Time doesn’t go backward, and that is a problem in current physics.
Why should the fundamental laws have that bizarre and problem-posing property, T invariance?
The answer we can offer today is incomparably deeper and more sophisticated than that we could offer 50 years ago. Today’s understanding emerged from a brilliant interplay of experimental discovery and theoretical analysis, which yielded several Nobel prizes. Yet our answer still contains a serious loophole. As I’ll explain, closing that loophole may well lead us, as an unexpected bonus, to identify the cosmological “dark matter.”
We are told, there are “grounds for optimism.” A theortical paarticle called the “axion”:
The theory of axions predicts, in a general way, that axions should be very light, very long-lived particles whose interactions with ordinary matter are very feeble. But to compare theory and experiment we need to be quantitative. And here we meet ambiguity, because existing theory does not fix the value of the axion’s mass. If we know the axion’s mass we can predict all its other properties. But the mass itself can vary over a wide range. (The same basic problem arose for the charmed quark, the Higgs particle, the top quark and several other others. Before each of those particles was discovered, theory predicted all of its properties except for the value of its mass.) It turns out that the strength of the axion’s interactions is proportional to its mass. So as the assumed value for axion mass decreases, the axion becomes more elusive.
Do axions exist? We still don’t know for sure. Their existence would bring the story of time’s reversible arrow to a dramatic, satisfying conclusion, and very possibly solve the riddle of the dark matter, to boot. The game is afoot. More.
We expect it will be going on for a while.
See also: Cosmologist tells us how time got its arrow Something is wrong here. Just recording, just recording…
Studying time’s arrow with philosophers
Did time’s arrow originate in a quantum source?
The bill arrives for cosmology’s free lunch
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