In the theoretical system constructed by science, we do not belong to this material world. We are not in this world, outside the world, we are only observers of the world. And the reason we think we're in this world is that our body is in this world, not just my body, but the friends around me, the cats, the dogs, the house, and all the creatures in this world, and that's the only way I can communicate with them.
— Erwin Schrödinger (1887–1961)
Figure 1 Schrödinger equation
There are many time-honored thought experiments in quantum mechanics, most of which are used to point out flaws in quantum mechanics. Erwin Schrödinger, a theoretical physicist and one of the pioneers of quantum mechanics, conceived of such an experiment of thought:
Keep a cat in a closed container containing radioactive sources and toxic gases. Radioactive sources have a certain chance of decaying per unit of time, and when the radioactive source decays, toxic gases are released and the cat dies; if the radioactive source does not decay, the cat survives.
The Copenhagen interpretation in quantum mechanics states that the properties of a physical system are not deterministic, that the properties of a system can only be measured in terms of probability in quantum mechanics, and that the behavior of the measurement has an impact on the system, causing the set of probabilities to shrink to one of many possible values, a situation known as wave-function collapse.
For example, before you look at the moon, the moon is an arbitrary state, such as a full moon, a half moon, a new moon, etc. But as soon as you look, the moon collapses into a possible state. Therefore, observation plays an important role in quantum mechanics. Consider the thought experiment above, which means that after a while, the cat is alive and dead at the same time. When you look inside the box, you see in an instant whether the cat is alive or dead, not both dead and alive.
The question here is when will the quantum superposition end and when will it collapse into one of the possible states? Quantum superposition does not apply to large objects, such as cats, because living things cannot live and die at the same time. Thus, Schrödinger decided that the Copenhagen interpretation must be inherently flawed. Schrödinger's cats try to point out just how strange this explanation really is by expanding the scale of the Copenhagen interpretation.
You have a box with a cat in it that satisfies the setting of a particle in a superposition state: when the particle is in one state, the cat will be poisoned to death; when the particle is in another state, the cat will be safe.
Since the cat's survival state is determined by the state of the particles, if the particles are in a superposition state, the cat must also be in a superposition state.
Fig. 2 Copenhagen interpretation
According to the Copenhagen interpretation, cats are actually both alive and dead, and when you open the box and look at it, the cat is in a state. Here's an important question about observing: Are cats observers?
In order to understand this, it is necessary to first understand the concept of "observer". The observer in quantum mechanics is closely related to the observer effect – in which the observed behavior necessarily interacts with the observed object, influencing the object's properties in the interaction. The cat has the same right to be called an observer as the person who opens the box, because it must be able to tell if a particle has decayed based on whether there is a toxic gas. The cat will continuously observe the air inside the box and will permanently collapse the vial containing the toxic gas into a "broken" or "complete" state.
This is not as easy to understand as it seems.
Here, both the person who opens the box and the cat inside the box can be seen as either an observer or a non-observer. According to this logic, can the bottle that releases the poison gas be counted as a measuring device for survey decay, so can it also be regarded as an observer or a detector? Isn't it constantly collapsing the wave function of the radioactive isotope? At which point of the chain reaction the superposition is broken?
This begs another question: What exactly is an observer? Can any object be considered an observer? If everything is an observer, why do we see quantum effects?
To solve this problem, the American physicist Hugh Everett proposed a beautiful interpretation called the Many-Worlds Interpretation of Quantum Mechanics, which was first published in his doctoral dissertation in 1957.
Figure 3 Hugh Everett (second from right) (from left to right: Charles Misner, Hale Trotter, Nils Bohr, David Harrison)
The interpretation holds that the general wave function is objectively real, and there is no wave function collapse. This means that all possible results of quantum measurements are physically implemented in some "world" or universe. Thus, this strange idea looks at the whole thought experiment from another perspective, showing that the "Schrödinger's cat" paradox in quantum mechanics is a series of events, each of which is a branch point. Even before the box was opened, cats were alive or dead, but these "living" or "dead" cats were real in different branches of the universe, but not related to each other.
Figure 4: The separation of the universe due to two superimposed and entangled quantum mechanical states
In 1935, the Austrian physicist Erwin Schrödinger, the Nobel laureate in physics, pointed out that there is an illogical contradiction between the nature and behavior of matter at the microscopic level and the nature and behavior of matter observed at the macroscopic level that ordinary people think is visible to the naked eye.
Figure V Erwin Schrödinger
For many years, Schrödinger's cat has been used as an analogy to illustrate the differences between emerging theories in quantum mechanics. In the theory of multi-world interpretation, for example, cats are both dead and alive, because observers and cats represent two kinds of reality: one cat is dead and the other is alive. Schrödinger himself is said to have said in his later years that he wished he had never seen the cat.
Particles and waves are equally important when observing a very microscopic world, and at this scale we cannot judge the laws of motion of things by known experience... All experience is wrong, and there are no similar physical examples to analogize what is happening inside an atom. At this time, the atom is like an atom, and there is no other metaphor.
—John Gribbin
Author: Physics History
Translation: Nuor
Reviewer: zhenni
Original link:
https://www.cantorsparadise.com/schr%C3%B6dingers-cat-the-most-bizarre-thought-experiment-in-physics-94fb3b0e962b
Science and technology have a very important impact on the development of human society. From the initial manufacture of tools, the construction of houses, to the formation of villages and cities, from simple means of transport to airplanes and spaceships, from simple words to a variety of languages, from primitive air shouting, to high-speed and convenient modern communication, from looking up at the stars, to deep exploration of the universe, all of which are closely related to the development of science and technology.
The translated content represents the author's views only
Does not represent the position of the Institute of Physics, Chinese Academy of Sciences
Edit: zhenni