Melissa Harris-Perry: Welcome back to the Takeaway. I'm Melissa Harris-Perry, with a question for you.

Speaker 2: What do you know about the multiverse?

[music]

Melissa Harris-Perry: Now the multiverse has been a very big subject this year in movies like Dr. Strange and Everything Everywhere All at Once.

Speaker 3: Across the multiverse, I've seen thousands of [unintelligible 00:00:23]

Melissa Harris-Perry: The existence of a multiverse is one of those big, big questions that make even nonscientists love physics. Could there be other universes where versions of ourselves are living different lives, or infinite versions of ourselves spinning off from every minuscule choice we do or don't make, like in this episode of Rick and Morty.

Speaker 4: Our time is fractured. You two somehow created a feedback loop of uncertainty that split our reality into two equally possible and possibilities.

Melissa Harris-Perry: Could all of time be happening all at once, or does that just happen in Doctor Who?

Speaker 5: People assume that time is a strict progression of cause to effect, but actually from a nonlinear non-subjective viewpoint, it's more like a big ball of wibbly-wobbly timey-wimey stuff.

Melissa Harris-Perry: How did the universe even get here?

Dr. Sabine Hossenfelder: In the beginning, Superstrings created high dimensional membranes. That's one story I've been told, but there are many others.

Melissa Harris-Perry: This is Dr. Sabine Hossenfelder, a Physicist at the Frankfurt Institute for advanced Studies in Germany.

Dr. Sabine Hossenfelder: Some physicists believe the universe started with a bang, others think it was a bounce, yet again others bet on bubbles. Some say that everything began with a network, some like the idea that it was a collision of thoughts or a timeless face of absolute silence or a gas of super strengths or a 5-dimensional black hole or new force of nature.

Melissa Harris-Perry: That passage is from her new book.

Dr. Sabine Hossenfelder: Which is called Existential Physics: A Scientist's Guide to Life's Biggest Questions.

Melissa Harris-Perry: I sat down with Dr. Hossenfelder, to tackle some of these questions. Before we get too far into all the possibilities of the universe, let's just establish some core definitions.

Dr. Sabine Hossenfelder: Physics is one of the most fundamental of the natural sciences. It describes many different processes. In a certain way, it describes the simple systems that we have, like the big fundamental laws, gravity electrodynamics, but also the universe as a whole, cosmology, galaxies, planets, solar systems, particles, all that kind of stuff.

Melissa Harris-Perry: What exactly is the multiverse?

Dr. Sabine Hossenfelder: The multiverse is a very speculative idea that has come up in the foundations of physics. It's broadly speaking the idea that our universe isn't the only one, but that there are many more.

Melissa Harris-Perry: Why the decision to write about these, in certain ways, extraordinarily complicated concepts within physics, but in a way that is digestible to all of us who only did moderately well in college physics?

Dr. Sabine Hossenfelder: I think physics is our best tool to find answers to some of the big questions like why do we only get older and not younger? Are there other universes? Will we ever know everything and so on? It's something that physicists shouldn't keep to themselves.

Melissa Harris-Perry: You begin in this first chapter with a truly existential question, one that perhaps particularly relevant or heartfelt after so much of the world has lost so much in the context of the COVID pandemic. You ask, does the past still exist? Help us to understand the content of that question and how physics goes about trying to respond to it.

Dr. Sabine Hossenfelder: This is a very interesting question to me because it's one that we are faced with as students in physics very early on. It comes up in the first year basically. The answer that we get from modern physics is basically, yes, the past still exists. The first time you hear of it, it just blows your mind. You're like, "Oh my God, how can it be?" Then you get so used to it that you forget how mind blowing it was. The reason we are fairly confident about it is that it comes out of Einstein's theory of space and time, which to make a long story short, it prevents you from defining a moment that you can plausibly call now.

We have this experience, like all of us, I think that we live in this present moment and we seem to share it, like your now is my now, but Einstein says, this is only because we're all moving fairly slowly compared to the speed of light. It's an illusion strictly speaking. If you want to rigorously define a notion of now, that we can all agree on, it's just not possible. The only logical conclusion that you can draw from this is that all moments exist the same way as this moment that we call now.

Melissa Harris-Perry: It is very hard for me to imagine what that means relative to a parent who I've lost, or a beloved who I haven't seen in a decade. How can the past still be existing? How could there be a moment still occurring when that beloved is still alive and still with me?

Dr. Sabine Hossenfelder: It's difficult to define what we mean by existence in the first place. I wouldn't say that the past exists. I would say that the past exists in the same sense as the present moment. You just can't distinguish between the both.

Melissa Harris-Perry: I want to talk a little bit more about the multiverse, in part, because it certainly has become such a critical part of our popular culture. I have an eight-year-old who pretty regularly seems to be pondering the existential physics questions as she's thinking about the multiverse. How different are the pop cultural representations of the multiverse compared to how physicists think about it.

Dr. Sabine Hossenfelder: They're much more interesting. The reason is that the multiverses that we deal with in the foundations of physics have other universes, but you can't visit them. They're there, but they're completely disconnected from ours. We can neither go there nor can we observe or experimentally test them in any way, and that makes for a pretty boring storyline.

