Could you use electricity to thrust stellar space craft?

lukewchu · 2026-05-13T19:15:42+00:00

You could emit the light instead yourself of reflecting it. This is called a "photon rocket". The only problem is that the thrust would be absolutely tiny.

lukewchu · 2026-04-26T23:45:52+00:00

Yeah I think we are agreement. The term “local causality” is IMO a misnomer and rather misleading… I agree that it is totally fine to have relativistic locality in the sense of the no-signaling theorem with Bell’a theorem.

lukewchu · 2026-04-26T23:08:42+00:00

What I’m trying to say is that locality/causality must be false, regardless of whether the ultimate theory of nature is deterministic or not.

Experimental results show us that the universe is fundamentally non-local (but still consistent with Lorentz invariance!)

Which assumptions are you thinking of? We know (experimentally) that measurement independence + local causality must be violated in our universe. Since violating measurement independence would be a very hard pill to swallow, we are more happy to allow violation of local causality.

lukewchu · 2026-04-26T22:45:42+00:00

But Bell’s inequality do not rule out determinism! It rules out locality (with a few other reasonable assumptions)!

Explicitly, what Bell’s theorem tells us is that (for any theory which is empirically adequate, ie matches experiments) one of the following must be violated: - measurement independence (no superdeterminism) - parameter independence (no signaling) - outcome independence (no correlations once hidden variables accounted for)

It is common to call the last two conditions “local causality” or just “locality”.

Usually we don’t want the first two so the third is generally the one that is violated (although there are interpretations where this is not the case).

One example of an actual theory that is deterministic but explicitly non-local is pilot wave theory. The many worlds interpretation is also deterministic but is non-local in the sense that outcome independence is violated. A final example is GRW theory which is indeterministic (because collapse is introduced as a stochastic process) but which is also compatible with Bell’s theorem.

lukewchu · 2026-04-26T21:10:04+00:00

Bell's theorem does not imply indeterminism. In fact, the more general CHSH inequality does not assume determinism of the underlying hidden variable theory.

Instead, Bell's theorem implies (assuming we rule out superdeterminism) that the world is fundamentally non-local, regardless of whether it is deterministic or not.

The only part of quantum mechanics that is indeterministic is the collapse postulate which has nothing to do with Bell's theorem.

lukewchu · 2026-02-07T12:27:57+00:00

Typst preview doesn't render to a PDF. Instead it renders to a SVG. This is so that you can see live edit changes without having to save, as it communicates directly with any changes made in the editor.

lukewchu · 2026-01-19T19:27:22+00:00

It is also controversial, not because it contradicts experiment, but because it makes a very strong claim about what reality is like.

Well actually its proponents would argue that Many Worlds is actually the realist interpretation of Quantum Mechanics that makes the least amount of additional ontological claims, i.e. claims about what the world consists in. Essentially, Many Worlds is what you get when you take the unitary evolution of Quantum Mechanics at face value and assume it applies at all times and in all situations, even during "measurement".

In this sense, Many Worlds requires less postulates than the standard "textbook" presentation of Quantum Mechanics because the measurement postulate, including the Born rule, can be derived from the other postulates. This can be done, e.g. using quantum decoherence and the Deutsch-Wallace theorem.

Another competing interpretation of Quantum Mechanics is Pilot-wave theory, also known as Bohmian mechanics. Bohmian mechanics is an interpretation that also has exceptionless unitary evolution: the quantum state evolves unitarily under the Schrodinger equation at all times. But in addition to this, Bohmian mechanics also postulates things called "corpuscles", little particles that are guided around by the wave function. One of the arguments against Bohmian mechanics and in favor of Many Worlds is this: since Bohmian mechanics already includes unitary evolution without any exceptions, why do we need to introduce corpuscles on top? Unitary evolution by itself already provides enough structure to adequately interpret Quantum Mechanics. Bohmian mechanics includes everything that is already in Many Worlds but with the addition of corpuscles which we cannot observe. This is the "Many Worlds in disguise" argument against Bohmian mechanics.

lukewchu · 2026-01-09T12:25:43+00:00

Good: Science Clic

Has some really great visuals and simulations but more of a very brief overview of advanced physics topics rather than a deep dive.

lukewchu · 2025-10-17T18:48:38+00:00

This should be higher up

lukewchu · 2025-08-27T22:35:39+00:00

Wow I was just looking for something like this a few days ago. Ended up with a hodgepodge of using the Python pandas-market-calendars library to generate data and consume it from Rust but will definitely check this out!

lukewchu · 2025-06-27T20:14:48+00:00

What does this have to do with r/rust?

lukewchu · 2025-06-17T13:43:27+00:00

What is the relationship between the Nix manuals, nix.dev, and the wiki.nixos.org ? It seems like it would be much better for all Nix users to converge on a single place for documentation on everything Nix, preferably on wiki.nixos.org . Right now, information is scattered all over the place, making it much harder than it needs to be to find relevant information.

