Why does time only go in one direction?
Why don’t broken dishes unbreak?
Why can I remember the past, but not the future?
There’s nothing explicitly written in the laws of physics that says the rules change
based on whether time runs forward or backward.
So, to explain why the arrow of time points in one direction,
scientists have relied on statistical arguments based on increasing entropy.
Now though, researchers believe they have demonstrated that fundamentally
time cannot be turned backwards in all cases
because things don’t always happen the same way in reverse.
To show this, the researchers simulated the orbits of three black holes,
which is much more complicated than I made it sound just now.
Two objects in orbit around a central point without outside influence move in stable and predictable ways.
But add a third object to the mix and things start to go haywire.
This is what’s known as the three-body problem.
Outside of special cases,
there’s no one analytic solution to calculate what the orbits of three similar bodies look like.
Their orbits are chaotic,
and I mean that in the scientific sense where small differences in initial conditions
can lead to huge changes in the paths they take.
Still, we can use supercomputers to simulate three objects in orbit
by calculating each body’s trajectory over a very short amount of time,
checking how the forces on all the bodies have changed, and calculating their new trajectories again.
The program the researchers used to simulate their three orbiting black holes was Brutus,
I assume because it uses brute computational force to work
and not because it was involved in the murder of Julius Ceasar.
They started the three black holes off at rest and let gravity do its thing.
The black holes hypnotically danced around each other for tens of millions of simulated years
before one was finally ejected and the two remaining settled into stability.
Then the researchers attempted the same thing, but in reverse.
They took the ending conditions and just flipped the velocities, then ran it again.
Ideally, that should lead to a retracing of the same orbits, all the way back to their same initial positions.
But that’s not what happened, at least 5% of the time.
5% of the time, the black holes ended up in completely different orbits.
Not immediately—for the most part,
they followed paths that were indistinguishable from their forward-time counterparts.
But once the black holes deviated, even a little, it was done.
Millions of years down the line even tiny disturbances radically changed their orbits.
How tiny? Literally the tiniest amount possible.
In quantum mechanics there’s a concept called the Planck Length,
conceptually it’s the smallest meaningful measurement.
It’s derived using measurements that don’t change with relativity,
namely Planck’s constant, the gravitational constant and the speed of light.
Practically, it measures out to 1.616 x10-35 meters,
or about 20 orders of magnitude smaller than a proton.
Even if the black holes running backwards in time experienced a deviation that small,
eventually the black holes would change their orbits and the symmetry of time would break.
Of course, this is all being run on a computer, so maybe limitations in computing resulted in the altered orbits,
but the researchers ran the calculations with over a hundred decimal places,
so they say that suggests what they were seeing was a fundamental product of nature...
and Brutus wasn’t stabbing them in the back.
So, even though equations that describe physics make no distinction
between time moving forward or backward,
it seems that in some as yet undiscovered way, the universe does.
Technically, you could keep adding more bodies to the problem to make the orbits even more complicated.
And this is what’s known as the n-body problem.
Want to see more weird ways simulations can play with the universe?
Then check out Maren’s video on the universe in a box.
If you like this episode then subscribe to Seeker, cause we come back to black holes time and again.
Thanks so much for watching, and I’ll see you next on Seeker.