Melissa Harris-Perry: Why do we get only older and not younger?

Dr. Sabine Hossenfelder: I have to admit that in the end the answer is, we don't really know. We do understand why we have two directions of time. One is forward, which is where we get older, and one is in the past where we can't go back, but if we try to write down a physical theory for it, then the answer is basically, it's because the universe started out in a very simple, almost perfect state, and since then it's gone downhills and continues to go downhills.

In physics we say the entropy increases. Staff breaks basically on its own, but it doesn't repair itself, but it brings up the question like why did the universe start in a very simple state with a very low entropy? To this, the answer is we don't know.

Melissa Harris-Perry: If there is an unstable or at least not in the way that we colloquially understand it past and present, what about the future?

Dr. Sabine Hossenfelder: The question, if the future exists already, like it's predetermined already is much more complicated than the past, because we have this additional ingredient to physics, which came after Einstein, which is quantum mechanics. In quantum mechanics, the future is not entirely determined by the past, but we have these unpredictable quantum jumps that bring in an element of randomness.

The future is not fixed because of this quantum randomness. Now you can ask, who gets to decide what's going to happen. That becomes very philosophical very, very quickly because it brings up the question like, what do we mean by making a decision? What do we mean by free will and so on and so forth. Strictly speaking, those random quantum jumps are not determined and not influenced by anything in particular, not by you, whatever you mean by that.

Melissa Harris-Perry: That it seems is perhaps among the most destabilizing contributions of this text, this idea of how, what you are laying out for us in this book does seem to mess with the notion of free will.

Dr. Sabine Hossenfelder: I would agree with you, but there are many different ways that people have defined free will. I would say if, you know that your brain is made of particles. The way that those particles behave is partly determined by the past, and then every once in a while there are some random jump and that's pretty much it. Then I would say it's fairly meaningless to talk about free will.

Other people, very important philosophers have proposed that you can speak of free will as a type of autonomy. You can talk about the autonomy of the decisions that you make of which humans have a lot as opposed to say your phone or a toaster which just executes what you want.

Melissa Harris-Perry: We like to think of humanness as a set aside identity, you phrase this in the chapter title as Are You Just a Bag of Atoms? Are we simply a bag of atoms?

Dr. Sabine Hossenfelder: Yes and no. We are simply a bag of atoms in the sense that this is what our body is made up of, but it's not the interesting part of us. The interesting part is how those atoms are put together, what they can do. It's basically the information that is in the configuration of the humans that I think is the important part.

Melissa Harris-Perry: You also write about the limits of physics, talk to us about the reasons we should or should not, or maybe how we should use physics to address deeply human spiritual questions.

Dr. Sabine Hossenfelder: I think some of the topics that we talked about address some of those big spiritual questions like how we connect with the universe for example, or how we can live our lives.

For example, if you think about the question of the multiverse, one of the peculiar things about the multiverse is that it's infinitely large, so they're infinitely many universes and they can be very, very similar to ours up to some really minute details. They could be copies of you in those other universes, and they would live your life pretty much exactly the same way except for one particular decision that you made differently. I think this is like a really spiritual thought. Like that there could be other versions of you living your life in different ways.

Melissa Harris-Perry: What's a core takeaway you want folks to take from this?

Dr. Sabine Hossenfelder: I think I've tried to make the book into a positive experience as good as I can, though I understand that there are some topics like the issue of free will that might be somewhat difficult for people, but I think what I'm trying to get across is that when we try to answer such big questions about our existence, we basically have three options that's religion, philosophy and physics. Of those three, I think physics has made the biggest progress in the past century. It's something that physicists don't talk about a lot.

I guess is for historical reasons, so scientists want to keep their distance from religion, but I think the downside is that this also distances us from humanity, and it's probably part of the reason scientists in general and physicists in particular perceived us somewhat cold and technocratic, because it seems that we don't care about those big questions, but I think physicists care about this as much as everyone.

Melissa Harris-Perry: Why is it so important for us to engage perhaps not only the sciences, not only philosophy, but learning more broadly and research more broadly sometimes with an eye to the practical, but often simply inquiry for inquiry sake?

Dr. Sabine Hossenfelder: Yes, I think there is a value in knowledge just for the sake of knowledge, and physics in particular I think it opens our minds to what is possible. For example, the multiverse that's something you might not even have thought about if it hadn't popped up in certain theories. In many cases it's physics that lets us imagine new possibilities in the first place.

Think about how often you see authors and screen writers borrow from physics. Black holes, time can slow down, parallel universes, [unintelligible 00:14:15], teleportation, it's all physics really, and it's just fascinating. I think physicists themselves don't appreciate this side of physics enough, this mind opening and inspiring side.

Melissa Harris-Perry: Dr. Sabine Hossenfelder, physicist research fellow at the Frankfurt Institute for Advanced Studies in Germany. She's authored a new book, Existential Physics: A Scientist Guide to Life's Biggest Questions. Thank you so much for joining us.

Dr. Sabine Hossenfelder: Thanks for having me.

[00:14:52] [END OF AUDIO]

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