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Why we think there's a Multiverse, not just our Universe (scienceblogs.com)
207 points by petercooper on Oct 29, 2011 | hide | past | web | favorite | 53 comments



As the only criticism on this article has been downvoted strongly, I feel compelled to repeat my explanation of that criticism (that believing in the multiverse is like believing in heaven or hell) at a higher level, in the hopes that people will be somewhat more critical of what esteemed scientists are peddling as truth:

There is not a shred of experimental evidence for the multiverse. In fact, experimental evidence is not even possible. That places it firmly in the pantheon of metaphysics.

That there is a multiverse could be an unescapable conclusion from an otherwise very successful theory, with many experimental verifications, but that isn't the case. The article neglects to mention that there are many alternative interpretations and theories that can equally well explain 'how it all started' and still result in inflation and all associated 'best' theories for the experimental observations of the universe. This article is all unwarranted extrapolation (back to the beginning of time and before).

The main pillar of the success of the multiverse theory is that it successfully appeals to the imagination. The main reason that it is being promoted is because the great physicists promoting the idea do not realize they are not as great philosophers as they are physicists.


Assertions that take the form "X is like religion" are not enlightening. When it is prefaced with "get the facts straight" then of course it's going to get downvoted. There is nothing being "peddled" here, there is no hidden agenda by "esteemed scientists".

We have experimentally demonstrated that the universe is a really, really weird place. Time and space can bend. Whoah. But hey, we proved that beyond a shadow of a doubt. Then there's quantum mechanics. Wave functions! Entanglement! Turns out the universe really works that way. Then the universe starts looking really non-deterministic. Then we figured out that the simplest interpretation of QM implies Many Worlds, that the universe splits and splits and splits into a bazillion near-copies. The quantum events just look non-deterministic from our perspective. Do we know everything? Hell no. But we know that QM is true and that it uses some form of Many Worlds. We know that the world is deterministic. We don't know for sure how the universe came into being (but we are sure the universe is expanding). We don't know if there really is such a thing as the multiverse, but as we figure out more about the big bang, about background radiation, about black holes and dark matter and the shape of our universe there's a good chance we'll figure it all out eventually.

Physics is treading on the domain of philosophy because we're getting better at physics. Philosophers debated since the dawn of time about free will and didn't get anywhere. Now we know that non-reductionist or dualistic world views make little sense. Philosophers debated for ages about morality. Now neuroscience is starting to deliver concrete answers. Philosophers debated for ages about religion and heaven/hell. And elementary Bayesian math shows the questions are just nonsensical.

Philosophy is about reasoning about topics we don't really understand. As our understanding of physics, math, neuroscience and biology grows philosophy has to make way.


  Then we figured out that the simplest interpretation of QM
  implies Many Worlds
Many physicists contend this point. They do not consider this interpretation as 'the simplest'. Things hinge on such contentious points as the physical reality of 'the collapse of the wavefunction'. Again, the major thing the Many World interpretation has going for it is that it easily appeals to our imagination -- even, or especially, to the imagination of non-physicists.

  We know that the world is deterministic.
Philosophers disagree. It's even the question whether this issue can possibly be settled by any experimental evidence, because of the way we, and our math, are wired to consider everything in causal chains. The evidence isn't really very compelling if you consider the fundamental problems surrounding the question.

  Now we know that non-reductionist or dualistic world views
  make little sense
Descartes has been critiqued extensively long before science could say anything about mind-matter interactions, because the main shortcomings of the duality argument do not depend on any experimental facts at all.

  Now neuroscience is starting to deliver concrete answers.
On the subject of morality, neuroscience cannot possibly deliver any concrete answers at all. You're engaging in the naturalistic fallacy if you think it does. What is the case in the world is not argument for what should be the case in the world. Neuroscience may explain why people hold certain ethical views. It can never explain what ethical view they should hold. What view is 'acceptable', 'the best', 'right'. The answer to that question cannot be reduced to physical facts about the universe.

  Philosophy is about reasoning about topics we don't really
  understand.
Then we don't really understand logic and math either, do we? The foundations of logic and math are still part of philosophy after all.

And, well, Hume didn't really understand causality, did he? But fortunately, thanks to modern physics, now we do?


> Many physicists contend this point [ ... ] appeal to imagination.

Not so. The vast majority of quantum physicists now agree Many Worlds has won, including Feynman, Gell-Mann and Hawking. The concept of 'collapse of the wavefunction' is silly and shouldn't even be considered for reasons outlined here: http://lesswrong.com/lw/q8/many_worlds_one_best_guess/

> Determinism; Philosophers disagree.

