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Curiosity: The Questions of Life - Season 1 Episode 1 Part 1 - YouTube
Curiosity: The Questions of Life - Season 1 Episode 1 Part 2 - YouTube
Curiosity: The Questions of Life - Season 1 Episode 1 Part 2 - YouTube
Curiosity: The Questions of Life - Season 1 Episode 1 Part 3 - YouTube
Will the universe end in a
big snap?
Focusing on a logical but gruesome end for the universe could reveal
elusive quantum gravity
IMAGINE one day you wake up and look at yourself in the mirror only to
find that something is terribly wrong. You look grainy and indistinct, like a
low-quality image blown up so much that the features are barely
recognisable. You scream, and the sound that comes out is distorted too,
like hearing it over a bad phone line. Then everything goes blank.
Welcome to the big snap, a new and terrifying way for the universe to end
that seems logically difficult to avoid.
Dreamed up by Massachusetts Institute of Technology cosmologist Max
Tegmark, the snap is what happens when the universe's rapid expansion is
combined with the insistence of quantum mechanics that the amount of
information in the cosmos be conserved. Things start to break down, like a
computer that has run out of memory.
It is cold comfort that you would not survive long enough to watch this in
the mirror, as the atoms that make up your body would long since have
fallen apart. But take heart, accepting this fate would without question
mean discarding cherished notions, such as the universe's exponential
"inflation" shortly after the big bang. And that is almost as unpalatable to
cosmologists as the snap itself.
So rather than serving as a gruesome death knell, Tegmark prefers to think
of the big snap as a useful focal point for future work, in particular the
much coveted theory of quantum gravity, which would unite quantum
mechanics with general relativity. "In the past when we have faced
daunting challenges it's also proven very useful," he says.
"That's how I feel about the big snap. It's a thorn in our side, and I hope
that by studying it more it will turn out to give us some valuable clues in
our quest to understand the nature of space." That would be fitting as
Tegmark did not set out to predict a gut-wrenching way for the universe
to end. Rather, he was led to this possibility by some puzzling properties of
the universe as we know it.
According to quantum mechanics, every particle and force field in the
universe is associated with a wave, which tells us everything there is to
know about that particle or that field. We can predict what the waves will
look like at any time in the future from their current state. And if we record
what all the waves in the universe look like at any given moment, then we
have all the information necessary to describe the entire universe. Tegmark
decided to think about what happens to that information as the universe
expands (arxiv.org/abs/1108.3080).
To understand his reasoning, it's important to grasp that even empty
space has information associated with it. That's because general relativity
tells us that the fabric of space-time can be warped, and it takes a certain
amount of information to specify whether and in what way a particular
patch of space is bent.
One way to visualise this is to think of the universe as divided up into cells
1 Planck length across - the smallest scale that is meaningful, like a single
pixel in an image. Some physicists think that one bit of information is
needed to describe the state of each cell, though the exact amount is
debated. Trouble arises, however, when you extrapolate the fate of these
cells out to a billion years hence, when the universe will have grown larger.
One option is to accept that the added volume of space, and all the
Planck-length cells within it, brings new information with it, sufficient to
describe whether and how it is warped. But this brings you slap bang up
against a key principle of quantum mechanics known as unitarity - that the
amount of information in a system always stays the same.
What's more, the ability to make predictions breaks down - the very
existence of extra information means we could not have anticipated it from
what we already knew.
Another option is to leave quantum mechanics intact, and assume the new
volume of space brings no new information with it. Then we need to
describe a larger volume of space using the same number of bits. So if the
volume doubles, the only option is to describe a cubic centimetre of space
with only half the number of bits we had before (see diagram).
This would be appropriate if each cell grows, says Tegmark. Where nothing
previously varied on scales smaller than 1 Planck length, now nothing
varies on scales smaller than 2 Planck lengths, or 3, or more depending
how much the universe expands. Eventually, this would impinge on the
laws of physics in a way that we can observe.
Photons of different energies only travel at the same speed under the
assumption that space is continuous. If the space-time cells became large
enough, we might start to notice photons with a very short wavelength
moving more slowly than longer wavelength ones. And if the cells got even
larger, the consequences would be dire. The trajectories of waves
associated with particles of matter would be skewed. This would change
the energy associated with different arrangements of particles in atomic
nuclei. Some normally stable nuclei would fall apart.
