With each passing moment in the Universe, we’re constantly
stepping forward in time. Each successive instant gives way to the next, with
time continuously appearing to flow in the same direction — forward — without
fail. And yet, it isn’t particularly clear exactly why this is the case. Still,
if we look for it, we can find that a number of things also happen to always
move in the same direction, from moment-to-moment, exactly the way time does.
Objects move through the Universe proportional to their velocity. They change
their motion due to the effects of gravity and the other forces. On large
scales, the Universe expands. And everywhere we look, the entropy of the
Universe always goes up.
As the story of our cosmic evolution continues, we think all
of these things will continue: the laws of physics will still apply just as
they do today, dark energy’s presence ensures that the Universe will keep on
expanding, and entropy will keep increasing, as dictated by the laws of
thermodynamics. Many have speculated — although there is no proof — that the
arrow of thermodynamics and the arrow of time may be related. Still others have
speculated that dark energy might evolve over time, rather than being a
constant, leaving the door open to the possibility that it might someday
counteract and reverse our Universe’s expansion. What happens, then, if we put
these speculations together?
We’d wind up imagining that perhaps the Universe will cease
expanding, that it will instead begin collapsing, and that we’d have to then
ask whether this means that entropy could decrease and/or time could even start
running backward? It’s a mind-bending possibility, and one for the laws of
physics to answer. Let’s see what they have to say about it all!
One of the most important symmetries in all of physics is known as time-reversal symmetry. Put simply, it says that the laws of physics obey the same rules whether you run the clock forward or backward. There are many examples where one phenomenon, if you run the clock forward, corresponds to an equally valid phenomenon if you run the clock backward. For example:
- • A purely elastic collision, like two billiard balls colliding, would behave exactly the same if your ran the clock forward and backward, right down to the speed and angle that the balls will go off at.
- • A purely inelastic collision, where two objects smash into each other and stick together, is exactly the same as a purely inelastic explosion in reverse, where the energy absorbed or released by the materials is identical.
- • Gravitational interactions work the same forward and backward.
- • Electromagnetic interactions behave identically forward and backward in time.
- • Even the strong nuclear force, which binds atomic nuclei together, is identical forward and backward in time.
The lone exception, and the only known time where that
symmetry is violated, occurs in the weak nuclear interaction: the force
responsible for radioactive decays. If we ignore that outlier, the laws of
physics truly are the same regardless of whether time goes forward or backward.
What this means is that, if you wind up at any final state at any moment in time, there’s always a way to get back to your initial state if you just apply the right series of interactions in just the right order. The only exception is that, if your system is complex enough, you’d have to know things like the precise positions and momenta of your particle to a better accuracy than is quantum mechanically possible. Leaving the weak interactions and this subtle quantum rule aside, the laws of nature really are time-reversal invariant.
But this doesn’t appear to be the case for everything we
experience. Some phenomena clearly display an arrow of time, or a preference
for a particular one-way direction. If you grab an egg, break it, scramble it,
and cook it, that’s easy; you’ll never uncook, unscramble, and un-break an egg,
though, no matter how many times you try. If you push a glass off the shelf and
watch it shatter against the floor, you’ll never see those bits of glass rise
up and spontaneously reassemble themselves. For these examples, there clearly
is a preferred direction to things: an arrow in which things flow.
Admittedly, these are complex, macroscopic systems, experiencing an extremely intricate set of interactions. Nevertheless, the combination of all these interactions adds up to something important: what we know as the thermodynamic arrow of time. The laws of thermodynamics basically state that there are a finite number of ways that the particles in your system can be arranged, and the one(s) that have the maximum number of possible configurations — the one(s) in what we call thermodynamic equilibrium — are the ones that all systems will tend toward as time goes forward.
Your entropy, which is a measure of how statistically likely
or unlikely a particular configuration is (most likely = highest entropy; very
unlikely = low entropy), always rises over time. Only if you’re already in the
most likely, highest entropy configuration already will your entropy stay the
same over time; in any other state, your entropy will increase.
My favorite example is to imagine a room with a divider down
the middle: with one side full of hot gas particles and the other full of cold
gas particles. If you remove the divider, the two sides will mix and achieve
the same temperature everywhere. The time-reversed situation, where you take a
room of even temperature and stick a divider in the middle, spontaneously
getting a hot side and a cold side, is so statistically unlikely that, given
the finite age of the Universe, it never occurs.
But what could occur, if you were willing to manipulate these particles intricately enough, is you could pump enough energy into the system to separate the particles into hot and cold, relegating one side to containing all the hot particles and the other into containing all the cold ones. This idea was put forth some 150 years ago, and goes all the way back to the person who unified electricity and magnetism into what we now know as electromagnetism: James Clerk Maxwell. It’s known, in common parlance, as Maxwell’s demon.
Imagine that you have this room full of hot-and-cold
particles, and there is a central divider, but the particles are evenly
distributed on both sides. Only, there’s a demon controlling the divider.
Whenever a hot particle is going to smash against the divider from the “cold”
side, the demon opens a gate, letting the hot particle through. Similarly, the
demon also lets cold particles get through from the “hot” side. The demon has
to put energy into the system to make this happen, and if you consider the
demon to be part of the box/divider system, the total entropy still goes up.
However, for the box/divider alone, if you were to ignore the demon, you’d see
the entropy of just that box/divider system go down.
In other words, by manipulating the system appropriately from the outside, which always involves pumping energy from outside the system into the system itself, you can cause the entropy of this non-isolated system to artificially decrease.
The big question, before we even get to the Universe, is to
imagine that along with these hot and cold particles, there’s also a clock
inside the system. If you were inside the system, had no knowledge of the
demon, but saw the gate opening and closing rapidly in various
places — seemingly at random — and experienced one side of the room getting
hotter while the other got colder, what would you conclude?
Would it appear that time was running backward? Would the
hands on your watch start ticking backward instead of forward? Would it appear
to you that the flow of time had reversed itself?
We’ve never performed this experiment, but as far as we can
tell, the answer ought to be “no.” We have experienced conditions where
entropy:
- • increased rapidly,
- • increased slowly,
- • or remained the same,
both in systems on Earth and for the Universe as a whole,
and as far as we can tell, time continues to always march forward at the same
rate it always does: one second per second.
In other words, there is a perceived arrow of time, and there is a thermodynamic arrow of time, and they both always point in the forward direction. Is this causation? While some — notably Sean Carroll — speculate that they are linked in some fashion, we should remember that is pure speculation, and that no link has ever been uncovered or demonstrated. As far as we can tell, the thermodynamic arrow of time is a consequence of statistical mechanics, and is a property that emerged for many-body systems. (You might need at least three.) The perceived arrow of time, however, seems largely independent of anything entropy or thermodynamics may do.
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