It's just basic reductionism. There are absolutely no fundamental problems surrounding this.

> On the subject of morality, neuroscience cannot possibly deliver any concrete answers at all.

All moral thought and all moral beliefs up to now have happened inside the human brain. Most moral thought is confused and self-contradictory. If we want to figure out why we are moral the way we are, what our "true" moral beliefs are, and how to build societies that flourish, we have to look inside the brain to see how it all works. Either morality has bearing on the real world and is therefore subject to measurement or it has no bearing on the real world and is therefore irrelevant.

> Then we don't really understand logic and math either, do we? The foundations of logic and math are still part of philosophy after all.

Where the line is drawn between math/philosophy isn't a very interesting question.

> And, well, Hume didn't really understand causality, did he? But fortunately, thanks to modern physics, now we do?

Nope. And yep. And thanks to Bayesian reasoning.


Concerning Many Worlds:

Firstly, [1] disagrees with you. Secondly, there is a difference between accepting MWI above other interpretations and actually believing in multiple parallel universes. A difference between accepting the physical and the metaphysical consequences. Thirdly and crucially, MWI depends on QM. Everyone acknowledges QM is probably not a definite description of the universe and non-locality, non-contextuality or another kind of modification would wreak havoc on MWI. If physicists subscribe to MWI but also acknowledge QM seems incomplete, they are being inconsistent.

Concerning determinism:

You say "it's just basic reductionism". But in fact it presupposes reductionism as a viable strategy to determine 'the truth' about determinism. It isn't [2]. Specifically, the question whether we can even determine whether our fundamental theories are deterministic is still wide open. None of the theories, from classical mechanics to quantum mechanics, is clearly deterministic and it remains a question whether it can even be determined whether they are deterministic.

Concerning morality:

No matter how well you can 'measure' my 'true' ethical views on abortion: no measurement will ever tell you what my ethical view on abortion should be. No collection of measurements will ever tell you what a governments policy on abortion should be. At most you can hope to be able to say: in this particular case we should allow it, because that will be best for everyone: the average happiness, productivity and insert-factor-of-importance will be highest if we allow it. However, that kind of judgment presupposes a certain moral system. You can never determine which factors should be important. At most you can measure which factors people rate as most important on average. But again, you presuppose a certain moral system if you choose to use that measurement to determine what to do. If you feel otherwise, there's a large philosophical nut left for you to crack.

I think you underestimate the complexity of these subjects and that your verdicts are premature.

[1] http://en.wikipedia.org/wiki/Many-worlds_interpretation#Rece...

[2] http://plato.stanford.edu/entries/determinism-causal/


BTW, you guys keep talking about MWI, while the original article is categorically not about MWI. The author describes a Tegmark Level I Multiverse[1], the most conservative of the multiverse models. MWI is Level III, and the author himself says in a subsequent comment[2]:

  What I've presented, above, is the argument for the first
  type of Multiverse, which I think is correct, and hence,
  which I believe in.
  
  I think that anything beyond that is too speculative to be
  believed at this point, at least with any sort of
  confidence. At least, by me.
[1] http://en.wikipedia.org/wiki/Multiverse#Level_I:_Beyond_our_...

[2] http://scienceblogs.com/startswithabang/2011/10/why_we_think...


The author responds to this point in one of the comments; I quote:

this is -- and I make no pretensions otherwise -- theoretical physics. It's not airy-fairy pie-in-the-sky speculation, but it also isn't proven the way you would prove that a * b = b * a. You may want to read this recent guest essay (http://scienceblogs.com/startswithabang/2011/10/guest_essay_...) that does a wonderful job of explaining, with some brilliant metaphors, what this sort of theory is (and isn't).


I could not agree more. We don't all believe in a multiverse.


Isn't this like having a function that explains very well data that is within an observable range, and then stating with confidence that that function also applies to data in a range that is unobservable to you?

Yes, that function may hold outside your observable space, but it may not. We can't simply state as fact the predictions that come from assuming that this function applies outside our observable space.


Spot on, and even much of what is 'observable' here is not even close to understood. Inflation and dark energy/matter are both concepts that have been built to explain observation but which are themselves not yet explained at least not with much consensus.


Potentially a stupid question -

How can the universe get "colder"? Does it just mean that the average energy-per-square-meter (or whatever unit) is less? If so, wouldn't that mean that saying "less dense" and "colder" mean basically the same thing at that scale?

Maybe I'm asking something stupid and obvious, but I keep thinking that energy can't be destroyed, ergo, there's exactly as much energy (and thus heat?) as before. Or is heat energy density? But doesn't that make "less dense and colder" a redundant statement, since they'd be the same thing?


