Jillian's Guide to Gravitational Waves : Sources

The shedding of energy in gravitational waves is much like the thing we call acceleration. Any mass that accelerates produces gravitational waves. Increasing speed and moving in an orbit are both types of acceleration, and there are quite a lot of examples of things that do both of these in space already. The reason the earth is not bathed in strong and quite painful gravitational radiation is due to the strength of gravity. Gravity is very weak on our scale compared to the other forces of the universe; for example it's quite easy for me to acquire a charge that equals and exceeds the gravitational attraction I have for an electron and push that electron away from me. However, on a much larger scale (like comparing stars and galaxies) gravity is the force to measure! Charges can cancel each other out leaving a very massive body with a very small net charge, but gravity does not behave like charge---it cannot cancel itself out. Gravity is cumulative and depends just on the amount of matter (or energy) present. Still, though, gravity is a subtle force even on the scale of suns and black holes. For this reason only the really massive accelerating bodies radiate gravitational waves, and even those produce signals that are insanely weak and extremely difficult to detect. What would produce these waves?

Accelerating masses: irregularities and inspiralling

Well, one would look for really big accelerating masses (otherwise known as stars). Standard stars like our sun don't really generate lots of gravitational waves. It takes a lot of matter compacted into a small area and moving around and high speeds to generate 'em.

Let's go back to physics. A collection of masses, such as two balls at each end of a stick, can be summed up in behavior like a single conglomerate mass at the center of gravity. If the two balls were equal in weight, the center of gravity would be in the center of the stick. Should one ball be heavier than the other, the center would be more towards the heavier ball. Imagine the center of mass of a binary with two neutron stars of roughly equal size. Yep, it's roughly in between the two stars. Now make the two neutron stars rotate about this point---that's the very simplistic model of a binary system. The total gravitational attraction of the system can be determined by this simple model, but a description of the actual gravitational field generated by these two stars is quite complicated. It's actually beyond my descriptive powers right now, but I thought it would be best to give you a rough idea of what a binary is and does.

The two neutron stars inspiral and orbit closer together.

I mentioned inspiralling, which is the gravitational wave generator for binary systems. What is it? It's when the two stars of the binary get closer and closer to one another and it's the norm. Everything that is gravitationally attracted to something inspirals to some degree---even if the movement over millions of years amounts to one inch. Inspiralling ties into something in physics called gravitational potential energy. The higher up in a gravitational field something is, the more potential energy it has. A jello mold five feet off the ground has more gravitational potential energy than a jello mold splattered on the ground. As something moves down into a gravitational field (down meaning in the direction of increasing gravity), it loses gravitational potential energy.

Irregularly-shaped mass as a neutron star binary system merges at the end of its lifetime.

As the stars gradually inspiral, they shed gravitational potential energy as gravitational waves. The two stars also star moving around each other faster and faster as they inspiral. As they get really close, such as towards the end of the binary's lifetime and just before the two stars coalesce into one blob, they radiate waves like crazy and are spinning around each other at sizable fractions of the speed of light. Yow!! I have a tough time wrapping my brain around something as dense as a neutron star (which is only a few kilometers in diameter) moving that fast. As the two stars coalesce, they're still radiating like crazy, even though they've technically lost all their gravitational potential energy. Why? I should explain some things about spheres and gravitational radiation.

Non-spherical Rotation with Regards to Supernovas

Well, another way of generating gravitational waves is by having a something that isn't perfectly spherical rotate rapidly. This happens in supernovas of massive stars, when the core of the star might become a neutron star or a black hole. Such supernovas (designated type 2) are such violent things that the core of the star (should it survive the process) deforms into something distinctly NON-spherical and rotating rapidly---thus generating gravitational waves. This stellar core remnant goes through a trimming down process in which the irregularities are thrown off or smooth down into a more spherical shape, generating more gravitational waves.

How and why does it radiate? Since the shape is non-spherical, there are some portions that are further away from the..well, the center of the mass for lack of a better description. Since those portions are further away from the center, they're higher up in the gravitational field of the mass. Since they're higher up, they've still got gravitational potential energy locked in them. Gradually, the imperfections are either thrown off due to an instability in the mass or they smooth down and radiate away their gravitational energy, and the resulting shape becomes more and more spherical. The refining process forms the remnant into a spherical shape and usually generates a loud burst of gravitational waves which then fade off. Supernovas aren't really understood too well.

When the two neutron stars coalesce, they form this distinctly NON-spherical blobby shape that's spinning at a fraction of the speed of light. Again, some parts are higher in the gravitational field than others, and gravitational radiation is shed as the combined lump of neutron star becomes more and more spherical. If we're really lucky, the coalesced form will be too big and form a black hole.

Binaries of two black holes would do much the same, except they're a lot more interesting and radiate a lot more gravitational waves. Why would this be? Neutron stars are typically a few kilometers in diameter. They can only get so close to each other (and therefore only go so fast) before they touch and being to coalesce. Black holes are smaller than neutron stars by far and can get much closer, spinning much faster, before they being to coalesce. The closer they get and the faster they spin around each other generates a stronger wave.

The difference between a neutron star binary and a black hole binary is in the coalescence stage. Instead of mushing lots of matter together like the neutron stars, the black holes merge their even horizons just like two soap bubbles joining. I'm not to clear on what the singularities do, but that's nothing new. Singularities are quite strange (<----an understatement).

Other Sources

It is very possible that certain events that occurred fractions of seconds just after the big bang (for those of us who support the theory!) would produce a background of gravitational waves just like the cosmic microwave background radiation detected by COBE. There are certain events in the big bang theory that would generate gravitational waves, things called phase transitions, when some important aspect of the nature of the universe changed. A good example of a phase shift (although not one that would generate a gravitational wave) is when the universe became translucent to light. See, before that the universe was such a hot soup of particles and energy that photons just got bounced around and could barely travel in a straight line. After the phase transition the universe had cooled enough to the point where light could, indeed, travel respectably enough to be called light rays. There are other more exotic proposed transitions, but I'm not too good with the details of the big bang or any other transitions (most of which probably scream quantum mechanics, which I don't understand yet).

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