Can you propel a rocket by combining an electron and positron in a controlled chamber?

Take a small battery, and put a switch between the positive and negative terminals. Now press the switch. What happens?

If your answer is “the battery gets warm”, you might notice that what you didn’t say is “the battery moves dramatically across the surface of the table”. The energy released is a fraction of a joule- if the battery has about 100 milliamps of current, and one volt, then it releases about a tenth of a joule.

But if you were looking at a microscopic level, this is because a huge number of electrons suddenly go from “hanging out” to accelerating through the circuit. An electron-volt is 1.6 x10⁻¹⁹ joules, and that tenth of a joule represents the energy in quadrillions of electrons being set in motion.

Compare that joule or so of reactive energy to that of antimatter. Take potassium-40:

The energy released from an electron-positron annihilation event is about 5 x 10⁵ electron-volts, and there’s hundreds of these occurring within your body, as naturally occurring Potassium-40 (about 0.012% of natural potassium) in your body decays. You eat about 4g of potassium salts every day, which means that there are about 6 x 10²² atoms of potassium, and so 7 x 10¹⁸ atoms of potassium-40 are pushed into your body each day. It decays with a half life of 10⁹ years, which seems extremely long, but if you think about it, if half of those atoms are going to decay over that period (about 10¹⁶ seconds), that means (roughly speaking) about 100 atoms of potassium decay every second. Out of those decays, about 0.001% of the time, the potassium releases a positron, so every 20 minutes or so a positron is being formed in your body from the potassium you eat every day. BANG.

Did you notice anything?

Well, in the time that it took you to read and understand this answer, it’s likely that a positron annihilated inside you somewhere, just from the potassium you eat every day, or one of the many other sources of positrons such as other radioactive elements or cosmic rays from space. But the total energy from that annihilation comes to less than 10⁻¹³ joules, a trillionth of the energy involved in the experiment with the battery above. An absurdly small amount that is only significant when you have a lot of it going on around you.

But also, take a look at the picture above. Notice what happens when the electron and the positron annihilate: they emit two gamma rays in exactly equal and opposite directions. So thinking about the conservation of energy, you know, “every reaction has an equal and opposite reaction”, would you expect there would be any net movement, no matter how small?

This is the challenge of the so-called “antimatter engine”. Yes, it releases energy, and if you use that energy to heat a gas in a container and then expel that gas into a vacuum you might get some useable thrust, but the amount of thrust from “an electron” and “{a} positron”, as stated in the text of the question:

Can you propel a rocket by combining an electron and positron in a controlled chamber?

is such a trivial amount of energy as to be meaningless when talking about the energy to create usable thrust in a rocket motor. It is also a reaction that creates no net movement due to the equal and opposite release of photons, so by itself it is unusable as a source of net thrust, even if you could collect enough of them to have a macroscopic amount of energy.

question originally answered on Quora at

matt harbowy is a scientist, activist, and data management expert. He is one of the founders of the non-profit Counter Culture Labs, working to bring fairness and egalitarian ideals to people interested in learning about science and biotechnology. He is also a top writer on the question and answer site, Quora.

no job too dirty for the f*%&ing scientists. --Burroughs