The idea that certain things, events or people can “cause” other things to happen plays
a huge role in human life.
We constantly desire to know “why” things happen, in science, love, sports, philosophy,
and so on.
But because the underlying laws of physics don’t care about the direction of time,
cause and effect don’t have the same meaning at a fundamental level.
It’s not that anything goes.
The basic constituents of the universe -- the particles and forces of modern physics -- behave
in predictable ways according to the laws of nature.
In principle, you can just as easily know their past paths as their future ones.
The current momentum and position of a particle determine its movement forward in the next
second, but they also determine how it was moving in the previous second.
Neither is really a “cause” preceding an “effect”, there’s just a pattern
that particles follow.
Kind of like how the integer after 42 is 43, and the integer before it is 41, but 42 doesn’t
“cause” 41 or 43 – there’s just a pattern traced out by those numbers.
At a fundamental, microscopic level, all we can say is that there are patterns between
events.
The macroscopic, human-scale concepts of cause and effect only emerge when you have larger
collections of particles, like humans!
As we know, time does have a direction for larger-scale systems, and we can indeed talk
about a spark causing oxygen and hydrogen to turn into water and an explosion.
“Spark plus oxygen plus hydrogen” and then “water plus explosion” is a sequence
the universe follows, and it only happens in one direction.
You never see a reverse explosion where water spontaneously splits into oxygen and hydrogen
gas and then at the very end emits a little spark.
One way of thinking about causes is that the tiny spark has great “leverage” over the
future.
If you hadn’t lit the spark, we wouldn’t have seen a giant explosion.
It doesn’t work the other way: removing or changing a tiny part of the giant explosion
doesn’t imply that there wasn’t a preceding spark.
When a small change to the present implies a big change to the future, the small thing
we’re changing is generally thought of as a “cause”.
Leverage can also go the other way.
Take this new pencil: the wood it’s made of contains trace amounts of radioactive carbon-14,
created by nuclear bomb testing.
If the pencil didn’t contain that carbon-14, that would imply that no nuclear bombs had
been detonated in the last 80 years; while if you removed a pencil-sized amount of one
of the atomic bombs, this pencil would still be basically the same.
In this case, the fact that the pencil has lots of carbon 14 means that it has a lot
of “leverage” over the past.
Instead of calling the carbon 14 in this pencil a “cause” of the earlier detonation of
hundreds of nuclear bombs, we call it a “record” of the bombs.
In general, when a small change to the present would imply a big change to the past, the
small thing we’re changing is thought of as a “record” or a “memory”.
So the distinctions between “cause” and “effect”, “records” and “predictions”,
aren’t fundamental to underlying physics – they only really make sense on the large
scale, because of the direction of time.