It makes your head spin when you try to imagine that it’s possible for something to be and not to be at the same time in quantum mechanics.
There are cats locked up in quantum physics. In fact, they’re Schrödinger’s cats. The theory says that these animals are in a box. As long as it isn’t opened, it would seem that they are alive and dead at the same time.
These felines aren’t flesh and blood but born out of the imagination of Erwin Schrödinger, an Austrian physicist who lived from 1867 to 1961, who included them in a famous thought experiment to demonstrate a paradox. It goes as follows:
You have an atom that can emit radiation, but you don’t know when it will do so. So, as long as no one’s observing the particle, it’s in two possible states: one in which it has already emitted radiation and another in which it hasn’t yet.
The key step is that you can only know whether it’s emitted radiation or not when you make the measurement. It isn’t possible before that point.
Schrödinger then imagined the atom next to a Geiger counter so that the exact moment it emits radiation could be measured.
“The instrument would be connected to a hammer that breaks a jar filled with poison, which would be placed in a box along with a live cat,” explains Shahen Hacyan, a researcher from the Institute of Physics at the National Autonomous University of Mexico (UNAM), in an interview with Tec Review.
Remember that according to quantum mechanics, the atom would be in two possible states: one in which it’s already emitted radiation and one in which it hasn’t. Therefore, the cat would also be in two possible states: alive and dead.
“Only when we open the box can we find out if the cat is alive or not. So, that paradox has given rise to a lot of philosophical discussions,” Hacyan says.
To think that the cat is alive and dead at the same time is contradictory. It’s like saying “to be and not to be.”
In fact, if this were so, it would violate the Aristotelian principle of non-contradiction, which Hamlet made famous when he said: “To be or not to be.”
The problem with this thought experiment, according to Hacyan, is that it connects the quantum world (that of the atom) with the macroscopic world (that of the cat), which causes theoretical inconsistencies.
This is because the mathematical equations to describe both worlds are different from each other. Some (those of the atom) have to do with quantum physics and the others (those of the cat) with classical physics. That’s the problem.
“The cat is a macroscopic being composed of 10 to the 30 (1 followed by 30 zeros) atoms, and they interfere with each other and erase any quantum effects, which can manifest themselves at the level of a few atoms,” says the UNAM scientist.
According to Hacyan, experiments have already been done in which it’s possible to put an atom in two distinct positions. Although it isn’t actually seen in two places, everything makes it appear as if the same atom were in two different places.
When explaining, Hacyan refers to Einstein himself, who used the phrase “God does not play dice,” to refute the idea that quantum physics had no single deterministic explanation without the need to be based on dual probabilities.
Mind over experience
The aim of the thought experiment devised by Schrödinger was to show that Einstein was right.
In the absence of experimental demonstrations, science sometimes resorts to imaginary scenarios to support or refute a theory.
This had already been done by Galileo Galilei, who proposed thought experiments in the 17th Century to convince people that it was more sensible to imagine that Earth orbited around the Sun than to continue to believe it was fixed in place.
Practice leads to theory in a traditional experiment with physical measurements, but it’s the other way around in a thought experiment, according to Camilo Camhaji García, a mathematician at UNAM, who adds the following in an interview for Tec Review:
“It’s a reverse process. You do a thought experiment to see what scope an already given theory may have.”
So, it’s about first thinking and then experimenting, if possible. That’s what Galileo did when he imagined Earth orbiting around the Sun according to astronomical data but never seeing it directly, as there were no astronauts in his time.
“What’s more, it could be that something you imagine never actually happens. Schrödinger’s cat plays with the ability to think up hypotheses without needing to test them experimentally.”
The revolution of quantum mechanics began in the late 19th Century, when theories based on Newton’s laws that accurately predicted the result of experiments had already matured, these being deterministic theories.
“Newton’s laws tell us that if we know the position and velocity of particles at one time, then the equations will tell us precisely where they’ll be and what their velocity will be a certain time later,” says Juan Carlos López Vieyra, a researcher at UNAM’s Institute of Nuclear Sciences, in an interview with Tec Review.
However, quantum mechanics proposed an interpretation through a wave function that provides information on a particle’s position and velocity, but only in probabilistic terms.
“Quantum mechanics tells us that we can only understand the world to a certain level of probability,” says López Vieyra.
In this context of divergent interpretations, Tec Review posed the following question to the researcher: “Do you, unlike Einstein, consider that God does play dice?” This was his answer:
“Yes, God plays dice, but we can understand how he plays them. At the end of the day, quantum mechanics is also a deterministic theory, However, it doesn’t determine positions and velocities, but probabilities.”