多重字宙的可能性

A few months ago the Nobel Prize in physics was awarded to two
teams of astronomers for a discovery that has been hailed as one of
the most important astronomical observations ever. And today, after
briefly describing what they found, I'm going to tell you about a
highly controversial framework for explaining their
discovery,namely the possibility that way beyond the Earth, the
Milky Way and other distant galaxies, we may find that our universe
is not the only universe,but is instead part of a vast complex of
universes that we call the multiverse.

Now the idea of a multiverse is a strange one. I mean, most of us
were raised to believe that the word "universe" means everything.
And I say most of us with forethought, as my four-year-old daughter
has heard me speak of these ideas since she was born. And last year
I was holding her and I said, "Sophia, I love you more than anything
in the universe. "And she turned to me and said, "Daddy, universe
or multiverse?"

But barring such an anomalous upbringing, it is strange to imagine
other realms separate from ours, most with fundamentally different
features, that would rightly be called universes of their own. And
yet,speculative though the idea surely is, I aim to convince you
that there's reason for taking it seriously, as it just might be
right. I'm going to tell the story of the multiverse in three
parts. In part one, I'm going to describe those Nobel Prize-winning
results and to highlight a profound mystery which those results
revealed. In part two, I'll offer a solution to that mystery. It's
based on an approach called string theory, and that's where the
idea of the multiverse will come into the story. Finally, in part
three, I'm going to describe a cosmological theory called
inflation, which will pull all the pieces of the story together.

Okay, part one starts back in 1929 when the great astronomer Edwin
Hubble realized that the distant galaxies were all rushing away
from us, establishing that space itself is stretching, it's
expanding. Now this was revolutionary. The prevailing wisdom was
that on the largest of scales the universe was static. But even so,
there was one thing that everyone was certain of: The expansion
must be slowing down. That, much as the gravitational pull of the
Earths lows the ascent of an apple tossed upward, the gravitational
pull of each galaxy on every other must be slowing the expansion of
space.

Now let's fast-forward to the 1990s when those two teams of
astronomersI mentioned at the outset were inspired by this
reasoning to measure the rate at which the expansion has been
slowing. And they did this by painstaking observations of numerous
distant galaxies, allowing them to chart how the expansion rate has
changed over time. Here's the surprise: They found that the
expansion is not slowing down. Instead they found that it's
speeding up, going faster and faster. That's like tossing an apple
upward and it goes up faster and faster. Now if you saw an apple do
that,you'd want to know why. What's pushing on it?

Similarly, the astronomers' results are surely well-deserving of
the Nobel Prize, but they raised an analogous question. What force
is driving all galaxies to rush away from every other at an ever-
quickening speed? Well the most promising answer comes from an old
idea of Einstein's. You see, we are all used to gravity being a
force that does one thing, pulls objects together. But in
Einstein's theory of gravity, his general theory of relativity,
gravity can also push things apart.

How? Well according to Einstein's math, if space is uniformly
filled with an invisible energy, sort of like a uniform, invisible
mist, then the gravity generated by that mist would be repulsive,
repulsive gravity, which is just what we need to explain the
observations. Because the repulsive gravity of an invisible energy
in space -- we now call it dark energy, but I've made it smokey
white here so you can see it -- its repulsive gravity would cause
each galaxy to push against every other, driving expansion to speed
up, not slow down. And this explanation represents great progress.

But I promised you a mystery here in part one. Here it is. When the
astronomers worked out how much of this dark energy must be infusing
space to account for the cosmic speed up, look at what they found.
This number is small. Expressed in the relevant unit, it is
spectacularly small. And the mystery is to explain this peculiar
number. We want this number to emerge from the laws of physics, but
so far no one has found a way to do that.

Now you might wonder, should you care? Maybe explaining this
number is just a technical issue, a technical detail of interest to
experts, but of no relevance to anybody else. Well it surely is a
technical detail, but some details really matter. Some details
provide windows into uncharted realms of reality, and this peculiar
number may be doing just that, as the only approach that's so far
made headway to explain it invokes the possibility of other
universes -- an idea that naturally emerges from string theory,
which takes me to part two: string theory.

