The Ultimate Question - Why is there something rather than nothing?

The big problem is that "nothing" is simply a place-holder for *undefined*. And as such, you're not going to get much of a sensible answer from this angle. I'm not sure I agree, based on my point above.

there is no reasoning, i'm simply stating a fact. Or perhaps you have a definition of "nothing" of which I am unaware?

The latter Oh, I'm sorry. I'll just ask all my clients to call back later whilst I compose some longer replies.

Done.So, what is "nothing"? I guess there are layers upon layers of concepts that may be thought of as "nothing", but all of which to me, as a theoretical physicist, are certainly "something". To me, "nothing" is absence of existence. But I don't even understand what I mean by that, as I don't understand "existence". We have no knowledge nor experience of non-existence."Nothing" is something with no parameters, no properties, no asscociations - it cannot be used as a base on which to build more structure, such as the various zeros and identities of mathematics, as they are by their very nature "something" as defined by their properties.We often spend time patiently explaining to others that there is nothing before the Big Bang (in the classical Big Bang comsology), so one cannot talk about something causing the Big bang; but we don't actually mean "nothing" - others simply walk away with an idea of a big empty space - what we mean is that "before the Big Bang" is an undefined concept. It cannot be talked about because it does not exist. Similarly with ideas concerning what is "outside" the Universe.If this nothing is so devoid of properties, then we cannot make even trivial sounding claims such as "nothing cannot give rise to something" because to claim anything about this "nothing" you need some handle on its properties. And by its definition, it has none. Were it to have, then it would be something - that something that is defined by those properties.

And "things" being? Matter, metric, topology, etc, etc?Is nothing the thing you add these things to in order to get something?

We have plenty of evidence of "something". We have no evidence for "nothing". So why would we postulate that before something there was nothing?

Bingo Good too see that at least someone appreciates this rather obvious fact.Unfortunately (as Straggler indirectly inferred) Ontology tends to trail current understanding by a time period measured in large fractions of centuries.

This is a good idealisation, but what happens when your definition of "energy" breaks down? Causality is an internal property of the Universe in those areas with a well-defined time dimension. If there is a minimum time (at the Big Bang), then you have found at least one point in which there is not a well defined time dimension - and hence a point at which naive concepts of causality cannot apply. Well, you have managed to find a point for which there is no clear definiton of energy, nor a clear sense of causality. So I would have to ask: what do you mean by "created" and "destroyed"?

Funny you should say that - as soon as I had posted it, I reread it and realised (again) that the problem with "teaching" through a written forum is that all the nuances of voice and expressions are lost. My method of teaching is to encourage my students to find their own answers and help them discover their own level of understanding. Knowing what you don't know is essential to progress. So please understand that I'm trying to encourage rather than belittle. No - not necessarily. If there is no "before", then there is no change and no other reality, definable or not. I picture it from the outside, but with no definition to the outside. I also picture the Universe as a surface, not a volume, so there is no temptation to extend beyond its boundary. My most common picture, which I have shared here many times, is that of a globe. The North Pole represents the Big Bang; the circles of latitude represent spatial cross sections of the Universe at different times, with the growth of the circles as you move south showing the expansion of the Universe. At the equator I can either continue around the globe with a collapsing Universe back to the Big Crunch at the South Pole; or I can let the circles of latitude start to grow again, expanding ever outwards which is a more realistic model of our current understanding.In this picture, there is obviously no "before the Big Bang". Following the lines of longitude back in time towards the North Pole/Big Bang simply leads you back onto the same lines now going forward in time down the globe. Causality is simply a function of position on the surface. It is impossible to apply this concept to the surface itself. No - energy is not a "thing". It is simply a measurement, a check on constistency. The "stuff" in the Universe is not "made" of energy. And this energy does not need to come from somewhere else. More later...

Well, I suppose so. But as a *theoretical* physicist, I think of that as a secondary consideration. Both scenarios (and all others) need to be understood to get the big picture. The principle tool we use to study the Universe as a whole in General Relativity, and it is essentially a universe-generating machine. Slight tweaks to the parameters fed into the GR mathematics result in the different types of universe we are investigating as possible models of our own Universe. The difference between a recollapsing universe and one with undergoes accelerating expansion is exceptionally minor. Well, you touch on some intersting ideas, some of which are close to some of Penrose's thoughts - and I'll get back to those - but no, I would not talk about nothing - something - nothing.Let's take a Big Bang to Big Crunch universe. I picture a ball, with BB at one pole and BC at the other pole. The ball floats in nothingness. The nothingness sitting above the BB point is the same nothingness surrounding the ball and is the same as that sitting above the BC point. And it all signifies absolutely nothing. There is no "before" or "after". There is just the ball. All possible times (from T=0 to T=end) are contained within the ball, and from this perspective you are seeing all of them at once. This "brute fact" appeal needs a bit more definition. A universe that has always existed has as much (or as little) need of explanation as a universe that has finite "duration", with a Big Bang and/or Big Crunch. I can (and often do) picture a universe that has infinite past and infinite future using exactly the same ball floating in nothingness as described above. The question of finite/infinite extent of the space-time dimensions is a fairly minor point in the scheme of things, though you might find this hard to believe given the space devoted to such questions.

Well, the inputs are essentially the matter content and maybe some topolgical information. For example, to get the space-time around a planet, star, or other "round" object, we just specify the spherical symmetry and zero mass/energy content outside the object. GR then spits out the Schwarzschild space-time along with, if the object is sufficently dense, the event horizon, and all the interesting space-time topology of the Einstein-Rosen bridge and the other universe sat the other side of the bridge!In the case of the cosmological space-times, we specify unformly distributed matter and radiation content, and also a comsological constant term. If we set the CC to zero, then we get the traditional Big Bang scenarios of open, flat, and closed, and if we let the CC take some small positive value, then we get the accelerating expansion space-times that we are now having to consider.