物理，但是不仅仅是物理。
Sabine Hossenfelder的这本Existential Physics从物理出发，拓展到我们思考世界和人生的方式。行文流畅有趣，涉及到日常关心的许多方面，也有寓教于乐在其中，是不可多得的一本好书。
It is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring.
—Carl Sagan
Preface
this is specific to physics. The disconnect between experts and non-experts is so widespread that
People don’t care much whether quantum mechanics is predictable; they want to know whether their own behavior is predictable.
Science has limits, and yet humanity has always sought meaning beyond those limits.
you do not need to silence rational thought to make space for hope, belief, and faith.
1 Does the past still exist?
Now and Never
当过去、现在和将来割裂，而只是成为一种感知上的“错觉”，那么人要如何理解时间？
Since Einstein, now is merely a convenient word that we use to describe our experience. The present moment is no longer of fundamental significance because, according to Einstein, the past and the future are as real as the present.
human experience is not a good guide to the fundamental laws of nature.
the physics of Einstein’s special relativity does not allow us to constrain existence to merely a moment that we call “now”
British comedian John Lloyd, “Time is a bit like a landscape. Just because you’re not in New York doesn’t mean it’s not there.”
Eternal Information
时间能否逆转？从信息的角度，因为现在的我们没有办法推算出关于过去的完整信息，所以难以穿越回到过去，信息总是越来越混乱的。
Time-reversibility merely means that, given the entire information at one moment, we can calculate what happened at any moment before that and what will happen at any moment after that.
two exceptions to time-reversibility: the measurement in quantum mechanics, and the evaporation of black holes.
A measurement is any interaction that is sufficiently strong or frequent to destroy the quantum behavior of a system. Only what it takes to destroy quantum behavior can be (and, for many examples, has been) calculated.
Quantum behavior just gets destroyed too easily.
just because something is out of sight doesn’t mean it has stopped existing.
Nevertheless, if you trust our mathematics, the information is still there, somewhere, somehow, spread out over the universe, but preserved forever. It might sound crazy, but it’s compatible with all we currently know.
Transcendent Math
这么一说，数学的确是过于“不可理喻”，因为许多匪夷所思的结论的源头是严谨的一丝不苟的数学。而物理化学生物，则是实验学科了。
mathematics is “unreasonably effective
“非科学”和“不科学”是两个概念。比如神学和宗教学的许多部分，我们不能直接用科学的法则全盘否定，因为他们并不落入科学的范围内。不要当长臂警察。
however you feel about it, it’s not wrong; it’s just not scientific
According to the currently established laws of nature, the future, the present, and the past all exist in the same way. That’s because, regardless of exactly what you mean by exist, there is nothing in these laws that distinguishes one moment of time from any other. The past, therefore, exists in just the same way as the present. While the situation is not entirely settled, it seems that the laws of nature preserve information entirely, so all the details that make up you and the story of your grandmother’s life are immortal.
2 How did the universe begin? How will it end?
What Does It Mean to Explain Something?
自然科学的法则仅仅是做出预测。
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.
Modern Tales of Creation
如上所述，如果我们真的要回溯过去，而所有的过去现在和未来都在一个无止尽的循环里呢。
—we can go back in time and deduce that the matter must have been squeezed together
a hot and almost entirely smooth soup of elementary particles, called a plasma.
The most popular cyclic models are conformal cyclic cosmology, proposed by Roger Penrose, and the ekpyrotic universe, originally proposed by Justin Khoury and collaborators.
Penrose glues the late phase of the universe to the early phase of the next universe, whereas Khoury and friends imagine that the universe was created in an extradimensional collision of high-dimensional surfaces, which can happen repeatedly. A Big Bounce without a cycle also happens in some approaches that aim to unify gravity with quantum mechanics, like loop quantum cosmology.
All our current theories rely on simple initial conditions. This isn’t optional; it’s essential for our mode of explanation to work. If you have to make the initial conditions complicated, even the simplest evolution law will not give your theory explanatory power.
