Gravity Waves: Chance of Gravity Tsunami?

I used to have time to follow astronomy news very closely, but those days are long gone. I must find time to read the articles on the whole gravity wave “thing”, but I understand one black hole swallowed another proving Einstein’s theories on gravity waves? That said, should we expect a gravity tsunami resulting from this phenomenon?

The wave we detected is the only wave we’re going to get from this. I’m not sure what you mean by a “gravity tsunami”: Gravitational waves are devilishly difficult to detect even with extremely sensitive instruments (that’s why this is a big story); there’s no chance at all that an ordinary lump of rock would respond in any detectable way.

We’ve already experienced the wave. It’s gone past us at the speed of light. When you read the articles on it, you’ll find that the “tsunami” was much smaller than the diameter of a proton. If you didn’t feel the burn then, then you’re out of luck.

Perhaps you’re asking if it’s possible for a gravitational wave event to be close enough to damage the Earth in some way.

The short answer is that yes, it’s possible, but if it happens you have much bigger things to worry about. A pair of black holes would have to pass close to the Earth–perhaps of the same order as the distance to the Moon. At best, such an event would cause massive disruption to Earth’s orbit and destroy all life.

Fortunately, these events are incredibly rare. The odds of one happening in our own galaxy on human timescales is already virtually zero. The odds that such a pair of infalling black holes would actually pass close to the Earth is even less.

There are other sources of gravitational waves, such as binary neutron stars, that are more common. But they are also less powerful, and still have the problem that if they pass close enough to Earth that the gravity waves have a noticeable effect, then you have much bigger things to worry about.

Indeed if you’re worried about sudden instantaneous destruction of life on earth we have much bigger issues, namely:

a) planet killer asteroid of more than 50 km diameter
b) gamma ray burst
c) collapse of false vacuum

Sleep well!

I guess in a sense we can’t rule it out.

The earth gets hit by cosmic rays that carry more energy than our models can account for at this time, and there are still many unknowns when it comes to dark matter, dark energy and “quantum gravity”. Maybe there are extremely rare events even more turbulent than two black holes merging?

But they’d have to be a hell of a lot more turbulent to cause physically noticeable effects on earth, even at close distances, plus of course saying “it can’t be ruled out” is scientifically meaningless…we can’t rule out the earth being knocked out of its orbit by a giant slice of pizza.

I’m curious about this one, coremelt, can you elaborate? The other two were obvious

There is a chance that our current vacuum state is not the real lowest energy level. if a real vacuum was to appear anywhere in the universe it would propagate at the speed of light destroying the universe as we know it.

It may have already happened and the true vacuum could be heading towards us right now at the speed of light. At least it would be quick…

In quantum mechanics, all fields have the notion of a ground state, where the energy is at a minimum. Higher energy states may be possible, but they are at best metastable. Think of a marble resting at the top of a volcano–it may be stable sitting in its hole, but if you bump it enough, it will roll over the lip and down the side with great energy.

The vacuum in QM is not empty–it is filled with various fields. It is thought that they exist in the ground state, which means that no amount of perturbation can cause them to fall into an even lower state. You can at most temporarily bump it to a higher state which collapses immediately.

However, we can’t rule out the possibility that the vacuum isn’t in the ground state, but rather in a higher metastable state. If so, either random fluctuations or forced perturbations could cause it to fall out of this state into the “true vacuum” ground state. A bubble of true vacuum would expand at the speed of light until it envelops the entire universe.

This would be rather catastrophic, to say the least. The laws of physics would change in a way unsuitable to life, or anything, really. A paper by the physicist Sidney Coleman had this to say:

The possibility that we are living in a false vacuum has never been a cheering one to contemplate. Vacuum decay is the ultimate ecological catastrophe; in the new vacuum there are new constants of nature; after vacuum decay, not only is life as we know it impossible, so is chemistry as we know it. However, one could always draw stoic comfort from the possibility that perhaps in the course of time the new vacuum would sustain, if not life as we know it, at least some structures capable of knowing joy. This possibility has now been eliminated.

