Question: How could you make a time machine?

Matthew Malek answered on 17 Mar 2014:

I think H.G. Wells knows something about this.

Actually, I have a very nice time machine. My partner bought it for me for Christmas. She was very thoughtful, it looks quite nice on my wrist! ;-D

As far as time travel goes, we can do it. I’m doing it right now, moving forward one second per second. Would you like to change the rate? That’s possible, too! According to Einstein’s theory of special relativity, moving clocks run slow. So you can dilate time by moving. Any motion at all will do the trick, though you have to go really fast for it to become noticeable. Try moving close to the speed of light for awhile, then turn around and come back. You’ll find that you have retained your youth, whilst your friends have all grown old!

Would you like to move backwards in time? Ah, but that’s a bit trickier. Probably impossible, even. When I was fifteen, I learned that I would never get the ability to travel backwards in time. I promised myself that if I ever did, I would come back to that very moment and let myself know. Then I waited for five minutes. No future-me ever turned up, so I knew that I would never travel backwards in time!

Even so, time reversal symmetry — the symmetry between going forwards and backwards in time — is very important in physics.

There are a few big symmetries that we look at. For instance “parity” (or “P”) is mirror symmetry. If it were true, it says that physics in a mirror-image universe is the same as in ours. We used to believe it was true, but this was disproved in the 1950s by my quantum mechanics teacher, CN Yang and a couple of others (TD Lee and Madame Wu). For this, Lee and Yang got the 1957 Nobel Physics prize.

Then there is also the symmetry between matter and anti-matter (called “charge symmetry” or “C”). This says that the laws of physics are the same for matter and anti-matter. We now know this is not true, either!

After “C” and “P” were disproved, people thought that maybe putting them together would work — giving us “CP” symmetry. This would say that matter in our universe behaves like anti-matter in a mirror universe. For awhile, people liked this idea of CP symmetry. However, in 1964, Jim Cronin (who I would work with decades later on the Pierre Auger Observatory) proved that CP symmetry is also broken, or violated. For this, he received the 1980 Nobel Physics prize.

I mentioned time reversal symmetry (or “T”) earlier. This says that the laws of physics are the same whether you are going forward or backward in time. We now believe that “T” is not a good symmetry by itself. For instance, entropy always increases — if you drop a glass goblet, it will shatter… but shards of glass will not spontaneously join together to form a goblet! But we do believe that time reversal symmetry works when joined to the other two (“charge” and “parity”) to make what we call “CPT symmetry”.

In effect, CPT says that the laws of physics for matter moving forward in time are the same as anti-matter moving backwards in time in a mirror universe!