Thread: Is there such a thing as a Planck time period.

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Is there such a thing as a Planck time period.

The shortest time period that could be possible.

PO'T

2. Planck time is defined as roughly 5.39106x10^-44 seconds. It is so small, I am not even sure I could understand it so I included the wiki link.

I did see that there are other quantities of extremely short time periods such as Chronons and time measurements by a huge number of Hertz (1121015393207857.4(7)Hz for a quantum clock.) It appears chronons may be a different way of thinking/modeling short time periods, but that may be a gross error on my part.

I have never heard of Chronons and barely understand Planck time, so perhaps someone else could provide more (and better) details.

3. My understanding is that if two events, A and B, occurred more than Planck time apart, it would, in principle, be possible to say which happened first, even if the time difference was far too short for our instruments to measure. (According to the linked article, 10^-17s is the current limit.)

On the other hand, if events A and B occurred less than Planck time apart, it would be meaningless to say that A was before B or vice versa. And if the events were linked in some way, it would be impossible to distinguish between "A caused B" and "B caused A".

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In terms of the uncertainty principle it is the smallest uncertainty you can have in a strong measurement. I am not aware of any such constraints on weak measurements - I do wonder if it is possible to reduce the variance in the expectation value below this value in finite time? I'd guess not but I am not aware of any rigorous theoretical reasons why. That is probably my ignorance shining through though. I'm guessing once you factored in the strong QM interactions required to remake the state to make the weak measurements it would still be a limit.

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Originally Posted by Paul Beardsley
On the other hand, if events A and B occurred less than Planck time apart, it would be meaningless to say that A was before B or vice versa. And if the events were linked in some way, it would be impossible to distinguish between "A caused B" and "B caused A".
If it is meaningless to say "A was before B or vice versa" then what is the meaning of "A and B occurred less than Planck time apart"?

6. Originally Posted by Paul Wally
If it is meaningless to say "A was before B or vice versa" then what is the meaning of "A and B occurred less than Planck time apart"?
What I mean here is, if our technology advanced to the extent that we could pinpoint the time of events that were only Planck time apart, then if two events A and B occurred and we still couldn't tell them apart, then it would be meaningless to ask what order they occurred.

7. Originally Posted by Paul Wally
If it is meaningless to say "A was before B or vice versa" then what is the meaning of "A and B occurred less than Planck time apart"?

My understanding of this is far from complete, but can't it be likened to the Planck Length? In quantum-mechanical models (at least, the ones I know about) you can't measure something smaller than the Planck Length because quantum effects that happen at those distances make the idea of length meaningless. Could you say something similar for time?

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Originally Posted by Paul Beardsley
What I mean here is, if our technology advanced to the extent that we could pinpoint the time of events that were only Planck time apart, then if two events A and B occurred and we still couldn't tell them apart, then it would be meaningless to ask what order they occurred.
For me a statement is "meaningful" when it makes logical sense. For example in Special Relativity it is meaningless to ask whether two events really occurred simultaneously, because it depends on your frame of reference. If "order of events" doesn't have meaning (in the Planck time case) then I cannot see how time itself can have meaning, and perhaps it doesn't.

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Originally Posted by Paul Beardsley
My understanding is that if two events, A and B, occurred more than Planck time apart, it would, in principle, be possible to say which happened first, even if the time difference was far too short for our instruments to measure. (According to the linked article, 10^-17s is the current limit.)

On the other hand, if events A and B occurred less than Planck time apart, it would be meaningless to say that A was before B or vice versa. And if the events were linked in some way, it would be impossible to distinguish between "A caused B" and "B caused A".
Is it possible that the universe expands in planck increments? Planck length per planck time? Or am I way off?

10. Originally Posted by potoole
Is it possible that the universe expands in planck increments? Planck length per planck time? Or am I way off?
(Shrugs) I dunno!

Actually I don't think it does. Planck time is the smallest resolution possible, but I'm pretty sure that doesn't mean time consists of Planck-length frames.

11. Originally Posted by Paul Wally
For me a statement is "meaningful" when it makes logical sense. For example in Special Relativity it is meaningless to ask whether two events really occurred simultaneously, because it depends on your frame of reference. If "order of events" doesn't have meaning (in the Planck time case) then I cannot see how time itself can have meaning, and perhaps it doesn't.
It's just a matter of resolution, as I understand it.

