Something I've never understood and would greatly appreciate understanding: could someone ELI"5" how the concept of "parallel universes" will not always be a metaphysical instead of a scientific question?

If there are truly separate "universes", then by definition wouldn't it be the case that there would be no way to test (or falsify) whether they exist or not, so would the question of their existence not be in the realm of philosophy and metaphysics rather than the sciences? What am I missing (I assume it has to do with my understanding of the word "universe")?

At this point, if we discovered something resembling the current conceptions of parallel universe, we would probably just re-define "universe" to refer to what we traditionally think of as a universe. In much the same way that we do not have any problem with talking about sub-atomic particles; even though that is sub-atomic is a concept that does not make any sense.

In this case, from the article, it sounds like there is only a very weak interaction. In fact, since it is only "evidence" left behind in background radiation, it is possible that there is no ongoing interaction, but rather some remaining artifact from when the universes did interact.

For now, we can define the current Univers as a set of universal constants that rule it.

Other constants would actually lead to a different universe.

Now, if we strech our imagination, let's have an "space" (meta universe ) where universes are just regions where this contants are active, and there is a slow transision from one "unverse" region to the other. Much like arctic reagion vs. tropical. However, this transision is unlike anything, could be a spacetime/quantum level/?? transition. At this moment, this transition is not measurable.

>even though that is sub-atomic is a concept that does not make any sense.

Can you please explain this? How is sub-atomic nonsensical? It just means "constituent particles of an atom."

That's exactly it. Atom comes from atomos, meaning indivisible; if you take the literal meaning, it has no constituent parts.

But that's pretty unreasonable as a line of argument; the derivation of a word from a long dead language has no bearing on the semantic in modern discourse. Atoms have been known to be divisible for over 100 years, the consequences of this knowledge are everywhere in our society!

It was in modern usage prior to being adopted in chemistry, it isn't just the derivation.

But that's just the point, instead of using a new word for atoms when sub particles were discovered, the existing meaning was set aside.

we still use atomic to mean indivisible in the context of concurrent programming, and the broader concept of atomicity is used in the same sense. the physical atom is the real edge case here; it just happens to have captured the attention of the general public.

It's not unreasonable because everyone understands the state things are in. Once upon a time atom was meant to be descriptive, and time has proven that original description insufficient. We could call atoms something else to address this, but it seems like we've decided "atom" stays, its meaning has just evolved over time in specific usages.

This reminds me of how the prefixes we use in computing hardware aren't right either. Your PC might have 8 gigabytes of RAM, but it doesn't literally have 8,000 megabytes or 8 billion bytes, and the same problem with mega- and kilo- too. Somehow we manage and when we're talking gigawatts giga means 10^9, and when we're talking gigabytes in computing we know it has a special meaning, 2^30.

Same prefix, two different usages depending on context. The only thing I can promise you though is we're never going to adopt gibi, mebi and kibi, at least not in my lifetime anyway.

Understanding the history and evolution can be interesting or informative as to why things are the way they are. So yeah, one upon a time "sub-atomic" would make no sense, it makes sense not of course. We're familiar and used to the idea. Wasn't always the case though.

but thing is we called it that because we thought there was nothing smaller than a atom.. although there is something smaller.

atoms are made of protons neutrons and electrons each which is made of 3 quantoms.

And similarly, we called it a universe because nothing observable can be outside it. Unless...

The analogy is quite apt, really, just switching out bigness in place of smallness.

Same with universe, we think it encompasses everything but according to our current definition it might now.

I think it's commonly held that an entities Universe is everything that has or could physically interact with that entity. So, everything that is out of the light cone for you is external to your universe.

> atoms are made of protons neutrons and electrons each which is made of 3 quantoms.

Electrons are currently thought to be indivisible, and protons and neutrons are made of three quarks each.

I've never heard of the word "quantom" before... where does it come from?

An etymological physics argument is new!

“Atom: late 15th century: from Old French atome, via Latin from Greek atomos ‘indivisible’, based on a- ‘not’ + temnein ‘to cut’.”

