Monday, September 27, 2010

Distinguishing Our Universe From Other Similar Universes In The Multiverse.

Srednicki and Hartle have raised an interesting concern recently about a limitation on the predictive power of multiverse theories. They observe that in multiverse theories, exact snapshots of our universe happen several times in different places. So if we want to have a physical theory that describes our universe, the one we live in, then the question arises: how can we tell which one it is from all the others?

From the paper:
Theories of our universe are tested using the data that we acquire. When calculating predictions, we customarily make an implicit assumption that our data D0 occur at a unique location in spacetime. However, there is a quantum probability for these data to exist in any spacetime volume. This probability is extremely small in the observable part of the universe. However, in the large (or infinite) universes considered in contemporary cosmology, the following predictions often hold. 
  • The probability is near unity that our data D0 exist somewhere. 
  • The probability is near unity that our data D0 is exactly replicated elsewhere many times.  An assumption that we are unique is then false.
This paper is concerned with the implications of these two statements for science in a very large universe... 
The possibility that our data may be replicated exactly elsewhere in a very large universe profoundly affects the way science must be done.
In order to solve this problem, the authors propose creating a "xerographic distribution" ξ.  Given the set X of all the similar copies of our universe in the multiverse, this xerographic distribution ξ gives a probability that we are the specific snapsot Xi of that set.

The authors claim that this distribution cannot be derived from the fundamental theory.  The fundamental theory can only predict the structure of the whole universe at large, not which snapshot in it we happen to be.  However, given a certain ξ, we can use Bayes Theorem to test which ξ appears to be most correct, and once that ξ is established, we have now a statistical likelihood hinting at which universe in the whole multiverse is ours.

So, given a fundamental physical theory T and a xerographic distribution ξ, the authors say:
We therefore consider applying the Bayes schema to frameworks (T,ξ). This involves the following elements: First, prior probabilities P(T,ξ) must be chosen for the different frameworks. Next, the... likelihoods P(1p)(D0|T,ξ) must be computed. Finally, the... posterior probabilities are given by
The larger these are, the more favored are the corresponding framework.
The authors then go on to give some examples of how this might work and solve issues with Boltzman Brains etc...

So, just to repeat:
  1. One glaring problem with multiverse theories is our universe happens several times in several places throughout the multiverse.
  2. However, we would like a good physical theory to make predictions about the snapshot we happen to live on.
  3. The fundamental theory of the multiverse cannot tell us which snapshot we are.
  4. However, creating a xerographic distribution ξ we may be able to put probability estimates of which copy is ours using Bayes Theorem.
Some further thoughts and Questions.  I remind the readers, as crazy of a topic this paper covers, it did get published in a respectable journal: Physical Review D.  However, while reading the paper I had several thoughts come to mind and I would appreciate your own thoughts on these issues:
  1. How should we feel about multiverse theories given issues like this arise?
  2. Can only tenured professors get away with writing such articles?  IE... if a grad student wrote papers like these will universities take him/her seriously when applying for a faculty position?
  3. What is your "exact other" in the "other snapshots" doing right now? :) 
Srednicki, M., & Hartle, J. (2010). Science in a very large universe Physical Review D, 81 (12) DOI: 10.1103/PhysRevD.81.123524


  1. I must confess this is not a problem I lose much sleep over. My exact other is presumably writing this comment, no?

  2. Cervantes,

    True, I should have asked what you thought your exact other is doing tonight as you may make different decisions or be influenced slightly differently between now and then.

  3. "What is your "exact other" in the "other snapshots" doing right now? :)"

    I think I have several exact others, In fact, perhaps, an infinite number of exact others. I think surely one of those "copies" of me comprehended this post. If not, maybe the other infinite me's that will read it tomorrow might comprehend it. If not, the other infinite me's the next...

    So if M-theory only has 10**500 solutions (heh, only...) does that limit the number of total universes or just the limit of unique, or universes that share the same properties? How does that play into this concept?

