# Meaning of annihilate

As a naive person I thought annihilate meant destroy. Matter and antimatter destroy each other, cancel each other, nothing left over. This pure energy idea is a big surprise also it fits no category of energy as we know it, kinetic etc Can someone bring me to understanding with simplest possible maths, please?

• If you look up pair annihilation you will see that it produces photons as a product. This isn't some mysterious pure energy thing. Jul 28, 2023 at 0:35
• en.wikipedia.org/wiki/Annihilation Jul 28, 2023 at 5:16
• So for context, what happens can more closely be described as: they collide and photons (particles of light) or some other particles fly out at the other end - but the thing to understand is that these products are not coming from the "inside" of the incoming particles - they are not their constituent parts. The incoming particles really do destroy each other, and then new particles are created through some complicated interaction in the underlying quantum fields. Jul 28, 2023 at 12:36
• What "pure energy idea" are you talking about? Jul 30, 2023 at 19:12

## 2 Answers

As a naive person I thought annihilate meant destroy.

Yes, that is what it means.

Matter and antimatter destroy each other, cancel each other, nothing left over.

No, not necessarily.

First, you should know that "antimatter" does not have "anti-mass;" it has positive mass just like regular matter. It is called "antimatter" because it has the opposite charge. For example, the anti-particle to the electron is the positron. They have the same mass, but opposite charge.

Second, physical processes tend to conserve energy and momentum. So, if a (positive mass) electron annihilates with a (positive mass) positron, they have to produce some other particle with at least as much energy as their combined rest mass energy.

This pure energy idea is a big surprise also it fits no category of energy as we know it,

Who is "we"? What is "pure energy"? This part of your question seem to veer into your own flavor of mysticism.

• The “pure energy” concept contradicting boxing energy into “kinetic”, “potential”, “spring energy” etc. is something I really had to battle when, coming from highschool physics, starting my Physics studies. I was absolutely clear that some wobbly green glowing stuff is not helpful as mental model for “energy”, but it took me years to come to a better understanding of the idea of “energy”. What I want to say: It doesn’t need to be mysticism, just not yet your level of understanding the concept. Jul 28, 2023 at 11:25
• It comes from the formula E=mc^2 , it needs relativity theory to understand it, but all the energy you get from nuclear power plants comes from converting mass to energy. Jul 28, 2023 at 12:30
• Guys, those questions are rhetorical. (Just FYI.)
– hft
Jul 28, 2023 at 13:11
• Thank you for paying attention to my question. I thinking more in the context of the big bang, as I am reading The First Three minutes again, I think I read it pretty soon after it came out. I looked up a term on the web and fell into the pure energy surprise. Excuse me for saying we, I am not here in any mystical capacity. So is this pure energy is only existing right in the first fraction of a second, before the strong force, the electromagnetic, and the weak separate out, so it is undifferentiated. Jul 29, 2023 at 17:32
• I miss out gravity specifically because spacetime seems to be own thing within which energy and matter take place. I hope I am not a nuisance with my simple mind. Jul 29, 2023 at 17:32

I thought annihilate meant destroy.

The word "annihilation" comes from Quantum Field Theory. To make a long story short, this theory is based on creation and annihilation operators which create and destory particles, and a Largrangian which contains multiplications of these operations and basically says which interactions are possible - where each interaction destroys a few incoming particles and replaces them by newly-created outgoing particles. The interaction terms in this Largrangian are often visualized using a Feynmann diagram.

For example, in Quantum Electodynamics (QED, the Quantum Field Theory for the behavior of electrons, positrons, and photons), there is just one basic Feynmann diagram - an electron goes in, and emits a photon and another electron. If "rotated" in spacetime (as one does in Feynmann diagrams), this is a equivalent to an electron and a positron (an anti-electron) coming in, disappearing and generating a photon instead. One says that the original electron and positron were "annihilated" and a photon was "created" in this interaction.

In other examples of Quantum Field Theory, the inputs of the interaction do not have to be just particle and anti-particle, and the output doesn't need to be just a photon. For example, in Beta decay, an down-quark goes in (annihilated in this interaction) and out go (created) an up-quark and a W- boson. There is no photon involved in this interaction - if there is more energy or momentum in the inputs than the output, then the excess energy is carried by the kinetic energy of the ouputs (the up-quark and the W- boson), not in a photon.

Matter and antimatter destroy each other, cancel each other, nothing left over.

As I explained above, this isn't quite what happens - when matter and antimatter annihilate, the result is not "nothing" or "pure energy" - it is a photon, but not always a photon:

You need to remember that in any real, visible (not virtual) interaction, all the conservation laws you are familiar with need to be obeyed. Because a particle and anti-particle always have opposite conserved charges (e.g., electric charge) the result of the interaction doesn't need to be a charged particle, but conservation of energy and momentum means to be it needs to be a particle capable of carrying this energy and momentum. This can be a photon. But it doesn't have to be! A positron-electron collision can generate other uncharged particles or collections of particles, including other uncharged gauge bosons like the Z boson, and more eleborate collections of particles and particles (e.g. pair of W bosons). This was beautifully confirmed in the LEP experiment (Large Electron-Positron Collider) in the 1990s, where electrons and positrons were collided and instead of just producing very high energy photons, a large zoo of more interesting interactions were realized and measured.

• The word "annihilation" does not come from quantum field theory; it is an ordinary English word. It is used in quantum field theory and obviously has some particular technical meanings, but it generally has a similar meaning as the ordinary English meaning.
– hft
Jul 31, 2023 at 1:10
• So if an electron an positron collide, and out comes not nothing, not pure energy, but rather specific particles like a photon or (at higher energies) Z boson or something else, are you saying that this is "annihilation" at the "ordinary English" sense? It isn't. And thinking that this word doesn't have a specific, technical, sense, is what confused the OP in the first place. Jul 31, 2023 at 7:03