The Physics and Philosophy of Time – with Carlo Rovelli
Short and snappy thought provoking excerpts from the talk. Some of what is written below is verbatim and some of it is redacted to be more reader-friendly:
The past leaves traces so we can talk about the duration of time. But mass slows down time. Time goes faster the higher the altitude. Your head is older than your feet. We live in a region where the difference of time is so small that we are not used to this. If we lived near a black hole and you went to it and came back, everyone here would be much older than you. So time is not linear, it can depend if you are higher or lower. So there isn’t a single time, there are many ‘times’.
So when I look at you now, do I actually see you now? No. The light takes time to come to you and me. So I see you a little bit in the past, like a few nanoseconds ago. But if you were on Jupiter, I would see you 4 hours ago. If you were on the closest star I would see you a year ago. So there is no concept of the now outside of the proximation in which we can disregard the time for light to go back and forth. There is no meaning of time outside of this bubble. Proxima B the nearest star has its own ‘now’. You can send signals back and forth, but there is no meaning of ‘now’.
We have this distinction between the past and the future. The laws of mechanics, particle physics and basic equations of physics does not have this distinction. There is however one science which clearly sees this distinction. The 2nd law of thermodynamics: that entropy always grows towards the future. The higher the entropy the more disorder. But, order is in the eye of the observer. What might seem like a pattern to me could look to be in total disarray to another observer. The growing of entropy is tied to the approximation we can make in describing a system; the macroscopic way which depends on how we interact with that system. Who for instance prepared the order in the Universe in the past? It’s a mystery. And what does it have to do with time?
So the distinction between the past and the future is only that. So we observe this strange order in the past. Everything in our experience which is ordered in time is because of Entropy. There is nothing else that distinguishes this past and future except this entropy. So the reason we have traces in the past if nothing else is because of entropy. There had to be some heat, some disorder at some point. So a 15th century monk wrote a text, but if it were not for friction from the paper the ink would not have been captured. The friction produces a little bit of heat so there is disorder, but resulting from high order. High order in the past determines traces. If you like, a cause and effect. It only occurs because of the law of entropy, otherwise there would have been no dissymetry.
Every time-line in the Universe has its own temporality. So it makes no sense to talk of the ‘now’ of the universe. Also, the distinction between one direction of time and another direction of time has a strange macroscopic effect which you don’t see in Micro-physics.
There is a connection between time and gravity. Mass affects the speed of time. The Newtonian way of looking at time is that time just passes even if nothing happens (empty space). But Einstein refuted it by knowing there was a gravitational field which fills everything up; a 4 dimensional curve space-time. In quantum gravity the events are connected to one another; a local notion of events, but a minimal notion of time. That is the minimal time of quantum gravity. Forget time. A time variable is macroscopic. But down at the microscopic level you have many variables. We live in a region where gravity is very very weak as compared to a black hole which is really strong.
Where curve space-time is very flat reflects a Newtonian space and time. We can disregard the back and forth of the light because it seemingly moves at infinite velocity. So we have this surface of simultaneity where we may as well be in a bubble. We can talk about one single point in time. We look at the past and decide it was special because entropy was low. There was perceived order.
Your Brain is a Time Machine
We are special subsets of the universe where we interact in such a way that the past looks ordered to us. However there still seems to be something missing about time. When we refer to this clear feeling we have about the flow of time it is not found in quantum gravity, general relativity, quantum mechanics nor thermodynamics rather it’s in the specific way our brains work. Your brain is a time machine because it exploits Entropy by finding traces in the past and then computes this and anticipates the future. The brain continuously grasps to events. We are connected by traces of events and connected by our anticipation in the other direction.
The passage of time is not a rational contemplation, rather it’s something we live into. We are this passage of time. It’s hard to think of your reality without time. We are the time machine, not the universe. Our thinking cannot be done without time, a cultural memory, the endless lineages of events. It’s my knowledge that I have attained throughout my life. Time is not emotionally neutral to us. Life is suffering because we have lost a lot of things in the past and we will assuredly die. Our brain is designed to tell the story of the past.
The more we go looking into the Universe and build a general picture, the more time loses its pieces. Our emotional connection with time is what time is for us.