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February 21, 2023 - August 11, 2025
in the foundations of physics—where I do most of my research—the primary product is knowledge. And all too often, my colleagues and I present this knowledge in ways so abstract that no one understands why we looked for it in the first place.
Not all ideas about life and death and the origin of human existence are compatible with the foundations of physics.
If physicists don’t step forward and explain what physics says about the human condition, others will jump at the opportunity and abuse our cryptic terminology for the promotion of pseudoscience.
Science and religion have the same roots, and still today they tackle some of the same questions: Where do we come from? Where do we go to? How much can we know?
knowledge we gained in the past hundred years is now replacing old, belief-based explanations. One of these old explanations is the idea that consciousness requires something more than the interaction of many particles, some kind of magic fairy dust, basically, that endows certain objects with special properties.
If your belief conflicts with empirically confirmed knowledge, then you are not seeking meaning; you are delusional.
It is from observations on such simple systems that we can reliably infer the physical nature of time without getting bogged down by the often inaccurate interpretation that our senses add to the physics.
in special relativity the statement that two events happened at the same time is meaningless.
It sounds crazy, but the idea that the past and future exist in the same way as the present is compatible with all we currently know.
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.
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.
Black holes are regions where space-time bends so strongly that light is forced to go around in circles and can’t escape.
physics isn’t math.
the belief that reality is math is deeply ingrained into the thinking of many physicists who treat mathematics as a timeless realm of truth that we reside in.
Physicists may not consciously subscribe to the idea that math is real and when asked will deny it, but in practice they do not distinguish the two. This conflation has consequences, for they sometimes erroneously come to think their math reveals more about reality than it possibly can.
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.
creation stories can’t be falsified, we can’t tell if they are false, but being false is not their problem. The problem with these stories is that they are bad scientific explanations.
The simpler an explanation, the more useful it is. For a scientific theory, this explanatory power can be quantified in a variety of ways that come down to calculating how much input a theory needs to fit a set of data to a certain level of accuracy.
quantification can serve to remove doubts that conclusions were biased by human perception.
the God hypothesis has no quantifiable explanatory power. You can’t calculate anything from it. That doesn’t make it wrong, but it does make it unscientific.
The concordance model is an application of Einstein’s theory of general relativity, according to which gravity is caused by the curvature of space-time.
As the universe expands, the wavelength of the light stretches and so its vibrational frequency decreases. Because the frequency is proportional to the energy of the light, and the average energy determines the temperature, the temperature of the light drops with the expansion.
The concordance model, in contrast, does not need much information—neither in the dynamical law, nor in the initial condition—to explain several different observations. It makes things fit together. It has, in the words of the previous section, high explanatory power.
The numerical values that currently fit best to the collected data tell us that only about 5 percent of the universe is made of the same stuff as we are, 26 percent is thinly distributed dark matter, which we can’t see, and the remaining 69 percent is attributed to the dark energy of the cosmological constant.
Even inside stars, temperatures and densities do not exceed the ones we have produced on Earth. The only naturally occurring event we know of that can reach higher densities is a star that collapses to a black hole. Alas, in this case, we can’t observe what’s going on, because the collapse is hidden behind the black hole horizon.
Emergence from quantum fluctuations means that this creation can happen even in vacuum.
Physicists currently count four fundamental forces: gravity, the electromagnetic force, and the strong and weak nuclear forces.
Even if we imagine forms of life very different from ours
they will all ultimately fall victim to the same problem: life requires change, and change requires free energy, and there’s a limited supply of it.
It’s impossible to predict that nothing unpredictable will happen.
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.
Tim Palmer, one of the recipients of the 2007 Nobel Peace Prize for his work in the Intergovernmental Panel on Climate Change.
Tim and I, it turned out, had independently arrived at similar conclusions about the lack of progress in the foundations of physics. We both pointed the finger at physicists’ overreliance on reductionism, the idea that we gain deeper insights into nature by looking at shorter and shorter distances.
That’s Tim’s passion in a nutshell—space-time geometry and chaos theory combined.
“Well, I’m not religious, but I get slightly resistant to people who are adamant they can prove that God doesn’t exist.”
“So I thought if someone was brought up with this belief that creation happened some thousand years ago, here is an easy way out. Six thousand years ago, God created the universe, and before it was all just mathematical equations. And 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.
Free energy is the counterweight of entropy. As entropy increases, free energy decreases, and change becomes impossible.
The past-hypothesis says that the universe started out in a state of low entropy—a state that was very unlikely—and that entropy has gone up ever since. It will continue to increase until the universe has reached the most likely state, in which nothing more will change, on average.
This is why the universe has a direction forward in time, the arrow of time—it’s the direction of entropy increase; it points one way and not the other.
in one direction the evolution law brings us from an unlikely to a likely state, and that transition is likely to happen. In the other direction, the law goes from a likely to an unlikely state—and that (almost) never happens.
You can find many different diagnoses in death certificates, but they’re just details. What really kills us is entropy increase.
The initial state must have been something, but we can’t explain the initial state itself; we can only examine whether a specific initial state has explanatory power and gives rise to predictions that agree with observations. The past-hypothesis is a good hypothesis in the sense that it explains what we see. However, to explain the initial state by something else than a yet earlier initial state, we would need a different type of theory.
The initial state of a very low entropy universe
entropy increase is also often described as the destruction of order.
counting microstates and comparing their numbers becomes tricky if a theory has infinitely many microstates, and that’s the case for all continuous-field theories.
according to our current theories, matter in the universe starts out as an almost evenly distributed plasma. That plasma must have had low entropy according to the past-hypothesis. But I told you earlier that the smooth batter had high entropy.
It’s because of the different role of gravity that the batter and the early universe are two very different cases, and why the one has high entropy, the other one low entropy.
to make this case quantitative, we’d have to understand how to assign entropy to gravity.
we still don’t really know how to do it, because we don’t know how to quantize gravity.
“we still don’t really know how to do it, because we don’t know how to quantize gravity.”
The universe didn’t start out with chemical elements in place, except for hydrogen, which was created a few minutes after the Big Bang, because making the atomic nuclei for the chemical elements requires substantial pressure. Heavy elements could be generated only once stars began to form from hydrogen clouds under the pull of gravity. In these collapsing clouds, gravitational pressure eventually ignites nuclear fusion, which merges the cores of light nuclei to increasingly heavier ones.
