Planck Time

Discussion of theories involving time as a dimension, time travel, relativity, branes, and so on, usually applying to the "real" universe which we live in.

Planck Time

Postby Jordan14 » Wed Jan 12, 2005 9:08 pm

I have discussed this many times but I have never got bored with it. The existance of both Planck Time and Planck Length.

I suppose this is a bit out of line in the relitivity forum but I just wanted to know what everyone else thought about discontinuosu time and length, and discontinous dimensions as a whole. :D
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Postby houserichichi » Wed Jan 12, 2005 9:31 pm

It's in line with relativity, as GR requires continuous spacetime while QM requires discrete. Unfortunately, IMO, these kinds of contradictions won't be answered until a proper quantum theory of gravity can be brought about so all we can do for the moment is speculate at the implications each would have on the other theory.

A quote from Brian Greene for all you string-lovers
Yes, so a real key question is, is space continuous or is it discrete, and what do I mean by that? If you take this piece of material, on everyday scales, it certainly looks continuous, right? But we know that if we look at it under a microscope, it's made up of individual threads. So if you wanted to imagine a microscopic ant walking on the fabric, if it was really tiny, it would have to leap from thread to thread to thread, even though you think, naively that it could continuously traverse the surface of this tablecloth. Could that idea be true of the universe? Namely, could space appear to be smooth and continuous, but if you examined it with adequate precision, could it be that space itself is more like a grid, where a microscopic entity, to move in space, actually needs to jump from grid line to grid line to grid line. We don't know if that picture is true or not. This is one of the open questions in physics quite generally, not just in string theory. String theory does give some indirect evidence that there should be a smallest length that you can possibly imagine. Could it be that that smallest length is really just the size of the grid? Could be, we don't know yet. We're investigating this, and more generally, what many of us are trying to do is work out whether we can pinpoint what those threads in space are.


Conseptually the Planck time and length exist, it's just a matter of whether we can do any measuring at those scales.
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Postby Jordan14 » Thu Jan 13, 2005 9:03 pm

I love the quote :D , and I have to admit that I am a String Theory lover and in context with String Theory it seems to make much more sense.

I think for any kind of proof however we will have to wait to 2007 at CERN. But I'm afraid that nothing will be found :?

A bit off but does anyone know about temperature being vontinous or discrete. I suppose if it exists in discrete time it has to be itself dicrete.

Any Opinions?
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Postby houserichichi » Fri Jan 14, 2005 12:44 am

Well temperature is the change of internal energy versus entropy of the system. At the quantum level energy is not continuous, so one would infer that temperature is also discrete, though as a whole it appears otherwise.
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Postby Jordan14 » Sat Jan 15, 2005 9:35 am

However anything that uses energy in a constructive way is going against the entropy of the system so in effect everything at a Quantum level is dicrete which make sense because in Quantum EVERYTHING is nonsense. :D
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Postby jinydu » Mon Jan 17, 2005 11:22 pm

Actually, according to what I've learned about Quantum Mechanics, the theory doesn't require that space-time is discrete. All it says is that in a system with appropriate boundary conditions, the energy levels are discrete. In fact, it is incorrect to say that QM claims that everything is discrete. QM requires the wave function of a particle/wave to be continuous (many equations in QM are derived based on that assumption). Since the wave function gives the probability density of finding a particle at a particular place and time; this would make little sense unless space-time were continuous. Its only later theories that attempt to replace QM that claim that space-time is discrete.
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Postby wendy » Tue Jan 18, 2005 11:18 pm

Planck devised an electrodynamic system of measures, based on G and h-bar. The units of this system implied a unit of length, mass and time that were mismatched.

None the same, because the principles of uncertianty says we cant exceed measure accuracies of a poduct < h, we have the basis of quantum gravity.

