Why is it you can break an egg, but not make the pieces spring back together again? To find out, we have to go back to the birth of the universe
There's egg on your face, literally. You tried to juggle some
eggs, it all went wrong, and now you've got to shower and change your
clothes.
Wouldn't it be faster to just un-break the egg? Breaking it only
took a few seconds, so why not do that again, but in reverse? Just
reassemble the shell and throw the yolk and the white back inside.
You'd have a clean face, clean clothes, and no yolk in your hair,
like nothing ever happened.
Sounds ridiculous — but why? Why, exactly, is it impossible to
un-break an egg?
It isn't. There's no fundamental law of nature that prevents us
from un-breaking eggs. In fact, physics says that any event in our
day-to-day lives could happen in reverse, at any time. So why can't
we un-break eggs, or un-burn matches, or even un-sprain an ankle? Why
don't things happen in reverse all the time? Why does the future look
different from the past at all?
It sounds like a simple question. But to answer it, we've got to
go back to the birth of the universe, down to the atomic realm, and
out to the frontiers of physics.
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Isaac Newton
Like many stories about physics, this one starts with Isaac
Newton. In 1666, an outbreak of bubonic plague forced him to leave
the University of Cambridge, and move back in with his mother in the
Lincolnshire countryside. Bored and isolated, Newton threw himself
into the study of physics.
He came up with three laws of motion, including the famous maxim
that every action has an equal and opposite reaction. He also devised
an explanation of how gravity works.
Newton's laws are astonishingly successful at describing the
world. They explain why apples fall from trees and why the Earth
orbits the Sun. But they have an odd feature: they work just as well
backwards as forwards. If an egg can break, then Newton's laws say it
can un-break.
This is obviously wrong, but nearly every theory that physicists
have discovered since Newton has the same problem. The laws of
physics simply don't care whether time runs forwards or backwards,
any more than they care about whether you're left-handed or
right-handed.
But we certainly do. In our experience, time has an arrow, always
pointing into the future. "You might mix up east and west, but
you would not mix up yesterday and tomorrow," says Sean
Carroll, a physicist at the California Institute of Technology in
Pasadena. "But the fundamental laws of physics don't distinguish
between past and future."
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Ludwig Boltzmann
The first person to seriously tackle this problem was an Austrian
physicist named Ludwig Boltzmann, who lived in the late 19th century.
At this time, many ideas that are now known to be true were still up
for debate. In particular, physicists were not convinced – as they
are today - that everything is made up of tiny particles called
atoms. The idea of atoms, according to many physicists, was simply
impossible to test.
Boltzmann was convinced that atoms really did exist. So he set out
to use this idea to explain all sorts of everyday stuff, such as the
glow of a fire, how our lungs work, and why blowing on tea cools it
down. He thought he could make sense of all these things using the
concept of atoms.
A few physicists were impressed with Boltzmann's work, but most
dismissed it. Before long he was ostracised by the physics community
for his ideas.
He got into particularly hot water because of his ideas about the
nature of heat. This may not sound like it has much to do with the
nature of time, but Boltzmann would show that the two things were
closely linked.
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Fire only makes sense if it's made up of atoms
At the time, physicists had come up with a theory called
thermodynamics, which describes how heat behaves. For instance,
thermodynamics describes how a refrigerator can keep food cold on a
hot day.
Boltzmann's opponents thought that heat couldn't be explained in
terms of anything else. They said that heat was just heat.
Boltzmann set out to prove them wrong. He thought heat was caused
by the random motion of atoms, and that all of thermodynamics could
be explained in those terms. He was absolutely right, but he would
spend the rest of his life struggling to convince others.
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Put ice cubes into water, and they will surely melt
Boltzmann started by trying to explain something strange:
"entropy". According to thermodynamics, every object in the
world has a certain amount of entropy associated with it, and
whenever anything happens to it, the amount of entropy increases. For
instance, if you put ice cubes into a glass of water and let them
melt, the entropy inside the glass goes up.
Rising entropy is unlike anything else in physics: a process that
has to go in one direction. But nobody knew why entropy always
increased.
Once again, Boltzmann's colleagues argued that it wasn't possible
to explain why entropy always went up. It just did. And again,
Boltzmann was unsatisfied, and went searching for a deeper meaning.
The result was a radical new understanding of entropy — a discovery
so important that he had it engraved on his tombstone.
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Ludwig Boltzmann's tombstone, complete with entropy equation |
Boltzmann found that entropy measured the number of ways atoms,
and the energy they carry, can be arranged. When entropy increases,
it's because the atoms are getting more jumbled up.
According to Boltzmann, this is why ice melts in water. When water
is liquid, there are far more ways for the water molecules to arrange
themselves, and far more ways for the heat energy to be shared among
those molecules, than when the water is solid. There are simply so
many ways for the ice to melt, and relatively few ways for it to stay
solid, that it's overwhelmingly likely the ice will eventually melt.
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You cannot un-break an egg
Similarly, if you put a drop of cream into your coffee, the cream
will spread throughout the entire cup, because that's a state of
higher entropy. There are more ways to arrange the bits of cream
throughout your coffee than there are for the cream to remain in one
small region.
