Many things can “happen” inside a quantum wave function, of course, including everything that actually does happen — formation of galaxies, origin of life, Lady Gaga concerts, you name it. But given a certain quantum wave function, what actual is happening inside it?
A surprisingly hard problem! Basically because, unlike in classical mechanics, in quantum mechanics the wave function describes superpositions of different possible measurement outcomes. And you can easily cook up situations where a single wave function can be written in many different ways as superpositions of different things. Indeed, it’s inevitable; a humble quantum spin can be written as a superposition of “spinning clockwise” or “spinning counterclockwise” with respect to the z-axis, but it can equally well be written as a superposition of similar behavior with respect to the z-axis, or indeed any axis at all. Which one is “really happening”?
Answer: none of them is “really happening” as opposed to any of the others. The possible measurement outcomes (in this case, spinning clockwise or counterclockwise with respect to some chosen axis) only become “real” when you actually measure the thing. Put more objectively: when the quantum system interacts with a large number of degrees of freedom, becomes entangled with them, and decoherence occurs. But the perfectly general and rigorous picture of all that process is still not completely developed.
So to get some intuition, let’s start with the simplest possible version of the problem: what happens inside a wave function (describing “system” but also “measurement device” and really, the whole universe) that is completely stationary? I.e., what dynamically processes are occurring while the wave function isn’t changing at all?
You’re first guess here — nothing at all “happens” inside a wave function that doesn’t evolve with time — is completely correct. That’s what I explain in the video above, of a talk I gave at the Philosophy of Cosmology workshop in Tenerife. The talk is based on my recent paper with Kim Boddy and Jason Pollack.
Surprisingly, this claim — “nothing is happening if the quantum state isn’t changing with time” — manages to be controversial! People have this idea that a time-independent quantum state has a rich inner life, with civilizations rising and falling within even though the state is literally exactly the same at every moment in time. I’m not precisely sure why. It would be more understandable if that belief got you something good, like an answer to some pressing cosmological problem. But it’s the opposite — believing that all sorts of things are happening inside a time-independent state creates cosmological problems, in particular the Boltzmann Brain problem, where conscious observers keep popping into existence in empty space. So we’re in the funny situation where believing the correct thing — that nothing is happening when the quantum state isn’t changing — solves a problem, and yet some people prefer to believe the incorrect thing, even though that creates problems for them.
Quantum mechanics is a funny thing.
I don’t know why some people are having trouble watching the video; it works fine for me. I just copy and paste the embed code that YouTube gives me. I suspect eigenman is right and some people need to upgrade their browsers.
Luke– I suspect the answer is “no.” An horizon-size volume with one particle is not maximum-entropy, but it’s extremely close. I don’t think there is room there for an entropy-increasing process that would split the wavefunction into multiple decoherent branches, one of which has a Boltzmann Brain in it.
I have a silly, slightly unrelated question (I got there through the Boltzmann Brains link).
Is entropy really increasing?
I mean, there certainly is many more ways how to arrange particles in a large box than there is to arrange them in a small box. And the universe is expanding.
Universe might have been extremely chaotic, at its highest possible entropy state right after the Big Bang. And ever since then the entropy is decreasing because the universe expands, making more and more microstates reachable. We only experience increasing entropy on our level because we don’t see and experience universe expansion, it is irrelevant to our small planet and our tiny experiments.
I have the latest Internet Explorer 11 for Windows 7 and Sean’s videos still don’t play whereas other videos from other sites do play.
I sprinkled magic dust and videos should be playing now.
Yes Sean, it now does play! Thanks for fixing it. Hmm! Could I buy a box of that magic dust? It sure would come in handy.
Can changes occur within a timeless Wheeler-deWitt solution?
“nothing at all “happens” inside a wave function that doesn’t evolve with time ” does this mean we can say that the net energy of the universe is non-zero?
It seems to me that the key word in Sean’s post is “decoherence”. For those of us trying to grasp the contradictions of quantum theory and its experimental results vs. the universe we actually interact with, decoherence gives us the explanation.
The best description I have read of decoherence came from Brian Greene in “The Fabric of the Cosmos”
In de Broglie’s double solution theory there is the physical wave which guides the particle and the wave-function wave which is statistical, non-physical and is used to determine the probabilistic results of experiments.
