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Bananas and warp drive

Published Sunday, February 05, 2023 @ 9:40 PM EST
Feb 05 2023

It's kind of neat when you're pushing 70 years old and random, seemingly unrelated things stuck in your brain merge into unexpected enlightenment.

Like matter and antimatter.

When particles of matter and antimatter collide, they annihilate each other. This releases energy. Given a large enough quantity of particles, a lot of energy- enough to power the futuristic warp drive engines in Star Trek, for example.

But antimatter is science fiction, right? We don't see it occurring in nature; we have to create it, and it's ridiculously difficult. Antimatter is the most expensive substance on Earth: generating one gram (0.035274 ounces) of the stuff would cost $62.5 trillion dollars

Surprisingly, matter/antimatter annihilations are not science fiction. You encounter them practically every day.

Consider PET scanners.

PET is an acronym for Positron Emission Tomography. Tomography is imaging by sections or sectioning that uses any kind of penetrating wave. Emission is something that has been emitted- released or discharged.

And positron?

A positron (or anti-electron) is the antiparticle, or the antimatter counterpart of the electron.

This gets interesting when you start talking about radioactive substances (radionuclides), those that are unstable because they have too much energy in their atomic nuclei.

When you get a PET scan, a radionuclide (tracer) is injected into your body. As the radioactive tracer emits its excess energy (decays), protons in the tracer's nucleus are converted into neutrons. That process releases various particles, including positrons: particles of antimatter.

When these antimatter positrons collide with the electrons of normal matter in your body, the particles annihilate each other. This generates electromagnetic gamma radiation, or gamma rays in the form of what are called, appropriately enough, annihilation photons.

The scanner detects the annihilation photons, which arrive at the detectors in coincidence at 180 degrees apart from one another. A computer analyzes those gamma rays and uses the information to create an image map of the organ or tissue being studied. The amount of the radionuclide collected in the tissue- how big the matter/antimatter annihilations are happening in the organs or tissues being studied- affects how brightly the tissue appears on the image, and indicates the level of organ or tissue function.

But isn't gamma radiation, like, deadly? It's the radiation produced by neutron stars and pulsars, supernova explosions, and regions around black holes. A gamma ray burst from one of those astronomical entities as close as a thousand light years could end life on Earth, its intense ionizing radiation ripping the ozone from the atmosphere and damaging the cells and DNA of all the planet's life forms.

But nature has a vast dynamic range. At the one end, cosmic apocalypse. Scale it down, and you have gamma rays generated by nuclear explosions and lightning.

Keep shrinking it, and you have PET scans. While it involves gamma rays generated within your body by matter/antimatter annihilation, it's on such a small scale and short timespan that it's of no concern. The radioactive half-life of the radionuclide used is under two hours, and it eventually decays into stable oxygen.

And matter/antimatter annihilations are probably happening on your kitchen counter right now. Bananas can contain tiny amounts of naturally occurring unstable radioactive isotopes, particularly potassium 40. Every hour or so, you could have a matter/antimatter annihilation going on in your kitchen.

I now have this image of Star Trek's Scotty furiously shoveling bananas into the Enterprise's warp engines...

(More on PET scans)

Categories: Antimatter, Bananas, PET scan, Positrons, Quantum mechanics, Star Trek


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