Tag: physics

Trigger Waves

Biomechanical interactions, rather than neurons, control the movements of one of the simplest animals. The discovery offers a glimpse into how animal behavior worked before neurons evolved.

We show a minimal mechanism — trigger waves — by which these walking cells may work together to achieve organism-scale collaboration, such as coordination of hunting strikes across 100k cells without central control.

The behavior of Trichoplax can be described entirely in the language of physics and dynamical systems. Mechanical interactions that began at the level of a single cilium, and then multiplied over millions of cells and extended to higher levels of structure, fully explained the coordinated locomotion of the entire animal. The organism doesn’t “choose” what to do. Instead, the horde of individual cilia simply moves — and the animal as a whole performs as though it is being directed by a nervous system. The cilia’s dynamics exhibit properties that are commonly seen as distinctive hallmarks of neurons.


2022-03-30: Physical gradients

Much of the research to date on self-generated gradients has looked at chemical signals, but cells can create gradients in other physical attributes, too, including mechanical properties. The recent paper analyzing migrating neural crest cells revealed a self-generated gradient of stiffness.


2022-07-13: Embryogenesis involves the extracellular matrix contracting

Mechanical forces induce embryonic chicken skin to create follicles for growing feathers. Just as surface tension can pull water into spherical beads on a glass surface, so too can the physical tensions within an embryo set up patterns that guide growth and gene activity in developing tissues. As an organism grows and develops, the cells in its tissues pull and push on each other and on the supportive protein scaffolding (extracellular matrix) to which they are intricately linked. Some researchers have suspected that these forces, coupled with changes in the pressure and rigidity of the cells, might direct the formation of complicated patterns. Until now, however, no studies were able to tease apart the effect of these physical forces from the chemical stew in which they simmer.

Thermodynamic Clocks

an ideal clock — one that ticks with perfect periodicity — would burn an infinite amount of energy and produce infinite entropy, which isn’t possible. Thus, the accuracy of clocks is fundamentally limited. “What we’ve done is to show that even if time is a perfect, classical and smooth parameter governing time evolution of quantum systems, we would only be able to track its passage” imperfectly, through stochastic, irreversible processes. “Could it be that time is an illusion and smooth time is an emergent consequence of us trying to put events into a smooth order? It is certainly an intriguing possibility that is not easily dismissed.”

Turing Patterns

Turing’s paper described a theoretical mechanism based on 2 substances — an activator and an inhibitor that diffuse across an area at different rates. The interaction between these 2 “morphogens,” as Turing called them, allows one to interrupt the effect of the other — creating a pattern of colored lines on a tropical fish, for example, rather than a solid color. Turing’s mechanism was indeed responsible for the stripes in the bismuth. And it demonstrated once again how robust and powerful Turing’s original insight was. Here, the stripe-forming process is driven by the forces at play among the bismuth atoms and the metal below. Bismuth atoms want to fit into particular spots on the molecular lattice of the metal. But these spots are closer together than the bismuth atoms find comfortable. Like a photograph that gets shoved into a frame that’s too small for it, the sheet of bismuth atoms buckles. The strain creates a wavy pattern that leaves some atoms raised, forming the stripes. The vertical shift — movement away from the plane of the crystal — acts as the activator in the Turing equations, while the shift within the plane acts as the inhibitor. The morphogens here are displacements, not molecules. When part of a Turing pattern is wiped out, it grows back. You might not assume that inorganic materials like bismuth crystals would be able to heal as animals do,but indeed, his team’s simulated bismuth crystal was able to mend itself.

Constructor Theory

Counterfactuals do appear in existing laws, but these laws are regarded as second class. They are not incorporated wholeheartedly. Constructor theory puts counterfactuals at the very foundation of physics, so that the most fundamental laws can be formulated in these terms. Concepts like work and heat can’t be captured fully with trajectories and laws of motion, because in the standard conception they are considered emergent and approximate. In constructor theory we can talk about them using exact statements about possible and impossible transformations.

2023-05-04: Assembly theory is an intellectual cousin

Assembly theory started when Cronin asked why, given the astronomical number of ways to combine different atoms, nature makes some molecules and not others. It’s one thing to say that an object is possible according to the laws of physics; it’s another to say there’s an actual pathway for making it from its component parts. “Assembly theory was developed to capture my intuition that complex molecules can’t just emerge into existence because the combinatorial space is too vast”.
Assembly theory makes the seemingly uncontroversial assumption that complex objects arise from combining many simpler objects. The theory says it’s possible to objectively measure an object’s complexity by considering how it got made. That’s done by calculating the minimum number of steps needed to make the object from its ingredients, which is quantified as the assembly index (AI).
Complex mixtures of molecules made by living systems — a culture of E. coli bacteria, natural products like taxol (a metabolite of the Pacific yew tree with anti-cancer properties), beer, and yeast cells — typically had significantly higher average AIs than minerals or simple organics.

