Short explanation of the physics Nobel Prize 2025

In classical mechanics you can know where a particle is and its momentum at the same time. In Quantum mechanics you can't. All information is in the wavefunction. Even if a particle is trapped, part of the wavefunction..

🧵1/7 ..is on the other side of the barrier (unless its an infinitely strong force field)

If you measure the position of the particle there is a finite probability it's outside the trap even if it never had enough kinetic energy to overcome the barrier. It 'tunnels' through the barrier

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Despite the 2012 Higgs boson discovery, we still don't know whether it is responsible for the masses of first generation fermions (up&down quarks and the electron)

Higgs couplings to gauge bosons, 3rd and 2nd generation fermions agree well with the Standard Model, but..

🧵1/5 ..we have never measured the Higgs boson decaying into first generation fermions.

The reason is the way mass generation via the Higgs mechanism works. The heavier a particle, the stronger it's interactions with the Higgs field, the more likely it is for a Higgs boson to interact with it

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In 3 (or more) dimensions, all fundamental particles are either fermions and bosons. But why?

This is a direct consequence of the properties of the configuration space for identical particles

🧵 1/14 If you have 2 indistinguishable particles, the configuration in which particle 1 is at r and particle 2 is at -r is completely equivalent to the configuration where they swap positions

So in 2D, the configuration space is R^2 with opposite points identified (and excluding the origin which corresponds to both particles being at the same place)

This space is a cone w/o a tip (e.g. upper half-plane glued along the x-axes)

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Gravity is weaker than all other forces, but is there a reason why it should be? Maybe.

This is a short thread on the weak gravity conjecture (minimal math)

1/5 There are two parts to the argument why gravity should be the weakest force: First, black holes can carry charges and can evaporate

Consider now a black hole with mass M and total charge Q. It can only fully evaporate if there exist particles with mass m and charge q so that M/m > Q/q

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Are neutrinos their own anti-particles?

They are the only fundamental fermions that could have this unique property. But they're so elusive, how could we measure that?

A brilliant experiment could answer this question without detecting a single neutrino

🧵1/11 You only need two facts to understand this experiment.

1. Neutrinos are produced when neutrons decay into protons (beta decay). That's how they were discovered.

Because of charge conservation a neutron decays into an electron and a proton

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Does quantum mechanics really need to be complex?

Why can't we just use real wavefunctions?

🧵1/15 *some math, non-relativistic QM First of all, not all wavefunctions are complex. The Schrödinger equation tells us that time-independent (stationary) solutions can be real. These are energy eigenstates, e.g. a bound state in an hydrogen atom

But even if you start with a real wavefunction, time-evolution introduces an imaginary part

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Renormalisation is a central concept in modern physics. It describes how the dynamics of a system change at different scales. A great way to understand and visualise renormalisation is the Ising model

(some math, but one can follow without it )

1/13 Start with a lattice and assign two possible values to each vertex (+1/-1 here) and an interaction between each point with its next neighbours. Mathematically this interaction can be written as a sum, where the parameter beta decides how strongly the vertices interact (beta=0 is no interaction)

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The most energetic neutrino ever observed just smashed into the Mediterranean and left a signal that was picked up by the K3Mnet neutrino telescope 2400m under the sea

At the moment we don't really know where this neutrino came from, nor should it really be there.

🧵1/10 Neutrinos that hit water or ice can kick and electron out of its atom with so much momentum that its speed exceeds the speed of light (in the medium). What happens than is the light equivalent of a sonic boom: Cherenkov radiation

The same effect makes nuclear reactors shine blue

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Really nice, short ASCI history of physics (from Piet Hut, IAS)

From the babylonians to Aristoteles:

🧵 1/12 Newton rings in the modern age ~1700

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Could we tell whether anyone in our galaxy uses a warp drive?

This sounds like a crazy question, but it can be answered using numerical GR

(one of the fun highlights of the annual theory meeting presented by Katy Clough)

🧵1/7 In GR, a 'warp drive' corresponds to a gravitational distortion of spacetime

In 94 it was shown that this is possible in principle by contracting space in front of of a spaceship and expanding space behind it (if you believe the aliens discovered negative mass) by Alcubierre

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We shall call the particles making up atoms electrons and quarks

-Quarks, sir?

Yes. Up and down quarks

-Will we call particles associated with forces quarks, too?

No, they shall be called photons and gluons

-What about the weak force, sir?

Those shall be called W and Z Won’t we use other letters?

-None of them, but greek letters shall be used for the heavy partners of the electron, the muon and tau

-What about other greek letters?