>If there are truly separate "universes", then by definition wouldn't it be the case that there would be no way to test (or falsify) whether they exist or not

This literally contradicts what the first paragraph of the article. It says that this paper "lays the theoretical groundwork for discovering a parallel universe". If you've discovered it, you've tested and proved that it exists.

Furthermore, the article states that it's a "mathematical paper which seeks proof of the 'multiverse' theory". Assume for a second that there is mathematical proof of the multiverse theory. In order to disprove this, you now need to disprove the math.

There cannot be a mathematical proof of a physical fact. You can have infinite number of theories that are all mathematically correct, yet contradict each other.

You need data from experiments to falsify the wrong ones. And I think that's why the article means - paper shows what experiment needs to be done to falsify some theories that assume multiverse.

Technically you might be able to do a proof by exclusion if you can show that only one of all the possible theories is sound and consistent.

I can personally assure you that String Theory is mathematically robust, and the only sound and consistent TOE; and as all other TOEs are excluded, therefore String Theory must be, and just has, been proven.

Isn't it the other way around? You start out with assumptions and a model (which usually involves some math), and then you show that you've made the correct assumptions by empirically testing it. You can't prove something in the mathematical sense about physical reality.

Of course you can; if you prove, given the indirect evidence, it's the only possibility.

You have to prove that math actually maps to reality. I don't think you can do that. With the possibility that it's all a simulation, our universe doesn't even have to be consistent.

Why not just simplify your whole position to "you can never prove anything"?

In Physics, this might be true, at least for generalizations (I can prove to myself that I currently write this text, for suitable definitions of "I, write, text". I might not be able to prove it to you.). In Math, it's not.

In physics it's absolutely true. You can only disprove theories. You can never prove them.

In math it's relatively true, because you can only prove theorems given a set of axioms and other assumptions you take as true.

In math, this is fine, as long as you can show that your assumptions are consistent.

If you disprove something in physics, you at the same time prove the negative of the something. Of course the negative of a theory is not really a theory by itself.

> Something I've never understood and would greatly appreciate understanding: could someone ELI"5" how the concept of "parallel universes" will not always be a metaphysical instead of a scientific question?

At this point it's all theoretical based on our best mathematical modeling of the universe/physics.

> If there are truly separate "universes", then by definition wouldn't it be the case that there would be no way to test (or falsify) whether they exist or not

By definition? No. Extremely difficult and maybe impossible. Yes.

Keep in mind that before powerful telescopes, the outer planets were "guessed" at by mathematical modeling. Astronomers mathematical calculations led them to believe that there should be planetary bodies like uranus and neptune out there. It took further advances in technology/physics to corroborate the mathematical calculations/model.

The same thing for hereditary genetics and the proton/electron/etc. When the first theoretical/mathematical models were first made, we had no conceivable way of "seeing" a proton or DNA.

> If there are truly separate "universes", then by definition wouldn't it be the case that there would be no way to test (or falsify) whether they exist or not.

Well, if the multiple universes theory provides falsifiable predictions about this universe and is the simplest explanatory model we have for this universe and said predictions / behavior, then the explanation would not be metaphysical at all. For example, if a multiple universe theory was a requisite basis to a universal theory of gravity and quantum mechanics then it would be perfectly within the realm of the physical sciences.

Yes, and that’s why such theories aren’t popular with theoretical physicists. If you can’t falsify a theory, it isn’t scientific.

The other extreme end, where there is significant interaction between parallel universes isn’t popular, either, because we would have noticed such universes if that were the case.

So, instead, theoretical physicists that think parallel universes may make sense look for mathematical models that either only have tiny interactions between those ‘separate’ universes or at least only have tiny interactions in the neighborhood (in time and space) where we live (the latter might even be a win; a theory that explains dark energy, dark matter or the Big Bang as detectable effects of paralllel universes would be a winner)

In the end, however, it all boils down to educated guessing, with good theoretical physicists being better (or luckier; the difference can be hard to spot if you need only one or two bright ideas/lucky guesses to build a career) at guessing and at building mathematical structures with some desired set of properties.

