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Martin Bauer @martinmbauer.bsky.social · 24/12/2025

I wish everyone a happy Christmas and some restful days at the end of the year! 12/12

Martin Bauer @martinmbauer.bsky.social · 24/12/2025

If a child which expects Santa to appear would succeed in observing the |Santa> state, it would be unprecedented direct evidence of Santa Claus, but it would collapse Santa's wavefunction and all presents would be deposited in his or her living room at once 11/12

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Martin Bauer @martinmbauer.bsky.social · 24/12/2025

Santa Mechanics described by a wavefunction α | Santa> + β | not Santa > with a very low expectation value for | Santa > provides a good explanation But a successful observation would have catastrophic consequences! 10/12

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Martin Bauer @martinmbauer.bsky.social · 24/12/2025

The leading theory today is the model of 'Quanta Claus'. Evidence for the quantum mechanical nature of Santa has been collected worldwide: 1. Santa is never directly observed, but indirect evidence of him abounds 2. 'Chimney Tunneling' 9/12

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Martin Bauer @martinmbauer.bsky.social · 24/12/2025

Alternative theories have been proposed: - 'There is no Santa' by Grinch et al. Phys. Rev. NP 001/1965 - 'There are no good children' by Ruprecht et al. Phys. Rev. NP 013/1816 - More speculative ideas… 8/12

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Martin Bauer @martinmbauer.bsky.social · 24/12/2025

86000 tons travelling at 4x10^6 km/h create enourmous air resistance This will heat the reindeer up They will absorb 40 billion Joule of energy per second each. The reindeer will be vaporised in a fraction of a second 7/12

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Martin Bauer @martinmbauer.bsky.social · 24/12/2025

Even if flying reindeer could pull four times as much as their cousins we’d need 144 333 reindeers to pull that sleigh 6/12

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Martin Bauer @martinmbauer.bsky.social · 24/12/2025

This ignores, that there is no known species of reindeers that can fly Also, if each household gets ~1kg worth of presents the sleigh is carrying at least 86 000 tons (not counting Santa) 5/12

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Martin Bauer @martinmbauer.bsky.social · 24/12/2025

Assuming evenly distributed houses with an average distance of 1.5 km, Santa will need to go 4 million km/h or ~1/250th of the speed of light (the corresponding relativistic length contraction has long been considered the secret behind Santa fitting through the chimney) 4/12

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Martin Bauer @martinmbauer.bsky.social · 24/12/2025

This works out to ~770 visits per second. Santa has ~1/1000 seconds to - park - hop out of the sleigh - jump down the chimney - leave the right presents - eat snacks - move to the next house 3/12

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Martin Bauer @martinmbauer.bsky.social · 24/12/2025

The classical theory of Santa Claus is highly problematic: - there are 2 billion children worldwide - about 15% are Christian - that makes 86 million households (with 3.5 children on average) -Santa has ~31 hours to work with 2/12

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Martin Bauer @martinmbauer.bsky.social · 24/12/2025

What would really be the implications of taking Santa Claus seriously? It's the time of the year to review the current state of the physics of Santa Claus 1/12

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Martin Bauer @martinmbauer.bsky.social · 10/12/2025

Really interesting event that would require black holes thatcan't be produced via any conventional mechanism!

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Martin Bauer @martinmbauer.bsky.social · 25/10/2025

A more careful formulation requires the embedding of the SM in a GUT (to get rid off the Landau pole) and can be found in the introduction of this nice paper (from which I also stole the figure) arxiv.org/pdf/1305.6939

arxiv.org

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Martin Bauer @martinmbauer.bsky.social · 25/10/2025

From this geometric perspective the Hierarchy problem is the question of whether there is a mechanism in some meow fundamental version of the Standard Model that selects or stabilises this very special trajectory 11/11

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Martin Bauer @martinmbauer.bsky.social · 25/10/2025

But from the point of view of the high energy fixed point, this trajectory is exponentially close to generic trajectories that fall directly into on of the RG attractors. The SM trajectory requires very specific, highly fine-tuned initial conditions to realise this trajectory 10/11

