One model refered to as the “Big Rip” theorizes (IIRC) that as the universe expands, the “dark energy” would essentially rip the universe apart, atom by atom. as the universe expands, the the distance between objects increases and the effects of gravity decrease. Eventually the dark energy that would pull stars from their galaxies, planets from their stars and even tear atoms apart as the gravitational, magnetic and even atomic forces are not strong enough to resist.
Or if dark energy isn’t strong enough, the universe would just go on expanding forever until it eventually ran down.
It’s not like “heat” and “cold” (which are really sloppy terms when you come down to it). Gravitation is a measure of how curved spacetime is in the area, and gravitational waves are like ripples in an otherwise smooth surface. All they carry is energy, and that energy has to come from the system which is radiating.
I suppose in a sense you could note that gravitational waves are caused by accelerations, and eventually the system would settle into a stable state as its energy is sapped by radiation. That’s the closest analogy to heat I can come up with.
I was wondering if anyone would bring that up. Yes, even with only gravitational interactions, orbits will eventually decay. But the question was about two-body interactions. If you’re considering the loss of energy due to gravitational radiation, then the approximation that the system is a two-body system fails: The gravitational field itself must be considered a “third body”.
David Simmons, how familiar are you with electromagnetism? Light is an electromagnetic wave, but that does not mean that it carries charges from one place to another. Charges can flow, but the flow of charges is not light. However, movement of charges can cause light to be radiated, which can travel far away from where the charges were moving. And if the light meets other charges eventually (it doesn’t have to, of course; it could just keep on going into the vasty deep), it can cause those charges to move. Likewise, there are gravitational waves, but they don’t move mass around. They’re caused by moving masses, and they can cause other masses to move, in much the same way as with light and electrical charges.
Cartooniverse, small black holes could presumably eat in just the same way as big black holes. A black hole a millimeter across would take a while to eat a whole star, but if it did, it’d then be a stellar-mass black hole, indistinguishable from one that was born that big. There would be some difficulties, though, with very small holes: For one thing, the Hawking radiation from a small hole would be much more significant than from a large hole, and might produce enough radiation pressure that things would have a hard time getting to the hole. It’s possible that some sort of weird quantum-gravitational effects might halt the Hawking radiation for sufficiently small holes, but that’s highly speculative, and wouldn’t apply to moderately-sized holes, anyway.
Mostly correct, but the way you phrased that suggests that the “Big Rip” mechanism is caused by the expansion of space. The idea is more that the dark energy, for whatever mysterious quantum reasons of its own, decides to get steadily stronger, and therefore causes the Universe to expand at an ever-increasing acceleration, until eventually, it’s able to overpower everything. I also hasten to add that Big Rip models are highly speculative, and that the simplest hypothesis, that the dark energy is truly constant, is still consistant with the data.
Man, this’ll teach me to let a black hole thread sit for a few days!
Eventually EM waves turn into heat which increases the temperature of something, somewhere. In that case, they merely are a part of the energy exchange between material objects that will result in all objects being at the same temperature. Each object will radiate away the same amount of energy as it receives. There will be plenty of energy but none of it will be available for use. Or at least that’s the current picture, or one of them, of the far distant future.
So for a two body system with nothing else in a universe, what’s the endpoint of their both having radiated gravity away?
Well, take a cold tool out of you toolbox and lay it in the sun. It will heat up won’t it? In fact its temperature will go up until it is radiating, condicting and convecting away as heat the same amount of energy it is receiving from the sunlight. The work is internal to the tool in that the atoms in its crystaline matrix vibrate more as a result of the energy input from the sun.
I believe that, if you think about it, you will see that the earth is receiving energy from the sun at all times without what you are calling a force being exerted. An electric cell produces energy from chemical reactions. Energy doesn’t * have* to be exchanged only by pushing or pulling on something and moving it.
Among the things that you find in the vast wide reaches of the universe, stationary may be the rarest thing of all.
Every object in the universe, or at least that portion of the universe which we can see, interacts with every other object. In fact, the things we call objects are never singular things, but rather groups of things like atoms, planets, gas coluds, quarks, and itty bitty tiny black holes. At a particular distance, Earth is one object. Up close, it includes other objects, like you and me.
It is not inherent in the properties of a black hole of any size that it will grow. Think of it as a target. If you put a target at the end of a very popular pistol range, then no matter how small the target is, it has a fairly good chance of getting hit now and then. Put it on a raft in the middle of the Pacific Ocean, not so much. Put it in a remote orbit around a tightly spiraled galaxy, say five hundred thousand light years out, really really not so much.
A black hole is just another object, with one peculiar property. It’s realllllly heavy, and has a really high escape velocity at its “surface.” The millimeter sized black hole we have been talking about has a mass somewhat, but not a whole lot smaller than the Moon. Anything that gets within one millimeter of its center is captured by the gravity of this tiny moon sized object. Anything near it falls toward it just like you would fall toward the moon. (Think of tides.) Now, the hard facts of life for black holes is, the big get bigger, and the small get Hawking Radiation. When you have a fifty or sixty light year Swartzchild Radius, well, stuff falls in fairly reliably, until the local space runs out of stuff. One puny millimeter? Mostly, stuff misses.
