Tuesday, March 6, 2012

TED 2012: Mystery of Multiverses

When I run into people who suggest that TED’s patented 18-minute talk is just too short a time to get across a big idea, I point them to Brian Greene’s  2004 talk on string theory.  A highlight of my first ever TED conference, Greene’s talk made string theory seem simple.  It was extraordinary.
This year, Greene once again brought the mystery of the cosmos to us earthlings, in particular the mind-blowing idea of multiverses, explained completely lucidly, supported by awesome visuals projected on the full wall behind him.  This was the first time TED has used a full video wall for projection and Greene's incredibly rich visuals did it proud; it gave one the feeling of hurtling through space as Greene described it. 

The theory of multiverses seems almost otherwordly, but we should pay attention to it, says Greene, because it just might be right.
When Hubble first realized that galaxies are rushing away from us, it was an idea that upset previous views of physics.  Hubble, and everyone else, believed that those galaxies must be hurtling away at an ever decreasing rate.  After all, gravity would be exerting a drag that would slow the universe down.  

However, the trouble is that once we got around to measuring the speed in the early 1990's, to the astonishment of the scientists, ethe measurements showed that the expansion of the universe is actually speeding up.   The theory of dark energy, a repulsive force pushing those galaxies apart, arose to explain that unexpected result.  Calculations showed that the amount of dark energy required to explain the expansion of our universe is astonishingly small, namely 1.28. (I don't actually know the unit of that answer, so I feel a bit as if I've fallen into the Hitchiker's Guide to the Galaxy, where I know the answer, but not its exact meaning).

Now take a hike back to Greene's original talk about string theory, which goes like this.  Inside particles, there are vibrating strings of energy.  Different sub-atomic particles  have different vibrations, giving rise to the 'cosmic symphony' that we are now familiar with.  But the mathematics of those strings does not work unless there are extra dimensions which determine how the strings vibrate.  That of course leads to the question – what shape would this extra dimension have?  Different amounts of dark energy would result in different shapes for those dimensions.

Soon physicists had generated many possible shapes for those dimensions – 10 to the 500th power  – far too many to tested and determine which was the right one.  Greene then proposed: What if all of these universes exist, each with a different shape for extra dimensions, each with a different amount of dark energy.  Maybe seeking the explanation of that one particular number was the wrong question.
Greene reflected back to Kepler who focussed on why the earth was 93 million miles from the sun.  But the right question was why were we on a planet 93 million miles from our sun.  The answer was because was the 'Goldilocks' effect – 93 million miles was just the right distance.  There were lots of other planets at different distances from the sun, but this was the distance where life could be sustained.

So Greene believes that we should be asking this new question about dark energy the same way.  Are there multiple universes out there,  each with a different amount of dark energy.  We find oursevles in this one because this amount of dark energy is the 'Goldilocks' zone for the formation of galaxies.  With less energy, the galaxies are blown away before they can form; with more energy, the universe implodes, precluding galaxies.   And without galaxies, we don't exist.

It's hard to test this theory, but there are certain phenomena we would see if the theory is correct.  There might have been evidence in the past that we missed (near the time of the big bang) that is lost to us now.  Greene mused philosophically that future astronomers could be in the same position in the future.  If galaxies speed up to spin away from us faster than the speed of light, then their light can never reach us.  Future astronomers could be looking out at a black inky sky.  

Greene's talk is already up on TED.com.  I hope you'll find it as fascinating as I did.

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