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Review: Physical Basis of the Direction of Time
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The Physical Basis of the Direction of Time by H.D. Zeh:
This is one of those books that I really love. I picked it up for fun in grad school and have had reason to go back to it several times since then. It is basically looking at the reason that time always seems to flow in the same direction -- forward.
The problem this has for physicists is that our most fundamental theories of tend to be time reversal symmetric: they operate just as well going forward as going backward -- as in the old idea that if you knew every physical fact about the universe right now, then you could predict the future with certainty. The same is true for the past: if you knew everything about the universe right now, then you could discern anything that happened in the past with certainty -- from the place and time that the first star formed in space to what Genghis Khan ate for dinner on his 26th birthday. This is true for classical mechanics, this is true for relativity, and this is true for the Schrodinger equation of quantum mechanics.
But it's not true for everything. Eggs don't unbeat themselves. This is what is called "irreversible," and Zeh goes through several places where irreversibility is found in physics, each supporting its own "arrow of time" that seems independent of the others:
(1) Advanced Green's functions don't exist.
(2) Entropy always increases.
(3) The collapse of the wavefunction destroys information.
(4) The speed of light defines our future.
(5) The statistical derivation of classical time from quantum gravity.
Which is the most fundamental kind of time, and how does one lead to the other?
This is one of those books that I really love. I picked it up for fun in grad school and have had reason to go back to it several times since then. It is basically looking at the reason that time always seems to flow in the same direction -- forward.
The problem this has for physicists is that our most fundamental theories of tend to be time reversal symmetric: they operate just as well going forward as going backward -- as in the old idea that if you knew every physical fact about the universe right now, then you could predict the future with certainty. The same is true for the past: if you knew everything about the universe right now, then you could discern anything that happened in the past with certainty -- from the place and time that the first star formed in space to what Genghis Khan ate for dinner on his 26th birthday. This is true for classical mechanics, this is true for relativity, and this is true for the Schrodinger equation of quantum mechanics.
But it's not true for everything. Eggs don't unbeat themselves. This is what is called "irreversible," and Zeh goes through several places where irreversibility is found in physics, each supporting its own "arrow of time" that seems independent of the others:
(1) Advanced Green's functions don't exist.
(2) Entropy always increases.
(3) The collapse of the wavefunction destroys information.
(4) The speed of light defines our future.
(5) The statistical derivation of classical time from quantum gravity.
Which is the most fundamental kind of time, and how does one lead to the other?