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What is the theory of relativity all about?

This is the century-old theory that has given us concepts like black holes and the big bang, writes Dr Cormac O’Raifeartaigh.

Dr Cormac O’Raifeartaigh Cormac O’Raifeartaigh (Cormac O’Rafferty) is an Irish physicist based at Waterford Institute of Technology.

Today marks the beginning of Science Week and for 100 years Einstein’s theory of relativity has been a pillar of modern physics. You’ve probably heard of it, but aren’t sure what the hell it means. Here is physicist Dr Cormac O’Raifeartaig’s super-painless guide to the theory that conquered the universe.

1. What’s the big deal?

2016 marks the centenary of one of the greatest breakthroughs in modern science, Einstein’s general theory of relativity. This is the theory that has given us concepts like black holes, time warps and the big bang.

2. Is it all just theory?

No, as the 20th century progressed, more and more evidence emerged in support of the theory, even its strangest predictions.

3. Can you give us a basic idea of relativity?

Students are always surprised to discover that there are actually two theories of relativity. In the first, the special theory of relativity, Einstein suggested that time and distance are not the same for all; an observer in motion relative to an object will measure the length of the object differently from an observer at rest relative to the object. Even stranger, the observer in motion will measure time differently from an observer at rest. The theory seemed strange at first, but experimental evidence soon emerged to support it.

shutterstock_173695178 E=mc2 changed our understanding of our world. Source: Shutterstock/Fernando Batista

4. And the second theory?

In 1916, Einstein published the general theory of relativity, a theory that predicted that space and time could be distorted by mass. In Einstein’s theory the sun doesn’t pull on the earth – the mass of the sun distorts the space around it, and the earth travels through that curved space. The theory also led to the prediction of strange phenomena such as black holes and the big bang. Strangest of all, time could be affected by gravity!

5. What is a black hole exactly?

In a black hole, a huge concentration of matter in a small amount of space warps the surrounding space so severely that the space curls back on itself so that nothing can escape – not even light.

6. Is there any evidence to support the general theory?

Yes. In 1919, British astronomers measured a bending of distant starlight by our sun, confirming a warping of space by the sun’s mass. Since then, many other effects have been measured, such as the discovery of a giant black hole in the centre of our galaxy.

7. Wasn’t there some big news on black holes recently?

Yes, last year saw the first detection of gravitational waves, a prediction of the theory. The waves are thought to emanate from the collision of two black holes a billion light years away.

8. I don’t suppose all this has any technological application?

Actually, GPS depends critically on general relativity. Because relativity predicts that gravity affects time, the satellite clocks have to be adjusted to take account of the fact that they are in a weaker gravitational field than earthbound clocks in order to the two sets of clocks. Without this correction, GPS would not work.

9. Where does the big bang come into the story?

General relativity predicts that our universe cannot be static, but must expand or contract. Astronomical evidence suggests that the universe has been expanding and cooling for billions of years, from an initial state that was much smaller and hotter – known as the big bang.

Galaxy Einstein connected the mass of an object with its energy and heralded a new world. Source: Shutterstock/PlanilAstro

10. Didn’t I hear that relativity has been disproven?

Newspapers love to publish ‘refutations’ of relativity from diverse commentators. Relativity scepticism is so common that most science journals and magazines have long since stopped accepting submissions on the topic. ‘Einstein wrong’ is a great story but amateur commentators usually ignore the vast body of supporting evidence for the theory.

11. What do you think causes scepticism about relativity?

Relativity scepticism is interesting because the theory is not in conflict with religious dogma, political worldviews or vested interests. I suspect the scepticism stems from the fact that relativity makes predictions that seem to conflict with our everyday ‘common sense’ experience of the world, while the supporting evidence is locked away in the rather inaccessible world of particle physics and cosmology. As Einstein himself once remarked, “Common sense is a collection of prejudices acquired by age 18.”

The Science Week talk on Einstein is on at 8pm tonight (November 14) in the Ed Burke Theatre in Trinity College Dublin and is open to the general public.

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About the author:

Dr Cormac O’Raifeartaigh  / Cormac O’Raifeartaigh (Cormac O’Rafferty) is an Irish physicist based at Waterford Institute of Technology.

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