MWI: Many Worlds interpretation of quantum mechanics explained | featuring Sean Carroll

The many worlds interpretation of quantum mechanics was put forth by graduate student Hugh Everett in 1957. It was considered preposterous at the time, but is now going mainstream. It requires us to change our paradigm about our experience of reality, and consider that there may be many worlds where every possible quantum outcome happens. And that we are living in just one of those branches of the universe at any one moment.

I spoke to the authority on this subject: Prof Sean Carroll of Caltech. That interview along with the explanation and comparison of the Many Worlds interpretation compared to the Copenhagen interpretation is featured here.

the Copenhagen interpretation was championed by Neils Bohr in the 1920’s. It suggests that objective reality doesn’t exist until it is observed or measured. The quantum world is governed by a set of probabilities as described by a wave function that evolves over time in the Schrodinger equation. The act of measuring forces the set of probabilities to randomly assume only one possible value.

But the many worlds interpretation says that the entire universe is in a state of superposition. A measurement may look like a particle has some set of properties, but that is not the overall reality. It posits that the world splits every time we THINK a quantum measurement is made. And that although we may see one thing in our world, there is another world in which another thing has occurred.

So for example, if we are trying to measure the spin state of an electron, the MWI says that our measuring device becomes entangled with the electron. This causes something called decoherence. The decoherence splits the universe in two such that in one universe the device measures spin up, and in the second universe, the device measures spin down. Both universes exist. We just happen to experience one of these universes. We could just as well be in either universe.

And similarly, this kind of decoherence is happening all the time. And in every instance this happens, the universe splits. So this is why it is called many worlds, because many such branches or splits of the world exist simultaneously.

In the Copenhagen interpretation, there are two sets of rules. One set of rules applies to systems prior to measurement, and a different set of rules applies for systems after measurement.

The many worlds interpretations says, no, this is not the way the universe works, that there are only one set of rules that abide by and evolve over time according to the Schrodinger equation. And there is no randomness because all possible outcomes are a branch of the many worlds.

The Good points about the MWI are:
1) it gets rid of measurement problem.
2) It lets us apply quantum mechanics to the entire universe.
3) there is no randomness.

The bad points about MWI are:
1) How does branching occur?
2) How is energy conserved.
3) Why do we see only one world, if the other worlds are equally present?

Branching occurs by decoherence, which is not quite like wave collapse. It can be thought of as a loss of information to the environment. When an isolated quantum system like say an electron gets entangled with its environment like photons and other molecules that may be present, this has the effect of a transfer of quantum information. All of the photons and atoms that bounce off the electron are agents of decoherence, and can fix its position in space and give it a sharp outline.
This is decoherence and causes the branching. This is how quantum systems can start behaving like classical system.
#quantummechanics
#manyworldsinterpretation
#quantumphysics

The total energy of the universe and all its branches is conserved analogous to the way that the all the individual probabilities inherent in the Schrodinger equation add up to one.

I asked Sean Carroll about that, and he, like me, hates that idea. I myself think that physics is more than calculations, it is a science that tries to get at the truth about what the true nature of reality actually is. That’s what we should really be after, I feel. And that pursuit at least on this channel will always exist.

[ArvinAsh]

Here are a couple of additional interesting points from Sean Carroll:
1) The theoretical total possible number of universes is not infinite, but is 2^10^122 (2 to the power 10 to the power 122). It is a lot, but is not infinite. How is this number derived you ask? It is is the total possible entropy of the universe based on its observable volume. Another way to calculate it - If the entire universe was one giant black hole, this number would be its entropy!

2) Sean Carroll pointed out to me that he would NOT say, like I do in the video, that "there are no probabilities." He'd say this: "...that the *theory* is deterministic (so there is no fundamental randomness), but there is uncertainty, namely uncertainty about which branch you are on when the universe splits. Otherwise you wouldn't be able to understand what we observe, which is that quantum events are unpredictable."




5) Did you see me eat the apple or the lollipop?

[theraven]

I’m still confused, but at a much higher level.

[amando]

Pretty crazy that in another universe he ate an apple.

[christosmakariou]

In the other universes my other selfs get to understand this completly!

[esra_erimez]

What is the flavor of the lollipop that I'm watching you eat right now?

[gurumage]

I don't think we should be carelessly splitting the universe with apps like that . Rick broke his universe that way and spawned a bunch of schrodinger's cats.

[taquiupa]

The problem here, I think, this theory of many worlds can never be proved and it will remain in the realm of speculation forever.

[frankfreaksout]

"Just shut up and calculate"
lol I like how even some scientists are enjoying shutting part of their brain off, so they can continue the life they're already living, and don't have an existential crisis.

[gregoryfloriolli]

I’m in the same boat as you were I came in as skeptical but ended up being a little less skeptical.

[Smrtcz]

So, there Is an universe in which I understand all this? Unbelievable 😅

[ditchweed]

Maybe we live in many worlds but think we're only in this one.

[LeViIain]

I'm no math expert but I know that you can create equations that work on paper but fall apart when tested in the real world because there's something you didn't take into consideration. In other words, you can't know if your equation is complete until you tested it in the real world.
So without physical proof of a parallel universe, you might not be able to prove that the many worlds theory is incomplete, but you also can't prove that it is complete.
I don't mean no disrespect and I'm most likely wrong, but this theory feels like they just took the problem elsewhere, "completed" the Copenhagen theory, and call it a day.

[colinmoffat]

I loved this video, I just finished Sean's book earlier this month and seeing you pop up with another amazingly detailed and open discussion about it with Sean! Amazing I don't know where I sit, I think I'm just as skeptical of both interpretations! Excited for more discoveries and human applications of QM!

[Hkari_]

Me, as someone who endorses determinism (even superdeterminism) and doesn't believe in free will, the many worlds theory just sounds like a lazy workaround for figuring out quantum mechanics. There is just too much that is almost impossible to prove. The mirror in that lab probably has structural characteristic that made that photon take a certain path at a certain time/angle.

[Inteli]

So we measure a particle and the "world", including us, branches? I am like John Snow, know nothing, but I think there's a huge gap in this logic.

[mr.markusi]

I'm not sure why, but all your videos or of deepest interest for me. Like they were made for me. Would like to have some Videos on Quantumcomputing.

[mrloop]

In another universe the emperor might wear some extravagant clothing. In this universe he wears nothing.

[Flyanb]

So many worlds is a feel good way for some scientists to get around super determinism? It’s still deterministic? Right? But not because every possibility that could happen does? It’s complex. I feel like KISS tells us super determinism is better? Thanks for the video! But, I don’t think we are any closer to the truth and my mind is still fried.

[davidtunstall]

Great video Arvin, I loved listening to the interview. You would make a great podcast host.

[LimbDee]

I don't know whether it is a comfort or torture that in one universe she will take my wrinkled hand one day and say: "we did have a wonderful life together, didn't we?" And I will reply: "yes my dove, we certainly had".