Less dense implies colder, as the volume is not constant, but expanding. Temperature and density are different things though - one is how much energy something has, the other is how far away from other things it is. Related, but different.

With respect to destroying energy - energy converted into matter is 'colder'. There's not the exact same amount of energy - it's not that it can't be destroyed, it must be conserved with matter (E=MC^2). Great example - kinetic energy is used in a particle accelerator to create new particles, and it takes a ton of energy to make even a tiny bit of mass, so the average temperature would go down as more mass is created (heat being the transfer of energy - once it's trapped in a particle, it's not transferred as freely).

(Caveat - I'm not a cosmologist...)


How does converting energy into mass make it "colder"? If you have 1 protons worth of energy in a volume, and you have 1 nonmoving proton in another area with the same volume, isn't there the same amount of energy in both regions? Or is it "colder" because you can't have a region with energy and nothing else, it must be something like kinetic energy, which gives heat?


Temperature is the average energy of the particles in a space. So if the proton isn't moving, it is at absolute 0 temperature even though it has some mass-energy left.


Here's a post from cosmologist Sean Carrol: "Energy is not conserved."

http://blogs.discovermagazine.com/cosmicvariance/2010/02/22/...

tldr: "Energy isn’t conserved; it changes because spacetime does."


> (Caveat - I'm not a cosmologist...)

That was really informative, thanks.

Man, physics is cool.


Yes, heat is energy density which is why pressure is basically the same thing as heat. That's why a pressure cooker can generate incredible pressure just by heating it. Similarly, your refrigerator cools things by expanding a liquid into a gas (state change is actually the dominant mechanism in a refrigerator but it would work in principle with out the state change)


Yes, less dense and colder is redundant.


A great TED talk on the design of the Universe, if any one is wondering about the glass shaped picture (the 5th image) in the essay:

http://www.ted.com/talks/george_smoot_on_the_design_of_the_u...


Fascinating article, but I'd still like to know more about that 8th figure on the inflation model. We are told that the y-axis represents energy, but what is the x-axis? It clearly isn't time because it's being measured in phi over something measured in GeV.


The attribution to Ned Wright takes you here:

http://ned.ipac.caltech.edu/level5/Narlikar/Narlikar3_2.html

which has a lot more information. Phi is defined as the Higgs scalar field.


This kind of science has always been hard for me to integrate. Thinking as a Bayesian, these arguments seem to be meant to influence my "prior" over possible initial states or natures of the universe. I'm not accustomed to that.

I'm used to science presenting me with evidence meant to influence my "posterior". I can handle that.


Are you aware that's irrational? Your assumptions about priors contain plenty of mistakes (all ideas do, mine too) and you should find a way of thinking which better allows for correcting those mistakes.


I think it's more unfortunate than irrational. I'd like to think I'm open to changing them--my priors started out (a couple of decades ago) pretty terrible. The process of how to do it well in response to arguments without experimental results is the challenging part.


Bayesianism is in the empiricist tradition where it's focussed on how to update ideas in response to evidence, but it doesn't actually provide guidance for how to deal with non-empirical ideas that can't be judged by evidence.

This (empiricism) has driven philosophers to things like positivism (where they declare everything non-empirical to be worthless or even meaningless). It's a problematic tradition.

The problems are made worse because it turns out that the large majority of ideas are assessed in a non-empirical way (not due to people being idiots; this is correct). Using evidence is the less common case (though it is quite important when relevant).

In _The Fabric of Reality_ by David Deutsch, he gives an example of the theory that eating a kilogram of grass will cure the common cold. This, he points out, should not be empirically tested, and will not be. Rather it is rejected without evidence because it is a bad explanation. Only good explanations are worth testing. (This does not make us miss out on any truths. If it really was true, someone could figure out some explanation of how it works, and then we'd test it.)

In his recent book, _The Beginning of Infinity_, Deutsch further explains that there's no point in testing any theory for which its details can easily be changed around in an ad hoc way, because you can never refute such things with evidence since they will just revise themselves endlessly. The only thing that can refute that sort of approach is philosophical criticism. Only ideas which survive some philosophical criticism and are therefore of higher quality are worthwhile to empirically test.

What makes it hard to change a theory around, ad hoc, to avoid refutation, is if there is some actual connection between the content of the theory and the problem it's trying to solve, so most changes to it would make it no longer address the problem as well.

Deutsch calls that quality "hard to vary" and says it is what makes explanations good. His books expand on this, and offer a version of Popperian epistemology with (relatively small) improvements.

