SEAN CARROLL: To a physicist, time is a label on the universe.It appeared to be a rerun from 2015.
RAZ: Sean Carroll is a theoretical physicist.
CARROLL: At CalTech.
RAZ: And physicists like Sean think about time very differently from you and me.
CARROLL: To a physicist, the universe is this thing, it's full of stuff, and it keeps happening over and over again. And this goes back to, you know, ancient astronomers. The Earth revolves around the sun, and it rotates around its axis 365 times. So the universe is filled with repetitive, cyclic moments, and time is just the label on those different moments.
RAZ: Those labels - one day, one month, one year - make up what Sean and other physicists call the arrow of time, meaning that time travels in one direction. That's why you were younger in the past, while you'll be older in the future, why you remember one and not the other. But here's the problem. That difference between past and future is nowhere to be found in the laws of physics because in physics, there is no arrow of time.
CARROLL: When modern physics came to be from people like Galileo and Newton up through Einstein, we realized something very gradually, which is that the deep down laws of physics don't distinguish between the past and the future. They treat them completely symmetrically as if they were just replaceable with each other. So as a physicist, there's almost more work to be done in understanding why time works the way it does than as a person on the street because you have to reconcile the fact that there is a difference between past and future with the fact that that difference doesn't appear anywhere in your fundamental equations.
RAZ: In the laws of physics, there is no intrinsic difference between the past and the future.
CARROLL: That is exactly right.
RAZ: Which theoretically means what?
CARROLL: So if you think about time going from the far, far, far past - let's say the Big Bang was not the beginning. Let's say there was sort of an infinite amount of time before the Big Bang, and there's an infinite amount of time after the present moment. It's possible there are regions of the universe where their notion of past and future are backwards compared to ours, where they call the past what we call the future and vice versa. That's completely allowed because the underlying laws of physics don't distinguish.
I don't know how a Caltech physicist can say such nonsense. He later acknowledges that entropy is increasing with time, but he argues that is just because the Big Bang was low entropy.
Of course the laws of physics distinguish past from future.