# Planck Time? Why does it exist?

1. ### Slem

12
I've recently heard of Planck Time.. I read that it is the minimum time interval that can EXIST. What exactly changes in time intervals that are less than Planck time? And if im not mistaken by the meaning of Planck Time; What is the reason that there is such a limit?
For Example, the universal speed limit of light exists because you ll simply require infinite energy to accelerate an object enough to reach the speed of light, which is impossible. SO , in the case of Time, what infinite quantity is required to measure below Planck Time. ( I do realize that Im most likely mistaken about my understanding of Planck Time).

And while you re at it , I ve also heard of Temperature limit, that objects cannot get hotter than that limit .. Or that physical laws change after that limit, again , why does such a limit exist in the first place? Please , enlighten me.

2. ### Danger

9,878
I don't even understand this stuff myself, but I'll try to help out a bit. A Planck length is essentially the smallest distance that two things can be apart from each other without being in the same place. (Maybe like proton packing in a nucleus. edit: neutrons in a neutron star would be a better example.) Planck time is the time that it takes light to traverse that distance.

### Staff: Mentor

Planck time is like motion picture time. When we see a film, it is shot in say 30 frames per second where each frame would represent a time unit. Its not continuous but it appears so when played.

Quantum Mechanics places limits of what we can measure and this has led physicist to conclude that perhaps the universe is pixelated both in space and in time with planck length the limit for space measurement and planck time the limit for time. Movement of a particle thru space and time would appear as a sequence of dots not as a continuous line if plotted.

4. ### Slem

12
It does make a bit more sense now, since light speed is the limit, and there is a limit to how close two thingscan be to each other. Then a time limit would make sense. But then again , why does Planck length exist. I assume that two objects can keep getting closer and closer Forever!

5. ### Slem

12
From Wikipedia:
"The physical significance of the Planck length is a topic of research. Because the Planck length is so many orders of magnitude smaller than any current instrument could possibly measure, there is currently no way of probing this length scale directly. Research on the Planck length is therefore mostly theoretical. According to the generalized uncertainty principle, the Planck length is in principle, within a factor of order unity, the shortest measurable length – and no improvements in measurement instruments could change that.
In some forms of quantum gravity, the Planck length is the length scale at which the structure of spacetime becomes dominated by quantum effects, and it would become impossible to determine the difference between two locations less than one Planck length apart. The precise effects of quantum gravity are unknown; often it is suggested that spacetime might have a discrete or foamy structure at Planck length scale."

That would mean then that Planck Time is the shortest MEASURABLE time interval, not absolutely shortest. IN this case , it does all make sense.

6. ### Slem

12
But That would mean that a curve length is made up of A FINITE amount of dots. Which kind of isnt the way i understand Calculus .

7. ### Danger

9,878
That's a sticking point for me as well. By my thinking, two events of one Planck time interval each, occurring out of phase with each other, would result in a 1/2 Planck time interval between them. At that point, I quit paying attention.

8. ### phinds

9,037
Slem,

The Plank length is, as I believe you have come to understand, the smallest MEASURABLE length (probably) but it is NOT the smallest length (well, it MIGHT be, but that is currently unknown).

The Plank time is the time for light to travel one Plank length, but it is NOT believed to be the smallest time, if there is such a thing. It has been hypothesized (here on this board as I recall) that if time is quantized, the quanta is MUCH smaller than a Plank time.

9. ### Slem

12
Thanks, so basically the definite answer to my question, is practically still unknown.
By the way, do you happen to know the answer to Why there exists a universal Temperature Limit?

10. ### Trollegionaire

10
Mathematically, dimensional space is internally infinite. Imagine it simply as a ball being shrunk down to size, indefinitely. Physically, however, we assume that Planck length is the smallest unit of length, simply because physicists hate infinity. Kaku would tell you that on the results of formula's concerning singularities.

11. ### Slem

12
haha That is what I've come to realize.

12. ### phinds

9,037
I've never heard of such a thing. Do you have a legitimate reference for such?

13. ### Slem

12
It is Planck Temperature.
http://en.wikipedia.org/wiki/Planck_temperature

I kind of understand it though, since kinetic energy equals m*V^2 and V has a limit of c , light of speed , then kinetic energy of an object has a maximum. Thus , temperature has a maximum

14. ### phinds

9,037
Interesting. Thanks. I see that according to Wiki, this is not yet known to be fundamentally a limit and might change, but it makes sense as stated.

15. ### bahamagreen

666
Seems to me that as the particles approach c their energy increases without bound (old way of thinking is mass increase). There is no upper limit on that, so temperature (kinetic energy) goes up with the mass/energy increase.
With quantum gravity, unknown effects might come into play as the individual particles' mass/energy magnitudes increase enough that mutual gravitation becomes a significant influence, so who knows?

16. ### Slem

12
Oh physics, i love you...