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Martin Bauer @martinmbauer.bsky.social · 25/10/2025

The SM as realised in nature corresponds to a very special trajectory in this theory space It stays for very long RG-time almost on the scaleless trajectory only to branch off into the broken phase 9/ 11

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Martin Bauer @martinmbauer.bsky.social · 25/10/2025

Importantly, both the mu^2>0 and mu^2<0 asymptotic low-energy theories are RG attractors But the mu^2=0 theory is a repulsive fixed point. It is unstable (Technically this is defined by the sign of the beta function). 8/11

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Martin Bauer @martinmbauer.bsky.social · 25/10/2025

the mu^2<0 branch corresponds to a broken electroweak symmetry phase The mu^2=0 option is special, because it corresponds to the trajectory on which all particles remain massless and the theory remains scale-invariant (all operators are marginal) 7/11

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Martin Bauer @martinmbauer.bsky.social · 25/10/2025

But there're different low energy fixed points. They correspond to the three different options for the Higgs mass term mu^2<0, mu^2=0 and mu^2>0 The mu^2>0 branch corresponds to the SM in which the electroweak gauge symmetry remains unbroken 6/11

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Martin Bauer @martinmbauer.bsky.social · 25/10/2025

One can then view the Standard Model as a specific trajectory in theory space from the high-energy fixed point to a low energy fixed point 5/11

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Martin Bauer @martinmbauer.bsky.social · 25/10/2025

Now imagine the SM in the opposite limit: almost 0 energy. In the effective field theory approach you have successively integrated out every massive particle and you're left with a theory of only massless states, which is also scale-invariant. 4/11

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Martin Bauer @martinmbauer.bsky.social · 25/10/2025

In the 'map of theories', this is a fixed point. Because it doesn't matter what the values if the mass parameters of the theory were different, in the limit of very high energies, physical observables would all look the same 3/11

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Martin Bauer @martinmbauer.bsky.social · 25/10/2025

For this it is important to understand the SM at different energy scales. At very, very high energies, all the mass terms in the SM are irrelevant. The SM looks scale invariant in this limit, because all effects from dimensionful parameters (scales) can be ignored. 2/11

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Martin Bauer @martinmbauer.bsky.social · 25/10/2025

The hierarchy problem (finetuning) of the Standard Model of particle physics is often presented as the question 'why the weak scale is so much smaller than the Planck scale' But there is also a geometric interpretation that can be understood with little maths (but a lot of physics) 🧵1/11

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Martin Bauer @martinmbauer.bsky.social · 08/10/2025

The Nobel laureates were the first to show this effect for systems containing billions of Cooper pairs. 'Macroscopic' objects described by a collective wavefucntion. Their work laid the foundations for superconducting qubits They also showed other properties like energy quantisation 7/7

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Martin Bauer @martinmbauer.bsky.social · 08/10/2025

In superconductors you can build have states where many, many Cooper pairs (pairs of electrons) are described by the same wavefunction, which allows them collectively to tunnel through thin insulator barriers called Josephson junctions 6/7

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Martin Bauer @martinmbauer.bsky.social · 08/10/2025

But in quantum mechanics one wavefunction doesn't mean one particle. Many particles can be described by the same wavefunction. Entanglement is an example in which multiple particles are described by one wavefunction 5/7

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Martin Bauer @martinmbauer.bsky.social · 08/10/2025

It is also responsible for field electron emission, used in electron microscopes and tunnel diodes, nuclear fusion in stars, etc 4/7

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Martin Bauer @martinmbauer.bsky.social · 08/10/2025

This effect isn't uncommon in nature It explains radioactive alpha-decay where a whole Helium nucleus is emitted from a heavy decaying element even if the binding forces wouldn't allow this process classically 3/7

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Martin Bauer @martinmbauer.bsky.social · 08/10/2025

..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 2/7

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Martin Bauer @martinmbauer.bsky.social · 08/10/2025

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

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Martin Bauer @martinmbauer.bsky.social · 05/10/2025

This night 102 years ago, Hubble discovered that there're stars outside our galaxy by observing a Cepheid variable star 1 million light years away Until barely a century ago, no human ever knew whether there was more than one galaxy in the Universe. Think about that!

