If quantum physics states matter doesn't exist when it's not being noticed, then, without life, would the universe cease to exist? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.

If quantum physics states matter doesn't exist when it's not being noticed, then, without life, would the universe cease to exist?

This question demonstrates the dangers of trying to comprehend modern physics from popularizations.

It is absolutely not true that states of matter do not exist when they are not noticed. If someone tells you this, he is talking nonsense.

The correct statement is that a quantum system usually exists in a state that is not at all like the states of the classical world. Simply put, it cannot be described by ordinary numbers.

This is in contrast with classical objects, which can be described with ordinary numbers. That is to say, their position, their velocity, and other physical properties can be described using simple numerical values.

Now it is true that there is no such thing as a truly classical object: that even people, cats, cannonballs or planets are made of a large number of quantum particles. However, in these systems, all the quantum behavior is averaged out, so to speak, so their behavior is indistinguishable from that of classical systems.

Every once in a while, some properties of a quantum system may be describable by ordinary numbers. When a quantum system is (briefly) in such a classical state, it is said to be in an eigenstate (“eigen” means “own” in German); the value of the property would be the corresponding “eigenvalue”.

This is precisely what happens when a quantum system interacts with a classical system; i.e., when we “measure” something. During the interaction, the quantum system is confined to an eigenstate.

Let me explain this through a common thought experiment: the infamous two-slit experiment. The experiment involves an electron gun that emits electrons which can go through one of two slits before impacting a screen. We find that an interference pattern emerges on the screen; this is true even when we fire the electrons one at a time. You may have heard that this means that every single electron goes through both slits. True, but there is a better way to understand this. When the electron finally arrives at the screen, the screen (a classical object) measures the electron’s position: the electron is now in a “position eigenstate”. En route, however, the electron is not in a position eigenstate. It does not have a position in the classical sense at all. So yes, it goes through both slits and does many other things that classical objects just cannot do (and it is wrong even to try to visualize the electron when it is in this quantum state). But it does not mean the electron’s existence is conditional. It exists all along… its position is just not describable by ordinary numbers. Nor is there any need for consciousness to be involved. The fluorescent screen has no consciousness at all, but it does confine the electron to an eigenstate because it itself is a classical object (or so close to being classical as to be indistinguishable from it).

So no, the existence of the universe does not depend on the presence of life. And many things in this universe are in eigenstates every once in a while simply because of the way they interact with other things which are, for all practical intents and purposes, classical.

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