The Tee Shirts belong to Steven Hawking. Well, one of them still does, the other one now belongs to Demetrios Christodoulou. In the high stakes world of cosmology, and theoretical physics, sometimes you have to bet it all, Steven did. He lost. So, Demetrios got the shirt. However, there is a new shirt, now, and it will take someone who can balance a cosmos sized pencil on the pointy end to win that one. The bet is tangentially concerned with black holes.
PS: If we get tangential with black holes, I am gonna need help from the actual physicists. T
That’s nothing special about EM waves, though. Eventually, everything is just heat (though it takes some things longer to get there than others). But heat isn’t so much a kind of energy, as a way that energy can be organised. You can have thermal EM radiation, and you can have thermal gravitational radiation (though thermal light sources are much more common than thermal gravitational radiation sources).
Trisk, do you happen to remember what the subject matter of that t-shirt bet was? There’s a lot of those sorts of wagers in theoretical physics (though most of them involve Kip Thorne as one of the parties). Amusingly, of all the Thorne wagers which have been resolved, Kip has lost every single one which depended on human intervention (predictions of when we’d detect some phenomenon, or of when he’d publish a particular book, etc.), but won every one which did not depend on human intervention.
Well, sort of, in a lay personish way, without any math.
Turns out Hawking (who bases his entire view of reality on the “no boundaries” model of cosmology) put forth the proposition that the nature of the universe itself prevented the existence of a naked singularity. In fact, he says, he will bet his shirt on it. Well, not even a full year later, Christodoulou rares back and mathematicizes up a possibility that a naked singularity could be formed by forces currently believed to exist. Now, this is one seriously hypothetical set of circumstances, and Hawking is reluctant to cough up his shirt for it. So, various mathematics and physics type begin ranting and raving back and forth, and finally, Stevie boy gives up a Tee Shirt that has printed on it, “Nature abhors a Singularity.”
Well, as you can imagine, the mathematics boys are nonplussed. It’s a payoff with a caveat! How droll. Everyone gives Hawking a tough time. So, he puts up another shirt, this time with a promised no quibble guarantee that no one can show a means whereby a singularity could be formed that did not require the giant pencil balanced on God’s nose sort of mathematics that Christodoulou’s example required.
Now, the math boys are still mumbling about a “True Scotsman” sort of quibble, but we notice they ain’t come up with a counter example so far. Odd that you should mention Kip Thorn, because he is pretty much beating the drum that this time Hawking’s shirt is safe in the closet. I don’t know if that makes Steve feel good about it though.
I know that this description has some shortcomings as far as scientific rigor goes, but hey, I ain’t got the branes to do it any better. I only read this stuff because it is important to continue to study stuff you don’t understand. Cosmology is right there in the middle of “stuff I don’t understand.”
I think you’re reversing what I’m saying (or maybe I’m just misunderstanding you). My point wasn’t that you need a force to have transfer of energy – it was that you don’t need to expend energy to exert a force.
You seemed to be saying above that the objects would eventually no longer be able to exert a gravitational force on each other because they’d have no more useable energy (i.e., energy that can be turned into work). But my point was you don’t need to do work to apply a gravitational force. I.e., gravity doesn’t require a “power source”, it just requires both objects to have mass.
But anyway, I agree with Omphaloskeptic. Forever is a long time. A lot of random *(&^) can happen in that time. So it’s hard to believe that a system couldn’t possibly wander into a lower energy state during that time.
The model is not reality. Modeling jupiter and the sun as theoretical point masses is useful for most purposes, but it’s not reality. Each object has lots and lots of particles interacting with eachother in different ways. Over a long enough time period, the model may lose predictiveness.
In the same way, being 150,000,000km from a solar sized black hole is not the same as being 150,000,000km from the sun. The gravitational forces exerted may be extremely similar, but they are not the same. Over a long enough time period, this may make a difference.
Space is far from empty isn’t it? In three dimensions there is a lot or room between dust particles. However as viewed from any point in space you see the dust particles, planets, etc., etc. projected on the surface of a sphere. I suspect that sphere’s surface is pretty well taken up by a physical object that can absorb light.
I don’t understand and therefore am dubious about “gravitational waves.” With EM waves, physical objects block them. When the moon gets between the sun and the earth the sun’s light is blocked. However the sun’s gravitational effect on the earth is undisturbed. Now these gravitational waves might be such long wavelength that it will take centuries to detect that one of them is passing by, but still …
You’re confusing gravitational waves with gravity itself. The sun’s effect provides the background spacetime curvature. If you wiggled the sun, it would send out ripples in the fabric. If the moon were in the way, yes it would interfere with these waves.
Besides, you’re trying to reason about something that’s only dimly understood in theory by introducing practical considerations like “there’s dust out there”?
My point is that “there’s dist out there” explains where the EM wave energy ends up. It’s absorbed and raises the temperture of the object that absorbs it. It’s my understanding that’s what is meant by the heat death of the universe. Energy radaiated from hotter objects and absorbed by colder ones so that ultimately everything winds up at the same temperature.
And the sun does wiggle. The common center of rotation for the sun-Jupiter pair is just outside the surface of the sun.
This is getting awfully close to a “They are there but we can’t detect them.” claim. Sort of like God.
If the waves fall off as the square of the distance, although the sun’s wiggle is small, we are awfully close. It might be that they foll off as the distance like the electric or magnetic field strength and not as the square.