So hold the mystery of the dark energy in the back of your mind as I
now go on to tell you three key things about string theory. First
off, what is it? Well it's an approach to realize Einstein's dream
of a unified theory of physics, a single overarching framework that
would be able to describe all the forces at work in the universe.
And the central idea of string theory is quite straight forward.It
says that if you examine any piece of matter ever more finely, at
first you'll find molecules and then you'll find atoms and
subatomic particles. But the theory says that if you could probe
smaller, much smaller than we can with existing technology, you'd
find something else inside these particles -- a little tiny
vibrating filament of energy, a little tiny vibrating string. And
just like the strings on a violin, they can vibrate in different
patterns producing different musical notes. These little
fundamental strings, when they vibrate in different patterns, they
produce different kinds of particles -- so electrons, quarks,
neutrinos, photons, all other particles would be united into a
single framework, as they would all arise from vibrating strings.
It's a compelling picture, a kind of cosmic symphony, where all the
richness that we see in the world around us emerges from the music
that these little, tiny strings can play.

But there's a cost to this elegant unification, because years of
research have shown that the math of string theory doesn't quite
work. It has internal inconsistencies, unless we allow for
something wholly unfamiliar -- extra dimensions of space. That is,
we all know about the usual three dimensions of space. And you can
think about those as height, width and depth. But string theory
says that, on fantastically small scales, there are additional
dimensions crumpled to a tiny size so small that we have not
detected them. But even though the dimensions are hidden, they
would have an impact on things that we can observe because the
shape of the extra dimensions constrains how the strings can
vibrate. And in string theory, vibration determines everything. So
particle masses, the strengths of forces, and most importantly, the
amount of dark energy would be determined by the shape of the extra
dimensions. So if we knew the shape of the extra dimensions, we
should be able to calculate these features, calculate the amount of
dark energy.

The challengeis we don't know the shape of the extra dimensions.
All we have is a list of candidate shapes allowed by the math. Now
when these ideas were first developed, there were only about five
different candidate shapes, so you can imagine analyzing them one-
by-one to determine if any yield the physical features we observe.
But over time the list grew as researchers found other candidate
shapes. From five, the number grew into the hundreds and then the
thousands -- A large, but still manageable, collection to analyze,
since after all, graduate students need something to do. But then
the list continued to grow into the millions and the billions,
until today. The list of candidate shapes has soared to about 10 to
the 500.

So, what to do? Well some researchers lost heart, concluding that
was so many candidate shapes for the extra dimensions, each giving
rise to different physical features, string theory would never make
definitive, testable predictions. But others turned this issue on
its head, taking us to the possibility of a multiverse. Here's the
idea. Maybe each of these shapes is on an equal footing with every
other. Each is as real as every other, in the sense that there are
many universes, each with a different shape, for the extra
dimensions. And this radical proposal has a profound impact on this
mystery: the amount of dark energy revealed by the Nobel Prize-
winning results.

Because you see, if there are other universes, and if those
universes each have, say, a different shape for the extra
dimensions, then the physical features of each universe will be
different, and in particular, the amount of dark energy in each
universe will be different. Which means that the mystery of
explaining the amount of dark energy we've now measured would take
on a wholly different character. In this context, the laws of
physics can't explain one number for the dark energy because there
isn't just one number, there are many numbers. Which means we have
been asking the wrong question. It's that the right question to ask
is, why do we humans find ourselves in a universe with a particular
amount of dark energy we've measured instead of any of the other
possibilities that are out there?

And that's a question on which we can make headway. Because those
universes that have much more dark energy than ours, whenever
matter tries to clump into galaxies, the repulsive push of the dark
energy is so strong that it blows the clump apartand galaxies don't
form. And in those universes that have much less dark energy, well
they collapse back on themselves so quickly that, again, galaxies
don't form. And without galaxies, there are no stars, no planets
and no chance for our form of life to exist in those other
universes.

So we find ourselves in a universe with the particular amount of
dark energy we've measured simply because our universe has
conditions hospitable to our form of life. And that would be that.
Mystery solved, multiverse found. Now some find this explanation
unsatisfying. We're used to physics giving us definitive
explanations for the features we observe. But the point is, if the
feature you're observing can and does take on a wide variety of
different values across the wider landscape of reality, then
thinking one explanation for a particular value is simply
misguided.

An early example comes from the great astronomer Johannes Kepler
who was obsessed with understanding a different number --why the
Sun is 93 million miles away from the Earth. And he worked for
decades trying to explain this number, but he never succeeded, and
we know why. Kepler was asking the wrong question.