Chemical reactions would get messed up too, since these depend critically
on the energy associated with configurations of electrons and ions that
would be altered by the large granularity of space. Living things would
regrettably cease to function. "It would probably kill us at the point where
the nuclear physics gets messed up," Tegmark says. "Many of the atoms of
which we're made would disintegrate."
Does a gruesome future await? Maybe not. If the big snap is really what's
in store, we should already be seeing signs of it - and thankfully we are
not.
In an expanding universe with a finite lifetime, most of the volume - along
with its stars, galaxies, and planets - shows up for the final curtain, simply
because that is when the universe has grown to its largest size. If we
assume the early universe expanded at an extremely rapid pace, as posited
by the widely accepted theory of inflation, we are most likely to be just a
few billion years away from the big snap. In that case, the granularity of
space should already be large enough to skew the arrival time of photons
of different wavelengths in gamma-ray bursts. Yet observed gamma-ray
bursts, powerful stellar explosions that can be seen from extremely far off,
show no sign of such an effect.
Therefore, for the universe to end in a big snap, we either have to reject
inflation altogether. Or alternatively assume we are very atypical beings,
and do in fact occupy a special place in the universe in violation of the
Copernican principle. Both options are anathema to cosmology. "There's
something here that's just very wrong," Tegmark says.
Raphael Bousso of the University of California in Berkeley and Andreas
Albrecht of UC Davis, both agree. A big snap in the universe's future
"somehow can't be right", says Bousso.
That's a relief. But what does happen to information in an expanding
universe, then? Tegmark hopes that a complete theory of quantum gravity,
which would describe how the tiniest regions of space and their associated
information behave, might change the whole picture in a way that avoids
the big snap.
"A lot of people in quantum gravity have gotten a little depressed," he
says. There is a sense that progress cannot be made without building
particle accelerators to probe space down to the Planck length, which is so
far beyond today's technology that it seems out of the question.
Pondering the big snap, however, could stimulate new ways of thinking.
Tegmark says: "I suspect there might be other ways of learning about
quantum gravity without building impossible machines."
Sign in to read: Will the universe end in a big snap? - space - 22 September 2011 - New Scientist
Curiosity: The Questions of Life - Season 1 Episode 1 Part 2 - YouTube
Curiosity: The Questions of Life - Season 1 Episode 1 Part 2 - YouTube
Curiosity: The Questions of Life - Season 1 Episode 1 Part 3 - YouTube
Will the universe end in a
big snap?
Focusing on a logical but gruesome end for the universe could reveal
elusive quantum gravity
IMAGINE one day you wake up and look at yourself in the mirror only to
find that something is terribly wrong. You look grainy and indistinct, like a
low-quality image blown up so much that the features are barely
recognisable. You scream, and the sound that comes out is distorted too,
like hearing it over a bad phone line. Then everything goes blank.
Welcome to the big snap, a new and terrifying way for the universe to end
that seems logically difficult to avoid.
Dreamed up by Massachusetts Institute of Technology cosmologist Max
Tegmark, the snap is what happens when the universe's rapid expansion is
combined with the insistence of quantum mechanics that the amount of
information in the cosmos be conserved. Things start to break down, like a
computer that has run out of memory.
It is cold comfort that you would not survive long enough to watch this in
the mirror, as the atoms that make up your body would long since have
fallen apart. But take heart, accepting this fate would without question
mean discarding cherished notions, such as the universe's exponential
"inflation" shortly after the big bang. And that is almost as unpalatable to
cosmologists as the snap itself.
So rather than serving as a gruesome death knell, Tegmark prefers to think
of the big snap as a useful focal point for future work, in particular the
much coveted theory of quantum gravity, which would unite quantum
mechanics with general relativity. "In the past when we have faced
daunting challenges it's also proven very useful," he says.
"That's how I feel about the big snap. It's a thorn in our side, and I hope
that by studying it more it will turn out to give us some valuable clues in
our quest to understand the nature of space." That would be fitting as
Tegmark did not set out to predict a gut-wrenching way for the universe
to end. Rather, he was led to this possibility by some puzzling properties of
the universe as we know it.
