Lawrence Krauss said this on an Australian Q&A programme.
"...when you apply quantum mechanics to gravity, space itself can arise from nothing as can time..."
Can you elaborate on this please?
It's hard to search for!
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The point is that spacetime can be emergent, i.e., you don't put it in by hand from the start, but it sort of pops out at you along the way.
Sometimes the dynamical degrees of freedom (the variables of the theory, if you like) can be different depending on how you describe the system. The best known example is string theory, where you start with an ordinary conformal field theory in 2 dimensions (no gravity, not really space-time per se), and out pops a 10-dimensional theory of quantum gravity. Oh, and stare at it from another end, and it looks like a theory of infinite dimensional matrices (no space, just time).
The AdS/CFT duality is another, related example, where on one hand you have a field theory in 4 dimensions, but no gravity, and on the other hand you have a theory in 5 dimensions, plus gravity, and you're really just looking at two parts of the same elephant.
There is also loop quantum gravity, causal dynamical triangulations, etc., but these are not as well established nor as successful.
The relevant search terms are induced gravity, emergent gravity, emergent spacetime, etc.
The word "nothing" here is used somewhat loosely. What Krauss means by nothing is no particles. But, there is still the vacuum state of the quantum fields, which is something. And those vacuum states can give rise to phenomena out of "nothing".
This is not so much an answer as a comment and caution: ex nihilo nihil fit (from nothing nothing is produced).
It is important, however, to recognize what a physicist may mean by the word nothing. Some physicists, such as Lawrence Krauss, define nothing as an unstable quantum vacuum that contains no particles. This is incompatible with the philosophical definition of nothing, since it can be defined by certain properties such as space, and is governed by physical laws. Indeed, many philosophers criticize these physical explanations of how the universe arose from nothing, claiming that they merely beg the question.
Think of the uncertainty principle. According the the uncertainty principle, over a period of time energy fluctuates and combining this notion with relativity, this translates to virtual particles popping in and out of existence from the 'nothingness' of the vacuum. If we have a quantum theory of gravity (which we don't yet), space and time can behave like virtual particles and become quantum mechanical entities and hence pop in and out of existence due to the uncertainty principle.
We still don't know how to combine general relativity with quantum mechanics. Such a stark incompatibility between the two arises because of the Uncertainty principle; in general relativity, perfectly flat space arises in the absence of a significant mass (such as the quantum vacuum) and thus the value of the gravitational field should be exactly zero. However, the Uncertainty principle needs only a mean value of zero and so the value of the gravitational field can fluctuate in a random 'foam'; therefore we get infinities (or ultraviolet divergences). Quantum gravity is currently nonrenormalisable
You are probably expecting an answer about mutual particles and anti-particles spontaneously appearing in the quantum vacuum. However, philosphically, 'nothing' has been described as "the purest indeterminate possibility of everything possible"
, which is, in the ordo cognoscendi, prior to the quantum vacuum.
As for the notion of everything accumulating from spontaneous manifestions in the quantum vacuum, I would suggest you watch Roger Penrose's lecture "Before the Big Bang?" on YouTube, or read up on it, eg
. His "smooth" state of the universe between Big Bangs is suggestive of the quantum vacuum, (though I expect not the same). Penrose's theory proposes an alternative to one-time spontaneous manifestation, in that his model is cyclic, so whatever exists (whether jagged or smooth) may have always existed, and therefore has only truly come from 'nothing' in the philosophical sense.
"As the universe nears the end of its expansion, the remainder of its black holes will evaporate or gobble one another up, thus setting things back into a state of order. During this period, the universe would begin to revert back into a similar state it was in at the big bang, when the geometry of spacetime will become smooth again, which is in stark contrast to its current jagged form. According to Penroses' model, this will usher in the new aeon, AFTER the universe is no longer able to expand any further. So it collapses back in on itself as a highly ordered system. One that that is ready to trigger the next "big bang.'"
Space and time, as ordinarily understood, came into being after the Big Bang and after the temperature of the universe decreased to the point that the particles (matter) created could not revert back to energy.
When two "permanent" particles were created, space (distance between the particles) was created, and the relative motion between them, created time.
Even going back to Newton, space and time are the consequences of measurement.
From the Principia's Scholium
Relative time is a measure of duration by the means of motion; Relative space is a measure of the absolute spaces determined by the senses.
So they came from measurement. Not from nothing.