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Martin Bauer @martinmbauer.bsky.social · 09/09/2025

These results use Run 2 data. By the end of Run 4, the much larger dataset would push the significance beyond discovery thresholds, if the central value holds In the plot below we were at the blue, we are now at the red and could be as sensitive as the green markers 6/6

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Martin Bauer @martinmbauer.bsky.social · 09/09/2025

This type of new physics would have the properties of a new vector boson (like the Z boson) or a leptoquark, a completely new state that connects color-charged quarks with color-neutral muons 5/6

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Martin Bauer @martinmbauer.bsky.social · 09/09/2025

If it is indeed physics beyond the standard model, the data points toward the same types of interactions that have already been shown to consistently explain several other anomalies in B-meson decays 4/6

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Martin Bauer @martinmbauer.bsky.social · 09/09/2025

It's an open question whether there is a systematic effect here that shows up in both experiments in the same way, or a theoretical uncertainty that has been underestimated or whether this is a genuine signal that the Standard Model cannot account for 3/6

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Martin Bauer @martinmbauer.bsky.social · 09/09/2025

The statistical significance is 4.1 standard deviations. Intriguingly, CMS, another experiment at the Large Hadron Collider, has also measured this process and sees the same anomaly, too 2/6

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Martin Bauer @martinmbauer.bsky.social · 09/09/2025

Today LHCb presented a new measurement of the decay of a B meson into a K* and a μ+ μ- pair Previous results disagreed with the Standard Model prediction for the branching fraction and angular distribution The new measurement has almost twice as much data and still disagrees! 🧵1/6

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Martin Bauer @martinmbauer.bsky.social · 01/08/2025

And if you consider the limit of x_1 -> x_2 you can see how the Pauli principle follows for fermions 14/14

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Martin Bauer @martinmbauer.bsky.social · 01/08/2025

The number of different ways to exchange particles and ending up in exactly the same configuration corresponds to different ways a WF transforms under particle exchange. There are only two classes in 3D, so only fermions and bosons. In 2D instead there're anyons (that can have any phase) 13/

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Martin Bauer @martinmbauer.bsky.social · 01/08/2025

In 2D this can be any complex number with a different phase factor for each winding number. In 3D+ it can be only +1 or -1 12/

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Martin Bauer @martinmbauer.bsky.social · 01/08/2025

In Quantum mechanics the question is how does the wavefunction describing two identical particles transform under this exchange. This corresponds to an operator O(\lambda) with is a 1-dimensional unitary representation of the fundamental group acting on the WF 11/

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Martin Bauer @martinmbauer.bsky.social · 01/08/2025

So the fundamental group of the configuration space in 3D+ is the group with exactly 2 elements 10/

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Martin Bauer @martinmbauer.bsky.social · 01/08/2025

Here is a representation of a non-contractible loop that can't be continuously transformed into a point 9/

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Martin Bauer @martinmbauer.bsky.social · 01/08/2025

Here is a representation of how the contractible loop can be continuously transformed into a point (=no exchange of the two particles) Again, the bar on the right part is the line connecting the two particles 8/14

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Martin Bauer @martinmbauer.bsky.social · 01/08/2025

And closing the q2 loop twice is equivalent to a contractible loop 7/14

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Martin Bauer @martinmbauer.bsky.social · 01/08/2025

Here, there're only 2 different classes of loops that can't be transformed into each other. Contractible loops (q1) and non-contractible loops (q2) 6/14

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Martin Bauer @martinmbauer.bsky.social · 01/08/2025

Now in 3D (or higher) the situation is different. Here, the configuration space is a half sphere, or a disk with opposite points identified 5/14

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