# What mass can become negative

## Can mass be negative?

Researchers have created something in the laboratory that they call "negative mass" - but the test facility has still not become a microgram lighter

Often the simplest things are the hardest to explain. The mass, for example: everyone has an intuitive understanding of it. "What's heavier, a kilo of feathers or a kilo of lead?" Only preschoolers fall for this joke question. And yet, of all things, the concept of mass has so far only been roughly understood by physicists. The electric charge made it a lot easier for us. After all, since the discovery and proof of the Higgs mechanism, we have known how mass is created. But could there be such a thing as negative mass?

If we take a look at the universe, analyze its behavior and try to explain it with the current state of our knowledge, then the answer is: there must be negative mass. We attribute the accelerating expansion of space to "dark energy", a mysterious field with negative energy density, to which a negative mass density must also be assigned via the Einstein formula.

So there should be "things" where the mass in kilograms takes on a negative value. At the moment, however, it is impossible to say more than that it is "things". After all, we can already consider how such a negative mass would behave, also in relation to positive mass. The good news: Both would get along very well, because it has nothing to do with the matter-antimatter duality. Antimatter has a positive mass!

### "Runaway Movement"

After all, the Einstein equations show that positive mass attracts other positive masses, but also negative masses. Negative mass, on the other hand, repels any other negative mass as well as positive mass. This means that two equally large masses with different signs would generate a constant acceleration of the entire system in the direction of the positive mass.

The system would become faster and faster without any external influence, but would have a constant total impulse of 0 and would therefore not violate any of the conservation laws ("runaway movement"). In a gas that consists of particles of negative and positive mass, the positive part would increase its temperature to infinity, but the negative part would have to reach negative temperatures.

It becomes particularly complicated when two identical masses with different signs and different momentum meet, i.e. a collision occurs. If they are in one place, they would have to annihilate each other without leaving any mass (and thus energy). But that would violate the conservation of momentum.

Masses with different signs can only coexist in the form of the runaway movement, which leads to the assumption that there can be no negative mass after all. But what does the expansion of the universe say about it?

### Inert mass in the model system

It's complicated, that much is clear. The nature of dark energy is still completely unexplained; generating it in the laboratory in order to be able to research it seems utopian at the moment. Therefore, as the second best alternative, it would of course be nice to be able to work with systems that behave as if they had a negative mass.

This is common practice with other physical quantities. For example, astronauts on board the ISS are researching the effects of micro-gravity, even though they are only attracted to our planet so close to the earth as they are to its surface. They just do not feel the - existing - gravity because they are in free fall together with the space station. If there was really no gravity here, the ISS would spin out into space.

Researchers at Washington State University have now created a similar model system in the laboratory. Her paper was published in the Physical Review Letters, but is also available on arxiv.org. In doing so, they aim at the aspect of mass, which is known as inert mass - that is, mass as a proportionality factor between force and acceleration.

Usually, in Newton's formula, F = m * a the force F and the acceleration a the same direction. A negative mass here understandably leads to the acceleration being reversed. If you hit a billiard ball of negative mass, it would move in the exact opposite direction to the direction you hit.

### The real feat

In order to construct such a system in the laboratory, you have to go to some effort. To do this, the researchers cooled a cloud of around 10,000 rubidium atoms to temperatures close to absolute zero. The atoms are then in the ground state, in which they can no longer be distinguished from one another and become a Bose-Einstein condensate (BEC). This can be described by a common wave function, no longer by the movement of individual particles.

Now comes the real trick. With the help of lasers, the researchers create a kind of virtual container with special properties for the BEC, from which the atoms can only break out under certain behavioral patterns.

They manipulate the dispersion relation under which the particles dictated by their common wave function can spread (a very rough comparison for this would be the effect that arises when you observe a teaspoon placed in a glass of water, which then apparently gets a kink). In fact, the superfluid behaves exactly as expected, as if it had an effective negative mass, and moves in the opposite direction to an external impulse.

Note the word "effective" which major international media have omitted from reporting on the attempt. The entire apparatus has not become a microgram lighter, just as the ISS cannot switch off gravity. So no negative mass has been created, but a system that, under very specific circumstances, pretends to have negative mass.

This is still very helpful and does not reduce the performance of the researchers, as it can be used to investigate many properties of negative mass that we otherwise have no access to.