Monday, March 8, 2010
We take time for granted. It's simply there and flowing forward at a steady and unstoppable pace. In fact all of modern science is dependent on it because it is necessary to define causality: the notation that one things "causes" some other thing to happen. This apparently happens only in one direction. However, there is a huge paradox luring behind the corner, because all fundamental laws of physics are time reversal invariant. The reason for the observed asymmetry of time is the second law of thermodynamics which says that entropy can only increase. That is actually not quite stated correctly. It should say that there is an overwhelming probability that it increases but there can be random fluctuations that make it temporarily decrease.
Where does the second law of thermodynamics come from? Fill a swimming pool with a red fluid and a blue fluid separated by a wall. Now remove the wall and you will notice that the colors mix. The opposite will never happen. Yet the underlying laws are symmetric. What's going on? The issue is that there are very (very very) many more states with colors mixed up that look the same to us than there are states which separate the fluids. So we can imagine the state-space as being build up from cells where all states in a cell all look the same to us. As we randomly wander around in this space we will move from cell to cell but since some cells are so hugely much bigger than others we always tend to wander into those. So, perhaps surprisingly, the second law is "subjective", it depends on us not being able to to distinguish the many states with mixed colors.
How would the world look like if all possible states would look equally different? Imagine just looking at 2 marbles moving around in a box. They reflect off the walls and so now and then reflect off each other. If we play the movie backwards it looks exactly the same. In this world we would have no features available to tell the directionality of time. In such a world the concept time might not even exist for creatures living in it. Hard to imagine isn't it?
Let's go one step further. If entropy is increasing since the conception of the universe, it must have been very small to begin with. In fact, this is exactly, what Roger Penrose proposes in his book "the Emperor's New Mind" (in the less controversial chapters of it). The universe was in a very low entropy state when it was created and has been steadily increasing ever since. The notion of time and the reason we perceive it must be sought at the "time" of the Big Bang. Nobody knows why this is true. In fact, one can easily imagine an opposite scenario where (for some reason) the universe must end in a very low entropy state (perhaps a bizarre version of a Big Crunch) but started out in a high entropy state. In such a universe effects are followed by their causes. Broken glasses magically assemble themselves into whole glasses. In such a world the future looks much more certain then the past.
Try to imagine living such a world. My feeling is that time would be experienced in reverse, but we wouldn't really notice it, because time is all just an illusion anyway. The reality is that the universe is simply there from beginning to end. We occupy a small window of that universe and perceive it as flowing in a certain direction. Very Buddha.
Now for some very controversial experiments that cast doubt on how we might understand time. It seems that there have been quite a few experiments where an experimenter would repeatedly show subjects pictures that were either extremely disturbing or very nice and beautiful. The subject is hooked up to some device measure his/her level of excitement (say fMRI or simply resistance in skin). Disturbing pictures will evoke a much different response then nice pictures after you have seen them. That's not strange. What's strange is that the subjects seem to have a significantly different response before the pictures have been shown. In other words, they seem to anticipate whether a picture is disturbing or not.
I am assuming that the experimenters very carefully ruled out any learning effects (although I haven't actually seen this mentioned). This is important because people seem to be able to learn very complicated patterns completely unconsciously without even knowing it. It's easy to control for this though, because you simply produce a random sequence, or even better show a complicated correlated (i.e. predictable) sequence and then reverse the correlations. If the signal stays you rule out that the subjects were able to predict what image would come next based on correlations. I also assume they did their statistics right of course.
However, if all these things were done correctly it presents a huge puzzle. These subjects seem to know the future, i.e. they "remember" the future. This leads immediately to a paradox.
"suppose one can know the future.
Then one can take action so that that future will not happen.
So it doesn't happen.
So no-one can know the future"
Unless, one could not use the information to change the future.
Ok, all of this is really bizarre. And the first impulse for any scientist will be to reject this out of hand. But I think we should keep an open mind. The notion of time is very strange and paradoxical in itself. And there are other strange cracks in our scientific theories that concern causality. Take the quantum mechanical phenomenon of the collapsing wave-function. If you take two entangled particles with opposite spin and shoot them off in opposite directions of the universe and then measure the spin of one of them, the other one is instantly known on the other side. You cannot explain this by assuming that they already were in a certain spin orientation but that you simply didn't know which one. No, the spin direction is genuinely undetermined until you measure it.
Measuring the orientation of this strange entangled state implies that some kind of signal must be traveling from one end of the universe to other to inform the other particle that it must now be opposite to the value we just measured. If it travels faster than light however, it means that you can identify two observers in the universe traveling with high velocities in different directions for which the causal relationship between the events is reversed! For one observer spin A collapsed first and sent a signal to B, while for the observer spin B collapsed first and sent a signal to A. Now these signals (if they are indeed signals) can never be used by somebody to actually send information because it would result in causal loops again. And indeed, if you try to figure out a way to send information you find that nature is just a bit too smart and prevents this.
One things seems clear from these paradoxes. Our current theories are clumsy when we try to explain these quantum mechanical phenomena. Nobody knows why the wave function collapsed, yet it is ingrained in quantum mechanics and widely accepted. In fact, there are no good explanations for these paradoxes. So keep an open mind, even when it come to things that are at odds with everything you have learned in class. Be critical, but don't dismiss too early. Breakthroughs only happen when the unimaginable become reality.