Originally shared by John Baez
Three eyes is better than two
Until recently, gravitational waves had only been seen by the two LIGO detectors in Washington and Louisiana. But there are other detectors – and the best is Virgo, shown below. It’s near Pisa, in Italy. Laser light bounces back and forth through two very long pipes. When a gravitational wave zips past, it stretches and squashes space… affecting the time it takes for light to move through these pipes… so Virgo can detect it.
And it did! It happened on August 14th, but it was just announced 3 days ago.
This event was also seen by the two LIGO detectors. This is really great! With previous gravitational waves, we had trouble telling where they came from. But if you know how long it takes a wave to reach three different locations, you can be almost sure which direction it came from. A third detector also reduces the chance of mistaking random noise for a real event.
The first paper on this event only gives a few basic facts. It seems that two black holes of masses about 25 and 30 solar masses collided with each other in a distant galaxy about 1.6 billion light years away. We don’t know which galaxy, but we can pin it down to a much smaller region of the sky than we could with just LIGO.
The detector in Louisiana was the first to receive the signal. The detector in Washington saw it 8 milliseconds later, and Virgo saw it 14 milliseconds later. These differences in timing let us triangulate the source, giving a very accurate estimate of its location, with a 90% chance of it being in a region of of just 60 square degrees – 20 times more accurate than if we just had LIGO.
This signals the full arrival of gravitational wave astronomy. Science demands replication. Now different teams of scientists can detect gravitational waves and check each other’s work.
It’s been a long time coming! I’ve been hearing about Virgo for decades. It’s almost strange to hear it’s finally working. In 2011 they turned it off and spent 5 years improving it. Now it’s much more sensitive – not as good as LIGO, about 1/3 as sensitive, but still very good.
One cute fact: Virgo contains the biggest chunk of almost nothing in all of Europe!
The laser beams in Virgo bounce back and forth in two pipes 3 kilometers long and 1.2 meters in diameter. The air in these pipes has been sucked out very carefully, so changes in temperature and such don’t affect the light. So this is the largest ultra-high vacuum installation in Europe, with 6,800 cubic meters of space at one trillionth the ordinary air pressure!
I bet the only thing better on this planet is LIGO, where each installation has two pipes 4 kilometers long.
Here’s the paper. If you work on gravitational wave astronomy, it help to have a last name beginning with A:
B. P. Abbott, R. Abbott, T. D. Abbott, F. Acernese, K. Ackley, C. Adams, T. Adams, P. Addesso, R. X. Adhikari, V. B. Adya, C. Affeldt, M. Afrough, B. Agarwal, M. Agathos, K. Agatsuma, N. Aggarwal, O. D. Aguiar, L. Aiello, A. Ain, P. Ajith, B. Allen, G. Allen, A. Allocca, P. A. Altin, A. Amato, A. Ananyeva, S. B. Anderson, W. G. Anderson, S. V. Angelova, S. Antier, S. Appert, K. Arai, M. C. Araya, J. S. Areeda, N. Arnaud, K. G. Arun, S. Ascenzi, G. Ashton, M. Ast, S. M. Aston, P. Astone, D. V. Atallah, P. Aufmuth, C. Aulbert, K. AultONeal, C. Austin, A. Avila-Alvarez, S. Babak, P. Bacon, M. K. M. Bader, S. Bae, P. T. Baker, F. Baldaccini, G. Ballardin, S. W. Ballmer, S. Banagiri, J. C. Barayoga, S. E. Barclay, B. C. Barish, D. Barker and 1048 more authors, GW170814: A three-detector observation of gravitational waves from a binary black hole coalescence, Physical Review Letters 116 (September 25, 2018), 061102. Available at https://arxiv.org/abs/1709.09660.