What do physicists mean when they talk about nothing?

August 15, 2018 by Martin Rees, The Conversation
Galaxy history revealed by the Hubble Space Telescope. Credit: NASA

Philosophers have debated the nature of "nothing" for thousands of years, but what has modern science got to say about it? In an interview with The Conversation, Martin Rees, Astronomer Royal and Emeritus Professor of Cosmology and Astrophysics at the University of Cambridge, explains that when physicists talk about nothing, they mean empty space (vacuum). This may sound straightforward, but experiments show that empty space isn't really empty – there's a mysterious energy latent in it which can tell us something about the fate of the universe.

Q: Is empty space really the same as nothing?

A: Empty seems to be nothing to us. By analogy, water may seem to be nothing to a fish – it's what's left when you take away all the other things floating in the sea. Likewise, empty space is conjectured to be quite complicated.

We know that the universe is very empty. The average density of space is about one atom in every ten cubic metres – far more rarefied than any vacuum we can achieve on Earth. But even if you take all the matter away, space has a kind of elasticity which (as was recently confirmed) allows gravitational waves – ripples in space itself – to propagate through it. Moreover, we've learned that there is an exotic kind of energy in empty space itself.

Q: We first learned about this vacuum energy in the 20th century with the rise of quantum mechanics, which governs the tiny world of atoms and particles. It suggests that empty space is made up of a field of fluctuating background energy – giving rise to waves and virtual particles that pop into and out of existence. They can even create a tiny force. But what about empty space on large scales?

A: The fact that empty space exerts a large-scale force was discovered 20 years ago. Astronomers found that the expansion of the universe was accelerating. This was a surprise. The expansion had been known for more than 50 years, but everyone expected that it would be slowing down because of the gravitational pull that galaxies and other structures exert on each other. It was therefore a big surprise to find that this deceleration due to gravity was overwhelmed by something "pushing" the expansion. There is, as it were, energy latent in empty space itself, which causes a sort of repulsion which outweighs the attraction of gravity on these large scales. This phenomenon – dubbed dark energy – is the most dramatic manifestation of the fact that empty space is not featureless and irrelevant. Indeed it determines our universe's long term fate.

Q: But is there a limit to what we can know? At a scale of a trillion trillion times smaller than an atom, quantum fluctuations in spacetime can give rise to not just virtual particles, but to virtual black holes. This is a range that we cannot observe, and where we have to combine theories of gravity with quantum mechanics to probe what happens theoretically – something that's notoriously difficult to do.

A: There are several theories that aim to understand this, the most famous being . But none of these theories have yet engaged with the real world – so they are still untested speculation. But I think nearly everyone accepts that space itself could have a complicated structure on this tiny, tiny scale where gravitational and quantum effects meet.

We know that our universe has three dimensions in space: you can go left and right, backwards and forwards, up and down. Time is like a fourth dimension. But it's a strong suspicion that if you were to magnify a little point in space so that you were probing this tiny, tiny scale … you would find that it is a tightly wound origami in about five extra dimensions that we don't see. It's rather as when you look at a hosepipe from a long way away, you think it is just a line. But when you look closer, you see that one dimension was in fact three dimensions. String theory involves complex mathematics – so do the rival theories. But that's the kind of theory we're going to need if we are to understand at the deepest level the nearest to nothingness that we can imagine: namely empty space.

Q: Within our current understanding, how can we explain our entire universe expanding from nothing? Could it really just start off from a bit of fluctuating vacuum energy?

A: Some mysterious transition or fluctuation could have suddenly triggered a part of space to expand – at least that's what some theorists think. The fluctuations intrinsic to quantum theory would be able to shake the entire universe if it were squeezed to a sufficiently tiny scale. That would happen at a time of about 10-44 seconds – what's called the Planck time. That's a scale when time and space are intertwined so that the idea of a clock ticking away makes no sense. We can extrapolate our universe with high confidence back to a nanosecond, and with some confidence right back much closer to the Planck time. But thereafter, all bets are off because … physics on this scale has to be superseded by some grand, more complicated theory.

Q: If it is possible that a fluctuation of some random part of empty space gave rise to the universe, why couldn't exactly the same thing happen in another part of empty space – giving birth to parallel universes in an infinite multiverse?

