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  • What is a gravitational wave?

  • It’s a ripple in the fabric of space and time.

  • Imagine that space is a giant sheet of rubber:

  • things that have mass cause that rubber sheet to bend, like a bowling ball on a trampoline.

  • The more mass, the more that space gets bent and distorted by gravity.

  • For example, the reason the earth goes around the sun is that the sun is very massive, causing

  • a big distortion of the space around it.

  • If you just try to move in a straight line around such a big distortion, you will find

  • yourself actually moving in a circle.

  • That’s how orbits work: there’s not an actual force pulling the planets around, just

  • a bending of the space.

  • Gravitational waves are produced whenever masses accelerate, changing the distortion

  • of space.

  • Everything with mass and/or energy can make gravitational waves.

  • If you and I started to dance around each other, we would also cause ripples in the

  • fabric of space and time.

  • But these would be extremely small. Practically undetectable.

  • Now gravity is very weak in the scale of the other forces in the Universe, so you need

  • something really, really massive moving very, very fast, to make the big ripples that we

  • can detect.

  • How would you observe a ripple in space?

  • If the space between you and me stretched or compressed, we wouldn’t notice it if

  • we had made marks on our metaphorical rubber sheet, for example, using equally spaced rocks.

  • Because these marks would also get stretched further apart.

  • But there is one ruler that doesn’t get stretched, one made using the speed of light.

  • If the space between two points gets stretched, then light will take longer to go from one

  • point to the other.

  • And if the space gets squeezed, light takes less time to cross the two points.

  • This is where the LIGO experiment comes in.

  • It has 4 kilometer long tunnels and uses lasers to measure the changes in the distance between

  • the ends of the tunnels.

  • When a gravitational wave comes through, it stretches space in one direction, and squeezes

  • space in the other direction.

  • By measuring the interference of the lasers as they bounce between the different points,

  • physicists can measure very precisely whether the space in between has stretched or compressed.

  • And the precision needed is incredible.

  • To detect a gravitational wave, you need to be able to tell when something changes in

  • length by a few parts in 10 to the 23.

  • It’s like being able to tell that a stick one sextillion meters long has shrunk by 5mm.

  • The effect of a Gravitational Wave is so minuscule and easily confused with random noise you

  • need a smart data analysis technique.

  • Scientists hope to identify the patterns of gravitational waves by comparing the wiggles

  • they measure in the experiment to the wiggles they expect from Gravitational Waves.

  • That’s like trying to identify a song being hummed at a noisy party. A very, very noisy

  • party.

  • Imagine that your whole life you had been deaf until one day your hearing was restored.

  • You’d be able to explore the Universe in this whole new way.

  • That’s why detecting gravitational waves is so significant.

  • It’s a completely new way of studying the Universe.

  • Anytime there's a new way to observe the Universe we discover things that we didn't expect.

  • It's really about looking for new things that we didn't know existed, examining the extreme

  • edges of our knowledge of physics and testing our theories about how the Universe works.

What is a gravitational wave?

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Gravitational Waves Explained

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    羅紹桀 posted on 2016/02/14
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