In search of gravity waves

-February 19, 2013

When a charge accelerates, it either radiates or absorbs energy. In addition to changing speed, acceleration includes changing direction. X-rays, for example, are created at your dentist’s by accelerating a beam of electrons through 40,000 Volts, the electrons absorb that energy and their speed increases to about a third of the speed of light. That screaming electron beam is directed at a heavy element target like Tungsten. The heavy element is surrounded by a dense cloud of electrons. When the electrons hit that dense cloud of negative charge, they experience abrupt deceleration. The energy they lose in the process is radiated away as X-rays.

It’s the same story for the charges of every force in nature, so it must be for gravity. Where the charge of the electromagnetic force is “electric charge” (okay, that wasn’t very clever) and the charge of the strong nuclear force is the “color charge” (which is quite clever, since there are three different strong-force charges and three primary colors detectable by the human eye) carried by quarks and gluons, the gravitational “charge” is mass (who knew?).

Hence, when masses accelerate, they must emit gravitational radiation or “gravity waves.”

“Well,” you might say, “As the planets orbit the sun, they’re constantly changing direction and that is acceleration, so where’s this gravitational radiation?”

And my lame response is, “You’re right, they do emit gravity waves, but the gravitational radiation is too weak to detect.” Every experiment related to the particle physics of gravitation has to make the same excuse.

Of the four forces (strong nuclear, electromagnetic, weak nuclear, and gravity) gravity is the weakest by leaps and bounds (sorry about the pun, all the worse for being intended).

To generate observable gravity waves, you need a huge acceleration of two gargantuan masses. The collision of a pair of colliding black holes or neutron stars would be perfect!

“Well,” you might add, “I don’t have any neutron stars or black-holes handy.”

But yes you do!

The 1993 Nobel Prize was awarded to Hulse and Taylor for their discovery of a new type of pulsar that turned out to be a pair of orbiting neutron stars.

After modest-sized stars go supernova they cool, and as they cool, they collapse until all that’s left is a sphere of nuclear matter that weighs about as much as our sun, but has a diameter of about a dozen kilometers; a tremendous amount of mass—a.k.a., gravitational charge—in one place.

A pair of orbiting neutron stars can pack a huge gravitational punch. As they rotate around each other, they radiate away their gravitational energy, spiraling closer and closer until, when they collapse on to each other, the generate a burst of increasing frequency and energy gravitational radiation. It’s called “gravitational chirp,” because the sound waves of a bird chirping exhibit the same short-duration increase in amplitude and frequency, and lasts about two seconds.

Now, if Einstein was right—and he was right about pretty much everything except quantum mechanics—and if we have a sensitive enough detector, we should be able to detect the gravity waves generated by such systems.

Image: The spiral galaxy Caldwell 5, the two magenta spots are X-ray sources, probably black-holes (photo taken by the Nuclear Spectroscopic Telescope Array, NuStar, courtesy NASA)

When electromagnetic radiation hits a detector, like your eyeball, you see light. When a high intensity gravity wave passes through space, it warps the fabric of spacetime and in so doing changes the lengths of objects in its path. Detecting gravity waves amounts to observing those length variations. Since gravity waves are so weak, you need a device capable of measuring wicked-tiny changes in length; you need a  LIGO (the Laser Interferometer Gravitational-Wave Observatory).

Next time I’ll tell you how physicists at LIGO are trying to observe gravitational radiation.

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