The analysis is susceptible to false negatives — some products of living systems, such as Ardbeg single malt scotch, have AIs suggesting a nonliving origin. But perhaps more importantly, the experiment produced no false positives: Abiotic systems can’t muster sufficiently high AIs to mimic biology. If a sample with a high molecular AI is measured on another world, it is likely to have been made by an entity we could call living.
Cronin and Walker hope that assembly theory will ultimately address very broad questions in physics, such as the nature of time and the origin of the second law of thermodynamics. But those goals are still distant.

PDE AI

researchers at Caltech have introduced a new deep-learning technique for solving PDEs that is dramatically more accurate than deep-learning methods developed previously. It’s also much more generalizable, capable of solving entire families of PDEs—such as the Navier-Stokes equation for any type of fluid—without needing retraining. Finally, it is 1000x faster than traditional mathematical formulas. Now here’s the crux of the paper. Neural networks are usually trained to approximate functions between inputs and outputs defined in Euclidean space, your classic graph with x, y, and z axes. But this time, the researchers decided to define the inputs and outputs in Fourier space. Because it’s far easier to approximate a Fourier function in Fourier space than to wrangle with PDEs in Euclidean space, which greatly simplifies the neural network’s job. Cue major accuracy and efficiency gains: in addition to its huge speed advantage over traditional methods, their technique achieves a 30% lower error rate when solving Navier-Stokes than previous deep-learning methods.

Superwhite Paint

Superwhite paint will cool the earth

This paint would be both cheaper to produce than its commercial alternative and could save $1 per day that would have been spent on air conditioning for a 1-story house of 100m2. It would take 37 liters to paint a 100m2 house. The world makes 55m tons of paint a year. Saving $1 a day would mean a payback for 37 liters of paint in 6-12 months. If paint production was doubled and the new production was used for this superwhite paint then 100K square kilometers could be painted every year.

2023-04-04: Rethinking how paint works can get us even farther

Unlike pigments, which require a different base molecule—like cobalt or purple snail slime—for each color, the base molecule for this process is always aluminum, just cut into different-size bits that oscillate to light at different wavelengths. It’s the lightest paint in the world— both in terms of weight and temperature. The paint consists of aluminum flakes dotted with aluminum nanoparticles. A Boeing 747 needs 500kg of paint. This paint could cover the same area with 1.3kg. Unlike conventional paint, structural paint doesn’t absorb infrared radiation, so it doesn’t trap heat. It can keep surfaces 10 degrees Celsius cooler than conventional paint.

Black hole info paradox

The most famous paradox in physics nears its end

In a landmark series of calculations, physicists have proved that black holes can shed information, which seems impossible by definition. The work appears to resolve a paradox that Stephen Hawking first described 50 years ago. All this reinforces many physicists’ hunch that space-time is not the root level of nature, but instead emerges from some underlying mechanism that is not spatial or temporal. To many, that was the main lesson of the AdS/CFT duality. The new calculations say much the same thing, but without committing to the duality or to string theory. Wormholes crop up because they are the only language the path integral can use to convey that space is breaking down. They are geometry’s way of saying the universe is ultimately nongeometric.

2023-02-12: More on Feynman Path Integrals

The most powerful formula in physics starts with a slender S, the symbol for a sort of sum known as an integral. Further along comes a second S, representing a quantity known as action. Together, these twin S’s form the essence of an equation that is arguably the most effective diviner of the future yet devised.

The oracular formula is known as the Feynman path integral. As far as physicists can tell, it precisely predicts the behavior of any quantum system — an electron, a light ray or even a black hole. The path integral has racked up so many successes that many physicists believe it to be a direct window into the heart of reality.

Multi-species community assembly

Early on, Mehta thought Gore was ignoring important details about microbial environments, but Mehta concedes that these details have proved less important than he believed. “What’s interesting and impressive about the work is that they show that many, many different phenomena — pH changes, competition for nutrients — can all be captured using these simple phenomenological models, at least in systems where you have a few species. This is a true and tried reductionism of the sort that’s served well in physics.”