A: The idea that our Big Bang is not the only one and that what we see with our telescopes is a tiny fraction of physical reality is popular among many physicists. And there are many versions of a cyclic universe. It was only 50 years ago that strong evidence for a Big Bang first emerged. But there have ever since been speculations about whether this is just an episode in a cyclic universe. And there's been growing traction for the concept that there's far more to physical reality than the volume of space and time that we can probe – even with the most powerful telescopes.

So we've no idea whether there was one Big Bang or many – there are scenarios which predict many Big Bangs and some which predict one. I think we should explore them all.

Q: How will the universe end?

A: The most straightforward long range forecast predicts that the universe goes on expanding at an accelerating rate, becomes ever emptier and ever colder. The particles in it may decay, making the dilution proceed indefinitely. We would end up with, in a sense, a huge volume of space, but it would be even emptier than space is now. That is one scenario, but there are others that involve the "direction" of dark energy reversing from repulsion to attraction, so that there will be a collapse to a so-called "Big Crunch", when the density heads towards infinity again.

There's also an idea, due to physicist Roger Penrose, that the universe goes on expanding, becoming ever more dilute, but somehow – when it's got nothing in it apart from the photons, particles of light – things can be "re-scaled," so that after this huge dilution, space becomes in a sense the generator of some new Big Bang. So that's a rather exotic version of the old cyclic universe – but please don't ask me to explain Penrose's ideas.

Q: How confident are you that science can ultimately crack what nothing is? Even if we could prove that our universe started from some strange fluctuation of a vacuum field, don't we have to ask where that vacuum field came from?

A: Sciences try to answer questions, but every time we answer them, new ones come into focus – we'll never have a complete picture. When I was starting research in the late 1960s, it was controversial whether there had been a Big Bang at all. Now that's no longer controversial, and we can say with about 2% precision what the was like all the way back from the present 13.8 billion years to a nanosecond. That is huge progress. So it's not absurdly optimistic to believe that in the next 50 years, the challenging issues about what happens at the quantum or "inflationary" eras will be understood.

But of course this raises another question: how much of science is going to be accessible to the human brain? It could turn out, for instance, that the mathematics of string theory is in some sense a correct description of reality, but that we will never be able to understand it well enough to check it against any genuine observation. Then we may have to await the emergence of some kind of post-humans to get a fuller understanding.

But everyone who ponders these mysteries should realise that the physicist's empty space – vacuum – is not the same as the philosopher's "nothing."

Explore further: Solving one of nature's great puzzles: What drives the accelerating expansion of the universe?

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julianpenrod
1 / 5 (2) Aug 15, 2018
A fact of the matter is that many have an illegitimate concept of effective physical nothingness.
Ask them what they think of and they will essentially describe a huge room with no light, no particles.
But to be a room with delineated edges of any size indicates the presence of a metric to give it a size and shape, and the presence of a physical metric requires the presence of something to define and measure that metric. Which means the area cannot be empty.
The fact is, true nothingness may have zero size and shape, too. That may be effectively even smaller than a mathematical point.
saltofthesea
1 / 5 (1) Aug 15, 2018
Zero size, zero shape, the idea of black being a color... It is extremely difficult to understand true nothingness, but it is not impossible. The telescope of understanding needs to be turned around so you can look through the other end, i.e. logically figure out what the true start of the universe had to be and construct yourself forward. And, no I'm not talking about mysticism.
Try this: https://therealro...3/16/15/
antialias_physorg
3.4 / 5 (5) Aug 16, 2018
Physicists never deal with absolutes (only as a limit that can be reached arbitrarily close...like 0K or vacuum). So when a physicist talks about 'nothing' one should not confuse this with a metaphysical/mathematical definition of 'nothing'.
dsylvan
2 / 5 (2) Aug 16, 2018
Physicists never deal with absolutes (only as a limit that can be reached arbitrarily close...like 0K or vacuum). So when a physicist talks about 'nothing' one should not confuse this with a metaphysical/mathematical definition of 'nothing'.


What about a -universal constant-? Not that we know its absolute value or even that it absolutely exists, but that if we assume it does, it is by nature inviolable, i.e. absolute?
antialias_physorg
2 / 5 (3) Aug 16, 2018
Universal constants are mathematical constructs. But if you want to know you always have to measure them by something (i.e the value of the constant is always bound to some observable). Since you cannot measure anything to the ultimate degree (sooner or later you run into uncertainty of one kind or another) then the universal constants can only be defined within a certain error bound.

