Friday, May 22, 2009

Gravitational Waves or the Hidden Whisperings of the Universe

Information is a seriously contemplated topic in the realm of theoretical physics. It's absolutely everywhere. When I look out my window here at my desk, light hits my eyes telling me there is a sparrow perched on the bush outside (I know I am incredibly lucky to have a desk with a window). All the leaves, the trunks of the trees, the blue sky, the grassy field beyond, the inidividual blades of grass, information about all these things are simultaneously being transmitted to me via electromagnetic radiation - light. It would seem light is the loudest kind of information we have. There is light everywhere in our lives. The next loudest would be sound. So long as we are surrounded by enough matter, we will hear sound. My computer is humming away, the air-conditioner is churning and every now and then, I hear a truck drive by. This is further information. Sound travels through the medium of matter. Massive objects have the ability to propagate sound. Light travels through the medium of the electromagnetic field. The electromagnetic field spans all space and tells charged particles how to behave. Changes in the electric field occur through this radiation. If a particle moves, a little signal is sent through the electromagnetic field as a light wave. Thus I see the leaves waving in the wind.

There are other more difficult kinds of information to notice. Radio waves and infrared signals are both electromagnetic radiation - just a different wavelength than light. There's many more classifications among those. Over the last century, we've gotten pretty goo at detecting them. We've even found this Cosmic Microwave Background radiation that seems to be telling us that the universe was once expanding in this incredibly hot fireball that grew at an amazing rate. There is, however, a kind of signal that is very hard to hear. It's almost the secret code that permeates the universe. We've actually not heard it before, but we have very good reason to believe it exists.

A lot of people have heard of Einstein. You might have heard of energy being mass times the speed of light squared? Well, that's a simplification of what he said in Special Relativity. The really important thing he said in Special Relativity is that information cannot travel faster than the speed of light (approx. 300,000 km/s). This is one of those fundamental laws of physics that every first year student wants to show is wrong and yet we never find a situation that upon careful examination does not comply with. The problem is, Einstein saw something that apparently did not comply with it..

As I said before, information is carried by light. It takes light about eight minutes to get from the Sun to the Earth. That means, if the Sun were to suddenly cease to exist, we would still have light for about eight minutes. This doesn't sound like a problem with the speed of information barrier. What is a problem is when you examine gravity. Over a hundred years ago, the prevailing theory of gravity is the same one we still teach in highschool physics today - Newton's law of gravity. It goes something like, the force of gravity between two objects is proportional to the masses multiplied together divided by the square of the distance in between. Look at what I just said carefully and you'll notice that I never mention time in that statement. What that suggests is that if the sun magically ceased to exist, we would instantaneously go flying off into space. We would still see the Sun, but we would know we were doomed because we wouldn't be orbiting it anymore.

Enter Einstein's General Theory of Relativity. The theory says that gravity is curvature. If you imagine a sphere with two people starting on the equator, imagine them walking north. Initially they are walking on parallel lines but the curvature of the sphere brings those two paths, always walking north, together. If you imagine north as the direction of time, you could then imagine how curvature brings two objects together. The obvious question is, what is being curved? My less obvious answer is space and time itself. Gravity is the curvature of space and time determining the direction objects move. Further, objects and their movements determine the shape of space and time. The next question is why does this fix our little problem with information? The answer to that is when an object changes in some way, that change is registered in the curvature of space-time and a ripple that propagates out from the object at the speed of light. Now, if the Sun were to disappear, we'd be in the dark figuratively speaking until we were in the dark litterally. It would take about eight minutes before we'd start feeling the lack of gravity from the Sun.

Einstein came up with this theory in the early twentieth century and we still haven't detected gravitational waves. We have good reason to believe this theory, however. If this theory is true, then gravity should divert light, which it does. In fact, gravitational lensing hasbeen used to get more information using our best telescopes. Effectively, large galaxies in between us and what we want to see bend the light from what we're looking at and refocus it like a lense does. Further, GPS systems make use of a correction term determined from General Relativity. Without the term, we could not have GPS guided airplanes. So we definitely think this theory has merit - we just haven't detected gravitational waves yet...

...but we're close. You see, gravitational waves are what's called a weakly interacting signal. That's physics jargon for gravity being weak compared to other forces. While we all know the feel of being stuck to the ground, the only reason that force is so immense is that there is so much matter to the Earth. We don't feel the electric pull so much because the Earth is mostly neutral. However, if you have ever been near where a lightning blot hits, I'm sure you recognize the emmense power of electricity. All this means that gravitational waves are very hard to hear. It takes very massive objects moving very quickly to create gravitational waves of the sort to produce a signal we could measure effectively. Then it takes some very careful equipment.

An interferometer is a device that fires a laser down one path and down another at a right angle. The beams bounce off a mirror back down the same paths. The beams are recombined and small differences in the length of the paths can be measured by the interaction. Well, a gravitational wave sends a little ripple in space and time itself. This can stretch one arm of an interferometer while shrinking the other. Remember, however, I said gravity is weakly interacting. I can't just do this expiriment on a table top. To detect gravitational waves, we're working on a project called LIGO or Laser Interferometric Gravitational-Wave Observatory. There are two locations for this project, one in Livingston, LA and the other in Hanford, WA. Just google LIGO to see some pictures. These are interferometers that fire lasers down four kilometer long vacuum chambers and are looking to detect a change in distance a thousandth the size of a proton. Now, I'm on the theoretical side of things so I can't accurately give you all the details on what they're doing but needless to say this is one of the most sophisticated scientific observing expiriments ever created.

What we expect to hear is the really amazing part. Perhaps the most likely source of gravitational waves to be detected by LIGO will come from orbiting binary black holes around thirty times the mass of the Sun. Black holes are objects so massive that the gravity doesn't even let light escape. We have only seen hints of black holes - places in the centers of galaxies where there doesn't appear to be anything except that other stars are orbiting those locations and pulsars which seem incredibly consistant with the idea that a black hole is powering them. This would be the first direct evidence of a black hole - and it could imply two of them. Think about it. These massive objects that can change the way galaxies behave orbiting togetehr and emitting powerful gravitational waves that due to the weak interaction, we have to go to extremes to see. It is as if some of the most fantastic information to be gathered about some of the most massive objects in the universe is hidden in a code that requires extreme effort to find. That is why this is titled, Gravitational Waves or the Hidden Whispering of the Universe.

1 comment:

  1. Sometimes equations look much scarier in words than they do in "normal" form (ie E=mc^2 looked intimidating the way you wrote it)

    And then Newton's equation... whoa.

    When will LIGO be up and running? Who/what is funding LIGO??