Werner Heisenberg, Erwin Schrödinger, and Georg Ohm are in a car and they get pulled over. Heisenberg is the driver and the police officer asks him:

“Do you know how fast you were going?”

“No, but I know exactly where I am,” Heisenberg replies.

The cop says: “You were doing 55 in a 35”. Heisenberg throws up his hands and shouts: “Great! Now I am lost!”

The police officer thinks this behavior is suspicious and orders him to pop open the trunk. He checks it out and says: “Do you know you have had a dead cat back there?”

“We do now, a..hole!” shouts Schrödinger.

The police officer moves to arrest them. Ohm resists.

## Explaining the “Heisenberg, Schrödinger, and Ohm are in a car” joke

### Heisenberg’s Uncertainty Principle

Werner Heisenberg’s dialogue with the police officer is a reference to the Uncertainty Principle formulated by Werner Heisenberg in 1927. The uncertainty Principle is one of the most famous aspects of quantum mechanics. It basically states that “one cannot assign exact simultaneous values to the position and momentum of a physical system.”

Momentum is the product of the mass of a particle and its velocity, and since the mass is constant for speeds negligible in comparison to the speed of light, we can translate the above proposition as “one cannot assign exact simultaneous values to the position and speed of a physical system.”

For example, let’s assume we are trying to determine the exact position and speed (actually, momentum which is mass x velocity, p = m x v) of an electron. We can do that by observing it, which means sending light (photons) onto it.

Problem is, these photon particles have a measurable mass and velocity and come into contact with the electron in order to achieve a value in its position. As two objects collide with their respective momenta, they impart these momenta onto each other. When the photon contacts the electron, a portion of its momentum is transferred and the electron will now move relative to this value depending on the ratio of their mass.

So, we lost the real speed info of the electron.

To measure a proton’s velocity (momentum), we can send photons much less frequently. We can measure its velocity by dividing the distance covered by the timespan between two photons, but the problem here is we lost the exact position of the electron. And, albeit sent less frequently, these photons will still change the momentum of the electron.

In summary, the final result of measuring an object’s position leads to a change in its momentum and vice versa.

Of course, for large objects like a car, this effect cannot be observed because it would be too tiny (or even a small pebble is too large for this). A car’s (or a pebble’s) mass is way too big than a photon’s mass. Yes, its momentum still changes because of the contact with photons, but that change would be very small, it’s practically unmeasurable. There is a huge exaggeration in the joke.

### Schrödinger’s cat

The dead cat in the trunk is a reference to Schrödinger’s cat, a thought experiment in the field of quantum mechanics. It was devised by the Austrian physicist Erwin Schrödinger in 1935 in a discussion with Albert Einstein. Schrödinger, one of the founders of quantum mechanics, posed this famous question:

“If you put a cat in a sealed box with a device that has a 50% chance of killing the cat in the next hour, what will be the state of the cat when that time is up?”

Schrödinger’s cat illustrates a paradox of quantum superposition. Quantum superposition is a fundamental principle of quantum mechanics that states that a particle can exist in multiple states or locations simultaneously. This means that, for example, a quantum particle such as an electron can be in two places at once, or can have two different properties (e.g. spin) at the same time.

The superposition principle is responsible for many strange and seemingly paradoxical quantum phenomena, such as interference and entanglement. When a particle is observed or measured, it collapses into a single state, choosing one of the multiple possibilities defined by its superposition.

Back to the cat paradox. Common sense suggests that at the end of the hour, the cat would be either dead or alive.

But, Shrödinger goes one step further: in his thought experiment, a cat, a flask of poison, and a radioactive source are placed in a sealed box. If an internal monitor (e.g. Geiger counter) detects radioactivity (i.e. a single atom decaying), the flask is shattered, releasing the poison, which kills the cat.

Now, because of the quantum superposition, the atom can be in multiple states: decayed, or not decayed. We can’t really be sure until we look. Hence, since the release of the poison depends on the atom decaying, the cat can be both alive, or dead. We can’t know until we look at the box.

This poses the question of when exactly quantum superposition ends and reality resolves into one possibility or the other.

### Ohm’s law

Ohm’s resistance to the police officer is a reference to Ohm’s law.

Georg Ohm (16 March 1789 – 6 July 1854) was a German physicist and mathematician. As a school teacher, Ohm began his research with the new electrochemical cell, invented by Italian scientist Alessandro Volta.

Using equipment of his own creation, Ohm found that there is a direct proportionality between the potential difference (voltage) applied across a conductor and the resultant electric current. This relation is called Ohm’s law, and the ohm, the unit of electrical resistance, is named after him.

V = I x R

Above is the formulation of Ohm’s law, where I is the current through the conductor, V is the voltage measured across the conductor and R is the resistance of the conductor. More specifically, Ohm’s law states that the R in this relation is constant, independent of the current.