First, why can’t grains of space-time be smaller than the Planck length? I thought that was more a theoretical barrier to understanding than an absolute limit.
Then the article continues with a non-explanation explanation.
Can someone decode this?
Or is the whole thing just a long-winded way of saying, “we had some data which we couldn’t really do anything good with, but we played around with it this way and got a paper out of it”?
Chronos, in past threads, has told me that the Planck length/time are not a hard limits. It may be the limit us humans could theoretically ever probe to (we are a long way from even that) but there could still be smaller slices of time/space.
Sounds like a philosophical question with physics. Can we presume an infinite progression of smaller and smaller slices? Does that put us into Zeno’s Dichotomy Paradox if it is that way?
Or is the universe grainy? If it is grainy what determines the smallest slice? If not the Planck length/time then what and why?
The origin of the Planck length as a size limit is really just that beyond it, you can’t probe spacetime any more, because to do so would require energy greater than the Planck energy, which, when put into the space of a Planck length, creates a microscopic black hole, obviously destroying any chance of resolving anything smaller. (In fact, you’ll get larger and larger black holes in the attempt to resolve smaller and smaller distances.) This doesn’t necessarily imply spacetime graininess at that (or any) scale, however.
As for Rovelli’s comment, he’s a researcher in the field of Loop Quantum Gravity, in which there exists a smallest quantum of area, which is the Planck length squared, so that may be where he’s coming from.
Regarding the comment on a ‘very specific’ model of quantum gravity that was used to derive the result in the analysis, I’m not sure how to parse that – I just quickly skimmed the intro of the paper referenced as the origin of the theoretical machinery used, and I think a point of contention may be that that paper considers Loop Quantum Gravity a Lorentz invariance violating theory, which many of its proponents hold it isn’t, or not any more, at least. (Lorentz invariance, incidentally, is what’s really being probed here: it’s the symmetry that relates different frames of reference in special relativity to one another, and theories in which spacetime is ‘grainy’ are typically expected to ‘break’ it at some suitably high energy. It’s similar to how a circle drawn on a computer with poor resolution is only perfectly round – rotationally invariant – if one looks from far enough away; when one gets closer, one can ‘see the pixels’, and a small enough rotation of the circle does not result in a congruent image, i.e. it’s not a symmetry of the ‘circle’.)
Personally, I think that the question of whether spacetime is discrete or continuous is, in that form, naive; it echoes debates on whether light is wave or particle, to which the answer is: no.