Existential Physics: A Scientist's Guide to Life's Biggest Questions

by Sabine Hossenfelder

The word relativity in special relativity means there is no absolute rest; you can merely be at rest relative to something.

Once you agree that anything exists now elsewhere, even though you see it only later, you are forced to accept that everything in the universe exists now.

the block universe

A measurement is any interaction that is sufficiently strong or frequent to destroy the quantum behavior of a system.

We understand fairly well what constitutes a measurement, but the fact that we need to update the wave function upon measurement makes quantum mechanics both indeterministic and time-irreversible. It is indeterministic because we cannot predict what we will actually measure; we can predict only the probability of measuring something. And it is not time-reversible, because once we have measured the particle, we cannot infer what the wave function was prior to measurement.

This means the measurement in quantum mechanics destroys information for good.

The destruction of information in black hole evaporation happens even before one measures the radiation. That’s a big problem because it means black hole evaporation is incompatible even with the evolution law of quantum theory. It is for this reason that most physicists currently think something is wrong about Hawking’s conclusion that black holes destroy information.

The arguments in the previous sections of this chapter do not depend on whether you believe in the reality of math. However, they implicitly assume that mathematics itself is timeless, that mathematical truth is eternal, and that logic doesn’t change. This is an assumption that cannot be proved, because what would you prove it true with? It’s one of the usually unstated articles of faith that our scientific inquiry is based on.

Science is about finding useful descriptions of the world; by useful I mean they allow us to make predictions for new experiments, or they quantitatively explain already existing observations.

the current theory for the universe, the concordance model

For 97 percent of all Wikipedia articles, if you click on the first link and repeat this in each subsequent article, you will eventually get to an entry about philosophy. Philosophy is where our knowledge ends, and the scientific method is no exception.

Charles W. Misner, Kip S. Thorne, and John Wheeler’s Gravitation greeting

Most physicists today ignore the entire debate and deal with quantum mechanics as a tool that makes predictions and that one shouldn’t overthink. This “shut up and calculate” attitude is the pragmatic way to go about it. It has led to great progress, so it shouldn’t be laughed off. However, many researchers who work in the foundations of physics feel that ignoring the problems of quantum mechanics is a mistake because we’d learn more from resolving them.

If you don’t want to actually change quantum mechanics, you can try to interpret the mathematics differently and hope that then it makes more sense. There are a number of such interpretations. For example, there’s the interpretation proposed by Niels Bohr, according to which the wave function just shouldn’t be considered real. It’s a device to make predictions for measurements, Bohr said, but if you are not making a measurement, it is meaningless to ask what is really happening. This is now often called the Copenhagen interpretation or just the standard interpretation, because it’s the most commonly taught one.

One can interpret the path integral as telling us that the particle takes each path in a different universe. Personally, I find this a rather meaningless statement—there isn’t anything in the math that says these paths are in different universes—but it isn’t wrong either. And I am all for different ways of looking at math because they can lead to new insights. So, OK.

The major difference isn’t in the math; the major difference is one of belief. Advocates of the many-worlds interpretation believe that all the other universes—the ones we don’t observe—are as real as ours.

For example, because deep down all our brain processes are quantum processes,

But that doesn't mean our decisions take place at the quantum level?

The problematic part of Bostrom’s argument is that he assumes it is possible to reproduce all our observations not using the natural laws that physicists have confirmed to extremely high precision but using a different, underlying algorithm, which the programmer is running.

Yes.

You can approximate the laws we know with a computer simulation—we do this all the time—but if that were how nature actually worked, we could see the difference.

the contemporary philosopher Wallace Matson called out as “the most flabbergasting instance of the fallacy of changing the subject.”

the future is still fixed except for occasional quantum events that we cannot influence.

much of the debate about free will in the philosophical literature concerns not whether it exists in the first place but how it connects to moral responsibility. The worry is that if free will goes out the window, society will fall apart because blaming the laws of nature is pointless.

It is easy enough to explain why we—as individuals and as societies—assign responsibility to people rather than to the laws of nature. We look for the best strategy to optimize our well-being.

We evaluate which actions are most likely to improve our lives in the future. And when it comes to that question, who cares whether philosophers have yet found a good way to define responsibility? If you are a problem, other people will take steps to solve that problem—they will “make you responsible” just because you embody a threat.

But it doesn’t prime people just to question free will; it primes them for fatalism—the idea that it doesn’t matter what you do.

Saying that the only value we have ever observed is “unlikely” is a scientifically meaningless statement.

Simply put, claiming that the constants of nature are fine-tuned for life is not a scientifically sound argument, because it depends on arbitrary assumptions.

Not sure about her logic here.

The best idea I have heard so far is that of David Deutsch, who has conjectured that the laws of nature are so that they will give rise to certain types of computers.

Sounds like a telelogical argument?

These ideas have in common the feature that to find better descriptions of nature, they don’t follow the trodden path of reductionism: toward shorter distance. Instead, they decouple ontological reductionism from theory reductionism, positing that a better theory might be found on large scales.

Frank Jackson

Idiot!

The advice of philosophers of science is certainly still needed in consciousness research to sort out what properties a satisfactory definition of consciousness must fulfill, what questions it can answer, and what counts as an answer. But the study of consciousness has left the realm of philosophy. It is now science.

So, really, when we are asking whether human behavior is predictable, we should be asking more precisely whether the probabilities of decisions are predictable. Insofar as the current laws of nature are concerned, they are—and to the extent they aren’t predictable, they aren’t under your control.

In summary: 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.

Some AI researchers have even argued that a body of some kind is necessary to reach human-level intelligence, which—if correct—would vastly increase the problem of AI fragility.

I suspect so also.

I want scientists to be mindful of the limits of their discipline.

Are these limits intrinsic? As Kant argues about the limits of pure reason.

Eventually, I think, we will have to accept some facts about our universe without scientific explanation, if only because the scientific method can’t justify itself. We may observe that the scientific method works, conclude that it’s to our advantage to continue using it, but still never know why it works.

Certainly our difficulty in keeping Earth’s climate in a comfortably habitable range raises severe doubts as to our cognitive ability to handle complex and partly chaotic systems.

I don't think the problem is understanding the problem per se, rather the ability to subsume short term self-interest to that of the common weal and to cooperate on a global scale to realise that end.

To me, my personal story is evidence that not only I but many of us have the desire to understand the universe—for no other reason than understanding the universe.

Yes, but I think the reason for our intrinsic motivation is that it does provide some long term survival advantage.

the universe is evolving toward a state in which it understands itself,

I suspect Godel's theirem comes in here and makes this impossible.