly material, the authors say.

 

Lead author Mathieu Touboul of the Swiss Federal Institute of Technology in Zurich said there is another theory that may explain its formation.

 

"Alternatively, the material from which the moon eventually formed was a magma disk, connected to the Earth by a common atmosphere," he said in a statement.

 

Material from Earth and the nascent moon could then be exchanged via a shared metal-vapor atmosphere.

 

By the time the two worlds had settled back down and begun drifting apart, their compositions would have been virtually identical.

 

"New simulations of such a process have recently shown that such exchange is possible," Touboul said.

 

And still more .....

 

Nice graphics on this site ...

 

From Universe Today

 

June 19th, 2008

 

http://www.universetoday.com/2008/06/19/new-instrument-could-reconstruct-planetary

 

New Instrument Could Reconstruct Planetary and Moon Origins

 

Excerpt :

 

One of the leading theories for how our Moon formed is the Giant Impactor Theory, which proposes a small planet about the size of Mars struck Earth early in our solar system's formation, ejecting large volumes of heated material from the outer layers of both objects. This formed a disk of orbiting material which eventually stuck together to form the Moon. Until now there's been no way to actually test this theory. But a new instrument that closely examines iron isotopes could possibly shed insight into the origin of the moon, as well as how Earth and the other terrestrial planets formed.

---

Merged post follows:

Consecutive posts merged

It gets better, Baby!* Within an empty universe, a tiny flea's own gravity can pull on a HUGE PLANET a trillion light years away.

 

 

Of course, the planet would pull it too, but so would another flea. So there.

 

 

 

 

* Your nickname was just WAITING to be (ab)used this way at some point, mah friend

 

 

Hmmmm. Interesting.

 

I wonder if two ultramassive ( and then some ) ancient black holes ( ones which formed shortly after the BB ) ... septillions of light years apart would ALSO pull on each other?

 

Lol. No. I suppose not.

 

I guess that would require an EMPTY universe ....

 

and no cosmic expansion.

 

Or would it ...... ??

 

---

Merged post follows:

Consecutive posts merged

Ok. How's this for bizarre?

 

I can make three ultramassive black holes, a trillion light years apart, merge into one ... with just two hydrogen atoms.

 

I think ... lol.

 

Let's see.

 

We start with three identical non-rotating, non-radiating UMBHs ( 50 billion sols ) positioned in a straight line on the exact same plane of the ecliptic, equidistant, in otherwise empty space.

 

The end BHs are rotating in perfect circular orbits around the center BH at exactly the same orbital velocity ... say ... 1000 k/s. ( Just a guess. It doesn't matter.)

 

At the exact same time, the end BHs collide dead center with an atom of hydrogen 'at rest'.

 

If I understand inertia, angular momentum, and gravity ....

 

The end BHs' orbital velocities will slowed by a nearly infinitely small amount.

 

But they will be slowed. And they will begin to 'fall' toward the center of gravity.

 

Wind the clock forward 10^500 ( give or take ... lol ) years, and those three BHs will have merged into one.

 

From just 2 atoms of hydrogen.

 

Please correct me if I am wrong.

Edited February 4, 2010 by pywakit
Consecutive posts merged.

[Sisyphus]

That's kind of a screwy scenario to begin with, with three equal bodies. What is the point of having three and not two?

 

Anyway, no, they don't fall into one another. They just have very very very slightly modified orbits. If you take a circular orbit and reduce its velocity at some point, it becomes an elliptical orbit with its apogee at the point where you reduced the velocity.

 

If you think about it, it's equivalent to just using more or less charge in Newton's cannon.

[mooeypoo]

Hmmmm. Interesting.

 

I wonder if two ultramassive ( and then some ) ancient black holes ( ones which formed shortly after the BB ) ... septillions of light years apart would ALSO pull on each other?

 

Lol. No. I suppose not.

 

I guess that would require an EMPTY universe ....

