ANTIGRAVITY: A RECENT TEST RESULT MIGHT HAVE RULED IT OUT

Hoverboards and flying cars may be off the cards... but dark energy may yet come to the rescue

- PROF JON BUTTERWORTH

According to first Newton, then Einstein, and now an experiment at CERN, gravity is an attractive force that exists between all objects in the Universe. That includes objects that have no mass, because gravity acts on energy, and mass is just one form of energy (as Einstein's most famous equation states, energy is equal to mass multiplied by the square of the speed of light). This is why even massless photons of light, travelling from distant stars, have their paths bent as they pass massive galaxies on the way.

Antigravity is a hypothetical repulsive gravitational force. In some ways, it sounds obvious that it should exist. There are both attractive and repulsive electric forces, so why not the same for gravity?

The difference is that electric charge comes in two types, positive and negative. Different charges (a positive and a negative) will attract each other, while charges that are alike (two positives or two negatives) repel each other. The equivalent of 'charge' for gravity is energy, and it only comes in one type: positive.

As these positive energies attract each other there doesn't seem to be room for antigravity, which is a pity because it would be a great way of flying around without the need for rockets, jet engines or even wings.

However, there is (or was, until this month) a possible get-out clause for antigravity: antimatter.

Antimatter isn't hypothetical, it's very real. Particles such as electrons have an antimatter equivalent.

The antiparticle of the electron is the positron, and it has not only been observed, but is regularly used in hospitals for diagnostic purposes.