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My identical twin went to space and now we’re not the same age!

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I’m a pure mathematician through and through – I love proofs, logic, algebra, number theory etc. But that doesn’t mean I can’t appreciate the beauty of some of Einstein’s discoveries.

Consider the ‘twin paradox’ in Einstein’s theory of Special Relativity. Special relativity looks at the relationship between space and time and how an object appears to be moving relative to an inertial frame i.e. relative to another observer. In its very simplest terms, the twin paradox goes like this. Say you have two identical twins on Earth (and ignore the seconds/minutes between their births, assume they are identical in age). Then send one twin into space in a rocket travelling at close to the speed of light and leave the other one on Earth. If the twin in the rocket travels for 2 years in their rocket, they are 2 years older. But…2 years in this rocket equates to 30 years on Earth, so the twin back on Earth has aged 30 years. Weird right?!

This can get a lot more complicated, and indeed as stated thus far doesn’t present a ‘paradox’. The paradox comes when you consider who the ‘inertial observer’ is. Is it the twin on Earth, or the twin in the rocket? Who can apply time dilation? And actually, is either of them in an inertial reference frame? What about gravity. For a good summary of everything involved in this, have a read of

https://www.einstein-online.info/en/spotlight/twins/

The crazy thing is, that when you consider concepts like ‘time dilation’ and ‘length contraction’, to the non-physicist it seems a bit bonkers, and you tend to think that it must just be theoretical musings. But the particle accelerator in Cern has shown just how right Einstein was. I don’t think there is yet a universal understanding of how all the elements of physics fit together, but looking at the individual elements certainly gives you some interesting examples.

There are other fun thought experiments you can do, such as the ‘ladder paradox’. This involves using special relativity to show that a ladder that is longer than a garage can nonetheless fit wholly into the garage when moving at close to the speed of light…it somehow gets shorter. Have a read of this to see more!

https://www.phys.unsw.edu.au/einsteinlight/jw/module4_pole_paradox.htm

You don’t need to understand relativity to see that it warps your sense of how the universe works!

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