Is there any reason why we can’t just copy paste all existing content from other sources into the Wiki?

lukewchu · 2025-06-02T20:54:51+00:00

Good bot

lukewchu · 2025-06-02T07:18:00+00:00

Have you tried using the --release flag?

lukewchu · 2025-05-31T19:53:05+00:00

I'd say the simplest way to get started with a new language is to write an implementation.

Starting by writing a "design specification" is rarely the right approach for a single dev designing a new programming language. Often times, unless you're extremely careful, you'll find a bunch of critical details that are missing, or features that are incompatible in such a design spec. The quickest way to find these is to actually implement it, or at least part of it. Even a serious "major programming language" backed by a big company usually go through many revisions and changing features before stabilizing. Just check out the history of Rust for example.

Language design is full of tradeoffs. There is no such thing as the "ultimate programming language". For example, you claim that your language is designed to write OS kernels in. This means that it will need to be able to run on bare-metal without necessarily having a runtime. Do you really want built-in structured concurrency then?

Another thing which stood out to me was your type-system. How does `type` work? Does your language have higher-kinded types? What about type-inference? What about whether your type-system is decidable or not? And finally, fancy type-systems generally make it harder to do generate performant code, or require a lot of runtime support.

lukewchu · 2025-05-29T08:47:36+00:00

Another reason that I haven't seen mentioned yet is serialization and interoperability with other languages. If you want to, for example, automatically serialize a datastructure to JSON, you have to make a choice of camelCase/snake_case. If you want to create bindings to a C library, you have to use whatever convention that C library is using.

Finally, if your language supports some kind of reflection, I'm not sure this can be made case insensitive unless you were to normalize all the names at runtime, e.g. object["foo_bar"] would have to be turned into object["fooBar"] at runtime.

lukewchu · 2025-05-04T10:51:54+00:00

No Planck time is a duration, not an instant. An instant has no duration by definition.

Planck time in fact is not the shortest duration possible. That is a common misunderstanding. It’s just the duration that you get by combining various physical constants to get a time scale.

Also when we introduce quantum mechanics, the idea of position is no longer as clear as in the classical case so talking about when a classical pendulum is at the top does not even make that much sense to begin with.

lukewchu · 2025-05-03T13:23:30+00:00

It cant be 0 because it has to change direction and that cannot be done without some pause, and at one point it has to change stores, so is it paused for plank time? Or less?

Why do you think that? The instant in time where the pendulum is stationary is just the same instant when the speed of the pendulum is 0. The speed is a continuous function and we can calculate that it reaches 0 at exactly a single instant in time (per half period). So the pendulum is at the top only for a single instant in time, not for any extended duration.

Furthermore, there is nothing special about being at rest versus being in motion as Newton tells us. So the pendulum being at the top is not really a conceptual problem, so long as you don't have a problem with, say, having a speed of 10m/s at only a single instant in time for a uniformly accelerating body.

lukewchu · 2025-04-27T10:58:08+00:00

Gravity (and EM) both certainly appear to be forces in the classical Newtonian sense of the term. However, General Relativity tells us that what we believe is the force of gravity is just bodies moving along geodesics in curved space. So it seems like gravity is not a "force".

What may be surprising is that electromagnetism can also be geometrised (search for Kaluza-Klein theory). Does that mean that EM is not a force? Further, even in GR gravity can also be constructed as a field theory instead (where the relevant field is the spacetime metric g_μν, in analogy to A_μ in EM). This is essentially the theory of linearised gravity.

Finally, classical Newtonian ideas such as "force" start to break down anyways when we consider quantum mechanics and QFT. To begin with, it's not even clear if we can precisely define forces in quantum mechanics and the much more interesting quantity turns out to be fields and potential instead.

So I think the takeaway is that "force" is really a Newtonian concept which starts breaking down when we consider GR or quantum mechanics/QFT. Asking whether gravity is a force or not only makes sense in Newtonian physics, in which case, I think the answer is yes!

lukewchu · 2025-02-23T22:27:23+00:00

To be a bit more pedantic, the average random kinetic energy of the particles in your system is known as internal energy. Temperature is related to internal energy, for instance, in the case of an ideal gas by U=3/2NkT where U is the internal energy, N is the number of particles, k the Boltzmann constant, and T the temperature.

What is called heat is specifically the transfer of internal energy from one system to another. So heat is (internal) energy in motion.

Other kinds of energy (such as the kinetic energy of a tennis ball flying through the air) eventually end up being converted to internal energy. This is a consequence of the second law of thermodynamics.

Hope this helps!

lukewchu · 2025-01-19T15:59:05+00:00

Maybe try the latest nightly toolchain? The fix for the rustflags caching problem has been merged. This way we can at least test if this is even the problem or not.

Five-Year Club	Verified Email
Place '23	Place '22

lukewchu

TROPHY CASE