July 4, 2006


From the present to the past: Cambridge physicist Stephen Hawking and his CERN colleague Thomas Hertog have proposed a radical new approach to understanding the universe that studies it from the "top down" rather than the "bottom up" as in traditional models. The approach acknowledges that the universe did not have just one unique beginning and history but a multitude of different beginnings and histories, and that it has experienced them all. But because most of these other alternative histories disappeared very early after the Big Bang to leave behind the universe we observe today, the best way to understand the past, they say, is to trace our knowledge back from the present (Phys. Rev. D 73 123527). (Physics Web, 30 June 2006)

Most models of the universe are bottom-up, that is, you start from the well-defined initial conditions of the Big Bang and work forward. However, Hawking and Hertog say that this method is flawed because we do not and cannot know the initial conditions present at the beginning of the universe and that we only know the final state -- the one we are in now. Their idea is therefore to start with the conditions we observe today -- like the universe is 3D, nearly flat and expanding at an accelerating rate -- and work backwards in time to determine what the initial conditions might have looked like.

The new theory aims to get round a fundamental problem of string theory -- the most popular candidate for a "theory of everything" -- which is that it allows the existence of a multitude of different types of universes as well as our own. [...]

[H]awking and Hertog say that the universe did not take just one path through time to arrive in its present state, but took a multitude of paths, or histories. The "sum over all histories" is therefore the universe we observe today.

The new top-down theory could also explain why some constants of nature seem to have finely tuned values that have allowed life to evolve in our universe.

Some folks objected the other day to the description of physics as an ideology, but note that these contortions are just an attempt to explain away the obvious scientific fact of the Observer.

Posted by Orrin Judd at July 4, 2006 12:05 PM

... "we only know the final state -- the one we are in now."

Are they trying to tell us something? Should we start worrying?

Posted by: erp at July 4, 2006 2:34 PM

Mr. Hawking continues to flail around as one after another of his outrageous speculations turns out to be unworkable. It wasn't too long ago that his "imaginary time" was the answer to all questions - now it seems "top-down" string theory is the answer. What's next?

Posted by: jd watson [TypeKey Profile Page] at July 4, 2006 3:00 PM

It falls under the maxim that "90% of everything is crap". If 9 out of 10 of Hawking's ideas are worthless, it is still worth paying his salary to get that 10th idea.

Posted by: Gideon at July 4, 2006 4:26 PM

But working it "top down" is both literally presumptous and irrational.

Posted by: ghostcat at July 4, 2006 6:40 PM

Well, yes. But to pretend you're being perfectly rational about it is, er, untrue. A truly "top down" analysis must perforce proceed from faith, not reason.

Posted by: ghostcat at July 4, 2006 7:30 PM

All analyses proceed from faith.

Posted by: oj at July 4, 2006 7:36 PM

Assumptions must be made, yes.

Posted by: ghostcat at July 4, 2006 8:36 PM