Tuesday, 24 May 2016

Talk : Gravitational Waves I: From Newton's law to black holes and space-time ripples

Another guest post from Gav, who attended a BSA talk recently (with linkage from NSB).....

Back in February, a paper was released confirming that gravitational waves had been discovered at LIGO. Dr Thomas Sotiriou from the School of Mathematical Sciences and the School of Physics & Astronomy speaks at the British Science Association's Public Lecture Series on how we got from Newton to space-time ripples.

The signal that LIGO detected was from two black holes colliding a billion light years away. Each of the black holes was 30 times larger than our sun and the collision lasted for less than a second. 30 solar masses is equivalent to around 10,000,000 Earths (about 10 to the power of 32 kilograms) At the collision, the equivalent of 3 solar masses of energy was emitted - this is enough to power a billion Earths for a billion years. Gravitational waves travel at the speed of light so it took a billion years for the wave to reach LIGO.

Northern leg of the LIGO interferometer

Isaac Newton revolutionised physics - his laws of motion and his theory of gravity described how objects move in space. He also recognised that motion is a relative concept. So, while objects do not start spontaneously start moving, inertia also applies to things moving at a constant speed. Newton realised that acceleration was linked with force and forces tell bodies how to move in space. The key point though was the fact that his laws could be tested quantitatively.

James Maxwell came up with theories about light and magnetic fields. He believed that light was a wave while Newton had been convinced that it was a particle. Diffraction patterns verified Maxwell's theory but this behaviour meant that light contradicted Newton's laws of motion - it can propagate in a vacuum and it doesn't obey Galilean relativity.

Albert Einstein set out to solve these issues. He came up with a new relativity principle - The only way for two moving observers to measure the speed of light in the same way is if they are measuring time and space differently. It agreed with Newton's laws of motion for objects at low speeds and it agreed with Maxwell for objects close to the speed of light. He realised that different observers who move with respect to each other don't even agree on space and time - they can experience things at different times or in different places. Space and time are not fundamental entities so Einstein came up with space-time.

However, he wasn't happy with his special theory of relativity because it didn't agree with Newton's theory of gravity (which Newton had created to work with his laws of motion) So, Einstein chose the most drastic way to deal with this - he tossed gravitational forces away completely. Where Newton's forces steer bodies, Einstein realised that you don't need forces to steer. If mass could curve space and another body existed in that space then it would be forced to follow that curve. Curvature = matter distribution.

Dr Thomas Sotiriou

This was Einstein's general theory of relativity. Formulated in 1915, it was verified in 1916. It improved observations of planetary orbits. For example, it predicts the precession of Mercury which Newton did not. Newton also thought that light would not be effected by gravity but Einstein predicted that it would follow the curvature of space-time. This was verified during a solar eclipse when Arthur Eddington saw changes in the known locations of stars.

As bodies move, the curvature that they create in space-time also moves - this creates waves - any body moving in space-time will produce gravitational waves. It takes energy to produce these waves - this energy is lost to the system so objects move closer together. This effect is tiny but it does mean that planets in the solar system are moving closer together - no orbit is really stable. Gravitational waves in the solar system are too small to detect so we need to look at something else. The way to get a lot of gravity is to pack a lot of mass into a very small space.

Which leads us nicely onto the life cycle of stars. Through nuclear fusion, stars create heavier elements out of hydrogen. Up to the formation of iron, this always creates energy. As the energy increases the star wants to expand but gravity keeps it in check. However, once iron tries to fuse with iron, the reaction loses energy and then the star starts to collapse. If it becomes small enough, its density will prevent any further collapse and you're left with a white dwarf. Stars with more mass keep compacting until the density reaches such a level that electrons and neutrons combine. At this point one of two things happen: either it stops collapsing and you have a neutron star or it keeps collapsing and becomes a black hole.

Now we know that not even light can escape a black hole but what does that actually mean. Well, with a big enough sling you could throw stones at the moon ie there is a speed that you can fire something at to make it leave Earth. This is the escape velocity. With a black hole, not even something travelling at the speed of light, the speed limit of the universe, can escape. John Michell first proposed the idea of "dark stars" back in 1784. He even gave a way to detect them - since you can't see them, you must look for their effects.

While the LIGO announcement is huge news, a Nobel prize was awarded in 1993 for the discovery of a new type of pulsar. A pulsar is a neutron start with a very powerful magnetic field and it emits radiation down its magnetic axis. If you are in the right location, you can detect this radiation. If there is another star orbiting around the pulsar, the gravity of this second star will affect the emission of the pulsar's radiation. This gives an indirect way of detecting gravitational waves. Of course, the LIGO discovery trumps this as it is a direct observation.

What does the discovery of gravitational waves mean for the future? Well, it's like a new pair of eyes for us to look through at the universe. Rather than using electromagnetic telescopes, we can now use gravitation waves to experience the universe in a completely different way.

The next talk in the Public Lecture Series is by Dr Helvi Witek on June 16th at 6pm at the University of Nottingham and the topic is Gravitational Waves pt II - How are they detected? For more information, check out the website: http://www.nottingham.ac.uk/physics/outreach/science-public-lectures.aspx

Image sources
Dr Sotiriou via Gav

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