Imagine you’re leading a game of 20 questions but halfway through you forgot the thing
you chose
You have to keep answering yesses and nos and hope that you think of something that’s
consistent with all your previous questions before the game is done. Well it could be
that’s what the entire universe is doing as it creates our reality. I hope it thinks
of something good before we run out of questions.
Scientists are fairly attached to the idea that the universe is real. We’re fond of
the realist viewpoint: that there’s an objective reality out there that’s entirely independent
of our subjective experience of it. That seems innocent enough, and it served physics awfully
well. But take this viewpoint to the extreme and we imagine the scientist as this perfectly
detached observer of the world, capable of monitoring and modeling physical reality without
ever disturbing it.
But then came the 20th century, and quantum mechanics. In quantum mechanics, not only
does the act of measurement profoundly influence the thing being measured, but it seems we
can’t even assign physicality to the world between measurements. As Niels Bohr put it
“no phenomenon is a phenomenon, until it is an observed phenomenon”. Bohr led the
charge with this observer-dependent view, encapsulated in his Copenhagen interpretation
of quantum mechanics, which insisted that the world only became meaningfully defined
through the act of measurement. Others were desperate to maintain a physical and realist
universe, including Albert Einstein himself. We’ve talked about this debate between the
founders of quantum theory before, and of some of the supposed resolutions, which I’ll
come back to. Today I want to begin a discussion of a family of quantum interpretations which
say that the universe exists not so much in physical particles and quantum fields, nor
solely in the mind of the observer, but rather in the interaction of the two.
To give a better sense of why we need to go to these crazy lengths to explain the universe,
let’s review some quantum weirdness. We’ll start with the good ol’ Schrodinger’s
cat thought experiment, devised by ardent realist Erwin Schrodinger to highlight the
absurdity of the Copenhagen viewpoints. We have a cat in a closed box that is either
killed or not killed by a random quantum process. For a scientist running the experiment, the
cat is in a twin state of both alive and dead until the box is open - a superposition of
states. Or so quantum mechanics seems to suggest. This seems even more absurd when you add the
so-called Wigner’s friend extension, where a second scientist waits on the report from
the first, rendering both cat and first scientist into a superposition of states. And doesn’t
the cat get to observe its own state? Perhaps to the cat, the scientists are in a similar
superposition,
The heart of the problem was a simple question: in a world where observation plays a role
in defining reality, how is it possible for separate observers to share a single, consistent
reality? Solutions to this seeming paradox spanned
the spectrum. For example, the Hungarian physicists John von Neumann and Eugene Wigner doubled
down on the observer-centric view, in which the act of observation in some way creates
reality. Others found schemes by which a real, physical universe could be interpreted in
the behavior of the wavefunction, such as de Broglie-Bohm pilot wave theory or objective
collapse interpretations. Still others sought to explain the observer’s influence as a
sort of selection bias - for example, Hugh Everett’s many worlds interpretation and
the ideas of quantum decoherence. And then we have Richard Feynmann’s “no interpretation”
interpretation of quantum mechanics, in which he suggested that his colleagues quit philosophizing
and to “shut up and calculate”.
In the end, most physicists took that advice. They were eager to put the whole mess behind
them and get on with the job of actually using quantum mechanics to invent the modern world.
But a very select few physicists refused to let it go. One of those was John Archibald
Wheeler, a prolific physicist whose students included Feymnann, Everett, Jacob Bekenstein,
and Kip Thorn among others. Wheeler was never completely satisfied with any of the proposed
quantum interpretations. Although he started out as a pure realist, he came to believe
that the observer must in some way be fundamental.
And yet he couldn’t accept what he saw as the solipsistic view of von Neumann and Wigner,
in which the observer was in a sense the primary causal agent and center of its own reality.
After decades of thought, Wheeler came to a view that is summarized in his expression
“it from bit.” In his words, “Every it — every particle, every field of force,
even the spacetime continuum itself — derives its function, its meaning, its very existence
entirely… from the apparatus-elicited answers to yes or no questions, binary choices, bits.”
In other words, he came to believe that the universe is fundamentally informational.
Wheeler felt that reality must have its primary existence not in the observer nor in the subject of
observation, but rather in the connection between the two. One of the thought experiments
that led Wheeler to his view is his delayed-choice experiment, which is an analogy to the double-slit
experiment. Here’s one example of this. Imagine a single photon hitting a beam splitter
- a semi-reflective mirror that has a 50-50 chance of transmitting or reflecting the photon.
