Is nature hiding a universal speed limit? New physics offers clues

Leave a hot cup of coffee on the table, and it gradually cools. Put a bottle of milk in the refrigerator and, given enough time, it reaches the same temperature as everything around it.

This everyday process, known as thermalization, is one of nature’s most universal behaviors. No matter how complex a system is, it eventually settles into equilibrium. While there is no doubt about whether or not systems will settle into equilibrium, physicists have been trying to answer the question of how quickly systems will reach this state for many years.

A new study suggests there is a fundamental limit.

Researchers at the Universitat Autònoma de Barcelona, along with colleagues in Austria and Switzerland, have shown that quantum mechanics itself imposes a universal speed limit on thermalization. Their findings indicate that no physical system can reach thermal equilibrium faster than a timescale set by a handful of fundamental constants.

Linking quantum entanglement to cold coffee

For more than 20 years, physicists have suspected that thermalization is governed by a special timescale called the Planckian dissipation time:

τPl=ℏkBTtau_{Pl}=frac{hbar}{k_B T}τPl​=kB​Tℏ​

where ℏ is the reduced Planck constant, kB is Boltzmann’s constant, and T is temperature.

The relationship suggests a simple idea: hotter systems can thermalize faster than colder ones.

At room temperature, this timescale is about 25 femtoseconds, one of the fastest timescales encountered in nature. In ultracold quantum systems, it can stretch to milliseconds.

Scientists propose the ultimate limit for how thick a liquid can be

Scientists had found hints of this limit in exotic materials, including high-temperature superconductors. But proving that it represented a universal law of physics remained frustratingly difficult.

The challenge is that if a system’s microscopic rules are already known, it is theoretically possible to prepare a thermal state very quickly. That makes it hard to define a universal limit.

Instead, the researchers imagined a more realistic scenario: a hypothetical “thermalization machine” that must work for many different physical systems without knowing their internal details beforehand.

Using methods of quantum metrology, which studies how precisely we can measure physical characteristics, researchers showed that there are limits on how fast such machines can determine what system they are observing due to the rules of quantum mechanics.

That restriction leads directly to a minimum thermalization time. The result is remarkably simple:

τ≥τPl2tau ge frac{tau_{Pl}}{2}τ≥2τPl​​

In other words, thermalization cannot occur faster than roughly half the Planckian timescale.

What Happens Near Absolute Zero?

The researchers found that at extremely low temperatures, temperature itself is no longer the key factor.

Instead, the crucial quantity becomes the energy gap between a system’s lowest-energy state and its first excited state. In this regime, that energy gap determines the minimum thermalization time, linking the result to well-known principles of quantum mechanics.

At first glance, the result may seem highly theoretical. But thermalization plays a central role in many areas of physics, including quantum computing, superconductors, ultracold atomic gases, energy transport in materials, and condensed matter physics.

The new work suggests that the Planckian limit is not a peculiarity of specific materials. Instead, it emerges naturally from the fundamental rules of quantum mechanics.

The study reveals something profound about the way the universe works.

Nature is extraordinarily good at washing away microscopic details and driving systems toward equilibrium. Yet even this process has limits. No matter how efficient a system may be, quantum mechanics imposes a minimum amount of time required for it to settle down.

Even though a simple cup of coffee cooling off on your desk can appear to be an ordinary occurrence, it is actually governed by one of the most basic laws of the universe; that is, no matter how fast nature tries, it will never achieve absolute balance in an infinite amount of time.

Journal Reference:

1. Abiuso, P., Rolandi, A., Calsamiglia, J. et al. An information-theoretic proof of the Planckian bound for thermalization. Nat. Phys. (2026). 10.1038/s41567-026-03397-y