I guess it depends on whether the "universes" are really separate as you say. If we could detect the existence of other universes, it would mean that there's some sort of relationship among universes. If we cannot detect them at all, either they don't exist or they're completely separate, and our inability to detect them won't tell us anything about which of the two scenarios is true.

When we're talking about "parallel universes", of course the word "universe" no longer means the sum of everything that exists. It has to be redefined as a suitable subset of existence, perhaps similar to the way we've learned to distinguish the "observable universe" with the universe proper. How we ought to redefine it will depend on what we find out (or not) about parallel universes (or lack thereof) in the coming decades and centuries.

To attempt to clarify the question:

Are you saying "separate universes" would be part of the same "universe" if they could be interacted with (so interaction forms the "universe boundary")?

Pretty much, it's more that I'm saying that would have been my understanding of the word "universe" until now, but that may be actually an incorrect understanding of the word as Hawking means it.

If there are seperate planes of existens or universes, the properties thereof are unknown, and it can't be said if they would ever be observable or not untill someone finds out they are. (finding out they arent isn't possible, which up to now does make this very philosophical question i suppose)

I guess that if we can interact with (measure) a separate universe, then we should adjust our definition of "universe".

What if you can observe past interactions with it, but it is no longer possible to interact with it? Would you say it's still part of "our universe"?

I'm no physicist, but my impression is that the observable universe is already smaller than the universe.

Who cares. If you had clear proof of the existence it would be useful to describe it. People would argue about the words used and it wouldn't really matter. Pluto didn't get any smaller when they decided it wasn't a planet anymore.

Well, due to the universe being ~13 billion years old, there is no way to observe(/interact with) entities that are more that 13 billion light years away at present. That does not put these things in a different universe.

What’s the latency?

The definition of "separate" would be in similar trouble.

> If there are truly separate "universes", then by definition

Sometimes words "truly" and "by definition" don't play nicely together, if the established definition happens to be built on mistaken assumptions.

The "atoms" were named like that because they were thought to be elementary and indivisible. Later we learned better, but the name stuck; it would be confusing to try to change it now.

Similarly, the "universe" was believed to be all there is. Maybe it is not. But we already have a thing which has this name, and it would be confusing to rename it, just because we found later that some other things exist too.

The current definition, intuitively, is something like: If you can hypothetically fly there using a rocket (ignoring the light speed limit), it is a part of the universe. If there is no way to get there (even hypothetically ignoring the light speed limit), then it is not a part of the universe... and it is probably wrong to even say that it exists, because how could you know that?

But this is based on an assumption that you can either travel somewhere (ignoring the light speed limit) or there is absolutely no way you could interact with it. And maybe that assumption is true. But maybe it is not... and in such case the reasoning above would fall apart. If there are, uhm, places you could interact with in certain ways, but not in other ways, would they "by definition" be in our universe or not? Well, the definition does not speak about such cases either way, because they were not assumed to exist.

How could you possibly "interact with a place in certain ways, but not in other ways"? I can imagine a few ways, and the nature may still surprise us by doing something unexpected.

Imagine that there are different "regions" such that certain kinds of particles can travel from one to another, but other kinds of particles can't. So for example, we could send there light, and with a bit of luck have some of that light reflected back, but we couldn't travel there. And if in certain direction does not happen to be anything that would reflect the light back from that specific direction, there is nothing we could do about it.

Or imagine that different "regions" interact for certain amount of time, and then stop interacting. So while the parallel universe is connected, we can scientifically prove it, but after it disconnect, we cannot prove the existence of this specific parallel universe anymore. Or maybe the disconnection does not depend on time, but on the number of particles exchanged, so by the mere act of experimentally proving the existence of the parallel universe we are also pushing it away, irrevocably.

I haven't read Hawking's paper, so I don't know if his idea is similar to any of this. It could be something completely different.

Quite a lot of people worrying about the definition of the word 'universe' in this thread.

And with good reason. "Discovering a parallel universe" is nonsensical in the common acceptance of those words. Parallel lines never meet and the universe by definition contains all there is. So people are grasping for alternative definitions of the words that might make the statement satisifiable.