Can we rule the possibility that the bubble catastrophically expanding at the speed of light and enveloping the entire universe may simply represent a lower energy state instead of the “true vacuum” ground state? I think Coleman’s hypothesis allows us to conceive both the situation in which the current ground state should last indefinitely even though it is not the “true vacuum” ground state on the one hand, and the situation where the “true vacuum” ground should be reached through a series of energy collapses. Anything can happen to a marble resting on a hillside, can’t it?

Well, I suppose that if the difference in energy levels were extremely small, the bubble might have a hard time growing. There might be some other factor like surface tension that causes any protobubbles to shrink down to nothing before they really get going.

Black holes have a “safety mechanism” along these lines; very small ones evaporate more quickly than they can draw in new matter, so it’s probably impossible for humans to create a small one that eats the Earth. Maybe there is a similar mechanism with vacuum collapse. A negative factor proportional to surface area would do the trick, since small objects have a very high surface area to volume ratio, and vacuum collapse proceeds because there is an energy associated with the volume of vacuum.

Very interesting. As a piece of scientific theory, does this rank up there as ‘wildly speculative’ like string theory, or ‘expected but hard to prove’ like gravity waves?

Yes, interesting implications indeed.

I was also wondering why is it that gravity waves propagate with the speed of light. I mean, what is the physical necessity, not the mathematical one, that makes these waves run as fast as electromagnetic radiation. I can only assume that there must be a general condition that effects both, but beyond that I got nothing. :confused:

In the most general sense, c is just a conversion factor between space and time. Relativity works on spacetime, allowing for conversions between spatial dimensions and the time dimension. A conversion factor is needed to go from meters to seconds, and c is that factor. Its existence is an artifact of our system of units, which for historical and practical reasons distinguishes between space units and time units, but the universe does not make the same distinction (that said, time is a different sort of dimension than space, but not in a way that matters here).

Everything–from waves to matter–moves through spacetime at c. As it happens, stuff with mass cannot move through spatial dimensions at quite c, which means that they move through time at some nonzero rate. Massless stuff like EM and gravity waves have no such restriction, and in fact must move through the space dimensions at c, which has the implication that from their perspective, time does not move at all.

:smack: Of course massless particles and waves travel at c. Thank you for you time and patience to explain this.

What makes a tsunami a tsunami is the shoreline. Out in the open it’s just a long, shallow wave with a lot of energy.

There is no “shoreline” for a gravity wave to hit and build up into a “tall” powerful crest.*

Other odd types of waves like solitons also require some sort of containment/limit/special medium on the wave to hold or alter its shape.

  • Although it makes me wonder when two black holes merge and the wave hits a very nearby third black hole. The effects on the accretion disk/event horizon area might be interesting.

EM waves and gravitational waves are analogous and the similarity goes beyond merely the propagation speed. There is an important distinction in that em waves travel through space, whereas gravitational waves are features of the spacetime metric.

The basic (if slightly hand-waving) reason why they travel at c is the same: they must be independent of the speed of their source and c is the only propagation speed where this is possible.

And then depression set in. . .

I’m not sure I’d classify it on that axis. There’s no particular reason to believe that we live in a false vacuum, but on the other hand the laws of physics don’t disallow it. So it’s a bit like speculation that there’s a huge comet set to impact Earth. It’s possible, but it seems unlikely. Unfortunately, unlike comets and other disasters, I don’t think we have any way of estimating the probability of vacuum collapse happening. The best we can say is that it hasn’t happened yet.

Well, almost. During the period of cosmic inflation–the time just after the Big Bang when the universe grew to be many, many times its original size–the universe was in a false vacuum state. After a while, it decayed either to the ground state or to a lower false vacuum state and inflation stopped (at least in our own little patch of the universe). One might hope that there was enough energy around at the time that if the vacuum was going to decay, it would go all the way down to the ground state, but I don’t think this has been proven.

I was thinking for a bit that resonance could help. I don’t know what the the Q factor for Earth is, but it might be pretty high (1000 or so?) since it’s a dense lump of solid matter. Gravity waves that match the resonance frequency of Earth could be amplified by roughly this factor.

However, I think in practice this doesn’t get you anything; the resonance frequency of Earth is far lower than the really energetic GW sources. You could have some orbiting back holes that give off waves in the frequency band, but the power would be extremely low. You only get the strong chirp when the black holes have nearly collided and are orbiting each other at many times per second.