• Quick note - the problem with Youtube videos not embedding on the forum appears to have been fixed, thanks to ZiprHead. If you do still see problems let me know.

Is time continuous?

andyandy

anthropomorphic ape
andyandy
Joined
Apr 30, 2006
Messages
8,377
is time truly continuous? I ask, because planck time....

The 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. This is the ‘quantum of time’, the smallest measurement of time that has any meaning, and is equal to 10-43 seconds. No smaller division of time has any meaning. With in the framework of the laws of physics as we understand them today, we can say only that the universe came into existence when it already had an age of 10-43 seconds.
Click to expand...
http://www.physlink.com/Education/As...TOKEN=68885559

....would suggest it's discrete.

It was suggested in another thread that in current theories time is continuous, but that most models of quantum gravity imply that spacetime is discrete. Is this where the fault line lies? Where does planck time come into it? Does it?
 
Basically, time is continuous at every scale we have the ability to observe. Quantum theory suggests that space-time is discrete, but at such a small scale that there is no chance of us actually verifying this in the near future.
 
Just because there's a smallest physically meaningful interval of time does't appear to imply that time must be discrete. How long does it take a photon to cross a distance equal to one and a half Planck lengths?

Unless, of course, "half a Planck length" has no physical meaning either -- if, in other words, distance (and therefore space) is also discrete.

Respectfully,
Myriad
 
Yes, that's the whole point. Planck length and Planck time are the shortest intervals of space and time possible. There is no such thing as half a Planck length. It is the same as the situation with other quantised things. For example, a particle can have angular momentum that is a multiple of h/2 (with or without a factor of 2pi depending on which book you use). It is simply not possible for anything to have angular momentum of h/4. In the same way, if you have two particles seperated by one Planck length, it is not possible for there to be anything between them, because that is the smallest distance possible.

It's kind of hard to explain or imagine really. Think of space as graph paper with a regular pattern of dots. A particle can only be on one of the dots, not anywhere in between. Essentially, the space between them doesn't actually exist.

Edit : In fact, it's really the other way around. It isn't that there being a smallest unit of time implies that time is discrete, it is time being discrete that implies there is a smallest unit of time.
 
Myriad said:
Just because there's a smallest physically meaningful interval of time does't appear to imply that time must be discrete. How long does it take a photon to cross a distance equal to one and a half Planck lengths?

Unless, of course, "half a Planck length" has no physical meaning either -- if, in other words, distance (and therefore space) is also discrete.

Respectfully,
Myriad
Click to expand...

well, this is what got me wondering - i understood that a planck length was the smallest unit of measurement of spacetime....which would seem to imply that both space and time are discrete...
 
Is time continuous?
It has generally appeared so to date, though there seem to be some gaps in the early 1980s.

I can't help feeling that if there is a minimum natural "amount" of time and we are serious about the notion of spacetime, then either the associated amount of spacetime is incredibly small , or it must be the size of the entire universe.
When one finds this scale of error in one's assumptions, it seems wisest to withdraw from the debate.
 
Myriad said:
Unless, of course, "half a Planck length" has no physical meaning either -- if, in other words, distance (and therefore space) is also discrete.
Click to expand...

Well, of course, that would be the case. Everything suggests that the notions of distance, causality, anything that depends on a metric lose their meaning at the Planck scale. You can arrive at a resolution limit for distance measurements within String Theory, Loop Quantum Gravity, etc. Basically, it seems that
  • the uncertainty principle.
  • c finite and constant.
  • the equivalence principle.
put together imply this discreteness.

I'll give a simple argument to show this later.
 
Cuddles said:
Yes, that's the whole point. Planck length and Planck time are the shortest intervals of space and time possible.
Click to expand...

This is one supposition, but quantum mechanics actually makes no such suggestion. The planck scale issue is a question of what happens at the intersection of quantum mechanics and general relativity, and we really don't know the answer.

Quantum mechanics has a length scale (the wavelength of a photon with that energy) associated with any energy/mass, which gets smaller when the mass gets larger. If the size of your system is on the order of this length scale (instead of much larger, as everyday objects are), you can't treat it classically and ignore quantum effects.

General relativity likewise has a lengthscale (the Schwarzchild radius) associated with any energy/mass, which gets bigger when the mass gets larger. If the size of your system is on the order of this length scale, you can't ignore general relativity.

So if one length scale is getting larger and one is getting smaller as we increase mass, then there must be some point at which the two scales cross. The planck mass is the mass at which these two lengths coincide (which is in turn the planck length). If your system has a planck mass and is on the order of the planck length, then both quantum effects and general relativity should be important. And at that point, we don't know what to do because the two theories aren't compatible. Quantizing space and time at this scale is one proposed idea of how to deal with the problem, but the truth is that we just don't know. The resolution of this problem might have nothing to do with quantizing space and time.
 
If you want my opinion, discrete/quantized spacetime probably holds the key to quantized gravity; when we see that gravity itself is a matter of curved spacetime, it becomes intuitively clear that gravity quanta imply spacetime quanta. Proving it mathematically and then devising testable hypotheses is the big barrier.
 
andyandy said:
is time truly continuous? I ask, because planck time....


http://www.physlink.com/Education/As...TOKEN=68885559

....would suggest it's discrete.

It was suggested in another thread that in current theories time is continuous, but that most models of quantum gravity imply that spacetime is discrete. Is this where the fault line lies? Where does planck time come into it? Does it?
Click to expand...

