Tuesday, 28 March 2017

Talk - Gravitational Waves and Black Holes

Dr Thomas Sotiriou from the University of Nottingham recently gave a Café Sci (or Café Scientifique et Cultural to give it its full name) talk on Gravitational Waves and Black Holes - Einstein's Amazing Legacy. @Gav Squires was there and has kindly written this guest post summarising the event, with some linkage added by NSB.

Dr Sotiriou began by describing how scientific theories are replaced with better ones, starting with Newton's law of universal gravitation, which describes the gravitational forces between two bodies in terms of their masses and the distance between them - multiplied by a factor called the Gravitational Constant.

Newton's law of universal gravitation

Gravity doesn't just attract things, it determines how objects move in space. In Newton's time his theory unified how we understood gravity both on the scale of the solar system and also how it works on Earth. It was a theory that proved useful for 200 years. Then, in the 19th century, it was observed that the planet Mercury didn't really obey this theory. Its orbit was very slightly different to what was predicted but this didn't really concern anybody. At this time the outermost planet that had been discovered was Uranus and its orbit didn't match Newton's theory either. The observed orbit hinted that there was another planet that was affecting it - this is how Neptune was discovered. It was then thought that something similar must be happening to Mercury and so an innermost planet called Vulcan was predicted. In reality, it was Newton's theory that was incorrect.

Dr Thomas Sotiriou, with visual aid

Einstein was very interested in light (his Nobel prize was for the discovery of the photoelectric effect). At the start of the 20th century it was known that observers moving at different speeds who are measuring the speed of light get the same value. This is counter-intuitive - if you're running straight at the light then surely you'd get a different speed. Einstein knew that the only way to explain this was that people moving at different speeds must have a different view of time and distance. This was the basis for his theory of special relativity. Einstein realised that space and time were independent, this is where his idea of spacetime came from. To pin down an event you need to know where and when it happened. This revolutionised the way that people thought about physics. The theory of electromagnetism worked really well with special relativity but Newton's theory of gravity did not.

Special relativity only relates to observes moving at constant speed. Einstein knew that to say something about observers that were accelerating, he would have to say something about gravity. For 10 years, he tried to formulate a theory that included both accelerating observers and gravity. The result was his theory of general relativity and it explains how matter curves spacetime. If we know what the matter distribution is then we know how spacetime will curve and this curvature tells matter how to move. This theory accounted for the deviations in Mercury's orbit.

This didn't impress scientists, they wanted a prediction for some unknown thing that existed only in the theory. Einstein predicted the bending of light rays and a year after the theory was published Eddington proved it during an eclipse. This was not the only ground-breaking prediction. Special relativity told us that nothing could travel faster than light and general relativity showed that light is affected by gravity. If light feels this pull and has finite speed then if there was something of huge mass in a very small space not even light could escape its pull. This is what we call a black hole and was predicted by general relativity. People didn't believe it for a long time.

The day light was shown to be affected by gravity

What happens when objects move around in space time? When a boat moves in water in causes ripples - the same thing happens in spacetime and this is where we get gravitational waves. This emits energy and this loss of energy causes objects to get closer together. This is happening with the Earth and the sun but it would take billions of years for the Earth to plunge into the sun. These emissions of energy are very small but when you come to black holes, the gravitational waves are much larger. It took four decades to develop the technology required to detect gravitational waves. The LIGO detector discovered the gravitational waves caused by the collision of two black holes. The energy emitted from the collision was more than the energy from all of the stars in the universe at that moment. Even so, the movement that LIGO detected was the size of an atom over 4km.

The LIGO Black Hole collision

Unlike with Newton's theory, general relativity has nothing to do with mass or forces - this is why it works with photons. While we know that energy and mass are related (E=MC2) but we don't need mass to have energy - photons have kinetic energy. It is actually energy that causes the curvature of spacetime. The famous equation E=MC2 actually only relates to mass at rest.

General relativity is better than Newton's theory but could we eventually have an even better one? Are dark energy and dark matter the equivalent of the procession of Mercury for general relativity? General relativity isn't a quantum theory so it's possible that at some point we will get a new theory of gravity or maybe even a new theory of matter.

Café Sci returns to The Vat & Fiddle on the 13th of March at 8pm where Graham Harrison from the University of Nottingham will talk on Photobiology - Effects Of UV Radiation On Normal Skin. For more information check out the MeetUp site: https://www.meetup.com/nottingham-culture-cafe-sci/

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