# Area law vs Perimeter law

1. May 13, 2013

### jfy4

While reading through Kogut's lattice gauge theory introduction he goes through the area and perimeter laws for lattice gauge models. The result is something like this
$$\left\langle \prod _{l\in C}\sigma_{3}(l) \right\rangle \sim \exp(-P)$$
for low temperature, and
$$\left\langle \prod _{l\in C}\sigma_{3}(l) \right\rangle \sim \exp(-A)$$
for high temperature. He then says
However, I can certainly conceive of a couple different closed loops which have more perimeter units than area units. Is this relationship between area size and perimeter size more subtle, or have I missed something. Thanks.

2. May 13, 2013

### fzero

You probably have in mind some degenerate loops which have one dimension parametrically small compared to the other. These are not loops that contribute to the discussion of confinement. Recall that one piece of evidence of confinement is that the quark-antiquark potential increases with separation. For your degenerate loops, the small dimension must always be the spatial one, since we must take the time cutoff $T\gg L$ by the physical situation. Then, for fixed $L$, we are not separating the quarks at all, so we expect no significant difference between the confining and deconfining phases. It is only in the limit that $L\rightarrow \infty$ that we should expect to see a difference.

If you have something else in mind, please explain.

3. May 13, 2013

### jfy4

I was thinking more like this, I'm going to explain this in terms of coordinates, (x,y).

starting at (0,0), connect the dots: (1,0),(1,1),(2,1),(2,2),(1,2),(1,3),(0,3),(0,2),(-1,2),(-1,1),(0,1),(0,0).
In words, a cross. This seems to have perimeter 12, but area 5. This seems to be a possbile loop, and these crosses can be made arbitrarily large I think.

4. May 13, 2013

### fzero

OK, suppose we scale the grid by a factor $L$, so we have points $(0,0), (L,0), (L,L)$ and so on. Now the perimeter is $P=12 L$, but the area is $A=5L^2$. In the limit $L\rightarrow \infty$, these have the behavior that was claimed. For any loop, the perimeter always scales linearly with the length of the edges, while the area is quadratic.

5. May 14, 2013

### jfy4

When you say
should I interpret that as
?

6. May 14, 2013

### fzero

It's not directly related to an RG transformation, or at least, we are not using that directly. I just wanted to introduce a scale corresponding to the size of the loop that we can vary, since Kogut is discussing the behavior for large loops. Of course the RG behavior is intricately tied in anyway. For example, the fact that the coupling constant of QCD grows in the IR is also an indication of confinement. But we are not directly using RG arguments to compare the perimeter with the area.

7. May 14, 2013

### jfy4

I see what you mean. I had in mind though loops of the form:
(0,0),(1,0)(1,n),(n+1,n),(n+1,n+1),(1,n+1)...
and n can get as large as it wants. I guess I realize that there probably are large loops which don't follow these `laws' but they contribute little I suppose.

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