Entropy, according to Boltzmann, is about what's probable. Objects
with low entropy are tidy, and therefore unlikely to exist.
High-entropy objects are untidy, which makes them likely to exist.
Entropy always increases, because it's much easier for things to be
untidy.
That may sound a bit depressing, at least if you like your home to
be well-organised. But Boltzmann's ideas about entropy do have an
upside: they seem to explain the arrow of time.
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Is time built into the universo?
Boltzmann's take on entropy explains why it always increases. That
in turn suggests why we always experience time moving forwards. If
the universe as a whole moves from low entropy to high entropy, then
we should never see events go in reverse.
We won't see eggs un-break, because there are lots of ways to
arrange the pieces of an egg, and nearly all of them lead to a broken
egg rather than an intact one. Similarly, ice won't un-melt, matches
won't un-burn, and ankles won't un-sprain.
Boltzmann's definition of entropy even explains why we can
remember the past but not the future. Imagine the opposite: that you
have a memory of an event, then the event happens, and then the
memory disappears. The odds of that happening to your brain are very
low.
According to Boltzmann, the future looks different from the past
simply because entropy increases. But his pesky opponents pointed out
a flaw in his reasoning.
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Once done, this cannot be undone |
Boltzmann said that entropy increases as you go into the future,
because of the probabilities that govern the behaviour of small
objects like atoms. But those small objects are themselves obeying
the fundamental laws of physics, which don't draw a distinction
between the past and the future.
So Boltzmann's argument can be turned on its head. If you can
argue that entropy should increase as you go into the future, you can
also argue that entropy should increase as you go into the past.
Boltzmann thought that, because broken eggs are more likely than
intact ones, it was reasonable to expect intact eggs to turn into
broken ones. But there's another interpretation. Intact eggs are
unlikely and rare, so eggs must spend most of their time broken, very
occasionally leaping together to become intact for a moment before
breaking again.
In short, you can use Boltzmann's ideas about entropy to argue
that the future and the past should look similar. That's not what we
see, so we're back to square one. Why is there an arrow of time at
all?
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Messy universo
Boltzmann suggested several solutions to this problem. The one
that worked best came to be known as the past hypothesis. It's very
simple: at some point in the distant past, the universe was in a
low-entropy state.
If that's true, then the flaw in Boltzmann's reasoning disappears.
The future and the past look very different, because the past has
much lower entropy than the future. So eggs break, but they don't
un-break.
This is neat, but it raises a whole new question: why is the past
hypothesis true? Low entropy is unlikely, so why was the entropy of
the universe in such a remarkable state sometime in the distant past?
Boltzmann never managed to crack that one. A manic-depressive
whose ideas had been rejected by much of the physics community, he
felt sure that his life's work would be forgotten. On a family
holiday near Trieste in 1906, Ludwig Boltzmann hanged himself.
His suicide was particularly tragic since, within a decade,
physicists accepted his ideas about atoms. What's more, in the
decades that followed, new discoveries suggested that there might be
an explanation for the past hypothesis after all.
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We now know the universe is about 14 billion years old
In the twentieth century, our picture of the universe changed
radically. We discovered that it had a beginning.
In Boltzmann's time, most physicists believed that the universe
was eternal – it had always existed. But in the 1920s, astronomers
discovered that galaxies are flying apart. The universe, they
realised, is expanding. That means everything was once close
together.
Over the next few decades, physicists came to agree that the
universe began as an incredibly hot, dense speck. This quickly
expanded and cooled, forming everything that now exists. This fast
expansion from a tiny hot universe is called the Big Bang.
This seemed to support the past hypothesis. "People said
'okay, the trick is clearly that the early universe had low
entropy,'" says Carroll. "But why [entropy] was ever low in
the first place, 14 billion years ago near the Big Bang, is something
we don't know the answer to."
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Large clouds of gas condense into stars and planets
It's fair to say that an enormous cosmic explosion doesn't sound
like something with low entropy. After all, explosions are messy.
There are plenty of ways of rearranging the matter and energy in the
early universe so that it is still hot, tiny, and expanding. But as
it turns out, entropy is a little different when there's so much
matter around.
Imagine a vast empty region of space, in the middle of which is a
cloud of gas with the mass of the Sun. Gravity pulls the gas
together, so the gas will get clumpy and ultimately collapse into a
star. How is this possible, if entropy always increases? There are
more ways to arrange the gas when it's wispy and scattered.
The importance of being clumpy
The answer is that gravity affects entropy, in a way that
physicists still don't fully understand. With truly massive objects,
being clumpy is higher entropy than being dense and uniform. So a
universe with galaxies, stars and planets actually has a higher
entropy than a universe filled with hot, dense gas.
This means we have a new problem. The sort of universe that
emerged immediately after the Big Bang, one that is hot and dense, is
low-entropy and therefore unlikely. "It's not what you
would randomly expect out of a bag of universes," says Carroll.