In the End
metastable的亚稳定状态可以长时间存在吗？
我可以一直饱和但是不析出，一直饱和但是不凝结吗。
we cannot tell a truly stable vacuum from one that is merely very long-lived, or metastable
Other Voices #1: Is Math All There Is? (An Interview with Tim Palmer)
Chemistry is underpinned by physics, and that is underpinned by mathematics. So there is nothing in science that would say anything about this moment of creation.
this is not unscientific. It does not go against anything in our current scientific lexicon. I like to use the word ascientific. Science has nothing to say about it—at least, science in its current state. There are things we are really profoundly ignorant about. And this is one of them
mathematics to describe our observations, but we don’t know why some math describes reality whereas other math doesn’t
3 Why Doesn’t Anyone Ever Get Younger?
The Last Question
这里讲了我熟悉的exergy和entropy概念
Eventually, everything will be broken and unfixable:
as mixed up as it can be, with no more change, on average.
Equilibrium states have reached maximum entropy;
And Julian Barbour posits that the universe started from a “Janus point” at which the direction of time changes, so actually there are two universes starting from the same moment in time. He argues that entropy isn’t the right quantity to consider and that we’d be better off thinking about complexity instead.
The Problem of the Now
记忆是区分过去和未来的“器官”。
但是我们也有可能被记忆欺骗。
The property that allows us to experience the present moment as unlike any other moment is memory—we have an imperfect memory of events in the past and we do not have a memory of events in the future.
Memories in mica don’t fade like ours do. But, like us, mica has a memory of the past and not of the future. That means that at any particular moment, mica has information about what has happened but no information regarding what’s about to happen. It would be a stretch to say that mica has experience of any kind, but it keeps track of time—it knows about the Now.
时间是单向流动的，这和过去和未来的真实性以及独立性也不矛盾。
Such things just don’t happen. And the reason they don’t happen is that entropy increases in only one direction of time.
In the long run, of course, further entropy increase will wash out any memory.
Many people feel uneasy when they first realize that Einstein’s theories imply that the past and future are as real as the present, and that the present moment is only subjectively special.
Brains. In Empty Space.
无穷大和波动
because infinity is a really long time. In an infinite amount of time, anything that can happen will eventually happen—no matter how unlikely.
It’s that not all types of evolution laws give rise to all possible fluctuations
A theory in which any kind of fluctuation will eventually happen is called an ergodic theory
所有的理论都有局限性，当我们越了解一个理论的时候，其局限性越加彰显。
Boltzmann brains are a theoretical device to lead an argument by contradiction
The foundations of physics give us a closer look at reality, but the closer we look at reality, the more slippery it becomes.
The more the fundamental descriptions of nature have become divorced from our everyday experience, the more we must rely on mathematical rigor.
而真相仅仅是真相，而不是任何理论。
we can’t assign “reality” to any particular formulation of a theory.
“两面性里的两面性”
physics we have dual theories. Two theories that are dual describe the same observable phenomena in entirely different mathematical form e.g. gauge-gravity duality.
Structural realism has it that what’s real is the mathematical structure of a theory, not any particular formulation of it.
We get older because that’s the most likely thing to happen.
4 Are You Just A Bag of Atoms?
意识论
More Is More
承接上文
this idea of dualism—that the human mind is more than a complicated biological machine.
区分emergent和fundamental
We say that such properties and objects, which play a key role in the effective theory but do not appear in the fundamental theory, are emergent. Emergent properties and objects can be derived from or reduced to something else. Fundamental is the opposite of emergent. A fundamental property or object cannot be derived from or reduced to anything else. Two other terms I will use in the following is that the more fundamental layers are the deeper ones, whereas the emergent ones are higher levels.
according to the best current evidence, the world is reductionist: the behavior of large composite objects derives from the behavior of their constituents, but we have no idea why the laws of nature are that way. Why is it that the details from short distances do not matter over long distances?
One Bit at a Time
如何定义“一样”。或许诉诸热力学第零定律？
with Theseus’s ship, it depends on how you define you and the same
5 Do Copies of Us Exist?
Many Worlds
The key feature of the many-worlds interpretation is that each time a quantum measurement happens, the universe splits, creating what’s commonly called a multiverse.
Do We Live in a Computer Simulation?
想起来柯南剧场版《贝克街的亡灵》，二十年过去AI仍然没有进化到那个程度，那个孤独而绝望的少年从高楼一跃而下。
To begin with, quantum mechanics features phenomena that are not computable with a conventional computer in finite time.
The idea that copies of us exist in the multiverse is not scientific, because such copies are both unobservable and unnecessary to explain what we can observe
6 Has Physics Ruled Our Free Will?
A Quagmire of Evasion
决定论的观点是，人类在命运面前束手无策
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.
