# The nature of time.

First off, I’ve talked about time before. What I am not sure about is whether I’ve presented this to my lab before. Sorry Catherine if I have. After 5 years, I think I’m starting to recycle old ideas! I choose to talk about time because I’m busy, and time is a conversation that just comes naturally to me. Plus, we have so many new students, and I want to share this lovely idea with all of them.

## Special Relativity

I imagine most of my readers (or if I’m presenting this, listeners) are familiar with Einstein’s special relativity. The faster you go, the slower time ticks. One of my favorite ways visualizing this is by imagining you plot your movement through time (y axis) and space (x axis). The magnitude of your vector in space time is fixed; as you move through space, you rotate that vector away from time. Similarly, large gravitational sources also slow time. If you’ve seen the pretty good movie, Interstellar, then I imagine you know this idea. It was one of the biggest challenges to my concept of reality as a kid; grew up young earth creationist. It doesn’t disprove a god, but this was really when I began to see how mundane my early ideas were. The nature of reality is so much grander than we imagine.

## The Block Universe

Around the same time I learned about Special Relativity, I learned about the concept of the block universe. Sadly, I went about a decade before I really understood the concept. I loved the idea, but my confusion around it led to me doubting whether my idea of it was ever right. Which is what motivated me to return to time. I read Lee Smolin’s Time Reborn, then From Eternity to Here by Sean Carroll, then The Fabric of the Cosmos by Brian Greene. Interestingly enough, it was the PBS special for Brian Greene’s book that really got me hooked to all this.

The basic idea (or question) is whether the Universe is a all moments (past, present, future) or just the current one. We have this idea that the universe is one moment in time, but special relativity suggests that is not the case. The notion of now falls apart in relativity. Because of how time changes, your “now” becomes deformed. I recommend watching the video below to get this best. The basic idea is is that the universe can be thought of as a block, or a slice of bread, or a book of pages. We think of books or bread, as sliced in equal parts, but that loaf, that block of paper that is the universe is constantly sliced differently depending on the observer. This introduces the idea of the Block Universe.

(3:15-5:00 for precise point) Brian Greene from PBS special on fabric of the cosmos.

While I am not an expert, I don’t think I’m wrong in saying that the block universe is fairly well agreed to be the most reasonable way of thinking about the universe. However, it is somewhat philosophical, and so there is dissent. Lee Smolin, for example, insists on Presentism, the idea that that moments come into existence, and the past no longer exists. I need to reread his book (but I don’t want to 😦 ) because it was not effective. I found no reasonable arguments beyond. He made a lot of emotional based arguments which is ridiculous. Nevertheless, I think I have a better idea of it all now and would like to explore his dissent again.

## The Flow of Time in a Block Universe

This raises a question? Why do we remember the past but not the future? Let me start by recommending you check out PBS Spacetime which is a fantastic resource (see videos and playlists linked), and you can check out Sean Carroll’s book too which is where I first learned about this concept. I was confused at first, but I think I understand it enough to explain it? The quintessential idea is Entropy. Fundamentally, we know the past is constrained to a fundamentally less complex configuration while the future has the potential for a range of results. It’s not like we can’t postulate reasonably about the future, but the lack of information constraining it makes it difficult. PBS Spacetime does a good job exploring this concept. Fundamentally, the past leaves imprints, or records, that give us the ability to constrain the past. Whether it be geologic history or the various imprints in our mind. Honestly, it’s best to just watch the video to get the full idea, but for now, just entertain me.

The concept of Entropy introduces an interesting concept because it suggests the flow of time exists only because an entropy gradient exists. There is a much larger conversation to be had here about the history of the Universe because we have this idea of the origin of the universe being the origin of everything, but it is really just the origin of the universe as we know it. Sean Carroll explores this idea significantly with an entire chapter in his book dedicated to entropy. It reframes the idea of the origin of the universe from how it came to be to what triggered the high entropy state. I haven’t explored the idea recently enough to talk about it, but if you are intrigued by the concept, I recommend checking out these videos and Carroll’s book.

For more cool PBS Space Time videos, check out this playlist I made (or their channel).