Replies to This Discussion

Permalink Reply by Doone on March 6, 2011 at 4:23pm

Excellent idea - I shall drop by here by the hour.

Permalink Reply by Adriana on March 6, 2011 at 4:25pm

Doone, I think you should move your comments here (the ones you already made). Also, can we put a permanent link to this discussion so people an always find it? For example in a text box?

Permalink Reply by Michel on June 30, 2012 at 1:57pm

I'll put something on the Main page.

Permalink Reply by Doone on March 6, 2011 at 5:41pm

Distinguishing Our Universe From Other Similar Universes In The Mul...

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 welive 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 X_i 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

Permalink Reply by Marianne on September 28, 2011 at 8:38pm

Well, I'm far from being a scientist, but at some point I stopped reading science fiction... maybe the ideas about alternate universes  or other topics disturbed me a bit too much and when I started thinking that when I die my whole universe would also die but it didn't prevent from other universes from kicking in...

I do know though, that everybody perceive the universe we live in differently (and not only colours and sounds...).

So, I'm a bit lost with your statement and questions... AND I WISH TO BE ENLIGHTENED !!!!!!!!!

Permalink Reply by Michel on July 1, 2012 at 3:14pm

I may be multiple and exist elsewhere as well, but "I" don't and can't know that. Whatever illusion of free will I entertain is not affected by these other Michels. And vice-versa, I'm sure.

Unless of course one of these Michels happens in a universe from which people can interact with people of another universe.

Permalink Reply by Ruth Anthony-Gardner 4 hours ago

Since we can't effect what happens in the alternate universes, this entire premise makes no sense to me. What difference would it make if there were other exact copies of our universe at a particular time? At each second, aren't an infinite number of new possible universes branching off from the one we perceive as "our" solid classical reality? The universe as I understand it is a lot more quantum-fuzzy at the "edges" (very small, very short time period samples). As macroscopic beings with limited perception/brain/behavior speeds we collectively create a social construction as what counts as reality. I don't see how the multiple universes matter to our physics-limited experience of existence.

Permalink Reply by Doone on March 7, 2011 at 9:43am

Human Activity Displaces Predators More Than Prey
ScienceDaily (Mar. 3, 2011) — A new paper by University of Calgary researchers, published March 4 in PLoS ONE, demonstrates the edge given to prey in the "space race" by human activity.

More http://www.sciencedaily.com/releases/2011/03/110303141555.htm

Permalink Reply by Doone on March 10, 2011 at 7:26am

WHAT SCIENTISTS BELIEVE

The notion that science and religion are at war is one of the great dogmas of the present age. For journalists, it is a prism through which to understand everything from the perennial kerfuffles over teaching evolution to the ethics of destroying human embryos for research. To many scientists, religious belief seems little more than a congeries of long-discredited pre-modern superstitions. For many religious believers, modern science threatens a deeply held faith that man is more than a mere organism and that our status as free beings bound by natural law implies the existence of a transcendent deity. But this is not the whole story. Every year, countless new books try to reconcile the claims of truths revealed by divine inspiration and those that are the product of earthly reason. Foundational developments and arcane speculations from theoretical physics — from the latest findings of quantum mechanics to the search for a “Theory of Everything” — take on a metaphysical import in the popular mind. One of the best known examples involves the cosmologist Stephen Hawking, who famously concluded his 1988 bestseller A Brief History of Time with the suggestion that our search for scientific meaning may someday allow us to “know the mind of God.” More recently, Hawking has backed away from this statement. His new book, The Grand Design, which posits that the universe may have created itself out of quantum fluctuations, is but the latest in a long line of volumes by prominent physicists and cosmologists translating scientific theory for a popular audience. Along with volumes by biologists with a flair for explaining complex concepts, these books have become a locus of debate about the place of God and man in our understanding of the universe.

more from Peter Lopatin at The New Atlantis here.

Permalink Reply by Doone on March 10, 2011 at 7:37pm

THE SCIENTIFIC METHOD IS ALIVE AND WELL

Daniel Holz over at Cosmic Variance:

I’ve been on somewhat of an unintended hiatus for the past few months, as I try to wrap up some projects, and deal with a few other things in my life. However, I just read something that has given me a kick in the pants. And I don’t mean that in a good way. In late December there was an article by Jonah Lehrer in the New Yorker titled “The truth wears off”. Much more suggestive was the subtitle, “Is there something wrong with the scientific method?”. The story discusses the “decline effect”: an article is published with startling results, and then subsequent work finds increasingly diminished evidence for the initial unexpected result. It’s as if there’s “cosmic habituation”, with the Universe conspiring to make a surprising result go away with time. The last paragraph sums things up:

The decline effect is troubling because it reminds us how difficult it is to prove anything. We like to pretend that our experiments define the truth for us. But that’s often not the case. Just because an idea is true doesn’t mean it can be proved. And just because an idea can be proved doesn’t mean it’s true. When the experiments are done, we still have to choose what to believe.

I don’t particularly disagree with any of this. But it’s completely besides the point, and to untutored ears can be immensely misleading. The article is a perfect example of precisely the effect it seeks to describe (there must be a catchy word for this? Intellectual onomatopoeia?). The article gives a few examples of people finding interesting results, only to have them disappear on sustained scrutiny.

Posted by Robin Varghese at 05:26 PM | Permalink

Permalink Reply by Doone on March 11, 2011 at 7:56am

AGE AFFECTS US ALL

From PhysOrg:

Early acomparisons with rats, mice, and other short-lived creatures confirmed the hunch. But now, the first-ever multi-species comparison of human aging patterns with those inchimps, gorillas, and other primates suggests the pace of human aging may not be so unique after all. The findings appear in the March 11 issue ofScience. You don't need to read obituaries or sell life insurance to know that death and disease become more common as we transition from middle to old age. But scientists studying creatures from mice to fruit flies long assumed the aging clock ticked more slowly for humans.

We had good reason to think human aging was unique, said co-author Anne Bronikowski of Iowa State University. For one, humans live longer than many animals. There are some exceptions - parrots, seabirds, clams and tortoises can all outlive us - but humans stand out as the longest-lived primates. "Humans live for many more years past our reproductive prime," Bronikowski said. "If we were like other mammals, we would start dying fairly rapidly after we reach mid-life. But we don't," she explained.

More here.

Permalink Reply by Onyango Makagutu on October 5, 2012 at 6:41am

Interesting article!

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