According to quantum mechanics, every particle and force field in the
universe is associated with a wave, which tells us everything there is to
know about that particle or that field. We can predict what the waves will
look like at any time in the future from their current state. And if we record
what all the waves in the universe look like at any given moment, then we
have all the information necessary to describe the entire universe. Tegmark
decided to think about what happens to that information as the universe
expands (arxiv.org/abs/1108.3080).
To understand his reasoning, it's important to grasp that even empty
space has information associated with it. That's because general relativity
tells us that the fabric of space-time can be warped, and it takes a certain
amount of information to specify whether and in what way a particular
patch of space is bent.
One way to visualise this is to think of the universe as divided up into cells
1 Planck length across - the smallest scale that is meaningful, like a single
pixel in an image. Some physicists think that one bit of information is
needed to describe the state of each cell, though the exact amount is
debated. Trouble arises, however, when you extrapolate the fate of these
cells out to a billion years hence, when the universe will have grown larger.
One option is to accept that the added volume of space, and all the
Planck-length cells within it, brings new information with it, sufficient to
describe whether and how it is warped. But this brings you slap bang up
against a key principle of quantum mechanics known as unitarity - that the
amount of information in a system always stays the same.
What's more, the ability to make predictions breaks down - the very
existence of extra information means we could not have anticipated it from
what we already knew.
Another option is to leave quantum mechanics intact, and assume the new
volume of space brings no new information with it. Then we need to
describe a larger volume of space using the same number of bits. So if the
volume doubles, the only option is to describe a cubic centimetre of space
with only half the number of bits we had before (see diagram).
This would be appropriate if each cell grows, says Tegmark. Where nothing
previously varied on scales smaller than 1 Planck length, now nothing
varies on scales smaller than 2 Planck lengths, or 3, or more depending
how much the universe expands. Eventually, this would impinge on the
laws of physics in a way that we can observe.
Photons of different energies only travel at the same speed under the
assumption that space is continuous. If the space-time cells became large
enough, we might start to notice photons with a very short wavelength
moving more slowly than longer wavelength ones. And if the cells got even
larger, the consequences would be dire. The trajectories of waves
associated with particles of matter would be skewed. This would change
the energy associated with different arrangements of particles in atomic
nuclei. Some normally stable nuclei would fall apart.
Chemical reactions would get messed up too, since these depend critically
on the energy associated with configurations of electrons and ions that
would be altered by the large granularity of space. Living things would
regrettably cease to function. "It would probably kill us at the point where
the nuclear physics gets messed up," Tegmark says. "Many of the atoms of
which we're made would disintegrate."
Does a gruesome future await? Maybe not. If the big snap is really what's
in store, we should already be seeing signs of it - and thankfully we are
not.
In an expanding universe with a finite lifetime, most of the volume - along
with its stars, galaxies, and planets - shows up for the final curtain, simply
because that is when the universe has grown to its largest size. If we
assume the early universe expanded at an extremely rapid pace, as posited
by the widely accepted theory of inflation, we are most likely to be just a
few billion years away from the big snap. In that case, the granularity of
space should already be large enough to skew the arrival time of photons
of different wavelengths in gamma-ray bursts. Yet observed gamma-ray
bursts, powerful stellar explosions that can be seen from extremely far off,
show no sign of such an effect.
Therefore, for the universe to end in a big snap, we either have to reject
inflation altogether. Or alternatively assume we are very atypical beings,
and do in fact occupy a special place in the universe in violation of the
Copernican principle. Both options are anathema to cosmology. "There's
something here that's just very wrong," Tegmark says.
Raphael Bousso of the University of California in Berkeley and Andreas
Albrecht of UC Davis, both agree. A big snap in the universe's future
"somehow can't be right", says Bousso.
That's a relief. But what does happen to information in an expanding
universe, then? Tegmark hopes that a complete theory of quantum gravity,
which would describe how the tiniest regions of space and their associated
information behave, might change the whole picture in a way that avoids
the big snap.
"A lot of people in quantum gravity have gotten a little depressed," he
says. There is a sense that progress cannot be made without building
particle accelerators to probe space down to the Planck length, which is so
far beyond today's technology that it seems out of the question.
Pondering the big snap, however, could stimulate new ways of thinking.
Tegmark says: "I suspect there might be other ways of learning about
quantum gravity without building impossible machines."
Sign in to read: Will the universe end in a big snap? - space - 22 September 2011 - New Scientist
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