Tuesday 10th May 12:08 pm
Anti-gravity dream may take off
Tuesday 22nd March 2016 12:40 pm
The genius of Albert Einstein led us to gravitational waves — maybe someday another genius will work out how to make them!
LIGO is the name of the device that actually detected the gravitational waves. LIGO looks at what happens when two laser beams at right angles cross and create a light pattern. These laser beams are each four kilometres long.
(There are actually two LIGOs, one in the north-west of the USA, the other in the south-east. By having two separate observatories, and noting the time when the gravitational wave hits them, you can get a rough idea of where in the sky it came from.)
A gravitational wave alternately shrinks, and then expands, the space-time continuum. So when it passes through LIGO, the four-kilometre length of the laser beam shrinks and then expands. This then changes the light pattern where the two beams cross — and bingo! you’ve picked up your gravitational waves. The change in distance along that four-kilometre arm is tiny — around one-billionth the size of an atom.
Of course, building a device to do that kind of measurement wasn’t that easy. With typical honesty, the scientists said that the detection of gravitational waves was “possible” with the first version of LIGO, but “probable” with the later advanced version — and that’s exactly how it panned out.
It took decades to get the money — and decades to build it. The laser beams had to run through the largest vacuum ever constructed — some 10,000 cubic metres of “nothing”.
The laser beams bounced off mirrors at the ends of the four-kilometre-long tunnels — and these mirrors had to be isolated from vibrations. By using noise-damping suspensions and active feedback systems, the Louisiana LIGO could tune out, not just a nearby door slamming, but the impact of waves in the Gulf of Mexico hitting the beach, a hundred kilometres away. That’s absolutely mind-blowing stuff.
A new branch of astronomy
Now here’s something surprising. By detecting gravitational waves, we have invented a whole brand-new branch of astronomy — the third one so far.
The first branch is electromagnetic astronomy. It looks at electromagnetic waves that travel on top of the space-time fabric of the universe. It’s been running for millennia, beginning with naked-eye astronomy in the ancient world (Egyptians, Mesopotamians, Chinese, and so on). It really took off about four centuries ago, with Galileo and the telescope — but using visible light only.
Centuries later, we arched into other frequencies in the electromagnetic spectrum, such as radio waves in the 1930s, and more recently, infra-red, ultraviolet, gamma ray, X-ray, microwave and so on. All these other frequencies are still part of the electromagnetic spectrum.
The second branch is neutrino astronomy. Neutrinos were discovered only in 1956. They can be created by certain nuclear reactions, such as happen in the Sun. Every second, about 65 billion solar neutrinos pass through each square centimetre of your body. But thanks to their tiny mass and other factors, they don’t interact with your body — they just pass through. To have a 50 per cent chance of catching a neutrino, you need a few light years of lead. To put that in perspective, you need a few millimetres of lead to stop X-rays.
As an example of how hard neutrinos are to catch, back in 1987, a nearby star exploded into a supernova. It emitted trillions upon trillions of neutrinos. But down here on planet Earth, we detected just two dozen of them with our three early neutrino telescopes.
Today, the IceCube Neutrino telescope down at the South Pole is the most advanced, and should be joined by another two advanced neutrino telescopes by 2017. But, just like electromagnetic radiation, neutrinos travel across the space-time fabric of the universe.
This third, brand-new gravitational wave astronomy is very different. It doesn’t look at stuff travelling on top of the space-time continuum. No, it looks at actual travelling distortions in the fabric of space-time itself. But the two current LIGO observatories are sensitive only to frequencies roughly between 10 cycles per second and a few hundred cycles per second. With further improvements in sensitivity, it could pick up rotating neutron stars, or exploding supernovas.
But there’s a whole range of frequencies that LIGO is not tuned to (in the same way that an optical telescope does not pick up X-rays). With different gravitational wave telescopes, we could pick compact objects captured by supermassive black holes in the centres of galaxies, or a pair of supermassive black holes orbiting each other.
The braininess of Albert Einstein was truly impressive. Time after time, he went out on an intellectual limb. Not only did he lay down theoretical frameworks that gave us useful gadgets such as the laser, GPS and solar panels, he also made many predictions that were later proven correct — the most recent one being gravitational waves.
Now I’m going way out on a limb here. We have just learnt how to detect gravitational waves. Maybe somebody living today will give us the framework for being able to make gravitational waves — and perhaps the anti-gravity dream may finally get off the ground …
This blog first appeared on Dr Karl's Great Moments in Science
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