T
RAVEL anywhere in the known universe
and, like Coca-Cola, the laws of nature
always taste the same. That is a basic tenet
of physics called the cosmological principle,
which holds that our patch of the universe is
a representative sample of the rest.
This, as far as we can tell, is true. Certainly
in the bits of the universe that we can see, the
laws of physics are “uncannily the same”, says
Richard Bower at Durham University in the
UK. But an important caveat here is “the bits
we can see”. What about those we can’t?
There are bits of the universe that are out of
sight. Forever. These aren’t the exotic parallel
universes conjured up by string theory or
quantum mechanics, but an unavoidable
consequence of workaday cosmology. Because
the universe is expanding at breakneck speed
yet the speed of light is finite, the outer reaches
of the universe have disappeared over the
cosmic horizon, forever out of contact as light
from them could never reach us. The known
universe inside the horizon stretches about
46 billion light years in all directions. How
much there is beyond that isn’t known, but
it is possible that there are places beyond the
horizon where the laws of physics are different.
One reason for thinking this may be true
is that our laws are bizarrely and arbitrarily
conducive to life. Cosmologists call this
fine-tuning. If any of the laws of physics were
slightly different, we couldn’t exist. As just one
example, if the strong nuclear force, which
holds protons and neutrons together inside
atoms, were slightly stronger, the sun would
have exploded long before life got started
on Earth. There are many other examples
of fine-tuning, collectively known as the
“Goldilocks paradox” because so many of
the laws are just right. And paradoxical it is.
“There’s no explanation for why they are the
value they are,” says Bower. “You’ve just got
to go, ‘That’s the way it is’.”
The odds of a universe with the exact
specifications that can sustain life are so low
that many physicists argue that there must
be other places where the laws are different.
It just so happens that we live in a life-friendly
patch of universe because, well, it couldn’t be
any other way.
And that’s just in our universe. There aresmall, but infinitely bigger than the
nothingness required for the big bang.
This hourglass model is known as
the big bounce, and it has dramatic
consequences for reality. Because
theoretical calculations dictate that
the preceding universe must have
been similar to our own, its origin must
also be similar. That means it, too,
would have begun from the collapse
of a preceding universe, and so on
throughout eternity. “In our model,
space-time never vanishes,” says Ijjas.
In other words, reality has always
existed and there was no beginning.
That seems difficult to imagine.
“It’s somewhat counter-intuitive,”
concedes Ijjas. But the alternative –
the total absence of reality before
space and time came into existence –
is more difficult. “It’s infinitely more
difficult,” she says.What came before?
Oriti favours another alternative. For
him, the big bang represents not the
birth of the universe, but the moment
the universe assumed its current form,
with intelligible properties such as
space and time. He compares it to a
phase transition such as the moment
steam condenses to liquid water.
“All sorts of notions that you apply
as a fish in the water simply do not
apply to a gas,” he says.
Before this phase transition,
notions of space and time are
meaningless, and reality itself
becomes fundamentally indescribable.
Even the word “before” is inaccurate,
says Oriti. “The notion of time ceases
to apply.” What’s more, because all
phase transitions are, at least in
theory, reversible, the universe could
return to this timeless state again at
some point in the future, presumably
with dire consequences for us. If
“future” is even the right word.
This inability to talk about reality
in everyday terms seems incredibly
frustrating. “We get frustrated as
well,” says Oriti. “I sympathise, but
get used to it.” Reality, it seems, is
truly beyond words.
Gilead Amitalmost certainly others. Multiverses are a
consequence of many theories, including
black hole physics and string theory. Not
all produce different laws of physics, but
some do. String theory, for instance,
conjures up 10^500 universes, all with different
laws of physics.
Could we ever know if any of this is true?
The reality is that other universes, if they
exist, are probably forever inaccessible to us.
“Multiverse theories must in principle be taken
quite seriously, but proposals to test them
don’t get very far,” says Simon Friederich at the
University of Groningen in the Netherlands.
For now, we have to study what we can see.
But there is an upside to this: it makes reality
tractable. “If there is just one universe, then
we might have a good chance of discovering
basically everything about physics,” says
Tim Blackwell at Goldsmiths, University of
London. Unfortunately, that would be like
assuming you understood all of biodiversity
by cataloguing life on a small island. Reality
may well be different elsewhere, but the
cosmos is too big for us to know for sure.
Jason Arunn MurugesuIS REALITY THE SAME
EVERYWHERE?38 | New Scientist | 1 February 2020