Indeed, as Feynman points out, the difference between future and past manifests itself in us psychologically, even though we are unable to precisely describe why. We find it absurd to think that we could influence our past, yet still mostly believe in free will as it regards to the future.

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 believe that there is anything we can do to affect the past.

A lack of belief in our ability to affect the past manifests itself in our emotions too, such as in remorse and grief as well as in our faith in the converse, hopes and dreams. This perfectly distinguishes the past and the future for us, psychologically. However, how can we interpret the same distinction physically?

Irreversibility

“Things are reversible only in the sense that going one way is likely, but going the other way, although possible, would not happen in a million years”

Now if the world is made of atoms, and we too are made of atoms and obey physical laws, the most obvious interpretation of this evident distinction between past and future, and this irreversibility of all phenomena, would be that some laws, some of the motion laws of the atoms, are going one way - that the atom laws are not such that they can go either way.

As Feynman next proclaims “there should be somewhere in the works some kind of principle that Uxles only make Wuxles, and never vica versa, and so the world is turning from Uxley character to Wuxley character all the time — and this one-way business of the interactions of things should be the thing that makes the whole phenomena of the world seem to go one way”.

Yet, as he points out, we have not yet found what it is that does this. What is it in nature’s laws that provides the “arrow of time”?

“The movie should work the same going both ways, and the physicist who looks at it should not laugh”

The Law of Gravitation

Take the law of gravitation. “Every particle attracts every other particle in the universe with a force that is directly proportional to the product of their masses and inversely proportional ot the square of the distance between their centers”:

If I have a sun and a planet, and I start the planet off in some direction, going around the sun, and then I [record a video of it], and run the film 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. From the perspective of the law, playing back the wave forwards or backwards in time makes no difference. Nobody laughs.

The laws of electricity and magnetism are the same, also time reversible. As are the laws describing the nuclear reaction and as far as we can tell, beta decay.

Complexity

As Feynman points out, if we look a little bit closer, we however soon realize that the above interpretation does not capture all of the picture:

If we look at our planets moving around the sun more carefully, we soon find that all is not quite right. For example, the Earth's rotation on its axis is slightly slowing down. It is due to tidal friction, and you can see that friction is something which is obviously irreversible.

That is, the frictional effect is the result of the enormous complexity that arises from the interaction between the jiggling atoms of two substances coming together. Reversibility at this scale, in this domain, is not as straightforward as are the reversibility of the motion of planets.

Thought experiment: The Water Tank

Suppose we have blue water, with ink, and white water, that is without ink, in a tank, with a little separation, and then we pull out the separation very delicately. The water starts separate, blue on one side and white on the other side. Wait a while. Gradually the blue mixes up with the white, and after a while the water is 'luke blue', I mean it is sort of fifty-fifty, the colour uniformly distributed throughout.

Now, would we expect the two fluids to ever separate back to blue and white again? As Feynman points out, we could do something to separate them, but we would most certainly never expect them to do so spontaneously. This gives us some clue. If we run a movie of the experiment backwards, we are surprised to see a mixed substance separate.

Now, if we magnify the picture even more, what do we expect to see? We expect to see giggling molecules of (for the sake of the thought experiment) blue and white color, interacting in a jittery motion. The extent of their jiggering is dependent on their temperature: