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Old 02-24-2012, 07:22 PM
 
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Good Question NightBazaar,

The silent loner is definitely a good discovery. I haven't read much on that subject, but I'm aware of the concept, and I'm not surprised that one would be detected due to gravitational lensing. There are loads of astronomical searches looking for lensing effects from 'invisible' objects of all kinds of scales, from galaxies to black holes to tiny objects out past the orbit of Pluto.

Unfortunately, gravitational lensing and gravitational waves are two very different things. Lensing comes from a static gravitational field so it's always there. I don't know how much you know about how light is generated, but gravitational waves are pretty similar. In light, the wave is generated by accelerating a charge back in forth. This us usually done when an electron feels an electric field, but there are other ways. If the electron is just moving along at a constant speed, nothing happens, so the electron has to feel a force to make the light wave and it has to change speed or change direction.

Gravitational waves are similar. If you want to make a gravitational wave, you have to accelerate mass, so it has change speed or change direction, which is why our silent loner won't make any gravitational waves while it's still a loner.

I should mention that gravity is a little different than light for a couple of reasons. The first is that gravity is a lot weaker than electromagnetism. This is why it's easy for us to detect light but very, very difficult to detect gravitational waves. It's also why you need really big and dense things to make gravitational waves, like black holes and neutron stars. The second reason is that there is only one charge of mass. With electromagnetism (light), you have positive charges and negative charges, so you can move a negative charge with a positive charge, and from a long way away, you can see that charges are moving, but you can't quite do that with gravity.

It's a little hard to explain, but in order to move an object, it has to push against another object. So if you and I are both standing on an icy surface, and I shove you, we both move. You might think, "hey, I'm a mass, you're a mass, we both change speed, that must mean that we're both making gravitational waves!" Unfortunately, since we're both the same charge (both positive mass), but we're moving in opposite directions, our waves cancel each other out! So you can't have waves like light (called dipole radiation) with gravity. Luckily, it still works for the next level of radiation, quadrapolar radiation.

You get quadrapolar radiation when the acceleration looks as though one direction is stretching and the other is shrinking. So if you take a hoola-hoop and you squeeze it so that you're trying to press two opposite sides together, those sides get closer, but the bits in between get farther apart. If you then stretched it out, the parts in between get closer. So if you were squeezing your two sides in and out, the whole thing would go: horizontal oval, circle, vertical oval, circle, and so on. Believe it or not, motion like this will give you gravitational waves.

You might think it's not that natural for things to move that way, but actually, it's not hard to think of some natural examples. The most obvious way is if something is ringing like a bell. Bells have a pretty strong quadrapolar component, and things like black holes and neutron stars can ring like bells too. Although it's theorized that they would only ring for a short time after formation, so that doesn't help our silent loner. Orbits are also quadrapolar. If you imagine looking down on two stars orbiting one another, you can see how they are always opposite each other, and some times they are oriented in one direction (North-South), and 1/4 orbit later, another way (East-West), which is just the kind of motion we're interested in. Finally, you can have something like a mountain on a neutron star, which is really just like an orbit, but with the two bodies touching.

Anyways, I hope that answers some questions and I haven't bored you to death. Basically, you need really massive things to orbit and/or collide for you to get detectable gravitational waves. Luckily, there are expected to be loads of sources like that, from orbiting neutron stars in our own galaxy, to colliding super-massive black holes in the centers of distant galaxies.
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Old 02-24-2012, 07:44 PM
 
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Scientists have been building detectors to find gravitational waves at least since the '60s. AFAIK, absolutely no unambiguous gravitational waves have been detected. I'm surprised by this. I figured that once we got LIGO, we would soon detect gravity waves.

I can imagine that setting up the interferometer would be challenging. When I was working towards my AAS in Lasers, I had to do experiments using interferometers (including a white light interferometer, that I was not able to get to work correctly). Those must be much easier to set up than something designed to detect gravitational waves.
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Old 02-28-2012, 05:35 PM
 
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Originally Posted by Pooua View Post
Scientists have been building detectors to find gravitational waves at least since the '60s. AFAIK, absolutely no unambiguous gravitational waves have been detected. I'm surprised by this. I figured that once we got LIGO, we would soon detect gravity waves.

I can imagine that setting up the interferometer would be challenging. When I was working towards my AAS in Lasers, I had to do experiments using interferometers (including a white light interferometer, that I was not able to get to work correctly). Those must be much easier to set up than something designed to detect gravitational waves.
You're right Pooua, it's really hard. It's really amazing the kind of work the hundreds of scientists and engineers in the LIGO Science Collaboration do. Even if we never detect gravitational waves, the development of the detector is an amazing achievement.

Depending who you ask, it may or may not be surprising that GWs haven't been detected yet. When you're trying to get money for the detector, you always tell the funding people that your detector will detect something, so there has been a lot of talk about the possibility of a LIGO detection, but for those who have done the development, LIGO has always been a prototype. With LIGO's sensitivity, we'd expect about one detection every 3 years, if we're lucky. Well, we haven't been lucky, and we only ran at design sensitivity for about 1.5 years. Now, LIGO's getting an upgrade, which is where the real detections are supposed to come from.

The truth is, we needed practice and better technology before we could build a sensitive enough interferometer. When the new detector comes online as Advanced LIGO in 2014, it should have a detection rate of at least one per month, and if it doesn't detect something, a lot of people will have a lot of explaining to do. Basically, a lot of physicists and astronomers have bet their careers on this, and if the advanced detectors don't detect anything, the GW community will basically die, and the theorists will have to work extra-hard to come up with a theory of relativity that doesn't make gravitational waves.
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