As a lay person, this is one of those times where the topic sounds incredibly interesting, but I don’t think I quite understand (and wish I did!). Are you effectively saying that “nothing” is happening in the universe as a whole? I suppose my confusion lies in that, if we zoom out enough, do we consider the “universe” as a whole evolving through time, or do we think of time as a construct within the universe?
If anyone could give a for-the-public bullet point spin on this, I’d love it!
Sean Carroll says:
March 11, 2015 at 10:58 am
Bob– This isn’t directly tied to inflation. We’re talking about true de Sitter space, with no rolling inflaton. And the horizon doesn’t “measure” the quantum state of the vacuum; it’s stationary itself.
))))))))))
Sean I was finally able to watch the Video of your talk and I found it pretty enlightening. One area where confusion still reigns for me is your argument that what you’re saying brings into question the eternal nature of inflation. The graceful exit problem required new inflation where you have a slow roll of the inflaton potential. Doesn’t this require a measurement process , hence must involve quantum fluctuations, giving us eternal inflation and the Multiverse? Another question I have is assuming what you propose is true, how is the interesting model you proposed with Chen affected. I think you maintain that it’s not, but I don’t understand why you say this.
Mr. Carroll,
Why did you delete my question? Was it inappropriate? If so, how was it inappropriate?
Mr. Carroll,
I don’t think my question about quantum mechanics was inappropriate,
and I would greatly appreciate a response. Here it is again,
You said in a previous post: “If God exists but has no effect on the
world whatsoever ? the actual world we experience could be precisely
the same even without God ? then there is no reason to believe in it,
and indeed one can draw no conclusions whatsoever (about right and
wrong, the meaning of life, etc.) from positing it.”
Doesn’t the same statement apply to the many worlds in the MWI?
Anyway, if I’m missing something or misunderstand the difference,
please let me know! I’d greatly appreciate it, Mr. Carroll!
Yup,
exactly why we must assume that nature randomly jumps every ~planck time.
I’ll name this the Mini-Worlds Interpretation of Quantum Mechanics
(since the universe only exists in superposition for ~10^-43 secs – and yes people, the mathematics is otherwise identical to standard quantum mechanics, we still need the unitary evolution of the entire state-vector after each “jump” so that the rest of the universe knows what has happened – why unitary? Well it’s about the simplest way to update the entire universe with information on how a single state has just randomly changed, and keep things finite)
NB mathematically speaking we have superpositions until we do a measurement, and the quantum zeno effect etc all obey the mathematical formalism
The wavefunction is just an abstract object that represents the set of possible measurement outcomes for a certain state preparation. For stationary states, this means the probability of any value is unchanging in time. One could interpret this as meaning that the particle, over time, is bouncing around such that it takes on values of its observables according to its state probability distribution.
I think that’s unlikely though, as this implies the counterfactual claim that “when” you choose to measure a system affects the measurement outcome. Obviously this is true when you consider entanglement with the environment and measurement apparatus, but this interpretation claims that the time dependence exists even without these physical limitations. Even though, statistically, this would still obey time symmetry, any given individual measurement would not. This would imply time symmetry is an emergent property statistically, not an inherent one. This seems like a really inconvenient interpretation if not completely incorrect since arbitrary subsets of world lines through quantum realizations would lack time symmetry.
damn it, Rutgers.
35:30 on; sounds good. I’ll take option #2 on the menu.
>”Even though, statistically, this would still obey time symmetry, any given individual measurement would not. This would imply time symmetry is an emergent property statistically, not an inherent one.”<
eh…I think this is where the label "equilibrium" becomes important, Senor Kerr.
“eh…I think this is where the label “equilibrium” becomes important, Senor Kerr.”
Sure, but it’s kind of a big deal that any instance of the Galilei or Poincare group is only statistically going to follow those group symmetries. Nothing is constraining individual measurements in the formalism of quantum mechanics, but it seems philosophically problematic if these individual particles don’t obey the group symmetries counterfactually. I would like to think a single, identifiable free particle in the ground state of a harmonic oscillator, when left untouched, is not literally jumping discontinuously from momentum value to momentum value.