和哲学的连接
free will itself is an inconsistent idea
For your will to be free, it shouldn’t be caused by anything else. But if it wasn’t caused by anything—if it’s an “uncaused cause,” as Friedrich Nietzsche put it—then it wasn’t caused by you, regardless of just what you mean by you.
we don’t know the result of our thinking before we are done; otherwise, we wouldn’t have to do the thinking. As Ludwig Wittgenstein put it, “The freedom of the will consists in the fact that future actions cannot be known now
Because one can certainly go and define something and then call it free will.
Life without Free Will
自由是如此含糊，以至于任何人都可以对其下定义。
在这里，自由意志与决定论对立。
one shouldn’t count on personal impressions. It takes more than an impression to infer how nature really works.
I think the best way to deal with the impossibility of changing the future is to shift the way we think about our role in the history of the universe. Free will or not, we are here, and therefore we matter. But whether ours will be a happy story or a sad story, whether our civilization will flourish or wither, whether we will be remembered or forgotten—we don’t yet know. Instead of thinking of ourselves as selecting possible futures, I suggest we remain curious about what’s to come and strive to learn more about ourselves and the universe we inhabit.
We frequently associate free will with moral responsibility in this way, which is how it enters our discussions about politics, religion, crime, and punishment. Many of us also use free will as a reasoning device to evaluate personal questions of guilt, remorse, and blame. In fact, 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.
I find this worry silly. If free will doesn’t exist, it has never existed, so if moral responsibility has worked so far, why should it suddenly stop working just because we now understand physics better? It’s not as though thunderstorms changed once we understood they’re not Zeus throwing lightning bolts.
“the more people doubt free will, the more lenient they become toward those accused of crimes and the more willing they are to break the rules themselves and harm others to get what they want.”
it has become common to use passages from Francis Crick’s 1994 book, The Astonishing Hypothesis: The Scientific Search for the Soul. Here is an excerpt:
You, your joys and your sorrows, your memories and your ambitions, your sense of personal identity and free will, are in fact no more than the behavior of a vast assembly of nerve cells and their associated molecules. Who you are is nothing but a pack of neurons.
如果我们的人生都已经被决定好，那么我们的努力我们的挣扎，我们的爱恨别离又有什么意义呢。
7 Was The Universe Made For Us?
Imagine No Religions
为什么这个世界呈现出如此协调的样子？或许走投无路的物理学家——比如牛顿——只能诉诸上帝来获得答案。
the universe as we observe it requires an explanation, a god who fine-tuned the knobs. If not a god, then we need another explanation.
Do We Live in the Best of All Possible Worlds?
其实和2nd law 差不多
principle of least action
这倒是有点人类中心了。
anthropic principle, which says the constants are what they are because the universe gave rise to life.
8 Does The Universe Think?
Size Matters
宇宙无法思考，因为宇宙太大了。有意思。
In a nutshell, the universe can’t think, because it’s too big
Are Electrons Conscious?
Panpsychism （泛心论） has been touted as a solution to the problem of dualism, which treats mind and matter as two entirely separate things.
integrated information theory, IIT for short … In IIT, each system is assigned a number, Φ (Greek capital phi), which is the integrated information and supposedly a measure of consciousness. The better a system is at distributing information while it’s processing the information, the larger the phi. A system that’s fragmented and has many parts that calculate in isolation may process lots of information, but this information is not integrated, so phi is small.
9 Are Humans Predictable?
The Limits of Math
NS当然不fundamental。虽然他是改变世界的公式，但是现在我有足够的知识和信心 和我的笔记 来批判这个我再熟悉不过的流体力学公式。
But the Navier-Stokes equation is not fundamental; it emerges from the behavior of the particles that make up the gas or fluid. And we already know that fundamentally—on the deepest level—the gases are described by quantum theories again, so their behavior is predictable, at least in principle.
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.
Epilogue: What’s the Purpose of Anything Anyway?
Yet you may ask, “What’s the point?” If the universe is just machinery, a set of differential equations acting on initial conditions, and we are but blips of complexity in an uncaring universe, temporarily self-aware conglomerates of particles that will soon be washed away by entropy increase, then why spend time figuring out just exactly how insignificant our existence is? What’s the meaning of life if there’s no purpose to it?
拖了两天终于把这篇整理出来了。