If we somehow managed to remove absolutely everything from a box, the box would not be absolutely empty. No, Schrödinger’s Cat wouldn’t be there. There would always remain what physicists call Zero Point Energy.
Let’s imagine that we intend to empty a box. Truly and completely empty it. Remove all content visibleextract any gas and, applying some sci-fi technology, evacuate any invisible materiallike dark matter.
So, according to quantum mechanics, or what remains not inside?
It seems like a tricky question. And in quantum mechanics, you already know to expect a tricky answer. Not just the box still full of energylike all efforts to empty it they barely made a dent in quantity existing.
This inevitable residue is known as ground state energyor zero point energy. Presented in two basic shapes: the one in the box is associated with fields, such as the electromagnetic field, and the other is associated with discrete objects, such as atoms and molecules.
Can we attenuate the vibrations of a fieldbut we were unable to eliminate all traces of your presence. And atoms and molecules retain energy even if they are cooled arbitrarily close to absolute zero. In both cases, the underlying physics is the same.
The zero point energy is characteristic of any material structure or object that is at least partially confined, such as an atom held by electrical fields in a molecule, explains the physicist and author George Musser in an article on .
The situation is similar to that of a ball that settled at the bottom of a valley. The total energy of the ball consists of its potential energy (related to position) plus its kinetic energy (related to movement).
To cancel both componentswe would have to assign a precise value to both the position of the object and its speed, something that is prohibited by the famous.
What the existence of zero point energy reveals to us on a deeper level ultimately depends on the interpretation of quantum mechanics that we adopt.
The only undeniable thingWhat can be said is that if we place a set of particles in their lowest energy state and measure their positions or velocities, we will observe a dispersion of values. Despite being devoid of energy, the particles will appear to have been vibrating.
In some interpretations of quantum mechanics, they actually were. But in others, the appearance of movement is a deceptive trace of classical physics, and there is no intuitive way to visualize what is happening.
Zero point energy was first introduced by Max Planck in 1911. After that, “it was Einstein, I believe, who first took it seriously,” he said. Peter Milonnifrom the University of Rochester, a theorist who studies the quantum vacuum.
Einstein and Others Invoked Zero Point Energy to explain numerous phenomena, including the subtle vibrations of molecules and crystal latticeseven in its lowest energy states, and the fact that liquid helium does not condense into a solid at normal pressure, even at temperatures so low that one would expect the atoms to be locked in place.
A recent example was presented in a published in 2025 in the magazine Science by researchers at the European X-Ray Free-Electron Laser Facility, near Hamburg, among other institutions.
The researchers cooled down iodopiridinaan organic molecule consisting of 11 atoms, almost to absolute zero and bombarded it with a laser pulse to break its atomic bonds. The team found that the movements of the released atoms were correlated, indicating that despite its icy state, the iodopyridine molecule had been vibrating.
“That was not initially the main objective of the experiment”, says experimental physicist Rebecca Bollresearcher at European XFEL and co-author of the article. “It’s basically something we discovered“.
Maybe the best known effect of zero point energy in a field has been predicted by Hendrick Casimir in 1948, glimpsed in 1958 and definitively observed in 1997.
Two plates of electrically neutral materialwhich Casimir imagined as parallel metal sheets, although other shapes and substances will do, they exert a force on each other. Casimir stated that the plates would act as a kind of guillotine for electromagnetic fieldcutting off the long-wavelength oscillations in a way that would distort the zero-point energy.
According to the most accepted explanation, in a way, the energy outside the plates is greater than the energy between the platesa difference that pulls the plates toward each other.
Quantum field theory theorists typically describe fields as a oscillator collectioneach of which has its own zero point energy. There are an infinite number of oscillators in a field, and therefore a field should contain a infinite amount of zero point energy.
When physicists realized this in the 1930s and 1940s, initially doubted the theory, but quickly accepted the infinite. In physics, or most of it, the Energy differences are what really mattersand with care physicists can subtract one infinity from another to see what remains.
However, this doesn’t work for gravity. As early as 1946, Wolfgang Pauli realized that an infinite, or at least gigantic, amount of zero point energy should create a gravitational field powerful enough to do blow up the universe.
“All forms of energy gravitate“, it says Sean Carroll, physicist at Johns Hopkins University. “This includes vacuum energy, so we cannot ignore it.” The reason this energy remains gravitationally muted continues to baffle the physicists.
In quantum physics, this zero-point energy of the vacuum is more than simply an ongoing challenge, and it is more than the reason we never succeed. truly empty a box. Instead of being something where there should be nothing, it is nothingness imbued with the potential to be anything.
“The interesting thing about a vacuum is that every field, and therefore every particle, is somehow represented“, says Milonni.
Even if there is a single electron, the vacuum contains “electronity”. The zero point energy of the vacuum is the combined effect in every possible way of matter, including those we have yet to discover.
