"General relativity doesn't admit black hole solutions. It only admits *wormhole* solutions."

I have previously made this statement and had people get confused by it. So let me try to clarify precisely what this means, using a neat analogy to real/complex analysis. (1/17)
When you first encounter the sine function, you probably see it defined in terms of triangles, which means that it really only makes sense for x between -pi and pi (or -180 and 180 degrees). But later on, you learn that the "true" sine function is much larger. (2/17)

Energy, momentum, pressure, stress, etc. are all just different ways of quantifying the same basic thing: how our perceptions of space and time get distorted over time.

And once you internalize this, it allows you to think about these concepts in a much more general way. (1/12) Suppose that the room you're in defines a 3-dimensional coordinate system (x, y, z). If the walls in the x-direction get pulled apart in the x-direction, and those in the y-direction get pulled apart in the y-direction at the same rate, etc., then the room is expanding. (2/12)

"Oh, I'm a *pure* mathematician, I don't write code/do calculations/etc.."
"Oh, I'm a *theoretical* physicist, I don't do experiments/analyze data/etc.."
Etc.

These kinds of statements are typically uttered with an air of intellectual smugness. But what are they really? (1/6) In actuality, this classification of certain tasks as "worthy" (e.g. proving theorems, developing models) and others as "beneath oneself" (e.g. doing calculations, writing code) is a signal of a fundamental absence of intellectual curiosity. (2/6)

To illustrate just how different neutron star and black hole metrics truly are, I simulated perfect fluid accretion onto a black hole vs. onto a neutron star of identical mass and spin.

Source code and simulation details in thread below. (1/4)

https://twitter.com/getjonwithit/status/1878094729809723568

To produce this, I simulated supersonic accretion of a perfect fluid, obeying an ideal gas equation of state, initially onto a standard black hole metric in Cartesian Kerr-Schild coordinates, and then onto a physically realistic rotating neutron star metric due to Pappas. (2/4)

There's something which almost every major neutron star simulation gets wrong, and it's related to a widespread confusion about the behavior of rotating objects in general relativity.

The confusion goes back to one of the most beautiful results in GR: Birkhoff's theorem. (1/14)
Here's one way to think about Birkhoff's theorem: suppose that you have a perfectly spherically-symmetric object that is uncharged and not rotating. Clearly, by measuring its gravitational field (i.e. the extent to which it curves spacetime), you can determine its mass. (2/14)

I'm often described (due to my work on computable physics) as being an advocate for the Simulation Hypothesis.
But I'm not.
In fact, I think the Hypothesis is nonsensical, and that the most famous argument in its favor actually proves the opposite of what it purports. (1/10) First, it is crucial to distinguish two things: a computation (which we can formalize as the operation of a Turing machine), and a simulation (which is a specific type of computation, whose output states are interpreted as approximate states of some *external* system). (2/10)

The apparent "philosophical problems" of quantum mechanics are not unique to QM at all: they are in fact the same problems that arise whenever one attempts to construct an abstract model of reality. We can see these problems already in high school-level mechanics. (1/14)
When you throw an object into the air and try to calculate when it will hit the ground, you can do this (neglecting air resistance) using a SUVAT equation: by solving for t in s = ut + 1/2 at^2. But this equation is a quadratic, and so generically has 2 solutions. (2/14)

Lord Kelvin famously believed that atoms were "knots in the aether".

And he was onto something: perhaps not atoms per se, but knots can indeed tell us something quite deep about the nature of the universe, and the relationship between classical and quantum field theories. (1/16)
A "knot" is an embedding of a circle (S^1) in 3D space (R^3). A "link" is a collection of knots (potentially linked together, but not intersecting). And whenever you smoothly (topologically) deform a knot or link, some properties will always remain the same. (2/16)

Much more work is needed before this is really a robust result (paper forthcoming, hopefully by the end of the year), but the initial findings are clear:

Spacetime discreteness may be observationally detectable in things like quasar luminosities. (1/6)
In previous work (), I showed that black holes in discrete spacetime accrete matter more slowly than their continuous spacetime counterparts, for essentially ergodic reasons: with only countable degrees of freedom,... (2/6)arxiv.org/abs/2402.02331

This is a rock.

More precisely, it's a sample of my favorite mineral: Zn Cu3 (OH)6 Cl2, otherwise known as anarakite or Herbertsmithite.

Here's what it may be telling us about the fundamental structure of space, time, and the universe. (1/17) Suppose that you have a big collection of matrices, representing elements of some symmetry group. For instance, the elements of the 2D rotation groups U(1) or SO(2) can be represented by 2x2 rotation matrices. Call this collection a "representation" of the group. (2/17)

New paper alert!

