# Ask a Physicist Answers

## It looks like they are looking for positive mass particles at the LHC at CERN. Isn't it logical that there are negative-mass "anti-particles"? Does the LHC have the capability of measuring negative mass?

-Asked by Heidi H. from Tacoma, Washington

The CMS detector at the LHC during construction in 2007.
Image Credit: µµ via flickr

Here are three possible answers to your question:

(a) the LHC can, and does, detect anti-particles. According to the theory of relativity, every particle comes with a corresponding anti-particle. But particles and their anti-particles have exactly the same mass. An example of this is the electron and its anti-particle the positron, whose masses have been well-measured and are the same.

If the positron had negative mass, then electron-positron pairs could be created spontaneously with no input of energy. This does not happen. Another example is the mass zero photon, which is its own antiparticle.

Yet another example is the Higgs boson itself, which is also its own anti-particle, so you could say that the LHC has already detected both the Higgs and the anti-Higgs.

(b) Some people have suggested that even though particles and anti-particles have the same mass, they behave differently with respect to gravity, so that a particle will fall down but an anti-particle will "fall" up. Gravity here on Earth is a very weak force, so it's quite hard to measure its effect on an elementary particle.

The LHC certainly couldn't do it. But we can say that if an anti-particle (or anything else) were to "fall" up, it would violate a very deep principle of Nature, called the equivalence principle, which is the basis for Einstein's theory of general relativity.

One of the consequences of the principle is that all objects fall at the same rate. So don't bet on finding that anti-particles "fall" up.

(c) We can go one step further, and consider particles whose mass squared is negative (mass squared is just mass multiplied by itself; if mass squared is negative, the mass must be a so-called "imaginary number").

This may seem even more bizarre than negative mass, but such things are actually allowed by Einstein's theory of relativity. Particles with negative mass squared are called tachyons, and they always travel faster than light. If tachyons existed in Nature, they would probably allow signals to travel backward in time, thereby violating causality.

Tachyons have never been detected, although there was some excitement a few years ago when an experiment claimed to measure a kind of elementary particle, the neutrino, traveling faster than light. As it turned out, the experiment was wrong.

Answered by:

Alan Chodos, PHD
Associate Executive Officer (retired)
American Physical Society