By the varieties of Einstein-relativity, we have gravity = curvature of space-time, so the planck measures move into the qunatum-nature of the space-time itself.
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Postby Jordan14 » Fri Jan 21, 2005 7:11 pm

Below the level of Plack Time does exists time but not in the sense that we know it because the time below 3.3+10^-42 secs (I think that's the right value) follow Quantum Laws in the sense that time is unpredictable. So when I say discrete I mean discrete in the sense that below the Planck Time boundary time is nonsense, that is why Planck Time is the smallest value of time. :lol:
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Postby jinydu » Sun Jan 23, 2005 1:52 am

Jordan14 wrote:Below the level of Plack Time does exists time but not in the sense that we know it because the time below 3.3+10^-42 secs (I think that's the right value) follow Quantum Laws in the sense that time is unpredictable. So when I say discrete I mean discrete in the sense that below the Planck Time boundary time is nonsense, that is why Planck Time is the smallest value of time. :lol:


You have to be careful there. As I said in my previous post, QM, as originally formulated by Schrodinger, Bohr, etc. doesn't require space and time to be continuous, just matter and energy. In fact, Schrodinger's Equation requires taking the derivative with respect to time and position, which implicitly assumes that time and space are continuous.

Despite the great success of "conventional" QM at predicting things like atomic spectra and the properties of subatomic particles, it became clear that not everything was well. The way QM looks at the universe is very different from the way General Relativity, the other highly successful theory of the 20th century, looks at the universe. Furthermore, in the second half of the 20th century, the Big Bang theory came to be accepted, and this brought the problem that at the instant of the Big Bang, the Universe was predicted to be infinitely hot and dense, which caused problems. Also, in their never-ending quest for simplicity, some theoretical physicists looked for a way to unify all four known fundamental forces: gravity, electromagnetism, the strong nuclear force and the weak nuclear force. Attempts to unify electromagnetism and the weak force at high energies were ultimately successful; during an experiment in the 1980s, it was shown that unification does indeed occur at sufficiently high energies. But this wasn't enough for physicists; they wanted further unification with the strong force (known as grand unified theories) and, most challenging of all, gravitation (known as theories of quantum gravity, or more the more grandiose term, Theories of Everything).

As a result, theoretical physicists searched for decades for a solution to these solution. Several theoretically plausible theories have emerged such as string theory. In some of theories, thorny issues are resolved by postulating that there is an upper light to energy. Now, Heisenburg's Uncertainty Principle states that:

(delta E) * (delta t) >= h/4pi

where (delta E) is the uncertainty in energy and (delta t) is the uncertainty in time. If there was an upper limit on energy, this would imply an upper limit on the uncertainty in energy. By the Uncertainty Principle, there would then be a lower limit on the uncertainty in time, which would mean that intervals of time shorter than that lower limit (called the Planck time) would be uncertain. The Planck time multiplied by the speed of light would then give the Planck length, below which measurements of length would be unreliable. This solves some theoretical problems; for example, if we assume that this is true, then we remove the singularity at the Big Bang where infinite temperature and pressure occur. But since we've assumed that space and time are discrete, there can't really be a t = 0 moment, the earliest moment possible is just the Planck time.

This is all nice on paper, but is it a correct explanation of the way the Universe works? Theories like this make few predictions that are testable with today's technology, and most of the few predictions that are testable haven't been confirmed experimentally. For example, many grand unified theories predict that protons are not eternal, but have a very long half-life (I don't remember the exact figure, but its over 10^30 years) and eventually decay into other particles. Since then, giant tanks of water have been placed underground for, and the quantity of water has been measured by instruments precise enough to detect the decay of a single proton. So far, no confirmed cases of proton decay have been observed. This doesn't necessarily that all those grand unified theories are wrong, since some of them predict a half-life that is so long that even those experimental results don't rule them out.

As I've already said, another prediction is that there is a smallest unit of time and length, below which time and length as we know it don't exist. But are these claims correct? The Planck Time and Planck Length are far too small for almost all of our experimental techniques to detect. But a few attempts have been made. Here is one from a few years ago:

http://uahnews.uah.edu/read.asp?newsID=99

To sum it up, the researchers made observations that, according to their calculations, should have turned up evidence for the existence of a Planck Length. No such evidence was found, which casts doubt on the existence of the Planck Length. This is not to say that all scientists agree with them. But it does mean that the quantization of space-time shouldn't be considered as certain as, say, the Schrodinger Equation or the constancy of the speed of light.
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