Popperian epistemology, focussed on criticism not justification (which is impossible), and which applies to all types of knowledge without difficulty rather than being narrowly focussed on empiricism, is the solution.

One of the Popperian ideas is that because we are fallible and make lots of mistakes, and need to improve on them (by criticism), irrationality has to do with anything that hampers this process of correcting errors. So that's why I regard difficulty error-correcting priors as irrational. Deutsch even proposes that hampering the correction of mistakes is the most important criterion of immorality.


Your response is really exciting! I'm grateful--it's pointing at exactly what I was hoping to find. Deutsch seems to be talking about how to evaluate challenging arguments that come before evidence.

To contribute something, other than my thanks, here's a link to Deutsch talking about knowledge, evidence, and incredible cosmic relationships:

http://www.ted.com/talks/david_deutsch_on_our_place_in_the_c...


If you'd like to contribute more, you could contribute any thoughts you have while looking into it at the discussion group:

http://groups.google.com/group/beginning-of-infinity/

Thanks for the positive response! Good luck.


About the example of grass that you talk about, I think that amounts to a criticism not of empiricism but direct hypothesis checking. We have built up a model of human body and grass which tells us that it would be harmful. Now this background model has been subject to empirical checks at lots of times and has evolved in presence of new empirical discoveries. (Though of course, the model has not been derived axiomatically from empirical data).

Almost nothing in science is justified by direct checks. Think of any clinical trial or say, the LHC experiments. There are so many implicitly assumed background theories about how the instruments respond to their stimuli. Some of these instruments are based on principles which were discovered only in this century. So scientists from previous times wouldn't accept the way the results are derived unless they are shown the results derived from previous experiments justifying the way the instruments work. For example, the atomic hypothesis was controversial till 1900 or so. How would one even interpret LHC electron beams if one one doesn't believe in the atomic hypothesis? A useful analogy here, is minesweeper - a new opened square, whose information, via a very long chain can tell us if there is a mine on a far away square.


Can you give a statement of what you mean by "empiricism" that you wish to defend?

I agree with some of what you say, but I'm not sure what you're trying to vindicate.

Bear in mind that one can construct infinitely many theories logically consistent with every piece of evidence or empirical check done in the past, and which predict that the grass cure for the cold will work. These theories will consist of various disjointed assertions in a rather arbitrary and ad hoc manner. The problem with these theories is that they are terrible as explanations -- they are bad philosophically -- but they are not empirically refuted.


My goal was not to defend pure empiricism - that justifications for knowledge can come only from sense data. I just wanted to emphasize that, in your example, when we chose a good explanation instead of a bad one, the goodness of the explanation has a large empirical component.I am guessing that you agree with this but would focus on the non-empirical component. One non-empirical component of its goodness would be the complexity of its specification, the Kolmogorov complexity for instance.


You're not meant to retain your prior forever. When you update, you replace it. Think of it as a FOR loop over incoming evidence. On each iteration, you increment or decrement the probability of the proposition in proportion with the evidence.

Consider what the alternative would look like. You retain both your original prior and every piece of evidence you've ever seen, then recompute all of it each time you need the current value? No. That doesn't pass the intuition test either. What you now think of as your prior on the Universe is not actually the belief you held at birth. You've updated dozens or hundreds of times during childhood, and each time you discarded your old "prior".

Footnote: you do retain some of your recent evidence, for smoothing purposes. For more information, investigate how Bayesian networks incorporate time: http://en.wikipedia.org/wiki/Dynamic_Bayesian_network


Sometimes I think I was born too soon. To think the new knowledge we well discover with in the next 100, 200, 1000 years is really exciting.

I wouldn't be surprised if in the next 20-50 years with all the genetic engineer going on one could live for ever. Of cause as long as you can afford it.


Sometimes I agree with you, but on the other hand, you have to recognize how lucky you are. There is no guarantee that the future will be better than the present, and there are many plausible risks that it could be much worse. For example, highly disruptive consequences of climate change; nuclear war; massively destructive accidents (like the gray goo scenario); and so on.

Whenever I find myself getting wistful about the future portrayed in science fiction, I remind myself that I am better off than all the humans before me since the dawn of our species, and also better off than the vast majority of humans on the planet right now. Future generations may look back at this as a golden age. So enjoy it!

Edit: this inspired me to write an article:

http://news.ycombinator.com/item?id=3172469


Always true. Yet you have you to embrace now for what it is, as it's what ya got.


I envision a world where, due to technology and science, everyone stops aging after 25.

A clock will be constantly visible on your forearm and time will be the only commodity in the world, in which you'll get paid and make payments in.