 

and no cosmic expansion.

 

Or would it ...... ??

Yes. It would.

 

The entire point is that we expected the universe to slow down the expansion, and yet it doesn't. That's why there was a need for "dark energy" - the universe continues its expansion away and stronger from the forces of gravity.

 

So, no, two black holes will not swallow the entire universe.

Two massive black holes would exert force on one another even from "septillion" light years away, but - as was pointed out before in this thread and others - that force will be countered, and likely cancelled and overcome, by gravitational pulls from closer (even if smaller) stars and by the acceleration already existing on the object by the expansion of the universe.

 

 

Ok. How's this for bizarre?

 

I can make three ultramassive black holes, a trillion light years apart, merge into one ... with just two hydrogen atoms.

If the universe is empty and there are no other forces on those black holes then you can have twenty three and merge them, and you don't even need the hydrogen atoms; if NO OTHER FORCE exists on those objects their gravities would attract them together.

 

All items with mass attract one another, so you don't need a hydrogen atom; in fact, if there were ONLY three massive black holes and a hydrogen atom in space, then the force exerted by the tiny atom would be overcome a billion-billion-billion times by the force from each of the black holes, which means that for all intents and purposes, you can ignore it.

 

If there were no other items in the universe, that is.

 

But in reality, forces DO exist, and do COUNTER this force. We see it observationally as well as mathematically.

 

~moo

Edited February 4, 2010 by mooeypoo
Consecutive posts merged.

[swansont]

Well, if one takes into account GR, any orbiting bodies will radiate gravitational waves and the orbits will decay over time. No need for any external perturbation.

[mooeypoo]

swansont, but the universe is expanding faster than the gravitational pull slows it down, isn't it? The "big crunch" theory depends on the escape velocity of objects to be lower than the pull from gravity for the universe to collapse back on itself. And as far as I've read, though it's possible for "Dark Energy" to, eventually, perhaps, change signs (and allow for the 'big crunch') it's not what we're seeing at the moment, and the evidence we currently have do not fully support this.

 

Relying on dark energy acceleration to switch signs (and hence to allow for the "big crunch") is not supported by observation at least until we fully understand what causes dark energy and can describe its nature..

 

Correct me if I'm wrong here.

 

~moo

[Sisyphus]

This is in the classical physics forum, so I was assuming no gravitational waves, and no cosmic expansion. I let "black hole" slide, since point masses suit classical physics problems just fine.

[michel123456]

I was asking about initial velocity of an orbiting body like Earth or Moon. I don't think the creation of the Moon has anything to do with my question. The material of the accretion disk was orbiting before the Earth was created.

I was just interested to hear someone say that gravity is not enough to make bodies orbit. You need something else, like "initial velocity". And I am still asking, where does this "initial velocity" come from?

For example, the solar system is orbiting the galaxy: where does its initial velocity come from?

[Sisyphus]

So the question is why is there motion in the universe? Because the initial conditions of the universe were not in a stabile configuration. Why not? Because it was not completely uniform. Why not? Good question. Presumably because of random quantum effects.

[dalemiller]

michel123456: Gravitation toward any third body should do the trick.

[Hawkin'sDawkins]

However, let's say Earth and that planet were the only mass in existence, but separated by a hundred million light years. Also, no cosmic expansion.

 

Would the gravity of both Earth and the other planet eventually cause them to slowly draw nearer,

 

Assuming no cosmic expansion we would have to assume that at the moment of creation these 2 planets simply came into being. In this case, they would not initially affect one another.

 

The instant they appear, each one would send out gravity waves in all directions at the speed of light, so neither one would move at all for the first 100,000,000 years and would then start moving towards one another.

 

The point about 2 objects a septillion light years apart is a bit shaky as it would take a septillion years for gravity waves from these objects to reach each other. This is almost certainly longer than the lifetime of our universe and so no, these hypothetical objects would not interact.

 

Also, Hi, cool forum!