## What is "nothing", from a scientific point of view? (Beginner)

You say that before the big bang there was nothing. My question to you is, what is nothing? Would it really be possible for nothing to exist? By definition nothing is something that does not exist. Try to describe nothing and you will always get something. From a scientific point of view could there ever really be nothing?

I'm no philosopher, but I'll take a shot...

The Universe is everything that exists. Okay, so when does something exist? Some possible definitions are, entity A exists if:

1) Given an infinite amount of time and resources, some other entity that exists can sense or imagine it.

This is the broadest definition I could imagine. It is recursive (meaning that it refers to itself). To start it off, I assume that I exist. Because I can sense you, you exist. Because you can imagine a place where pi = 99 and square root(-1) = shoe, that place exists. And so on.

2) Given an infinite amount of time and resources, some other entity that exists can sense it. (Same as #1, except I removed the "imagine" bit.)

This is probably closest to the intuitive definition we use every day. By assumption, I exist. I can sense you, so you exist. Nothing that exists, however, can ever measure the circumference of a circle and come up with 2*99*(radius of the circle), so the land of imaginary shoes mentioned above does not exist.

So, to answer your question: According to #2, there is nothing before the Big Bang. According to #1, there is something before the Big Bang, because we can imagine it, whether or not it makes sense. This, I think, is the root of the confusion about the matter. We can only describe "nothing" as it exists in another, more general, definition of existence--this is the usefulness of #1.

For your information, here is another useful definition you may run across:

3) Given the time since the Big Bang and infinite resources, I can sense it.

This Universe has a name: astronomers call it the "observable universe". It has a finite size, unlike the previous ones. (I had to remove the recursive part to make it finite. If I left it in, the following would apply: I look up at night and sense a photon, so the photon exists. The photon came from (sensed) Sagittarius A, so Sagittarius A exists. Sagittarius A sensed a photon from some more distant star, so that star exists, and so on, so I can work my way out to an arbitrarily large distance, without any information travelling faster than the speed of light. With definition 3, I can never sense anything outside a certain sphere, so nothing outside of that sphere exists. Thus the Universe is finite, and it revolves around me. But of course I already knew that.) Wow, are you really still reading?

#### Sara Slater

Sara is a former Cornell undergraduate and now a physics graduate student at Harvard University, where she works on cosmology and particle physics.