Note that it isn't useful to treat such constants as 'variable on a tiny scale' because it introduces complexity that isn't relevant to physical processes. But it's not possible to ascertain full const-ness either.

E.g. you can take PI and declare it as the ratio of circumference of a circle to its diameter...but that only goes for euclidian space. The universe does not contain any truly euclidian space.
dsylvan
2 / 5 (2) Aug 16, 2018
Universal constants are mathematical constructs. But if you want to know you always have to measure them by something (i.e the value of the constant is always bound to some observable). Since you cannot measure anything to the ultimate degree (sooner or later you run into uncertainty of one kind or another) then the universal constants can only be defined within a certain error bound......


Yes you're right, universal constants are expressed as mathematical constructs--as a conceptual model or place-holder for the relationship being measured. And yes we can't measure these things to an ultimate degree. But isn't all of science based on the assumption that there's something real to be observed and measured? Some absolute context that is constant, predictable, and dependable enough to provide us a basis for any accuracy at all?
antialias_physorg
2.5 / 5 (4) Aug 16, 2018
And yes we can't measure these things to an ultimate degree. But isn't all of science based on the assumption that there's something real to be observed and measured?

Not really. It might have been in 100 years ago when people were still trying to save determinism...but at the latest with quantum mechanics in general and the Uncertainty Principle in particular the notion of an 'ultimate reality' that can be measured to the last degree went out the window.
torbjorn_b_g_larsson
1 / 5 (1) Aug 16, 2018
To add to what antialias said, there is also the theological/metaphysical "nothing" idea, which we have no definition for. As we look back we always see something, and that is what cosmology consider. Rees is making an approximation but not an equality.

The article reference to inflation is caught in decades old and rejected cosmology. As we all know there was no Big Bang singularity "before" inflation, the Hot Big Bang happened after: https://www.forbe...e5187d81 .

Another confusion is discussing energy of the universe. Inflation makes a perfectly flat universe with zero energy density; the observed eternal inflation process is consistent with a zero energy (eternal macrostate) universe.

And, no I'm not talking about mysticism.
Try this: https://therealro...3/16/15/


So why point to a mysticism page?
torbjorn_b_g_larsson
1 / 5 (1) Aug 16, 2018
And yes we can't measure these things to an ultimate degree. But isn't all of science based on the assumption that there's something real to be observed and measured?


Not really. It might have been in 100 years ago when people were still trying to save determinism...but at the latest with quantum mechanics in general and the Uncertainty Principle in particular the notion of an 'ultimate reality' that can be measured to the last degree went out the window.


I think this is mistaking uncertain observation with certainty of absolutes. The universe is zero energy, the universal speed limit is constant, Planck energy is constant, et cetera.

And as far as I understand large scale determinism of the block universe means the past and the future is decided and deterministic however quantum likelihoods play out, since it is the classic light cone that determines [sic] that.
saltofthesea
not rated yet Aug 16, 2018
...So why point to a mysticism page?


Are you saying logic is mystical? Or, are you saying that asking what is the beginning of the universe is mystical?

If not, what is your logical explanation for the beginning of the universe?
dsylvan
1 / 5 (1) Aug 17, 2018
Not really. It might have been in 100 years ago when people were still trying to save determinism....

I'm sorry to press the point, but I'm not arguing for determinism. I'm questioning your statement that physicists never deal with absolutes by raising a subtle and difficult point that relates to the article. Science is of the "measurable", even if the measuring can't be complete. Probability is founded on something predictable being probable. "Uncertainty" describes our capacity to measure and understand a phenomenon, not the phenomenon itself which may be beyond human measuring and understanding. In science, the study of "something" that can be verified as true, the idea of "nothing" is wholly irrational. But still we have that "something" and nothing other, so perhaps "absolute" or "ultimate" aren't such good words to use here, but "unequivocal" works. Otherwise why bother with science?
dsylvan
1 / 5 (1) Aug 17, 2018
I'm suggesting that ultimately physicists have nothing but absolutes to work with. :)
antialias_physorg
2.3 / 5 (3) Aug 17, 2018
Physicists deal with the *idea* of absolutes but not with absolutes in reality.

Natural laws aren't anything in particular (they have no existence in and of themselves). They are useful shorthands to group apparently (and sometimes only seemingly) consistent behaviors. There is no underlying rule-book with constants that the universe consults every time something happens. Some of these things are just very stable (like the speed of light).
So while you can define constant stuff (i.e. write down the idea) you can never determine whether the constant you defined is actually absolutely correct.