Imagine you took a photograph of two people, John and Susan, with a digital camera. They are quite close to you, so John, on the left of the picture, occupies several thousand pixels. Susan, on the right, occupies another several thousand pixels.

Now imagine you take a photo of them while they are standing close together, but they are so far away from you that they both occupy a single pixel. Now, from examining that one pixel, you cannot say whether it is John or Susan standing on the left.

Just because you can't say who is on the left on the basis of looking at one pixel, it doesn't follow that relative positioning is meaningless.

Going back to time, if you watch two runners in a race, and runner A finishes in 59 seconds and runner B finishes in 60 seconds, it's clear that A won, because there was a delay of about one second (~0.5-1.5s) between them.

If the delay between them was about one hundredth of a second, you probably wouldn't be able to make the distinction on your own, but if you had some very fast cameras filming the event, you'd be able to play them back at slow speed and then you could say who was first.

In principle, you could get faster and faster cameras (or other recording apparatus) to resolve shorter and shorter intervals, whether it's two people racing, particles decaying or whatever. If these events are in the same frame of reference (e.g. the two runners are the same distance from the camera) then clearly it's meaningful to say that one reached the finish line before the other.

But there's a limit to how far you can increase this temporal resolution, even in principle, and that limit is Planck time. It's the temporal equivalent of the pixel.

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Originally Posted by primummobile
My understanding of this is far from complete, but can't it be likened to the Planck Length? In quantum-mechanical models (at least, the ones I know about) you can't measure something smaller than the Planck Length because quantum effects that happen at those distances make the idea of length meaningless. Could you say something similar for time?
That's also my idea of it. Spacetime at such scales is so disturbed by quantum effects that it is meaningless. Maybe, when the time comes, we will reach a "Sub-Quantum Physics" era.

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Originally Posted by Paul Wally
For me a statement is "meaningful" when it makes logical sense. For example in Special Relativity it is meaningless to ask whether two events really occurred simultaneously, because it depends on your frame of reference. If "order of events" doesn't have meaning (in the Planck time case) then I cannot see how time itself can have meaning, and perhaps it doesn't.
Think of it this way Paul. The definition of Planck time is the the time it takes a photon (moving at c) to move the Planck length. Since c is the fastest speed and the Planck length is the shortest length, for time to have meaning at a scale smaller than the Planck length, you would either need some speed faster than c or a length smaller than the Planck length. Since, by current definition, neither of these is possible, time at scales smaller than the Planck time is not possible, as you stated at the end of your sentence. Now, if time doesn't have meaning, then you cannot determine whether one event caused another or whether that event came before or after the other.

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Originally Posted by Paul Beardsley
Actually I don't think it does. Planck time is the smallest resolution possible, but I'm pretty sure that doesn't mean time consists of Planck-length frames.
String theory and loop quantum gravity among others speculative theories do predict space time to be quantised like this. Main QM does not require it as I understand it.

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Here is something I was thinking about. People have said that galactic core black holes were safer to pass through (event horizon) due to the gravity well not being so immediately steep. Still, if you were to pass your hand through--you lose it in chops/slices as the particles can't wiggle back and forth to hold your hand solid.

But--

if your spacecraft is so fast that the time it takes to cross the event horizon is less than one plank unit--you live? Because there is literally no time for the black hole to cut you--as it were.

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It doesn't really work like that. An event horizon is not some point in spacetime which slices off anything that crosses it. What matters in your example is where your centre of mass is

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What would your mass be if you were traveling greater than c ? Infinite?...or non-determinate? ....or zero?

And how does this relate to space itself being drawn into the black hole at > c ?

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Originally Posted by potoole
The shortest time period that could be possible.

PO'T
Sure. If you consider the shortest observation of any possible system in physics exists within the so-called Planck Time. The Planck length has units of

If is measures in seconds, then is measures in meters, then . To measure it we need a clock with an uncertainty of can be no larger than . Time and energy uncertainty says the product of and can be no less than

so that

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What would your mass be if you were traveling greater than c ? Infinite?...or non-determinate? ....or zero?
It would have an infinite mass.

Infinities don't exist in nature though, which is why such a limit is imposed. It is non-physical.

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Well, actually... I am not technically right. If an object passes the lightspeed barrier, then it has infinite mass. A particle if it starts it's journey above the lightspeed barrier will then have an imaginary mass... the two are quite different.

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