# Plank time, Plank length

## Main Question or Discussion Point

Doesn't this digitize the Universe?

Does 1 Plank Time + 1 Plank Time = 2 Plank time?

Does 1 Plank length + 1 Plank length = 2 Plankies?

## Answers and Replies

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Maybe, but then where are the gridlines? Where is the origin? What happens when you transform your coordinate system and get irrational fractions of either a planck length or planck time?

Those values, as of now, are useful for considering where classical physics breaks down, but they're not yet the basis of any accepted quantum theory of spacetime since there is none. :)

Alfi
I'm amazed at the gap in scale from Plank size to, say sub atomic partical size.

Quarks are HUge ! :)

Quarks are HUge ! :)
How big are quarks?

pam
Doesn't this digitize the Universe?

Does 1 Plank Time + 1 Plank Time = 2 Plank time?

Does 1 Plank length + 1 Plank length = 2 Plankies?
Planck time and Planck length are just convenient (or inconvenient) sizes that imply no fixed discrete values or limits.

Quarks are believed to be point particles, so they have no volume per se. They are, however, very far apart relative to the Planck scale.

nrqed
Homework Helper
Gold Member
Doesn't this digitize the Universe?

Does 1 Plank Time + 1 Plank Time = 2 Plank time?

Does 1 Plank length + 1 Plank length = 2 Plankies?
In themselves, Planck units are just that: units!!

What you are asking is exactly the same as asking "Does one second + one second
= 2 seconds?" Of course!! But so what?

Now, some believe that the Planck units also represent the smallest allowed units of time, space, energy. But that's a whole different issue and it is speculative.

Well, I took the op's question to mean is spacetime divided into pixels like a computer momitor on the order of the Planck scale?

If the smallest possible distance between two objects is the PL, that's all well and good, but can the objects be 1.5 PL's apart? No current theory says they cannot, hence the "grid" analogy doesn't work.

LURCH
The theory is that PL is in fact the shortest length that can exist. So no, two objects cannot be 1.5 PL's apart, as it would require the existence of a ".5 PL". If that theory is correct, then yes, this "digitises" the universe.

And, like I said, what happens when you rotate your coordinate system and have irrational fractions of PLs? The "theory" has way too many problems to even be called a theory.

xantox
Gold Member
Doesn't this digitize the Universe?
Well, I took the op's question to mean is spacetime divided into pixels like a computer momitor on the order of the Planck scale?
The theory is that PL is in fact the shortest length that can exist. So no, two objects cannot be 1.5 PL's apart, as it would require the existence of a ".5 PL". If that theory is correct, then yes, this "digitises" the universe.
The answer is no : this does not imply that spacetime must be a "chessboard". It may be for example a "quantum chessboard", which is entirely different. As a proof, one may show a theory implying the opposite conclusion, I would suggest for that Wheeler-DeWitt cosmology, where the fundamental Planck length coexists peacefully with continuous space and time.

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A very basic question about planck length (and time). Are these universal constants like the speed of light which appear the same in all inertial frames of reference, and if so does this mean that someone travelling with very high velocity wrt me will appear from my perspective to have a different scale for their planck length and time regarding their quantum processes? Or are the units of plank time a universal unit counting some universal tick, which as a consequence of being universal would appear to be of different lengths in different referential frames, perhaps permitting one to define a special stopped referential frame? How irrespective of the answer does this affect the physics at the quantum level of "stuff" moving at a relative velocity to other "stuff"? Does relative velocity have to be factored into the math behind the physics?

xantox
Gold Member
A very basic question about planck length (and time). Are these universal constants like the speed of light which appear the same in all inertial frames of reference
It is not known, but it may well be that the Planck scale is invariant. If that was the case, then special relativity should be either modified at this scale (eg DSR, controversial) or cease to apply, or quantum effects avoid that observers are able to measure contracted Planck lengths.

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You should look up something called "extra special relativity" or "scale relativity." It postulates that the minimum distance, like the maximum velocity, is observer-independent and invariant, and is the Planck Length. There is no "chess board", but there is simply the notion that no distnace measurement in any reference frame (regardless of Lorentz transforms) can yield a value of less than the planck length, and no time measurement less than the planck time. One paper purports to derive the heisenberg uncertainty relations and the Schrodinger Equation (and hence virtually all of QM) from this postulate, though I do not know how accepted the derivation is. And it also makes predictions of the ratio of electron mass to charge. Very interesting stuff.

Thanks, I will check out the paper.

I think the reason Planck units are not more widely used is that the units of length, mass, and time are too small for ordinary use. To write measurements in Planck units would require scientific notation, not taught until 8th grade. To have body measurements running to 34 figures and weight in the billions would be unsettling, but that would happen with Planck. It would work well in AutoCad 2000, as it supports exponential (scientific) notation, but plotting would be more difficult.