We shall use rho, pion, kaon, eta, etc

Are those also heavy electrons sir?

-No, those are made from quarks

Are interstellar comets in our solar system?

There're comets drifting around space not in orbit of any star. On our way around the Milky Way our solar system moves past these rocks. How likely is it that one of them has been caught in orbit of the sun?

🧵 1/7 The density of rocks with 100m-1km size is surprisingly high in interstellar space

We know this from @PanSTARRS1 at the Haleakala Observatory in Hawaii, which produced the largest volume of astronomical data ever released, 1.6 pb

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What do you think was the color of the night sky when the Universe first turned transparent?

What do you think was the wavelength of the cosmic microwave radiation at the time when the photons decoupled?

1/5 You can calculate the temperature of the Universe at that time using the measurement of the CMB temperature today: 2.725 K, the time of decoupling 380000 yrs and the fact that the Universe was matter dominated

You'll find a temperature of 3000 K

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Quantum mechanics can be demystified to some degree by realising that it's probability theory with a squared norm. E.g. you'd add classical probabilities (real numbers) p1, p2, .. < 1 with

p1+p2+...+ pn = 1

In QM instead you have amplitudes and

|w1|^2+|w2|^2+...+|wn|^2=1

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https://twitter.com/ns_nikhil9/status/1827945682935976236

For classical probabilities the linear norm is preserved by acting on P=(p1, ..., pn) with stochastic or Markov matrices M (non-neg matrices where all entries in a column add to 1)

M P = P' and p'1 + p'2 + ... +p2n =1

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The strongest evidence for dark matter:

In hot plasma like the early Universe structure can't persist. Whenever matter is compressed by gravity it is driven apart by radiation pressure

The resulting oscillations wash out every seed of over density. Galaxies never form

1/6 Only if there's matter that does interact gravitationally but is *not* susceptible to radiation pressure, these oscillations can be suppressed

Overdensities of this 'dark' matter provide gravitational wells for visible matter (gas) to fall into

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What is a proton made of?

Early scattering experiments probing the substructure of the proton found an interesting result. Instead of being made just from point-like particles (quarks), there seemed to be more going on inside the proton

A🧵1/6 Zooming into a proton corresponds to colliding it with more and more energetic beams

Similar to probing a target with light using shorter wavelength, higher energy results in a better resolution

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General relativity is often called geometric, but special relativity is also a geometric theory

It explains moving at different speeds as rotations in spacetime, similar to the way we explain looking in different directions as rotations in space

Some math in this 🧵1/12

In Newtonian mechanics we can disagree about directions, but not about (relative) distances. E.g. if we stood back to back and asked about the distance to London we would agree on the distance, but not about the direction.

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Did Newton know F=ma?

Newton's 'Principia' uses geometric proofs, not calculus. He didn't give differential equations for his laws, but did he know them?

Exploring whether Newton knew diff expressions for forces is a fascinating rabbit hole

NB @rmathematicus

🧵1/9 Newton published the Principia in 1687 (2nd edition 1713, 3rd 1726). In it he doesn't mention that F = ma. In fact he doesn't use calculus

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Renormalization group without any math:

For the longest time we discovered more fundamental structures in nature. The further we zoom in the more new structure we see.

The discovery of quarks is a good example

Molecules → Atoms → nuclei → protons → quarks

🧵1/13 Will this go on for ever? Will we keep uncovering layer after layer of new particles?

We don't know the answer to that, but at the most fundamental layer we have discovered Nature has a trick that sounds counterintuitive at first

If you split quarks you find more quarks. 2/13

The basic idea of renormalisation: A sketch

In Quantum field theory with interactions, there're corrections to fundamental constants

The electron mass is corrected because of the presence of the photon field

So what is measured in an experiment is the 'corrected mass' mr

1/9 You can only ever measure mr

In order to measure m0 you'd need to be able to turn off the photon field. Not the presence of any number of photons, but the existence of a photon field overall -like a Universe where photons don't exist.

We can't do that: m0 is unobservable

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The Higgs can decay into a vector meson and a photon H → J/Ψ γ → μ+μ− γ

This process is so rare, it takes a quadrillion (10^15) collisions to see it once at the LHC!

It took 8+ yrs for CMS&ATLAS to see a few of these decays and now they test a never observed effect

🧵1/12 According to the standard model the Higgs boson interacts with all quarks with an interaction strength directly proportional to the quark mass.

Even though there're 6 types of quarks we've only good measurements for 2 of them : top and bottom

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