Counterfactually, I would hope a measurement done at time t2 instead of time t1 would yield a momentum value along the trajectory of the value we would have measured at t1. It’s meaningless to speak of such counterfactual trajectories if you assert that stationary states have no dynamics, but if you assert otherwise, then these counterfactual trajectories have to exist and have these weird properties in conflict with the group symmetries. At “equilibrium” these symmetries will hold in a statistical sense, but it just seems like a bloated belief to accept all of the above.
Spin is a funny thing. The mathematical equations of quantum spin describe an object that is actually spinning, but a lot of people then say that they do not actually spin. Then those people are often the same type of people that do not believe in higher dimensions. It makes one wonder how much more worth the mathematics have when they tell us one thing, but it is not actually supposed to be what is actually happening.
I guess the discovery of spin comes from finding out if a particle would be attracted to a negative or positive field and that can change, but it is always by the same amount, from reading a link about an experiment here a little while ago. That struck me as odd, because I assumed that particles always have a positive or negative charge from reading about them.
If you thought of a particle like a bar magnet spinning around, then it was expected the particle would be attracted to the positive or negative field by different amounts from getting caught halfway in a turn of a real spin. What if you had a bar magnet traveling the speed of light? The bar magnet would be contracted to zero in length, and if the magnets north or south pole wasn’t facing the field then the source of the field wouldn’t see anything. It would be like looking at the side of a 2-dimensional plane with no height. It would take a transfer of a photon to transmit the force of electromagnetism. If it can’t be seen from that angle, then it wouldn’t feel the force of electromagnetism either.
Oh, it just so happens that particles with spin travel at or close to the speed of light… Then a rotation would be a form of acceleration, so then we could know that it is the particle that is actually the one spinning.
Somebody should figure out how this many-worlds interpretation Carroll is excited about works in the Ontological Models framework.
Won’t SOMEONE answer my questions? Mr. Carroll?
I’m a layman, I don’t get it.
So Sean Carroll says, that after 10^120 years the universe / our hubble sphere by then will become a stationary vacuum, from which nothing ever will come (- if you don’t poke the fluctuation, there is none?). Whereas Lawrence Krauss claims, that the universe could come from “nothing”/ a dynamical fluctuating vacuum (- vacuum always fluctuates, wether you poke it or not?)
Are these vacua different (- past hypothesis / low entropy at the beginning, maximum entropy at the end), so that both hypothesies can co-exist, or is Sean saying, that Lawrence is wrong? Or did I misinterpret one or both of them?
To rephrase it in different terms: If the wave function is not the potential from which history brings into being reality, but is “empty” to start with, then the universe really came from nothing? Or is the wave function at the end of the universe different, than at the beginning (- nothing vs everything)?
I just have no grasp on words like “wave function” and vacuum it seems.
When there is a flow, there is something, when the flow stops there is nothing – the flow is “powered” by / “meassured in” entropy? It all sounds like “the prime mover” / infinite regress problem. I’m probably mixing a few things up here.
For as long as nothing fluctuates into being, is the time-part of spacetime zero, and only space exists? How can something fluctuate into being, when there is no time to fluctuate while? How can something fluctuate, when there is nothing there? I’m hopelessly classical, and hopelessly lost. There is definitely something in conflict about my conceptions.
How can you make meaningful statements regarding the quantum stasis of closed systems when to know so would require observation and, therefore, an opening of the system? If math and physics are inherently linguistic, is this apparent incongruity merely an artifact of the (in)ability of language to describe anything at all??
@Meleny
MWI does not posit many worlds, it predicts them. Unlike your other hypothesis.
An excellent analysis of what it is and how the wave function works.
My only suggested improvement is to the last comment on Quantum Mechanics- it is not “funny” but “strange”, i.e. dealing with things outside and beyond common experience. Even predicting things unimaginable like particles being in two places at once or General Relativity’s prediction of Black Holes in Space-Time. The provence of Extreme Physics is indeed “Strange”.
@Ravi Ivaturi
I don’t think you answered my question. If you replace every instance of ‘God’ with ‘many worlds’, don’t you get an equivalent statement?