Existential Physics: A Scientists Guide To Life’s Biggest Questions

I just finished Sabine Hossenfelder’s new book, Existential Physics: A Scientists Guide To Life’s Biggest Questions. I intended to do a full review but alas don’t have the time. Still, I wanted to share a few notes I made on key points she made in each chapter. So here goes:

Chap 1 – Does The Past Still Exist?

When our grandparents die info about them becomes irretrievable. We can’t communicate with them. “Nevertheless, if you trust the mathematics, the information is still there, somewhere, somehow,  spread out over the entire universe, but preserved forever. It might sound crazy but it’s consistent with all that we currently know.” (She’s alluding to the block universe.)

Chap 2 – How Did The Universe Begin? How Will It End?

Regarding the beginning of our universe – we just don’t know. all theories are speculative – “modern creation myths written in the language of mathematics.” As for the end of our universe, extrapolating from the laws of physics it will be dark because energy will eventually run out. However predictions about results trillions of years in the future are suspect. In the final analysis, “you shouldn’t trust physicists’ predictions for the end of the universe.”

Chap  3- Why Doesn’t Anyone Get Younger? 

The arrow of time results from the low entropy at the initial state of the universe and the increase of entropy since then. We don’t know why the universe was in this initial state although we have many hypotheses. As for the 2nd law of thermodynamics, H doesn’t think we can trust it to tell us about the fate of the universe because it’s only “based on how we currently understand the universe.” [emphasis mine.] And she doesn’t think “conclusions drawn from it today will remain valid when we understand better how gravity and quantum mechanics work.”

Our subjective experience of “now” makes it seem special, but objectively all moments are equally real. In short, H accepts the block universe of modern physics—the past and future are as real as the present. The reality we experience may be just a construction of our minds and sensory input has been around for a while  (think Descartes and “The Matrix.”) We may be brains in vats. That may be true but she doesn’t think believing it will make much difference.

Chap 4 – Are You Just A Bag Of Atoms? 

Reductionism, the idea that the behavior of an object (like you) can be deduced from its constituent objects, properties, and their interactions is”one of the best-established facts about nature. Fact is we have never observed an object composed of many particles whose behavior falsified reductionism …” Still, she admits that reductionism may fail in the future when we learn even more about nature.

However, what’s important about us are the relationships and interactions of the particles. This suggests that we could in theory replace those physical particles with silicon or some other substrate and if the replacement parts maintained the same functions then even our consciousness could be uploaded into a computer. This is not now possible but “it’s compatible with all we currently know.”

Chap 5 – Do Copies Of Us Exist?

H says that the parallel universes of the many-worlds interpretation of quantum mechanices—including the idea that there are many yous out there—is compatible with what we know scientifically. Yet, for the moment, science tells us nothing about whether any of this is true or not. Like other ideas of multiverses, they are ascientific ideas since other universes are by definition unobservable. As for the simulation argument, she is very skeptical. In the end, the idea that there are copies of ourselves is unscientific “because such copies are unobservable and unnecessary to explain what we can observe.”

Chap 6 – Has Physics Ruled Out Free Will?

Philosophers redefine free will (FW) so that it’s not what people usually mean. Nonetheless, “The currently established laws of nature are deterministic with a random element from quantum mechanics. This means the future, is fixed except for occasional quantum events that we cannot influence.” In other words, what we do today follows from the state of the universe yesterday and so on all the way back to the Big Bang. Still, we believe we have FW because we don’t know the results of our thinking before we have done it.

Strong emergence isn’t possible–higher-level properties of a system derive from lower levels (particle physics.) And there is no evidence that strong emergence is real; in other words, that the macroscopic can’t be derived from the microscopic. Yes, we may intuitively feel we have FW but analogous to how we understand, contrary to intuition, that the “now” is an illusion, FW is also an illusion. In fact “you’re running a sophisticated computation on your neural processor.”

