Brace yourselves for gravitational waves

[TheRealJoeFish]

Yes. What I don't understand is how the beams cancel each other out and how the distance they have to travel has any bearing on whether they cancel each other out or not at that ridiculous angle they meet at to begin with. But I think that just has to do with my lack of understanding of how light waves behave, and so on.

A little out of left field, but the pop-sci book In Search of Schroedinger's Cat by John Gribbin has a fantastic discussion of this, if my memory serves me correctly.

[Excited Penguin]

Yes. What I don't understand is how the beams cancel each other out and how the distance they have to travel has any bearing on whether they cancel each other out or not at that ridiculous angle they meet at to begin with. But I think that just has to do with my lack of understanding of how light waves behave, and so on.

A little out of left field, but the pop-sci book In Search of Schroedinger's Cat by John Gribbin has a fantastic discussion of this, if my memory serves me correctly.

I'll check it out, thanks.

I'm open to similar suggestions from anyone else as well.

[Brian37]

Maybe this description would make it easier for laypeople to understand. The ripple in space time can viewed like if you suddenly dropped a bowling ball onto a water bed, the sudden disturbance will create waves rippling outwards. The rubber box example with tennis balls listed here is one way. I just thought maybe if you view the photons as boats shot into the path of the waves no wave affects the speeding boats, like flat water, the boats come back at the same time. But if a wave affects one but not the other, the one boat will come back slightly slower, and the difference between the time it takes to get back is how we know a wave is affecting the boats(photons).

Really no different than knowing wind affects objects in it's path. Like a place kicker in the NFL tries for 3 points. No wind the ball goes right down the center. Lots of wind the ball gets blown off course. In the case of the photons the wave is bending the path of the photon causing it to come back slower than the other. The shortest distance between two points is a straight line, bend that line, it takes more time to get get back to the original point.

Hopefully my layperson's example helps rather than hurts.

[Excited Penguin]

I always understood that part(more or less, your example helped me think about it in a yet another way, though, so thanks). What I don't understand is the purpose of the odd positioning of the paths the two beams of light are supposed to travel and how can they possibly cancel each other out if they meet at a right angle, rather than a flat one.

[Brian37]

I always understood that part(more or less, your example helped me think about it in a yet another way, though, so thanks). What I don't understand is the purpose of the odd positioning of the paths the two beams of light are supposed to travel and how can they possibly cancel each other out if they meet at a right angle, rather than a flat one.

God did it.

[Alex K]

I always understood that part(more or less, your example helped me think about it in a yet another way, though, so thanks). What I don't understand is the purpose of the odd positioning of the paths the two beams of light are supposed to travel and how can they possibly cancel each other out if they meet at a right angle, rather than a flat one.

Oh, very good question! They indeed wouldn't cancel if they just met at a right angle - they come in at a right angle but hit something like a semipermeable mirror, and so on the other side of that you have one direction where both returning waves are added (or subtracted depending on design) and *in parallel* again, and that's where you see the cancellation (where it says detector in the pic)



Note the half arrows showing the directions of the beams

[Thumpalumpacus]

I am guessing. The fabric of space time wouldn't seem to me different than the motion of a water buoy in the ocean bobbing up and down left and right because of waves.

Meaduring lengths in relativity is a subtle thing because there is so much freedom to choose coordinates. The simplest way to understand it is that you have two freely floating test masses at a distance which get hit by a gravity wave. The wave changes the distance measure between the masses just as cosmic expansion increased the distance between galaxies.

... and because the Hubble Constant is a known factor, they can discriminate the additional motion and adduce it to gravitational waves?

[Thumpalumpacus]

Penguin - the idea here is that you have two signals (lasers) which, due to alignment, are 180 degrees out of phase with one another.   When you combine the two signals, they cancel each other out.  If you slightly modify the distance the beams have to travel, the signals are no longer out of phase and therefore not completely canceling out, making the perturbation apparent.

ETA:  I see that Alex and TRJF have provided explanations already.  Another doesn't hurt.  

Yes. What I don't understand is how the beams cancel each other out and how the distance they have to travel has any bearing on whether they cancel each other out or not at that ridiculous angle they meet at to begin with. But I think that just has to do with my lack of understanding of how light waves behave, and so on.

Think of it this way: in a pond, you drop two rocks, exactly the same weight, at exactly the same time. They each set up a ripple of their own waves, and at a certain point, those waves are identically strong, but headed in the opposite direction.

If the waves contact each other exactly half the distance between the rock-drops, they will be exactly as strong, and counter each other out. Any discrepancy beyond that may be adduced to the influence of another body.

[Alex K]

Meaduring lengths in relativity is a subtle thing because there is so much freedom to choose coordinates. The simplest way to understand it is that you have two freely floating test masses at a distance which get hit by a gravity wave. The wave changes the distance measure between the masses just as cosmic expansion increased the distance between galaxies.

... and because the Hubble Constant is a known factor, they can discriminate the additional motion and adduce it to gravitational waves?

Oh, I just meant that it seems to be a similar physical effect, but that's really a great question.
So I'd say even if the hubble constant were large enough to be seen in detectors (and I doubt that but with those astonishing sensitivities I have to check again) it would maybe result in a constant outward force on the mirrors which would shift the equilibrium point of the mirror suspension by a tiny amount... but I really have to check again. The effect is probably totally eclipsed by tidal forces etc, since the earth is a big mass itself and we are not at the center of mass, and it is rotating and moving around the sun etc. A good homework question for a cosmology lecture

[Aractus]

Says you! I'd only agree with this partially because of my species notoriously flawed perceptions, and you cant get to Einstein's level or Hawkings level without that skepticism. I think both are amazing. 

But I fail to see how he didn't build on Newton, he certainly thought there was more to it than Newton, but to get further he certainly had to learn what Newton did. QM doesn't negate Newton, it is just that you cant use classical physics at the QM scale. Just like we still have bicycles and still make them, but you cant apply a bicycle tire to a Lamborghini. Science always builds upon the past. Einstein simply made a correction in our perceptions. No Newton no Einstein to update Newton.

No, says everyone.

It isn't a continuation of classical physics. Quantum mechanics is a continuation of classical physics (although considered to be beyond classical physics in its own right), and to some degree special relativity could also be described as a continuation of classical physics also. General relativity, however, is not. It is a brand new theory. Einstein realised he couldn't continue developing his theory along the same path as Newtonian mechanics and special relativity. It can't be simplified into anything that came before it, it can't be reconciled with quantum mechanics either. And the reason is because Einstein removed gravity as a force in GR. In classical mechanics, gravity is a force. But in general relativity it is a result of the distortion of space-time.

Without this description of gravity, quasars and black holes would not be possible, and neither would a "big bang". And there is little to no evidence that anyone else in the past 100 years would have or could have come up with the theory, or one like it.

What we now know, and there's almost no uncertainty left about this, is that General Relativity is correct. Okay sure, you can't prove a scientific law, let alone a scientific theory, but consider this. All experimental evidence has so far proven the predictions of GR right. And these were made way back when Einstein did not have the capacity to make calculations on measurements that would differ significantly from Newtonian mechanics. There's a huge difference between NM and GR. NM simply points out interesting features of the universe, and provides a way to calculate those features. GR however doesn't point out the interesting features, but asks the question of "how does the universe fundamentally behave and function" and from there it poses solutions to the question "what features might I expect". One is based on replicating real-world data, the other was designed to go beyond it - and it has. It's gone way beyond the data available to Einstein in the early 20th century.