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Archive for August, 2012

From the Sydney Morning Herald (here):

Melbourne researchers rewrite Big Bang theory

Melbourne researchers believe they may be on the brink of rewriting the history of the universe.

A paper being published in a US physics journal suggests it may be possible to view “cracks” in the universe that would support the theory of quantum graphity – considered to be the holy grail of physics.

The team of researchers from the University of Melbourne and RMIT say that, instead of thinking of the start of the universe as being a big bang, we should imagine it as a cooling of water into ice.

… Their research rests on a school of thought that has emerged recently to suggest space is made of indivisible building blocks, such as atoms, that can be thought of as similar to pixels that make up images on a computer screen.

Mr Quach said the standing model for the origins of the universe, the big bang, needed to be rewritten. He hoped experimentalists would be able to find evidence to support the theory put forward by the Melbourne team of researchers, that would replace it. “The biggest problem with the big bang model is the bang itself,” Mr Quach said. …

Mr Quach and his fellow researchers theorise that if quantum graphity “cracks” do exist, they will bend or reflect light, which, if observed through a telescope would support their predictions.
“If they prove my predictions that’s really good evidence for the condensed matter model of quantum graphity in which case you can throw out all the other attempts.”

Here’s a few pointers for the layman trying to decipher this article.

  • Note how the claim of the title changes. “They’ve rewritten the big bang theory” becomes “they believe they’re about to rewrite the big bang theory” becomes “it may be possible to observe the consequences of a theory that might provide a model for the big bang”.
  • The name “quantum graphity” is a pun on the terms quantum gravity and graph theory [edit: 1/9/2012]. Quantum gravity is the “holy grail” of physics (to some). Quantum graphity is not. The journalist evidently didn’t get the pun.
  • Note that the article quotes Mr Quach. Not Dr or Professor. I love grad students, but claims that they are about to rewrite everything we know about the fundamental laws of nature and the entire history of the universe should be taken with a grain of salt.
  • The paper that the article refers to contains no cosmology. It doesn’t claim to. None of Mr Quach’s papers do. What the paper shows is that, if spacetime consists of these building blocks, and the blocks get put together imperfectly, then light will scatter of the imperfection. The paper concludes: “they produce intriguing scattering, double imaging, and gravitational lensing-like effects. Importantly this serves as a framework in which observable consequences of the QG model may allow it to be tested.”
  • It is difficult to express just how astronomically huge the “if” is in the sentence “if observed through a telescope”.  What observational signature should we be looking for? There are an awful lot of things in the universe that bend and deflect light. How would we distinguish between the observation of a graphity imperfection and other gravitational lenses? What unique predictions does the model provide? How many imperfections should we expect in the universe?  What astronomical targets should we aim at?
  • This idea isn’t new. The further we look in the universe, the more likely we are to see something funky along the way, so distant quasars have been used to test theories about interesting spacetime phenomena. So far: nothing. No evidence for quantum foam. No evidence for cosmic strings. No topological defects. Why would graphity defects be any different? (more…)

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Let’s begin by quoting from Radford Neal:

There is a large literature on the Anthropic Principle, much of it too confused to address.

I’ve previously quoted John Leslie:

The ways in which ‘anthropic’ reasoning can be misunderstood form a long and dreary list.

My goal in this post is to go back to the original sources to try to understand the anthropic principle.

Carter’s WAP

Let’s start with the definitions given by Brandon Carter in the original anthropic principle paper:

Weak Anthropic Principle (WAP): We must be prepared to take account of the fact that our location in the universe is necessarily privileged to the extent of being compatible with our existence as observers.

Carter’s illustration of WAP is the key to understanding what he means. Carter considers the following coincidence: (more…)

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