Short post of the month February 2016 – gravitational waves

The subject of February’s short post of the month is gravitational waves. These were predicted by Albert Einstein back in 1916, and after decades of searching have finally been detected. In an announcement made on 11 February at a Washington DC press conference David Reitze, the executive director of the LIGO Laboratory, said:

“Ladies and gentlemen, we have detected gravitational waves. We did it!”

(Castelvecchi and Witze 2016). Later that day president Obama tweeted his congratulations to the team:

What are gravitational waves?

Gravitational waves are ripples in space time. As gravitational waves pass through an object they cause it to move slightly. The Universe is believed to be awash with gravitational waves, because when massive objects move, such as the Earth orbiting around the Sun, they emit gravitational radiation. However compared to other forms of radiation such as light and radio waves, gravitational waves are very very weak, which is why they have proved so difficult to detect.

Image from NASA

The gravitational waves detected were due to the one of most violent events in the Universe, the merger of two black holes, which were rapidly rotating around each other.  Even so, the signal was still incredibly weak

How were they detected?

In the LIGO facility a laser beam is split into two and travels down two 4km tunnels which are at right angles to each other. The two beams then reflect back and forth many times  between two mirrors before they are eventually recombined at an electronic light detector.

The apparatus is very finely tuned so that the waves from the two light beams, shown as  A and B in the diagram below, are completely out of phase with each other and as a result cancel each other out completely when they recombine giving no net signal at the light detector C.  Some of you may remember from your high school science lessons this is known as “destructive interference”.

When gravitational waves pass through the LIGO facility, the waves cause the tunnels to change their shape by a minute amount.  As a result of this, the distance travelled by each beam of light also changes very slightly, so that they are not completely out of phase and when the beams recombine they no longer completely cancel out. This produces a small signal at the detector.

On 14 September the same signal was found at the two separate LIGO detectors: Livingston in Louisiana first and Hanford in Washington State 7 milliseconds later. The fact that the patterns of the signals were the same and that there was a time delay between the two detections provided the proof that the signals were due to gravitational waves.

What does this mean for astronomy?

Up until now astronomers have only been able to see the Universe by detecting electromagnetic radiation through telescopes which work at different wavelengths, for example those of visible light, radio waves and x-rays. The instruments at LIGO allow astronomers to observe the Universe in a whole new dimension, in effect to “feel” the Universe vibrating.

That view was reinforced by Stephen Hawking, who in an interview for BBC News (2016) said

“Gravitational waves provide a completely new way at looking at the Universe. The ability to detect them has the potential to revolutionise astronomy. This discovery is the first detection of a black hole binary system and the first observation of black holes merging.”

References

BBC (2016) Einstein’s gravitational waves ‘seen’ from black holes, Available at:http://www.bbc.co.uk/news/science-environment-35524440 (Accessed: 23rd February 2016).

Castelvecchi, D. and Witze, A. (2016) Einstein’s gravitational waves found at last,Available at: http://www.nature.com/news/einstein-s-gravitational-waves-found-at-last-1.19361 (Accessed: 21st February 2016).