Numerical general relativity is hard, complicated and computationally expensive. Here, we develop a radically new approach to doing it: cheat, and do special relativity instead...

arXiv link and explanation in thread... (1/11)
Link: 
In GR, a deep consequence of the equivalence principle is that, within any curved spacetime, there must always exist an observer (a worldline) that perceives it to be flat. A free-falling observer feels no gravity. (2/11)arxiv.org/abs/2410.02549

Functions like sine and cosine have some pretty nice properties: they're always differentiable and well-behaved (i.e. don't ever blow up to infinity). Apparently.

Yet it turns out that such functions are actually impossible, at least over the complex numbers. (1/9) As soon as we start extending sine and cosine to the complex plane, we see that they start blowing up - they don't remain nicely bounded like they do over the reals. We could try imposing bounds on them, but that would quickly break their differentiability. (2/9)

"Duality" is a deep concept in mathematics, but an intuitive way to think about it is in terms of tables.
When we lay out data in a table, we are familiar with the idea that each row represents a different "entity", and each column represents a different "property". (1/10) If we flip it over (swapping rows for columns), now we have a new data layout in which each "entity" is really a property, and each "property" is really an entity. It's clearly the same data, just represented differently; if we flip again, we get back to where we started. (2/10).

Gödel's first incompleteness theorem is commonly proved by means of a diagonal argument. But, in retrospect, we can see that what Gödel was really doing was proving that Peano arithmetic is Turing-complete, and then applying an argument from computational irreducibility... (1/15)

First, the standard proof. In Peano arithmetic (integers with multiplication), the fundamental theorem of arithmetic guarantees that all integers greater than 1 admit a unique prime factorization. So a sequence of integers can be encoded uniquely as a single integer... (2/15)

Moths are attracted to lights because of the same mathematics that underlies twistor theory and compactification in theoretical physics: projective geometry.

It all starts from a simple observation: translations are just rotations whose center is located "at infinity". (1/11) So if we take an ordinary space (like the 2D plane) and we adjoin a hypothetical point "at infinity", then the distinction between translations and rotations disappears: as the center of rotation moves further and further away from the object being rotated,... (2/11)

As this is pretty much the only statement of my personal philosophical outlook on metaphysics/ontology that I've ever made on here, I should probably provide a little further clarification.
It starts from a central idea from philosophy of science: theory-ladenness. (1/11)

https://twitter.com/getjonwithit/status/1813766177862353132

As argued by Hanson, Kuhn, etc., raw sense data is filtered through many layers of perception and analysis before it may be said to constitute "an observation". So making a truly "bare metal" observation of "reality" (uninfluenced by theoretical models) is impossible. (2/11)

At first glance, the idea that distributions of stars in the outermost regions of the universe directly affect physics here on Earth seems pretty absurd.
But, as it turns out, reconciling the concept of rotation with general relativity requires some pretty absurd things. (1/16) Relativity is based on the idea that physics is the same in all "inertial" frames, i.e. all frames of reference that are not accelerating. If I move every object in the universe one meter to the left, then I haven't changed the laws of physics: only the coordinate system. (2/16)

"Quantum mechanics is just thermodynamics in imaginary time."

It sounds like a hand-wavy, quasi-philosophical statement. But with a brief dive into the relationship(s) between hyperbolic and parabolic PDEs, it becomes possible to formalize it mathematically. (1/16)

(2nd-order) PDEs may be classified broadly as hyperbolic, elliptic or parabolic, based upon their relationship with time and causality. Hyperbolic PDEs have a notion of time/causality: a perturbation somewhere in the domain propagates out, wave-like, with a finite speed. (2/16)

In physics, one often thinks of space and time as being fundamental, pre-existing concepts, and proceeds to define everything else (energy, momentum, forces, etc.) in terms of them. But it doesn't need to be so - symplectic geometry shows us how to go the other way. (1/16) Suppose you have two functions, f and g, and you want to quantify "How much does f change as a result of a flow generated by g?"
Well, that's exactly what the Poisson bracket {f, g} measures. [We'll come on to precisely what a "flow generated by g" means in a moment…] (2/16)

Fibrations/bundles/sections/etc. are slightly opaque-sounding terms for an otherwise very intuitive idea: that you can parameterize a collection of spaces in terms of points from a different space, and then assemble new spaces out of points taken from that collection... (1/15)

Take the simple example of a curve on a 2D plane. That curve is a 1D space, and at every point along it there is a line (i.e. a different 1D space) that is tangent to it. So we can say that the collection of tangent lines is parameterized by the points on the curve. (2/15)

"The boundary of a boundary is always empty."
A huge amount of (classical) physics, including much of general relativity and electromagnetism, can be deduced directly from this simple mathematical fact.
Yet, on the surface, it doesn't seem to have much to do with physics. (1/10) Some spaces (like spheres) don't have boundaries. But, when the boundary exists, it's always one dimension lower (codimension-1). A disc is a 2-dimensional space, but its boundary is a 1-dimensional circle.
But what's the boundary of a circle? Well, it doesn't have one. (2/10)