In the September 2002 Scientific American, Plank time is briefly mentioned. It said that the Plank time is 10[sup]-43[/sup] second and that it is believed to be the possible shortest duration. Could someone please explain why this is?
Haj
Well, do we also think that the Planck distance is the smallest distance, and the Planck mass is the smallest mass?
No.
I’m inclined to side with RM Mentock on this one.
One can make some gestures to quantum nature, the Heisenberg Uncertainty Principle, and fooling around with units to get to the Planck time. Also, various stabs at Theories of Everything (TOEs) make use of deriving Planck units as the fundamental “pieces” of reality. As it is, Planck time and other Planck “measurements” stand as a particular formulation of a theoretical observational limit, but as far as we know it’s completely speculative because we don’t have decent observational probes for that level of precision let alone a self-consistent or falsifiable TOE.
Planck time is the time it would take a photon (travelling at the speed of light) to across a distance equal to the Planck length, which is .6 x 10**-35 m. This is point at which “classical” physics is replaced with quantum physics.
So, I think what is really meant when one says it is the shortest possible duration, they mean it the shortest before things get “quantum whacky”.
Thanks so far. If I understand correctly, Plank time is supposed to be a quantum, sort of like a quark is supposed to be the smallest particle. How was this derived? What occurs in one Plank time?
Haj
The word Atom derives from a Greek word meaning Indivisible, doesn’t it?
Is it just that we haven’t managed (even theoretically) to break anything into smaller pieces than Planck units, or is it that the universe really has some sort of pixellation effect?
Plank time is the smallest interval possible because people got bored trying to measure smaller.
Get it? Broed-BOARD… PLANK!!!
Haha I kill me…
Where’s that smashie smilie when you need it? 
Zev Steinhardt
this is a subject that chapped my butt when i read “A Brief History of Time”. basically, Hawking said (and I paraphrase):
“First, they thought substances were the smallest unit, and they were wrong. Then, they thought that atoms were the smallest unit, and they were wrong. Then, they thought that protons and electrons were the smallest unit; you guessed it, they were wrong. Quarks smallest? Wrong. Now, we think we have the smallest unit. WE are, of course, correct.”
jb
Okay, quantum mechanics works fine for large scales as it does for small scales.
GR works fine for large scales as it does for small scales…
trouble is that when you try to get GR and Quantum Mechanics to talk to each other (in a theory of everything) you will end up with some absurdities. For instance, the energy of the vacuum is naively calculated by combining constants from both theories to be just about 101 orders of magnitude higher than it is observed to be.
Bad news.
So, this is where this Planck time comes into play. It is on these scales where the universe is “grainy”. It is on these scales that string theory becomes important. Note I say “scales” and not “points” because the Planck time is just a haphazard way of putting the units together. It gives us a range estimate. It is not a fast-in-stone value the way, say, the speeed of light is.
It is a convenient conjecture about the nature of the universe, but thus far it is only a conjecture.
I don’t know that Planck Length, Planck Mass and Planck Time are absolute limits. It is possible that there are things smaller than those values.
I thought the Planck limits merely referred to the place at which we mere mortals haven’t a clue how to describe what is going on. Of course, at our present understanding things get funky for us long before the Planck limits are reached but supposedly we will never be able to peer byond the Planck limits no matter how good our technology is (sort of another concept like the Heisenberg Uncertainty Principle).
It’s mostly what Whack-a-Mole said: With current theories, we have no hope of describing things that happen at the Planck scale(s). This does not necessarily mean that we will never understand those scales, but to do so, we’ll need (at the least) a quantum theory of gravity. The problem is that quantum mechanics and General Relativity (Einstein’s theory of gravity) appear to be fundamentally incompatible. This is not a problem, for instance, if you’re talking about a stellar-mass black hole: Quantum mechanics is almost completely insignificant for such an object, so you can use pure GR. Similarly, it’s just fine if you’re talking about a hydrogen atom: Gravity is irrelevant there, so you can use pure QM. The thing is, though, that whenever you’re doing QM, the constant hbar (Planck’s constant) is significant, and whenever you’re doing gravity, the constant G (Newton’s constant) is significant. The Planck units are constructed from a combination of G and hbar, so whenever you’re doing physics at a Planck scale, both gravity and quantum mechanics are significant, and we don’t know how to handle that.
It’s interesting to note, by the way, that the Planck units are not always a lower limit to our understanding. The Planck mass, for instance, is about a microgram, comparable to the mass of a bacterium. When you’re doing particle physics (quantum mechanical), that’s an upper bound on what you can understand, but when you’re doing gravity, that’s a lower bound. But you can do Newtonian physics just fine at that scale.