18
time is not absolute— it is dynamic, stretching and twisting in response to
19
matter and energy. Not long thereafter, we learned that the universe is ex-
20
panding, which led to the prediction of a Big Bang singularity in the past.
21
In classical general relativity, the Big Bang is the very first moment in the
22
history of the universe. It is the beginning of time.
23
Then in the 1920s we stumbled across quantum mechanics. The “state
24
of the universe” in quantum mechanics isn’t simply a particular configura-
25
tion of spacetime and matter. The quantum state is a superposition of many
26
different classical possibilities. This completely changes the rules of the
27
game. In classical general relativity, the Big Bang is the beginning of space-
28
time; in quantum general relativity— whatever that may be, since nobody
29
has a complete formulation of such a theory as yet— we don’t know whether
30
the universe has a beginning or not.
31
There are two possibilities: one where the universe is eternal, one where
32
it had a beginning. That’s because the Schrödinger equation of quantum
33
mechanics turns out to have two very different kinds of solutions, corre-
34
sponding to two different kinds of universes.
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One possibility is that time is fundamental, and the universe changes as
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time passes. In that case, the Schrödinger equation is unequivocal: time is
02
infinite. If the universe truly evolves, it always has been evolving and always
03
will evolve. There is no starting and stopping. There may have been a mo-
04
ment that looks like our Big Bang, but it would have only been a temporary
05
phase, and there would be more universe that was there even before the
06
event.
07
The other possibility is that time is not truly fundamental, but rather
08
emergent. Then, the universe can have a beginning. The Schrödinger equa-
09
tion has solutions describing universes that don’t evolve at all: they just sit
10
there, unchanging.
11
You might think that’s simply a mathematical curiosity, irrelevant to
12
our actual world. After all, it seems pretty obvious that time does exist,
13
and that it’s passing all around us. In a classical world, you’d be right.
14
Time either passes or it doesn’t; since time seems to pass in our world, the
15
possibility of a timeless universe isn’t very physically relevant.
16
Quantum mechanics is different. It describes the universe as a superpo-
17
sition of various classical possibilities. It’s like we take different ways a clas-
18
sical world could be and stack them on top of each other to create a quantum
19
world. Imagine that we take a very specific set of ways the world could be:
20
configurations of an ordinary classical universe, but at different moments
21
in time. The whole universe at 12:00, the whole universe at 12:01, the whole
22
universe at 12:02, and so on— but at moments that are much closer together
23
than a minute apart. Take those configurations and superimpose them to
24
create a quantum universe.
25
That’s a universe that is not evolving in time— the quantum state itself
26
simply is, unchanging and forever. But in any one part of the state, it looks 27
like one moment of time in a universe that is evolving. Every element in the
28
quantum superposition looks like a classical universe that came from some-
29
where, and is going somewhere else. If there were people in that universe, at
30
every part of the superposition they would all think that time was passing,
31
exactly as we actually do think. That’s the sense in which time can be emer-
32
gent in quantum mechanics. Quantum mechanics allows us to consider
33
universes that are fundamentally timeless, but in which time emerges at a
34
coarse- grained level of description.
35S
And if that’s true, then there’s no problem at all with there being a first
36N
moment in time. The whole idea of “time” is just an approximation anyway.
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I’m not making this up— this kind of scenario is exactly what was con-
01
templated by physicists Stephen Hawking and James Hartle back in the
02
early 1980s, when they helped pioneer the subject of “quantum cosmology.”
03
They showed how to construct a quantum state of the universe in which
04
time isn’t truly fundamental, and in which the Big Bang represents the be-
05
ginning of time as we know it. Hawking went on to write A Brief History
06
of Time, and become the most famous scientist of the modern age.
07
08
•
09
The idea of the universe having a beginning— whether time is fundamental
10
or emergent— suggests to some people that there must be something that
11
brought it into being, and typically that something is identified with God.
12
This intuition is codified in the cosmological argument for God’s existence,
13
an idea that traces its lineage back at least as far as Plato and Aristotle. In
14
recent years it has been championed by theologian William Lane Craig,
15
who puts it in the form of a syllogism:
16
17
1. Whatever begins to exist, has a cause.
18
2. The Universe begins to exist.
19
3. Therefore, the Universe had a cause.
20
21
As we’ve seen, the second premise of the argument may or may not be
22
correct; we simply don’t know, as our current scientific understanding isn’t
23
up to the task. The first premise is false. Talking about “causes” is not the
24
right vocabulary to use when thinking about how the universe works at a
25
deep level. We need to be asking ourselves not whether the universe had a
26
cause but whether having a first moment in time is compatible with the
27
la
ws of nature.
28
As we go through our lives, we don’t see random objects popping
29
into existence. It might be forgivable to think that, at least with a high de-
30
gree of credence, the universe itself shouldn’t simply pop into existence. But
31
there are two very substantial mistakes lurking beneath that innocent-
32
sounding idea.
33
The first mistake is that saying that the universe had a beginning is not
34
the same as saying it popped into existence. The latter formulation, which is
S35
natural from an everyday point of view, leans heavily on a certain way of
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thinking about time. For something to pop into existence implies that at
02
an earlier moment it was not there, and at a later moment it was. But when
03
we’re talking about the universe, that “earlier” moment simply does not
04
exist. There is not a moment in time where there is no universe, and another
05
moment in time where there is; all moments in time are necessarily associ-
06
ated with an existing universe. The question is whether there can be a first
07
such moment, an instant of time prior to which there were no other in-
08
stants. That’s a question our intuitions just aren’t up to addressing.
