## Quantum-like Collisions Let me show what kind of strange things can happen in quantum mechanics, with a little artistic license using everyday objects as stand-in for quantum particles. ### Collision 3, again: $\mathbf{A}+\mathbf{B}\to \mathbf{C}$ >**Seriously:**

>Please write the above "formula" and draw the corresponding picture! Weird Happens in quantum mechanics. We know how to prepare initial quantum states, (a). Our particle beams do that for us. We know how to detect what comes out of a collision, (c). The detectors we build do that for us. It’s what happens at (b) when the collision actually happens that’s interesting and because quantum mechanics---and relativity---are involved. Here’s a quantum mechanical version of the glued-billiard balls:

TIME GOES FROM TOP TO BOTTOM IN THREE STEPS: What you put in may not be what you get out of a quantum collision.

A little different in two ways. First, what comes out is not a recognizable combination of $A$ and $B$, but something else entirely. Second of all, what happens at (b) is now the question:

Modeling how quantum particles interact when they're near one another is one of the goals of particle physics.

### Collision 4: $\mathbf{C} \to \mathbf{a} + \mathbf{B}$ >**Seriously:**

>Please write the above "formula" and draw the corresponding picture! A more common situation is sort of the opposite of Collision 3:

TIME GOES FROM TOP TO BOTTOM IN THREE STEPS: Collision 4: $\\mathbf{C} \\to \\mathbf{a} + \\mathbf{B}$, a quantum particle decay.

Here we've got a quantum particle sitting there minding its own business (the bowling ball) when suddenly it *disappears* and two (or more) *entirely different* quantum particles are created…seemingly out of nothing. This was quite the shock when "decays" like this were observed in the 1890's and we'll talk about some of these discoveries. Now we know how to describe this process but surprising decays are often the signature of brand new kinds of physics. A firecracker going off is the closest we to a quantum decay that have in everyday life. But firecrackers don't explode on their own and the results are not only two or a few objects. ### Collision 5: $\mathbf{a}+\mathbf{B}\to \mathbf{c}+\mathbf{D}$ >**Seriously:**

>Please write the above "formula" and draw the corresponding picture! Collision 5 is a standard quantum collision of two initial quantum particles which collide—doing something in the intermediate (b) state—and then completely transforming into two or more entirely different objects, (c).

TIME GOES FROM TOP TO BOTTOM IN THREE STEPS: Collision 5: $\\mathbf{a}+\\mathbf{B}\\to \\mathbf{c}+\\mathbf{D}$, a very standard quantum particle collision.

There is no everyday analogy for this. It's as if our safety and running backs collide and become a baseball pitcher and an Oldsmobile. Creating an obsolete GM car would be a surprise indeed, but that's how we discover new particles and study unusual forces of nature.

Quantum collisions can take ordinary matter and produce entirely different states of matter.

```{admonition} Please answer Question 1 for points: :class: danger What kind of collision is this? ``` ```{admonition} Please answer Question 2 for points: :class: danger What kind of collisoin is...this! ``` ```{admonition} Please answer Question 3 for points: :class: danger And...what kind of collision is this. ```