Great question, I've been wondering the same thing since I read about this the theory that spacetime is quantized.

An quantized spacetime is much more intuitive to me. In continuous space, there's the problem of how we ever move at all in space or time, which I haven't had explained to me yet in a way that would be intuitive.

This is tangential, but there are also built in cognitive metaphors about how our brains make quantized things seem continuous, such as pointilist paintings. Although I'm pretty sure we observe the universe directly in much larger quantized units than we can measure with today's technology, and it's those much larger quantized units of space and time (and spacetime) that are cognitively represented as continuous to us.
 
Last edited:
How about this ...

If space-time is quantized to Planck lengths then there should be a smallest possible volume, or 3D structure -- let's call it a Planck Space. As the universe expands are these Planck Spaces getting farther apart or are more coming into existence to fill the void created by the expansion?
 
Now, I never graduated from college, but while I was there I recall my Physics professor spending a class period deriving the uncertainty principle from a few axioms and Maxwell's Equations. The impression that I got was that the uncertainty principle is hard-wired into the universe as we have come to know it, and that these quantum quantities (from the Office of Redundant Tautologies) are either a true physical concept of our model of the universe (from the Office of Really Using Imprecise Language, but Doing so Authoritatively), or our basic model is wrong on the macroscopic level.
Again, I am no expert, but as I understand it, these limits are real, in the sense that beyond (or in the interior of) them, the physics that has defined our universe is possibly playing dirty pool, and it is doing so at a level of subtlety that we are not able to detect, although we can surmise that dirty pool is being played, and adjust for the effects that the cheating may have in our observable realm.
Please be gentle to my metaphor, as it is not responsible for any misconceptions or inaccuracies in this post. Please direct any anger at me. I'll be waiting in the quantum foam.
 
Just thinking said:
How about this ...

If space-time is quantized to Planck lengths then there should be a smallest possible volume, or 3D structure -- let's call it a Planck Space. As the universe expands are these Planck Spaces getting farther apart or are more coming into existence to fill the void created by the expansion?
Click to expand...

The trouble is that you can't really think of the universe as made up of little cubes. Things like Planck length only make sense when you are refering to something happening, like particle interacting, or even just floating around on their own. You don't need the cubes to get bigger or multiply because they only exist when there is something there to exist in them. Think of two people. You can measure them as being two metres apart. Do those two metres exist when the people aren't there?
 
Cuddles said:
The trouble is that you can't really think of the universe as made up of little cubes. Things like Planck length only make sense when you are refering to something happening, like particle interacting, or even just floating around on their own. You don't need the cubes to get bigger or multiply because they only exist when there is something there to exist in them. Think of two people. You can measure them as being two metres apart. Do those two metres exist when the people aren't there?
Click to expand...

I see we certainly get into trouble when taking the microscopic and begin to apply it to the macroscopic -- anyway, that's what some cat told me. As for your example, I would have to say yes, the two metres of distance still exist, at least in the same reference frame as the two people that were previously there -- but in absolute terms, no -- at least not as two metres. But even in a Lorentz contraction there should still be the same number of Planck Lengths between them -- only contracted somewhat. If not, then the matter of the moving mass (relative to you) has in part started to occupy some of the space between what the "at-rest" observer measures as Planck Lengths, unless of course, the Lengths have contracted along with the matter.

Am I making any sense? ;)
 
autumn1971 said:
Now, I never graduated from college, but while I was there I recall my Physics professor spending a class period deriving the uncertainty principle from a few axioms and Maxwell's Equations. The impression that I got was that the uncertainty principle is hard-wired into the universe as we have come to know it, and that these quantum quantities (from the Office of Redundant Tautologies) are either a true physical concept of our model of the universe (from the Office of Really Using Imprecise Language, but Doing so Authoritatively), or our basic model is wrong on the macroscopic level.
Click to expand...

The uncertaintly principle (in all its forms) is indeed central to and insepparable from quantum mechanics, and since quantum mechanics works so well it does appear that it's intrinsic to reality. But that uncertainty principle applies to wave functions only, not to space and time itself. Quantum mechanics assumes space and time are continuous, non-dynamic coordinates. It is only when you try to combine quantum mechanics with general relativity that people propose introducing quantized space-time and using these quantities as dynamic coordinates. But that's an unfinished project, and it's not clear yet whether that approach to reconciling the two is correct, or will even work.
 
andyandy said:
is time truly continuous?
Click to expand...


Nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnyes...
 
Just thinking said:
How about this ...

If space-time is quantized to Planck lengths then there should be a smallest possible volume, or 3D structure -- let's call it a Planck Space. As the universe expands are these Planck Spaces getting farther apart or are more coming into existence to fill the void created by the expansion?
Click to expand...

Good question for someone who knows more than me to answer. But although the universe seems to be expanding, it seems to be expanding from some point rather than all parts equally expanding away from each other. To build on another poster's example, I have a chair 2 meters away from me. 1 minute later, it's still 2 meters away from me. It's not moving away from me at the speed of light, although my understanding is that the edges of the universe themselves are expanding at that speed. But we seem to have local distance stability -I don't see the need for stretching or new Planck spaces coming into existance as necessary to explain things. Maybe at the edges of the universe it's required? Or maybe those are pre-existing Planck spaces that the universe is expanding to fill?

I don't know -I'm sure mostly because of my weaknesses on first principles regarding this question. It's a fun question to think about though.
 
You must log in or register to reply here.

Back
Top Bottom