09
Said another way: even if the universe has a first moment of time, it’s
10
wrong to say that it “comes from nothing.” That formulation places into our
11
mind the idea that there was a state of being, called “nothing,” which then
12
transformed into the universe. That’s not right; there is no state of being
13
called “nothing,” and before time began, there is no such thing as “trans-
14
forming.” What there is, simply, is a moment of time before which there
15
were no other moments.
16
The second mistake is to assert that things don’t simply pop into exis-
17
tence, rather than asking why that doesn’t happen in the world we experi-
18
ence. What makes me think that, despite my best wishes, a bowl of ice
19
cream is not going to pop into existence right in front of me? The answer is
20
that it would violate the laws of physics. Those include conservation laws,
21
which say certain things remain constant over time, such as momentum
22
and energy and electric charge. I can be fairly confident that a bowl of ice
23
cream isn’t going to materialize in front of me because that would violate
24
the conservation of energy.
25
Along those lines, it seems reasonable to believe that the universe can’t
26
simply begin to exist, because it’s full of stuff, and that stuff has to come
27
from somewhere. Translating that into physics- speak, the universe has en-
28
ergy, and energy is conserved— it’s neither created nor destroyed.
29
Which brings us to the important realization that makes it completely
30
plausible that the universe could have had a beginning: as far as we can tell,
31
every conserved quantity characterizing the universe (energy, momentum,
32
charge) is exactly zero.
33
It’s not surprising that the electric charge of the universe is zero. Protons
34
have a positive charge, electrons have an equal but opposite negative charge,
35S
and there seem to be equal numbers of them in the universe, adding up to
36N
a total charge of zero. But claiming that the energy of the universe is zero is 2 00
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something else entirely. There are clearly many things in the universe that
01
have positive energy. So to have zero energy overall, there would have to be
02
something with negative energy— what is that?
03
The answer is “gravity.” In general relativity, there is a formula for the
04
energy of the whole universe at once. And it turns out that a uniform
05
universe— one in which matter is spread evenly through space on very large
06
scales— has precisely zero energy. The energy of “stuff” like matter and ra-
07
diation is positive, but the energy associated with the gravitational field (the
08
curvature of spacetime) is negative, and exactly enough to cancel the posi-
09
tive energy in the stuff.
10
If the universe had a nonzero amount of some conserved quantity like
11
energy or charge, it couldn’t have an earliest moment in time— not without
12
violating the laws of physics. The first moment of such a universe would be
13
one in which energy or charge existed without any previous existence,
14
which is against the rules. But as far as we know, our universe isn’t like that.
15
There seems to be no obstacle in principle to a universe like ours simply
16
beginning to exist.
17
18
•
19
To the question of whether the universe could possibly exist all by itself,
20
without any external help, science offers an unequivocal answer: sure it
21
could. We don’t yet know the final laws of physics, but there’s nothing we
22
know about how such laws work that suggests the universe needs any help
23
to exist.
24
For questions like this, however, the scientific answer doesn’t always sat-
25
isfy everyone. “Okay,” they might say, “we understand that there can be a
26
physical theory that describes a self- contained universe, without any exter-
27
nal agent bringing it about or sustaining it. But that doesn’t explain why it
28
actually does exist. For that, we have to look outside science.”
29
Sometimes this angle of attack appeals to fundamental metaphysical
30
principles, which are purportedly more foundational even than the laws of
31
physics, and cannot be sensibly denied. In particular, the pre- Socratic Greek
32
philosopher Parmenides put forward the famous maxim ex nihilo, nihil
33
fit—“out of nothing, nothing comes.” Even Lucretius, the Roman poet who
34
was closer to modern naturalism than almost anyone else in the ancient
S
35
world, subscribed to a similar belief. According to this line of thought, it
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doesn’t matter if physicists can cook up self- contained theories in which the
02
cosmos has a first moment of time; those theories must necessarily be in-
03
complete, since they violate this cherished principle.
04
This is perhaps the most egregious example of begging the question in
05
the history of the universe. We are asking whether the universe could come
06
into existence without anything causing it. The response is “No, because
07
nothing comes into existence without being caused.” How do we know
08
that? It can’t be because we have never seen it happen; the universe is differ-
09
ent from the various things inside the universe that we have actually expe-
10
rienced in our lives. And it can’t be because we can’t imagine it happening,
11
or because it’s impossible to construct sensible models in which it happens,
12
since both the imagining and the construction of models have manifestly
13
happened.
14
In the Stanford Encyclopedia of Philosophy, an online resource written
15
and edited by professional philosophers, the entry on “Nothingness” starts
16
by asking, “Why is there something rather than nothing?” and immediately
17
answering, “Well, why not?” That’s a good answer. There is no reason why
18
the universe couldn’t have had a first moment in time, nor is there any rea-
19
son it couldn’t have lasted forever, even without the benefit of any external
20
causal or sustaining influences. Our job, as always, is to ask how well com-
21
peting theories account for the information we accumulate as we observe
22
the actual universe.
23
•
24
25
Our job, in other words, is to move from the first question, “Can the uni-
26
verse simply exist?” (yes, it can) to the second, harder one: “What is the best
27
explanation for the existence of the universe?”
28
The answer is certainly “We don’t know.” Understanding that time may
29
be emergent, and that the laws of physics are perfectly compatible with the
30
universe having a first moment of time, might help explain how the uni-
31
verse came to be, but it says essentially nothing about why. It says nothing
The Big Picture Page 35