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  • 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 immediatelyfor 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.

Why does time only go in one direction?

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物理学者が考えていたように時間は逆行しないかもしれない、その理由はここにある (Time Might Not Reverse Like Physicists Thought, Here’s Why)

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    Summer に公開 2021 年 01 月 14 日
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