(February 12, 2016 at 9:05 am)Excited Penguin Wrote: Brian, I think all you have to understand is how the interference pattern works, which is what I want to understand as well. This basic experiment with the laser beam I never understood even in highschool, not that I was trying, physics didn't interest me that much back then.

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.

(February 12, 2016 at 11:02 am)Aractus Wrote: The truly astonishing thing is not that a gravitational wave has been measured, but that Einstein worked out such counter-intuitive theories without the benefit of building on other's ideas (i.e. Newton's).

I'm pretty sure that Einstein built upon the ideas of his contemporaries, but your point is well-taken. His work was truly a milestone in physics.

It's pretty amazing in general that Einstein's theories have held up so well and that every prediction has been satisfied, if I am not too mistaken, making relativity pretty bombproof as far as theories go.

(February 12, 2016 at 11:02 am)Aractus Wrote: The truly astonishing thing is not that a gravitational wave has been measured, but that Einstein worked out such counter-intuitive theories without the benefit of building on other's ideas (i.e. Newton's).

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.

James Clerk Maxwell was an equal genius to Einstein. Einstein was in awe of him, and rightly so. His theories were inspired by Maxwell's equations on electromagnetism which also resulted in the speed of light. This is the guy who in the 19th Century mathematically figured out one summer at home that Saturn's rings were made up of small rocks, created the first colour photograph and laid the foundations for the modern world by figuring out electromagnetism. He died in his 40's.

I read both Maxwell's and Einstein's biographies to help better understand how they thought about things. It's a shame that many people have never heard of James Clerk Maxwell even though they recognise the names of Newton and Einstein.

(February 12, 2016 at 10:12 am)TheRealJoeFish Wrote:

(February 12, 2016 at 9:36 am)Brian37 Wrote: Just read about the "L", still not understanding it completely. They shoot lasers down both arms, and if one comes back slightly later, that is what detects the gravitational wave. That confirms what measurement is, the difference. But still don't understand the complete how.

Someone with more knowledge/smarts correct me if I'm wrong, but I'm'a try to come up with an interesting and informative analogy.

Imagine you're standing in the corner of a big box made of rubber.  (I know, I know, stay with me.)  You don't know exactly how big it is, so you take a ruler and, starting at your corner, mark off every foot on the wall with chalk.  You find out that the box is both 20 feet long and 20 feet wide, a perfect square, so there are 20 chalk marks on each wall.

You suspect that someone on the outside might be squeezing the box a little bit, distorting its shape by pressing or pulling on one of the walls.  You can't tell, though, because all you have to go on are the 20 chalk marks on the wall, and you're not fast or precise enough to measure if the box actually changes shape to say, 19 feet 11 inches by 20 feet 1 inch for just a second.

So, you devise a clever experiment.  Standing in your corner, you throw two tennis balls - one along the length (which you think is 20 feet) and one along the width (which you think is 20 feet).  You make sure the tennis balls are exactly the same, and you throw them at the exact same speed. 

Since they're both traveling at exactly the same speed, and presumably going the same distance, they should get back to you at exactly the same time.  However, one gets back to you a second later than the other one.  As far as you can tell, they both went exactly 40 feet (20 marks of chalk to the opposite wall, and 20 marks back).  However, the difference in time means that, indeed, someone must have been squeezing the box a little bit from the outside, making the one wall just a little farther than 20 feet away when the tennis ball hit it.


That was my best shot

(To clarify, the tennis balls are the lasers, throwing one along the width and one along the length is the "L" they're discussing, the room itself is... spacetime, the universe, etc, and the person pushing or pulling the room slightly... is gravity!)

I love you, man. You made it so easy for me to have a satisfactory understanding of this stuff.

The mathematics of general relativity is taken wholesale and with only minor modification from the works on differential geometry of CF Gauss and his pupil Bernhard Riemann, as well as some developments by Tullio Levi-Civita and Ricci . Einsteins genius was in developing a physical intuition what gravity should be like if it adheres to the equivalence principle, and then ask the mathematicians (in particular his old buddy M. Grossmann) what they have that fits his requirements. In the early phase, he got important physical inspiration from the ideas of Ernst Mach (the guy the speed is named after). In the final stages, he was almost beaten by David Hilbert, whi had a perfect grasp of the mathematics of his time and followed a more abstract approach to derive the field equation.


Of course he was based on Newton in the sense that certainly the very first thing he did with his theory ideas was to check whether they reproduced the Newtonian physics in the limit of weak gravity and low speeds.

(February 12, 2016 at 11:44 am)Mathilda Wrote: James Clerk Maxwell was an equal genius to Einstein. Einstein was in awe of him, and rightly so. His theories were inspired by Maxwell's equations on electromagnetism which also resulted in the speed of light. This is the guy who in the 19th Century mathematically figured out one summer at home that Saturn's rings were made up of small rocks, created the first colour photograph and laid the foundations for the modern world by figuring out electromagnetism. He died in his 40's.

I read both Maxwell's and Einstein's biographies to help better understand how they thought about things. It's a shame that many people have never heard of James Clerk Maxwell even though they recognise the names of Newton and Einstein.

I certainly knew of Maxwell but I didn't know he died in his 40s. I've read estimates that he was only months behind Einstein regarding special relativity. And Einstein's work on the photo-electric effect was definitely done while standing on Maxwell's shoulders. I doubt anyone but Einstein would have come up with general relativity in that era though. That was quite a conceptual leap and it blew his contemporaries away.

If Maxwell had trusted his own equations, he could have discovered special relativity...

(February 12, 2016 at 10:26 am)Alex K Wrote: The "how" goes like this in a simplified version: you arrange the laser light waves coming back from the two arms to exactly cancel at the point back in the middle where your light sensor sits - the peaks of one exactly meet the troughs of the other and no laser light hits the sensor. If one arm gets slightly shorter or longer, one of the waves will be shifted ever so slightly because it had to travel further, the peaks don't quite meet the troughs of the other returning laser wave and the cancellation will not be perfect any more->light reaches the sensor and you see a signal.

The mathematically complex part is to properly understand why grav waves change the length of the arms as measured by a light wave.

It seems to me like light behaves very strangely in this scenario. So I think I was right, I'd have to actually study physics to understand how this process works. I understood what you said but only at a very superficial level.

But also, it seems like you're saying the more important thing to understand is the mathematics of it all, not how light is tested in this way. Ok, I can get behind that. Thanks.

(February 12, 2016 at 11:12 am)Cthulhu Dreaming Wrote:

(February 12, 2016 at 9:05 am)Excited Penguin Wrote: Brian, I think all you have to understand is how the interference pattern works, which is what I want to understand as well. This basic experiment with the laser beam I never understood even in highschool, not that I was trying, physics didn't interest me that much back then.

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.