If we add a pair of detectors we see that each photon randomly arrives in detector 1
or detector 2, revealing whether it was transmitted or reflected by the beamsplitter. But remember
I said that the quantum world appears to live in a state of uncertainty until it’s observed?
In this case, quantum mechanics states that the photon is in a state of having been both
transmitted and reflected until we make an observation to check which happened.
We can see this if we add a second beamsplitter to scramble the beams so that we don’t know
which path brings the photon to detectors 1 versus 2. Now detector 1 always registers
a photon and detector 2 never does. Why? Well if the photon was traveling both paths at
the same time, then the combination of phase shifts in the beamsplitters causes the photons
wavefunction to perfectly line up in detector 1 - constructive interference, and to perfectly
cancel out in detector 2 - destructive interference leaving no signal . For that to work this
pair of “possible photons” had to travel both paths.
This was called the delayed choice experiment because, in principle, the second beamsplitter
could be put in place only after the photons passed through the first. After they’d made
their decision of a path to take. This experiment, and many clever variations of it, have now
been performed. They seem to verify that the universe really does exist in a superposition
of all possible states until the measurement is made, and the choice of that measurement
can be made after the superposition is formed. That means that the reality that emerges depends
in a profound way on the choice of measurement.
Here’s another experiment. If we measure the spin of one member of a pair of entangled
particles - the choice of the direction of the spin measurement seems to have an instantaneous
influence on its partner, even if that partner is on the other side of the lab - or on the other
side of the galaxy. We’ve talked about this entanglement stuff in a previous episode.
So it seems that when you interrogate the universe, the answers that you get depend
on the questions you ask. Wheeler became convinced that that process of interrogation in some
way creates reality.
To help explain what he meant, Wheeler invented a pathological version of the game 20 questions.
In the usual game, someone, let’s say Alice, thinks of an object, and another person, Bob,
asks yes/no questions to narrow down the possibilities until he arrives at only one possible answer:
the object that Alice had in her mind from the beginning.
But Wheeler’s version is a little different. He called it “negative 20 questions,”
and it works like this. Unbeknownst to Bob, Alice never thought of an object to begin
with. Instead, she answers the quest ions as they come, maybe completely randomly, maybe
according to some secret pattern. Every answer narrows the space of possible correct answer,
even if Alice still doesn’t know what the object is.
Let’s say the rule is that Alice will answer yes and no alternately no matter what the
question. So Bob asks, “is it smaller than a bread box?” “yes.” Then “is it living?”
“no.” This line of questioning has narrowed Bob to the realm of papercl ips and pebbles.
But had Bob reversed the order of the questions he’d have interrogated his way towards tigers,
trees and T-rexes. And depending on Alice’s rules, you can imagine scenarios where it
looks like Bob’s choice of question influence the past. But it doesn’t, his questions
are helping reality define itself on the fly.
Wheeler came to the view that the universe was one giant game of negative 20 questions,
in which the reality we observe is the only reality that is consistent with the questions
asked of it by all observers. The universe, according to Wheeler, was a giant, closed,
“self-excited circuit.” The relationship between the observer (us) and the observed
(the universe) brought *both* into existence. Take this yet further and you have a version
of the Strong Anthropic Principle: The only reality that can exist is one that can have
observers whose questions can make that reality real.
Wheeler called this the “participatory universe.” He symbolized his idea with a sketch of a
giant “U”. On one side of the U is the big bang. As the universe expands, the line
of the U gets thicker, until we reach the present day, the other side of the U. An eyeball
here symbolizes the observer. All the astronomers with their telescopes and observatories are
aimed back at the Big Bang.
Their observations *now* cause the Big Bang *then*. Well, sort of. As Wheeler once wrote,
“the past has no existence except as it is recorded in the present.” Only the self-consistent
now can have a real existence.
As I warned, this all sounds very mystical. Let me be clear: Wheeler wasn’t saying
that conscious minds have direct power over reality, as claimed by some of the worst practitioners
of quantum woo. But the role of consciousness was something Wheeler really struggled with.
He said “About no feature of “It from Bit’ do I feel less comfortable than whose
bit.” After all, information implies a thing to KNOW that information. But non-conscious
entities can, in a sense, know something if information of that something is recorded
inside their being somehow. He also once wrote that it was “Not consciousness but the
distinction between the probe and the probed” that was central to the act of observation.