So how did our universe start in such an unlikely state? It's not
even clear what kind of answer to that question would be a satisfying
one. "What would count as a scientific explanation of the
initial state [of the universe]?" asks Tim
Maudlin, a philosopher of physics at New York University.
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Perhaps our universe is one of many
One idea is that there was something before the Big Bang. Could
that account for the low entropy of the early universe?
Carroll and one of his former students proposed a model in which
"baby" universes are constantly popping into existence,
calving off from their parent universe and expanding to become
universes like our own. These baby universes could start out with low
entropy, but the entropy of the "multiverse" as a whole
would always be high.
If that's true, the early universe only looks like it has low
entropy because we can't see the bigger picture. The same would be
true for the arrow of time. "That kind of idea implies that the
far past of our big-picture universe looks the same as the far
future," says Carroll.
But there's no wide agreement on Carroll's explanation of the past
hypothesis, or any other explanation. "There are proposals, but
nothing is even promising, much less settled," says Carroll.
Part of the trouble is that our best theories of physics can't
actually handle the Big Bang. Without a way to describe what happened
at the universe's birth, we can't explain why it had low entropy.
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Physics still can't explain everything
Modern physics relies on two major theories. Quantum mechanics
explains the behaviour of small things like atoms, while general
relativity describes heavy things like stars. But the two can't be
made to combine.
So if something is both very small and very heavy, like the
universe during the Big Bang, physicists get a bit stuck. To describe
the early universe, they need to combine the two theories into a
"theory of everything".
This ultimate theory will be the key to understanding the arrow of
time. "Finding that theory will ultimately let us know how
nature builds space and builds time," says Marina
Cortês, a physicist at the University of Edinburgh in the UK.
Unfortunately, despite decades of trying, nobody has managed to
come up with a theory of everything. But there are some candidates.
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Maybe all matter is made of tiny strings
The most promising theory of everything is string theory, which
says that all subatomic particles are actually made of tiny strings.
String theory also says that space has extra dimensions, beyond the
familiar three, that are curled up to microscopic size, and that we
live in a kind of multiverse where the laws of physics are different
in different universes.
This all sounds quite outlandish. Nevertheless, most particle
physicists see string theory as our best hope for a theory of
everything.
But that doesn't help us explain why time moves forwards. Like
almost every other fundamental physical theory, the equations of
string theory don't draw a strong distinction between the past and
the future.
String theory, if it turns out to be correct, might not help
explain the arrow of time. So Cortês is trying to come up with
something better.
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Time only ever goes forwards, but no one knows why
Working with Lee
Smolin of the Perimeter Institute in Waterloo, Canada, Cortês
has been working on alternatives to string theory that incorporate
the arrow of time at a fundamental level.
Cortês and Smolin suggest that the universe is made up of a
series of entirely unique events, never repeating itself. Each set of
events can only influence events in the next set, so the arrow of
time is built in. "We are hoping that if we can use these types
of equations to do cosmology, we can then arrive at the problem of
the initial conditions [of the universe] and find they're not as
special," says Cortês.
This is completely unlike Boltzmann's explanation, in which the
arrow of time emerges as a kind of accident from the laws of
probability. "Time isn't really an illusion," says Cortês.
"It exists and it's really moving forward."
But most physicists don't see a problem with Boltzmann's
explanation. "Boltzmann pointed the correct direction to the
solution here, a long time ago," says David
Albert, a philosopher of physics at Columbia University in New
York. "There's a real hope that if you dig carefully enough, the
whole story is in what Boltzmann said."
Carroll agrees. "If you have that low-entropy Big Bang, then
we're done," he says. "We can explain all the differences
between the past and the future."
Inside the Large Hadron Collider
One
way or another, to explain the arrow of time we need to explain that
low-entropy state at the beginning of the universe. That will take a
theory of everything, be it string theory, Cortês and Smolin's
causal sets, or something else. But people have been searching for a
theory of everything for 90 years. How do we find one? And how do we
know we have the right one once we've got it?
We
could test it using something very small and very dense. But we can't
go back in time to the Big Bang, and regardless of what a recent
blockbuster movie suggested, we also can't dive into a black hole and
send information back about it. So what can we do, if we really want
to explain why eggs don't un-break?
For
now, our best hope lies with the largest machine in human history.
The Large Hadron Collider (LHC) is a particle accelerator that runs
in a 27km circle under the border of France and Switzerland. It
smashes protons together at nearly the speed of light. The phenomenal
energy of these collisions creates new particles.
The
LHC has been closed for repairs for the last two years, but in the
spring of 2015 it will turn back on — and for the first time, it
will be operating at full power. At half-strength in 2012, it found
the long-sought-after Higgs boson, the particle that gives all the
others mass. That discovery led to a Nobel Prize, but the LHC could
now top it. With any luck, the LHC will catch a glimpse of new and
unexpected fundamental particles that will point the way to a theory
of everything.
It
will take several years for the LHC to collect the necessary data,
and for that data to be processed and interpreted. But once it's in,
we may finally understand why you can't get that stupid egg off your
face.
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