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Physics' Greatest Puzzles Excerpted from George Johnson

INTRODUCTION
'WHO of us would not be glad to lift the veil behind which the future lies hidden; to cast a glance at the next advances of our science and at the secrets of its development during future centuries?'

One hundred years ago, with those inviting thoughts, the German mathematician David Hilbert opened his landmark address to the International Congress of Mathematicians in Paris, laying out 23 of the great unsolved problems of the day.

'For the close of a great epoch,' Hilbert declared, 'not only invites us to look back into the past, but also directs our thoughts to the unknown future.'

With another century ended - a whole millennium in fact - the pressure is all the greater to tabulate human ignorance with lists of the most enticing cosmic mysteries.

This August 2000, top physicists ended a conference on Superstring Theory at the University of Michigan with a session called 'Millennium Madness', choosing 10 of the most perplexing problems in their field.    It was like a desert island game, involving some of science's smartest people.

'The way I thought about this challenge was to imagine what question I would ask if I woke up from a coma 100 years from now,' said Dr. David Gross, a theoretical physicist at the University of California at Santa Barbara, as he unveiled the winners.

After weeding out unanswerable questions (like: 'How do you get tenure?), the judges came up with enough puzzles to occupy physicists for the next century or so.   There are no monetary prizes, though solving any one of these would almost guarantee a trip to Stockholm with its attendant award of a Nobel Prize!

In presenting his own list of mysteries, Hilbert allowed: 'It is by the solution of problems that the investigator tests the temper of his steel;  he finds new methods and new outlooks,  and gains a wider and freer horizon.'

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Top 10 Puzzles

#1.
Are all the (measurable) dimensionless parameters that characterize the physical Universe, calcuable in principle, or are some merely determined by historical or quantum mechanical accident and uncalcuable?

#2.
How can quantum gravity help explain the origin of the Universe?

#3.
What is the lifetime of the proton and how do we understand it?

#4.
Is Nature supersymmetric, and if so, how is supersymmetry broken?

#5.
Why does the Universe appear to have one time and three space dimensions?

#6.
Why does the cosmological constant have the value that it has? Is it zero and is it really constant?

#7.
What are the fundamental degrees of freedom of M Theory (the theory whose low-energy limit is eleven-dimensional supergravity and that subsumes the five consistent superstring theories) and does the theory describe nature?

#8.
What is the resolution of the black hole information paradox?

#9.
What physics explains the enormous disparity between the gravitational scale and the typical mass scale of the elementary particles?

#10.
Can we quantitatively understand quark and gluon confinement in quantum chromodynamics and the existence of a mass gap?

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