We now know that there are many planet sat a wide variety of
different distances from their host stars. So hoping that the laws
of physics will explain one particular number, 93 million miles,
well that is simply wrong headed. Instead the right question to ask
is, why do we humans find ourselves on a planet at this particular
distance, instead of any of the other possibilities? And again,
that's a question we can answer. Those planets which are much
closer to a star like the Sun would be so hot that our form of life
wouldn't exist. And those planets that are much farther away from
the star, well they're so cold that, again, our form of life would
not take hold. So we find ourselves on a planet at this particular
distance simply because it yields conditions vital to our form of
life. And when it comes to planets and their distances, this
clearly is the right kind of reasoning. The point is, when it comes
to universes and the dark energy that they contain, it may also be
the right kind of reasoning.

One key difference, of course, is we know that there are other
planets out there, but so far I've only speculated on the
possibility that there might be other universes. So to pull it all
together, we need a mechanism that can actually generate other
universes. And that takes me to my final part, part three. Because
such a mechanism has been foundby cosmologists trying to understand
the Big Bang. You see, when we speak of the Big Bang, we often have
an image of a kind of cosmic explosion that created our universe
and set space rushing outward.

But there's a little secret. The Big Bang leaves out something
pretty important, the Bang. It tells us how the universe evolved
after the Bang, but gives us no insight into what would have
powered the Bang itself. And this gap was finally filled by an
enhanced version of the Big Bang theory. It's called inflationary
cosmology, which identified a particular kind of fuel that would
naturally generatean outward rush of space. The fuel is based on
something called a quantum field, but the only detail that matters
for us is that this fuel proves to be so efficient that it's
virtually impossible to use it all up, which means in the
inflationary theory, the Big Bang giving rise to our universe is
likely not a one-time event. Instead the fuel not only generated
our Big Bang, but it would also generate countless other Big Bangs,
each giving rise to its own separate universe with our universe
becoming but one bubblein a grand cosmic bubble bath of universes.

And now, when we meld this with string theory, here's the picture
we're led to. Each of these universes has extra dimensions. The
extra dimensions take on a wide variety of different shapes. The
different shapes yield different physical features. And we find
ourselves in one universe instead of another simply because it's
only in our universe that the physical features, like the amount of
dark energy, are right for our form of life to take hold. And this
is the compelling but highly controversial picture of the wider
cosmos that cutting-edge observation and theory have now led us to
seriously consider.

One big remaining question, of course, is, could we ever confirm the
existence of other universes? Well let me describe one way that
might one day happen. The inflationary theory already has strong
observational support. Because the theory predicts that the Big
Bang would have been so intense that as space rapidly expanded,
tiny quantum jitters from the micro world would have been stretched
out to the macro world, yielding a distinctive fingerprint, a
pattern of slightly hotter spots and slightly colder spots, across
space, which powerful telescopes have now observed. Going further,
if there are other universes, the theory predicts that every so
often those universes can collide. And if our universe got hit by
another, that collision would generate an additional subtle
pattern of temperature variations across space that we might one day
be able to detect. And so exotic as this picture is, it may one day
be grounded in observations, establishing the existence of other
universes.

I'll conclude with a striking implication of all these ideas for
the very far future. You see, we learned that our universe is not
static, that space is expanding, that that expansion is speeding up
and that there might be other universes all by carefully examining
faint pinpoints of starlight coming to us from distant galaxies.
But because the expansion is speeding up, in the very far future,
those galaxies will rush away so far and so fast that we won't be
able to see them -- not because of technological limitations, but
because of the laws of physics. The light those galaxies emit, even
traveling at the fastest speed, the speed of light, will not be
able to overcome the ever-widening gulf between us. So astronomers
in the far future looking out into deep space will see nothing but
an endless stretch of static, inky, black stillness. And they will
conclude that the universe is static and unchanging and populated
by a single central oasis of matter that they inhabit -- a picture
of the cosmos that we definitively know to be wrong.

Now maybe those future astronomers will have records handed down
from an earlier era, like ours, attesting to an expanding cosmos
teeming with galaxies. But would those future astronomers believe
such ancient knowledge? Or would they believe in the black, static
empty universe that their own state-of-the-art observations reveal?
I suspect the latter. Which means that we are living through a
remarkably privileged era when certain deep truths about the cosmos
are still within reach of the human spirit of exploration. It
appears that it may not always be that way. Because today's
astronomers, by turning powerful telescopes to the sky, have
captured a handful of starkly informative photons -- a kind of
cosmic telegram billions of years in transit. and the message
echoing across the ages is clear. Sometimes nature guards her
secrets with the unbreakable grip of physical law. Sometimes the
true nature of reality beckons from just beyond the horizon.

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