After the age of 25, you'll get 1 to 75 years of "time" deposited in your account. The clock on your arm will decrement by default as time passes, and increase/decrease as you earn/spend.

When your time expires (the clock reaches zero), you die.

Most people will live day-to-day, only having 24 hours of time in any given moment. Others will have millions of years on their clocks.


      I envision a world where ...
You could try being a little more original: http://www.imdb.com/title/tt1637688/


'In Time' itself could try being a little more original - the main premise probably comes from either a great late-80s short called 'The Price of Life' (http://vimeo.com/16265933) or a 1970s Lee Falk story, 'Time is Money'.


I would only hope that there would be a place on earth where time moved slower than anywhere else -- call it something like 'New Greenwich'.


This plot doesn't make any sense.


Time is the ultimate currency.

Gold is only backed by the agreement of its scarcity; otherwise, it has no value (outside its physical usefulness as a material).

Fiat currency is only backed by the confidence of a nation; otherwise, it has no value.

Time is a currency that's backed by your life.

It is as real as it gets.


The difference between Time in this scenario and Gold, is that the value of time will be based on artificial scarcity.

The scarcity of Gold on the other hand is very real. Gold is limited and while it is possible to synthesize gold, the cost of doing that is great and does not scale.

Which is why I think the world you mention is impossible: we cannot even stop the piracy of music. Artificial scarcity is not sustainable. Once all the people will be able to live forever, they will live forever. What makes you think these "clocks" will be unbreakable?

And even if the technology will be perfect, do you really want to have on your hands a huge angry mob of people with less than 24 hours to live? Good luck with that ;-)


Why would time be of any value if nobody is ageing after 25?


I'm trying to fill in the gaps in the author's thinking, but I imagine it something along the lines that keeping you "aged" 25 requires resources. So time is sort of a proxy currency for the resources required to keep you alive. If you run out of time you don't have anything to purchase the resources to keep you living.


That didn't stop them making a movie about it: http://www.imdb.com/title/tt1637688/


I cannot recommend the writings of Max Tegmark on the multiverse enough. Among people doing science at the heights of their fields, he is perhaps a uniquely talented popular writer. He even keeps it readable on his scientific papers, which read mostly just like his popular papers except shorter and with more math.

http://arxiv.org/PS_cache/arxiv/pdf/0905/0905.1283v1.pdf is his overview of the levels of multiverses.

If that properly blows your mind, see Bostrom's Infinite Ethics. http://www.nickbostrom.com/ethics/infinite.pdf


If you're interested in MWI you should read David Deutsch's books:

http://beginningofinfinity.com/


I agree -- if you're interested in MWI then you should read Deutsch (in fact, you should read Deutsch anyway) -- but the OP isn't about MWI, it's about a different sort of multiverse, with lots of "pocket universes" even on a single Everett branch because of inflation. Many of the implications are pretty similar, though.


I did read the original link but honestly don't really understand. Seemed to be some kind of argument there has to be a multiverse to account for the expansion of the universe. That seems like a weak argument to me because in my understanding we're not that sure about what's happening to the universe and cosmologists keep changing their minds. Regardless, I didn't quite follow what sort of multiverse they said it implied and how it differs from MWI.

Going by your comment, I'm not sure there's as much difference as you may be suggesting. Deutsch's MWI is full of what we might (or might not) call "pocket universes", it's not focussed on just big universe sized branches.

I saw the article talking about the wave function spreading out which I think is the same concept Deutsch talks about in BoI with fungible instances of particles spreading out (a multiversal object is a collection of instances).


hah, its like a variable in a function making theories about what the whole program is all about, and that there might be many other functions similar to it. :D

take a look at this one too - http://en.wikipedia.org/wiki/Many-worlds_interpretation


I've always been put off by the many-worlds interpretation.

They take something that's perfectly explainable with a probability wave, and turn it into fantasy.

http://en.wikipedia.org/wiki/Wave_packet


Let's get the facts straight. Believing in a multiverse is no different than believing in heaven/hell.


* for a similar enough definition of heaven, hell and multiverse.

Have you read the article?


He may very well have. There is not a shred of experimental evidence for the multiverse. In fact, experimental evidence is not even possible. That places it firmly in the pantheon of metaphysics.

Now it could be an unescapable conclusion from an otherwise very successful theory with many experimental verifications, but unfortunately the article neglects to mention that there are many alternative interpretations and theories that can equally well explain 'how it all started' and still result in inflation and all associated theories.

That the multiverse theory successfully appeals to the imagination is the main pillar of its succes.

It's a shame he is being downvoted for succinctly stating a plain truth.




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