Again the future is fixed except for occasional quantum events that we cannot influence. Whether this eliminates FW depends on how you define FW.

Chap 7 – Was The Universe Made For Us?

Some say that the (supposed) “fine-tuning” of the universe is explained by a god; others say it is explained by the multiverse. H considers both ideas unscientific because “they postulate the existence of things that are unnecessary to describe what we observe.” More importantly, “claiming that the constants of nature are fine-tuned for life is not a scientifically sound argument, because it depends on arbitrary assumptions.”

The brief answer is that we have no reason to think the universe was made for us—with the caveat that “no scientific theory will ever be able to answer all questions.”

Chap 8 – Does The Universe Think?

The universe doesn’t think—if for no other reason than that it is 90 billion light-years across and it would take a lot of time for a thought to get from one end to another! Still “the idea that the universe is intelligent is compatible with all we know so far.”

As for panpsychism—the idea that all matter is at least minimally conscious—H is extremely skeptical. But could it be that every particle is proto-conscious and when arranged properly in brains they become fully conscious? No. The idea that elementary particles think, even minimally, conflicts with the evidence.

However, panpsychism is right in one way. It points to how consciousness (which she thinks of as systems like brains processing information) isn’t binary but a gradient property. Put differently, systems are more or less conscious “because some process more information, others less.” At a certain size and configuration of matter, consciousness emerges.

Mary’s room” is a thought experiment (she grew up in a black-and-white room and knows everything about the physics of color and the brain’s reaction to color.) Defenders of the argument claim that she will learn something new about color if she starts to see color and this shows that the experience of color is more than just brain states—instead, the mind has a non-physical aspect. H replies that this argument confuses knowing about the perception of color with actually perceiving color. The fact is we have no scientific evidence that human perception is non-physical. And the study of consciousness is now in the realm of science.

Chap 9 – Are Humans Predictable? 

Humans are largely predictable; but are they fully predictable in principle, if we have enough knowledge? Now human behavior is partly unpredictable because of those quantum events we talked of previously, but I could still make probabilistic predictions about what you will do. But what of undecidable problems in computer science (the domino and halting problems for instance) and mathematics (the incompleteness theorem)?  What of chaos theory?  H arguments here are complex but the bottom line is that “we have no reason to think human behavior is unpredictable in principle, but good reason to think it’s very difficult to predict in practice.”

Turning to AI, H grants that artificial general intelligence is possible and that we should be concerned. But as she sees it in the near future the problems will arise because of our ethics or lack thereof, not AI’s desire to kill us all.

Epilogue – What’s The Purpose of Anything Anyway?

Here she addresses questions of meaning with both insight and humility. She begins by admitting that science doesn’t have, and almost certainly never will have, all the answers to life’s biggest questions. But, in addition to its practical implications, Hossenfelder practices science to make sense of her life. And this leads to the book’s final question “What’s the meaning of life in the universe revealed by modern science?” She believes that each person must answer this question for themselves but she tells a simple story to explain how she thinks about the issue.

When she was young she asked her mother “What’s the meaning of life?” Her mother was a teacher and she replied that for her “the meaning of life is to pass along knowledge to the next generation.” At the time Hossenfelder thought her mother’s answer was “rather lame. [But] Thirty years later I have come to pretty much the same conclusion.” For most of her life, she has studied the laws of nature and still takes great joy in sharing that knowledge with others. She has found that many people want to know how the universe works because we want to make sense of ourselves and our place in the universe.

Ultimately, Hossenfelder writes that she is trying to do her part “to aid the universe’s understanding of itself.” As she concludes, “So, yes, we are bags of atoms crawling around on a pale blue dot in the outer spiral arm of a remarkably unremarkable galaxy. And yet we are so much more than this.”

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One thought on “Existential Physics: A Scientists Guide To Life’s Biggest Questions

  1. Sounds like a good read. Am reminded of something else recently read of:
    Planetary emergentism. Thanks, Dr.

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