So “Observation” could well be code for “interaction”, in which every time two
particles bump together and become entangled we have an act of measurement. If this all
seems a little vague and wishy washy, it’s because it is. Wheeler was still obsessively
thinking and writing about and trying to solidify these ideas until his death in 2008.
In John Archibald Wheeler’s version of an informational universe, he felt that the information
- the bits - resided in the answers elicited by acts of observation. It appears to pull
the comforting ground of realism from under our feet. But others have taken Wheeler’s
ideas and put them back on firm ground. It’s possible to think of the universe as being
“informational” without altogether giving up on realism. In the broad category of “participatory
realism” we have things like quantum bayesianism and relational quantum mechanics, in which
the universe emerges from the information that some set of real entities have about
each other. Crucially, these acknowledge the importance of the observer in defining the
observed, while still allowing that there’s some kind of substrate to reality. We’ll
talk about these ideas soon, in our long quest to find that substrate. And do that we just
need to keep asking the right questions of a participatory space time.
Hey Everyone. Before we get to comments, we have a small favor to ask. We want to check
in with all of you to see what episode ideas you have, what new topics you’d like us
to explore, and what ideas you’d like us to revisit. And in order to organize all this
great information, we’ve set up a very very short, 8 question survey. We love hearing
your ideas and thoughts and if you have a moment, there’s a link in the description.
Thank you!
It may be that we live in a participatory universe that’s self-generated by its observers.
Or that could be complete nonsense. But I can say with certainty that PBS Space Time
is generated by participation of its observers - that’s you guys - and most especially
by the participation of our wonderful supporters on Patreon. And today I want to give an extra
special thanks to Mark Evans, who’s supporting us at the big bang level. Mark, in creating
a universe or a show about the universe, every little bit counts, but there’s nothing bitsy
nor itsy about your generosity. Thanks for helping make it happen.
Today we're going to look at your comments from our two most recent episodes in which
we asked: Where is the center of the universe? (if it even has one) and could a universe
actually be inside a black hole.
In reference to the question “What is the universe expanding into?”, John Retherford
asks whether a fair answer would be “It’s expanding into the future.” I don’t know
how fair it would be, but I’m going to steal it anyway. But it’s at least as fair as
saying that the center of the universe and the location of the big bang are points in
time in the past. So, they’re fair as long as you explain what you mean by that - or
point them to PBS Space Time episode for the explanation.
Comrades of Perístanom asks if the geometry of the universe - close, open, flat - dictates
the “temporal shape”. As in does a closed universe necessarily collapse, and an open
one expand forever, and a flat one tend to zero expansion. The answer is absolutely yes
in the good old days when we didn’t have dark energy. If you just consider the positive
effect of gravity, then the spatial geometry dictates the final fate. But dark energy screws
everything up. The presence of dark energy tends to flatten the universe because it’s
a positive energy density, just like matter. But it’s negative pressure can allow even
a closed universe to expand forever. Thanks, dark energy.
Some of you asked about the possibility of detecting Hawking radiation in the case that
the universe is a black hole. Actually, we should expect a type of Hawking radiation
from the cosmic event horizon even if the universe isn’t a black hole. All event horizons
will lead to a disconnection in the vibrational modes of the quantum fields in a way that
looks like thermal radiation. That radiation has a wavelength that’s on the scale of
the event horizon. So the horizon radiation from the cosmic event horizon would have wavelength
roughly the size of the observable universe. Unfortunately that makes it not very detectable
without a universe-sized camera. It would also have such low energy that it wouldn’t
have any effect on any sane timescale.
Z argues that the universe can't be a black hole because the difference between the masses
of our largest black holes in the our universe and the whole universe is much too great.
That would mean for the universe, for the universe to be a black hole would require
very different physics. In a way that’s right. If the universe is a black or a white
hole then it formed by a different process than the black holes that we’ve observed
in our universe. The gap between the masses is too enormous for it to just be some extreme
outlier of the same process. But that’s not the same as saying it requiring different
physics - general relativity permits white or black holes of any size.
By the way - fun fact - if you calculate the size of a black hole with the mass of our
observable universe - adding together all the stars, dark matter, other black holes,
etc - then it’s event horizon is the same size as our cosmological even horizon. Sounds
too much of a coincidence, right? Well actually, this is just another way of saying that the
universe is just below the critical mass density to avoid recollapse. At least in the absence
of dark energy. Thanks dark energy. That “coincidence” itself may require some explanation, but the
explanation still isn’t that we’re in a black hole. And it’s not aliens or a
simulation either. Probably.