I read a short article in Discover magazine today, which can
be found here, about a new discovery that some physicists made. What exactly
happened was not very clear to me, and I think it would be confusing to the
public as well. So I thought I’d try to re-explain what I believe is going on,
using some relatable examples below. Hopefully I have the right idea now, and
you can appreciate this discovery a bit more!
To quote the article title: Physicists discovered, as
theoretically predicted, that time flows asymmetrically at the electron (very
tiny particle) level. But what does this mean? Well first, let’s review how we
perceive time. Time, to human beings, is a forward moving unchangeable entity.
Time passes for us everyday, and we can’t ever go backwards in time. If we were
to run time backwards, we should see ourselves re-experiencing everything that
happened that day, just in reverse. For example, if I video taped you driving
to work and walking into your office, then played the tape backwards, I should
see you walking backwards out of your office and driving home from work in reverse.
Therefore, for humans, time is symmetrical. What happens forward in time must
happen exactly the same way, but backwards, if time were played in reverse. So
what is this time asymmetry these physicists are talking about? Scientists at
the CERN particle accelerator have been smashing tiny particles together at
very high speeds, and watching what happens to them when they collide. They
know that when you smash two particles together, you should see the pieces that
the particles are made of as a result. Think of it like a car crash. You’re
driving down the road and you see two cars collide in a head on collision. When
these cars collide, the pieces of each car are strewn about the road. So when
you collide two things, you’re left with the bits and pieces of the inner
working of that object. So what does this have to do with time not being
symmetric? Ok, let’s run this car crash scenario again. You see a SUV and a
mini van crash on into each other. Time is flowing forward when the cars crash,
and lets assume both cars crash and all their constituent pieces are on the
road. Now, we can theoretically make time flow in reverse, by collecting all
the pieces of the cars and rebuilding them.
Putting the cars back together is similar to flowing time in reverse,
essentially arriving back at the pre-crash state with two unharmed and
functional vehicles. Now, you would expect that if I have all the pieces and
put both cars back together, I should end up with an SUV and a mini-van again.
This would be an example of time symmetry. But what if I put all the pieces of
the cars back together, and ended up with a small car and a truck, instead of a
SUV and a minivan? Al cars are made of (essentially) the same parts, so in
theory I could end up with two different cars than I started with. I’m
essentially running time in reverse, but I’m not arriving back at exactly the
same pre-accident state. The end result is two completely fixed vehicles, but
not the SUV and mini-van I started with. This is time asymmetry. Running time in
reverse does not get me back to my starting point. Physicists realized that if
they watch a specific particle, which likes to “change state” (you can think of
this like a coin, which can be heads up or tails up, depending on how the coin
is sitting), when it changes state (flips from head to tails) it does not
always change back to its previous state, if time is run in reverse (the coin
is flipped again).
They also realized that time has a preferential direction.
Let’s go back to our car crash scenario. The SUV and the mini-van crash. When
time is run forward, the crash occurs and the pieces for the mini-van and SUV
are on the road. When run in reverse (the cars are put back together),
sometimes you end up with a mini-van and SUV, sometime you end up with two
completely different cars. The fact that when time is run forwards, you almost
always see the SUV and mini-van collide, means that the laws of physics, as we
understand them, prefer time moving forward. Things don’t always “make sense”
when time runs backwards. So in this example, time preferentially moves
forward. It was unclear to me in the articles I read whether the scientists
findings showed forward or reverse time as preferential, but it’s fascinating
that they are not the same! Intuitively time running forward or backward should
yield the same result, but it does not! Now remember, this only happen in the
sub-atomic world. This doesn’t actually happen on car-size scales. But still,
this result changes how we perceive time at the smallest level, and ultimately
our understanding of the universe as a whole!