  4. How do we travel from one universe to another universe? If we do, would our double be happy to see us, or would think some imposters are on loose and try to get rid of us?

    Is there any practical physics left anywhere?

  5. Stan,

    If M-Theory is right, there can be an infinite number of total universes but only with 10^500 different types of fundamental physics. IE... the masses of the fundamental particles can take on 10^500 sets of values.

    However, even if you have two universes with the same underlying physics, you can still have different long term outcomes as in a similar way two identical twins can have the same DNA but 20 years down the road be fairly different in many respects.

  6. Ancient1,

    I would be happy to see my double. :) However, baring something like a wormhole such travel is for all practical purposes impossible.

  7. Well, if there are multiple parallel universes then it should be a common occurence to see energy disapear from our own universe and enter a different universe. Thermodynamics as we know it would be drastically different if, for instance, all of the energy that is exploding in your gasoline engine piston were to disapear into a parallel universe or if all of the energy from a car piston in a parallel universe were to enter your car piston thus giving you the appearance of having a turbocharger (even if you don't have one).

  8. I met this guy once who looked like he might have come from a parallel universe who claimed his ξ was deceasing. I never saw him again.

  9. JS,

    We can't even go the Moon safely...

    Extra-Universe Worm Holes! Why not faster than speed of light? Did not StarTrak guys stopped halfway to Andromeda?

    I think Prof. Conway is on to something... But I am still listening to his lectures...

  10. Quantum_Flux,

    Multiverse theories are not always such that the universes are parallel universes. That is just a special case and a very unlikely one for reasons you describe among others.

    If anyone is interested, the wikipedia article on multiveres puts forth the wide possibilities.


    That's funny. :)


    I'm glad your mind is open to many things. Maybe one day we can do all that you discuss but right now the laws of physics see to rule out traveling faster than the speed of light.

  11. Lat night on Coat to Coast, Michio Kaku talked about how gravity flows freely through the multiverse. Is this true? What evidence is there?

  12. Stan,

    Yes, I believe regardless of the type of Multiverse, gravity can flow freely throughout it. Except there is one caveat: gravity theoretically can only flow at the speed of light.

  13. Does that explain why gravity is a relatively weak force? Because it is "leaking" into another universe?
    Kaku also said that dark matter may be ordinary matter in another Universe exerting a gravitational force in this Universe. This seems so speculative.

  14. Stan,

    1. Actually yes... sort of. Well, at least in string models, yes sort of. (This may be true of other models too I just will stick to what I know or think I know.)

    In string theory the non-gravitational forces are all stuck to the 3D "brane" we see as being our 3D space around us. However, gravity is not confined to this brane and can leak out to other branes which may be other universes etc... This leaking is the reason why gravity is weak.

    2. *If* these brane world models from string theory are correct, *perhaps* dark matter can be explained by mass clustered on another brane whos gravity is leaking to our brane. I believe there are papers on this.

    However, most people I talk to believe this is unlikely and that dark matter being weakly interacting particles in our universe seems to fit the data better. But I admit it is a possibility discussed in the literature by some people.

    3. So yes, this is very speculative. But we all know Kaku is famous for being most interested in the most speculative stuff. :)

  15. JS,

    Travelling through wormholes is faster than speed of light, relatively speaking!

    Second issue is whether multiverses are connected through wormholes at all.

    Third is multiverses could each have specialized laws of physics operating in each verse...

  16. Oh, right, I was referring to multiverse in terms of "many worlds intepretation of quantum mechanics". However, apparently there is a lot of interesting ideas under that broader class of theories of multiverse. I do believe cyclic theory to be a given, and am open to the idea of the p-brane hypothesis of M-theory as well. Maybe the universal constants change from location to location as well, that seems to be a given as well to me, the idea of universal constants from local measurements is like extrapolating a local slope on a curved graphed.

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