# Vibrating black holes

"Every star in the universe vibrates or oscillates in some manner. Turbulence on the surface of the Sun causes well-known solar oscillations that are studied under the field of helioseismology. Cepheid variable stars pulsate with very regular periods and act as a valuable tool for measuring distances in the universe. Neutron star oscillations, studied under the field of asteroseismology, can be used to probe the interior of these compact objects. And even the extreme objects known as black holes vibrate when disturbed by an infalling object or a collision.

In fact, the oscillation properties of a black hole are surprisingly similar to those of a ringing bell or tuning fork. Just as a bell has a unique tone and timbre determined by its size and composition, each black hole rings with a unique fundamental pitch and duration that depends on its mass and rate of spin. These properties were described in 1973 by Saul Teukolsky who studied what would happen to a spinning black hole if it was perturbed from its stable configuration. He found that the black hole would oscillate with a collection of quasinormal modes, or characteristic frequencies, before eventually returning to a stable configuration."

How is this possible?

How can vibrating black holes emit gravitational waves? (sending information?)

• What do you mean "how is this possible?". Are you trying to ask How does gravity escape a black hole? Feb 20 '16 at 22:33
• How does the the gravitational waves escape a vibrating black hole? Feb 20 '16 at 22:37
• I mean, if the vibrating mass is inside the event horizon how could the waves escape? Feb 20 '16 at 22:49
• Maybe duplicate physics.stackexchange.com/questions/168503/… but how are the black hole vibrating? Feb 20 '16 at 23:10

There is a fairly simple answer: absolutely everything you interact with is from outside the event horizon. it is never the things inside (if any) that cause anything at all you see outside.

Why do you orbit a balck hole? Same reason you orbit a star. Back when it collapsed it left the spacetime outside itself curved in a particular way, and you respond to that curvature..

Even the thing you call the mass of the balck hole is referring to the kind of curvature you see outside the black hole. It is not the sum of the masses of the parts that make the star or black hole. And mass isn't the source of gravity anyway, and things without mass react to gravity anyway, so mass has nothing to do with it. When you have a kind of curvature that is bigger, you say it is a curvature of type $M$ with a larger $M.$

OK. So what kinds of things can you have? If you had a stationary (experts, please pardon the pun) spherically symmetric black hole, you might see the spacetime outside is curved in a way that doesn't change in time. But that means a moving person sees a curvature that is changing in time, it curves in a changing way around the star to look like it is not changing to the person moving with the star.

Super. So curvature around a star can change and be dragged around with it. A rotating star is basically a bunch of parts of star all moving in a spinning fashion. They drag spacetime around them. This dragging of spacetime around them is the "see it on the outside" version of them rotating.

When you had a non rotating star it left spacetime outside curved, and that outside curvature is why you orbited it. When it was rotating it left the spacetime outside curved and dragging around. And you react to that.

But since you too can curve spacetime and you too could drag spacetime you could try to eliminate the curvature and the dragging.

The dragging is easier to eliminate becasue you could just go any direction you want, same direction as the balck hole or the opposite direction. When you go the opposite direction you can eliminate some of that dragging.

Trying to eliminate the curvature entirely is harder. You and the star are both made of the same positive kind of energy so unlike the rotation where you could just go the opposite direction there isn't an easy way to just do the opposite.

Now let's get to waves. When you were moving around you were creating the kind of dragging and curvature that by itself would make more of that curvature in front of you but the parts behind you are a bit to one side and the parts in front of you are a bit to the other side and so you aren't cancelling ea H other perfectly. Some the curvature is always leaking out.

So the waves are constantly being emitted. They only get large as the parts squeeze a big change in themselves from their own interaction.