5.3912×10^−44 seconds.
Plank Time
There are more units of plank time in a single second... than there are seconds that have ever passed since the beginning of time as we understand it. (4.3478x10^17 seconds since the big bang)
*shudder*
Normally? It's a big numbers that give me that shiver, but this?
It's too damn small! It's so small it's creepy.
#physics , #bigNumbers , #time , #plankUnits
Planck time and planck length are very small, but the planck mass, defined as:
sqrt( hbar*c/G )
is surprisingly mundane. It's about the mass of a grain of sand.
@catselbow @futurebird That feels *so* wrong - I got the impression from a book (by Sean Carroll maybe, or Carlo Rovelli) that the Planck dimensions, or some of them, were at the scale for which physics breaks down if you go any smaller. I’ll have to go back and read the again, maybe it only applies in certain cases…
@MichaelPorter @catselbow @futurebird Yeah, but in the case of the Planck mass this is the mass of a single fundamental, point-like particle at which our theoretical understanding breaks down (because you need to apply both quantum field theory and general relativity at the same time, which we don't really know how to do). Viewed in that lens it is huge, compared to something like an electron.
@MichaelPorter @catselbow @futurebird Oh yeah, no issue with your questions at all. Actually, the only problem is that most of my professional work in Physics was in non-relativistic quantum physics/quantum information, so this topic isn't entirely in my wheelhouse. If we're lucky, someone better prepared may chime in.
So without getting myself into too much trouble speaking on things I haven't thought enough about, I'd make a few points:
1. AFAIK, Planck originally just came up with these as a set of natural units, ones that arise from only fundamental constants, not reference to specific objects, etc. And I think he came up with these before general relativity was formulated, so the phenomena that are now discussed in relation to them are not ones he would have known about.
2. OTOH, you might reasonably expect that if these ratios make quantities with these specific dimensionalities there would be at least *some* physical phenomenon that would be characterized by that scale, but not necessarily that it should be especially interesting.
3. We haven't measured phenomena anywhere close to these Planck scales (length, time, or single-particle energies), so our theories could easily be wrong way before you get there, but these correspond to places we're pretty sure there *must* be something else because our existing theories don't make sensible, internally-consistent predictions there at all.
4. You're right that it's not as though every mass is a multiple of the Planck mass (obviously); it's not a smallest unit of mass. It's perhaps more reasonable to think of it as a dividing line between the least massive black hole it makes sense to consider and the largest non-black-hole, point-like particle it makes sense to consider based on our current understanding of physics. So if you give single point-like particle (like an electron) that much energy then whatever happens requires new physics to describe, since it's at the boundary where two incompatible concepts meet.
5. There are other places where we see inconsistencies are that not directly related related to the Planck scale. The example of this I'm most familiar with are questions that arise from the black hole information paradox and related problems. Here the fundamental issue is that stuff falling into a black hole seems to be an irreversible process that destroys information whereas quantum mechanics demands that all processes are reversible and conserve information. Unfortunately, since the questions here relate to what happens at the event horizon of a black hole and what happens when it evaporates (something that would only happen in the very late universe for ordinary black holes), these are also not very accessible to experiment/observation. Some apparent, purely theoretical progress has been made in this area, though.
@internic @catselbow @futurebird Ah, okay. I like #4. (Confession, in the same way I like cats - I don’t understand them, but something attracts me 😄).
I just got through Brian Cox and Jeff Forshaw's recent book on black holes, but I struggle greatly with ADHD, so it's not going to stick until I'm through it at least one more time. It sounded like some interesting new ideas were being developed.
@MichaelPorter @internic @catselbow @futurebird I don't know if this will help, but I once answered a question on Physics Stack Exchange on roughly this topic. https://physics.stackexchange.com/q/37521/124
@diazona @internic @catselbow @futurebird Thank you - as usual, I find my understanding grows with exposure. I can’t say I understand the finer details, but I’m getting a feeling about it 😊
@MichaelPorter @internic @catselbow @futurebird in non-GR QM, a particle can have any amount of *mass-energy*. If a point particle had this much (and, yes, it is *huge*) then you have to start thinking about GR, and we don’t know how to do that.
So: shorter than Planck length or time is problematic, *larger* than Planck mass is problematic.
@nickbarnes @internic @catselbow @futurebird The whole idea of a *point* particle with lots of mass is what is finally clarifying this for me. I missed it when @internic@goto.org brought it up the first time.
With respect to the caveat that we’re in messy QR+GR territory, I’m assuming that the concept of a point location is fuzzed out by uncertainty? Otherwise *any* point particle with mass would have to be a black hole? We must also be in Hawking radiation territory… Hmm. Okay, my brain is exploding with half-baked ideas.
I’ll spare you all the questions that are cropping up (for now) - I fear that the answer might be “Go back to school, do your doctorate in quantum gravity, and get back to us” 😄
@internic @catselbow @futurebird I guess I keep coming back to the question of *why* combining fundamental constants would necessarily produce a condition (for any of the Planck units) or particle that causes the breakdown.
If you combine these fundamental constants, you can then define a “fundamental” length unit, or time unit, or mass unit. The length and time units are extraordinarily small – but would we run into the QM/GR issue before we got to that small a scale? And given how large the Planck mass is compared to fundamental particles, it’s clearly not an indivisible mass - so why assume a particle must exist with that mass?
You’re the theoretical physicist, and I don’t even play one on TV, so please take these questions as respectfully coming from someone looking to learn, not as any kind of criticism.