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Lorna Mills and Sally McKay
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Richard Feynman said this:
excerpts from The Distinction of Past and Future
from The Character of Physical Law, 1965
It is obvious to everybody that the phenomena of the world are evidently irreversible...The past and the future look completely different psychologically, with concepts like memory and apparent freedom of will, in the sense that we feel that we can do something to affect the future, but none of us, or very few of us, believe that there is anything we can do to affect the past. Remorse and regret and hope and so forth are all words which distinguish perfectly obviously the past and the future. ...
[HOWEVER] In all the laws of physics that we have found so far there does not seem to be any distinction between the past and the future. ...
If I have a sun and a planet, and I start the planet off in some direction, going around the sun, and then I take a moving picture, and run it backwards and look at it, what happens? The planet goes around the sun, the opposite way of course, keeps on going around in an ellipse. The speed of the planet is such that the area swept out by the radius is always the same in equal times. In fact it just goes exactly the way it ought to go. It cannot be distinguished from going the other way. So the law of gravitation is of such a kind that the direction does not make any difference; if you show any phenomenon involving only gravitation running backwards on a film, it will look perfectly satisfactory. ... If you have a lot of particles doing something, and then you suddenly reverse the speed, they will completely undo what they did before. ...
[Experiments a few months ago indicated that] there is something the matter, some unknown about the laws, [suggesting] the possibility that in fact beta-decay may not also be time reversible, and we shall have to wait for more experiments to see. But at least the following is true. Beta-decay (which may or may not be time reversible) is a very unimportant phenomenon for most ordinary circumstances. The possibility of my talking to you does not depend on beta-decay, although it does depend on chemical interactions, it depends on electrical forces, not much on nuclear forces at the moment, but it depends also on gravitation. But I am one-sided - I speak, and a voice goes out into the air, and it does not come sucking back into my mouth when I open it - and this irreversibility cannot be hung on the phenomenon of beta-decay. In other words, we believe that most of the ordinary phenomena in the world, which are produced by atomic motions, are according to laws which can be completely reversed. So we will have to look some more to find the explanation of the irreversibility.