At one point "Split an atom" was by definition nonsensical because atoms were conceived of as the smallest, unsplittable bits of matter. But then we found out that we could split these pieces of matter we'd mistakenly called "atoms" and rather than renaming them we just changed the definition of "atom" to correspond to what we'd always been calling "atoms". In practice I don't think this was unreasonable and I wouldn't strongly object to doing the same with what we've been calling "The universe".

Might we think of a new word to describe a "sub-universe", like the one containing all our galaxies? This would allow for "universe" to be the thing that contains many or infinite sub-universes? Okay.. I'm done, I just confused the hell out of my brain.

Just leave it be. We don't worry about atoms not actually being atomic anymore either.

The concept of another universe is pretty commonplace - see 'Star Wars expanded universe' and similar. The fact that we haven't found a real one doesn't mean the concept is nonsensical.

Wouldn't we just use the word, "Multiverse"?

>Our conjecture strengthens the intuition that holographic cosmology implies a significant reduction of the multiverse to a much more limited set of possible universes.

Until it's published and reviews there isn't much to say. But it's exciting to me, hope I can read it soon.

As the article mentions, you can read it already: https://arxiv.org/abs/1707.07702v2

Oh, I was a bit too quick there it seems. Thanks! :-)

Looking forward to more and more discoveries and theories coming up in this field. Apart from providing a satisfactory answer to the quandary of "fine-tuned Universe" problem, there has been some observed anomalies for which multiverse theory provides explanation.

https://en.wikipedia.org/wiki/CMB_cold_spot

https://en.wikipedia.org/wiki/Dark_flow

:)

The problem with physics is, that it went into the wrong direction in the last 100 years and as a result, everybody which looks beyond pop science papers knows, that it does not fucking fit. The fine-tuned Universe is a Problem of the model and the requirement of dozens of free chose-able variables required to make the model fit observations should give you a red sign. And even then it's a bad fit, in very aspect. 10%+ difference in calculations vs observations is a pretty strong hint that your model is far from reality.

The model I'm following does not have dark matter, because it does not require it. I have also expansion of the universe, without abstract higher dimensions etc. But of course, this can not be done without breaking paradigms and this is the problem.

Physicists have shown to be tho most short sighted and ignorant scientists of all:

http://www.pnas.org/content/112/24/7426.full.pdf

And then he leaves. Hmmm, suspicious.

Hopefully to a parallel universe. :)

Imagine the look on Stephen Hawking 1011156B's face when he finds out Stephen Hawking 7086C published the exact same paper that he did, died two weeks later, and the combination of those events compelled scientists in that Universe to eventually master interdimensional travel, when 1011156B only got laughed at.

How is the Hawking ID generated? Which universe is used for synchronization? Do we need a new UUID level? Will the NISTs of all universes ever come to an agreement? So many questions...

Multiversally-Unique ID's are going to be so long...

I was going to disagree because our universe seems to be very compressible and a lot of the properties are just natural. I bet it could be defined in 0 - 100 bits at the "earliest" point.

But over time, QM seems to make lots of arbitrary decisions and those also seem to create different universes if they end up contributing to macro behavior.

Depends on how many of them there are. If there is infinite, there is no MUID, if it's finite and below 2^64 then a traditional UUID should be enough to assign a UUID per universe and then assing a UUID for objects within.

> If there is infinite, there is no MUID

Formally, you could still have MUIDs even if there are infinite universes so long as they are countably infinite. Practically, the system would work as well so long as you could allocate new qbits to boost your information exponent.

Well, no, as long as you want a finitely long MUID since a finite number of digits can only express a countable but not infinite number of elements.

We don't want a finitely long MUID, otherwise we couldn't represent infinite universes. A quantum bit represents infinite states. The "fiction" part is that we (currently) need to collapse quantum bits.

Well, to be fair… if he suffers from the same ailment, I quite easily can imagine his look, actually.

Some people's senses of humor...

This sounds like something Rick Sanchez would say

Truly amazing. Thank you Professor Hawking! RIP.

Or maybe it's a metaphorical expression of sadness over the recent death of a respected idol?