As the old saying goes: The map is not the territory.
saltofthesea
not rated yet Aug 17, 2018
@ dsylvan @ antialias

Physicists deal with the *idea* of absolutes but not with absolutes in reality.

As the old saying goes: The map is not the territory.


It seems as though you agree with each other, but perhaps it needs to be said that science is a world entirely of ideas, of concepts, while the natural world does what it does no matter what you believe. Honeybear don't care!

The problems of discussing these things (science and nature) arise when we forget (perhaps because the scientific descriptions sound so good) that they always are and will remain separate things.

Correspondence is in the eye of the beholder.

Cognitive development seems to be the determining link between the two. But, what do I 'know', heh, heh.
dsylvan
1 / 5 (1) Aug 17, 2018
Physicists deal with the *idea* of absolutes... The map is not the territory.

Well said. I (absolutely) agree. But--

There is no underlying rule-book with constants that the universe consults every time something happens.

Do you realize how unsupportable this statement is? --in the context of everything that so reliably and persistently happens? There is indeed some sort of rule book. This is what science is trying to discern. The territory will always determine how accurate the map is--never the other way around.
There's reality -- and then there's the meta-reality of ideas. We routinely conflate the two as saltofthesea points out. I'm just suggesting that the "rule book", the territory, is the only trustworthy context we have to judge the veracity of our ideas--let alone the capacity to have an idea--and that the rules are non-negotiable, even as we persist in debating what the rules actually are. This is why I love science.
antialias_physorg
1 / 5 (2) Aug 17, 2018
Do you realize how unsupportable this statement is?

Well, if we can agree that the universe has a finite age and that the laws are specific to the universe then it's already supported, no?

I agree that we should use science because it's the best possible tool we have. We should strive to describe the universe with the simplest theories we can. The uitility of this approach is enormous.
But we should not forget that science is a utility based approach - not a search for 'ultimate truth' or somesuch (...and I say this as a former scientist)

It's a bit like the search for perfection. Let's say you find something that seems to be perfect: How can you *know* it's perfect? There's nothing to gauge it against (i.e. no gold standard). You can define one. E.g. defining speed of light as a constant - which works extremely well - but you'll never know if it holds true to the last decimal place.
saltofthesea
not rated yet Aug 17, 2018
@anti & @dsylvan
I think you guys are talking past each other. What I understand dsylvan to be saying is that reality is the ultimate arbiter, but for some reason he chooses to call that the "rule book". That threw me until I worked out what he means to say. To extend the metaphor he's using, I would say that reality, "the territory", is more like the judge. i.e. reality will whack you if your understanding is wrong. While anti seems to want to have a limit on what can be called science. While I agree with both of you in many ways, it seems the issues of epistemology, cognition and agreed terminology are tripping up the conversation.

May I suggest reading the essay I posted the link to above: https://therealro...3/16/15/

It is not exhaustive, but it deals with many of the issues you both have raised. In its very nature, the essay will challenge some concepts that you each put on the table, but you might find it useful to continuing this conversation.
saltofthesea
not rated yet Aug 17, 2018
To the moderator/monitor of the phys.org comment section:
Yes, you must have some kind of size limit on comments, but only 1000 characters for a physics website? Really??? I think I might be able to find German WORDS that contain 1000 characters. Is there some reason not to raise it to something reasonable, say 2500 characters? I often find I'm just hitting my stride and I have run out of room for my main idea. What say you? And, why not simple html tags? Scientists have feelings too.
dsylvan
1 / 5 (1) Aug 17, 2018
Well, if we can agree that the universe has a finite age and that the laws are specific to the universe then it's already supported, no?

I'm sorry. I didn't mean that your statement is unsupportable. I meant that the idea that there's no underlying rule book governing the universe is inconsistent with a universe that seems to follow specific rules. Yes, the universe as is, is well supported by what we've discerned of it. How could it be otherwise?
Perhaps there's a prejudice inherent to the human mind against a search for the "ultimate truth" if some part of that truth is yet beyond the mind's grasp. If the truth is synonymous with what actually is, the reality we inhabit, than a search for the "absolute" truth is nothing other than the attempt to discern reality.
Yes, science is a utility based approach--using the ultimate utility=reality. it's the only truly rational approach. Much of the motivation for science is the discovery of reality's absolute (inherent) perfection.

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