% The following is a LaTeX source file for the paper % (appeared in Phys Rev A 45 618--625 (1992)) % `Finite Size Effects and Shock Fluctuations in the Asymmetric Simple % Exclusion Process' by Janowsky/Lebowitz % % Can use ams.sty to get correct `\lessapprox' character; see below. % BODY \documentstyle[11pt]{article} \textwidth 6.25in \textheight 8.6in \topmargin -.25in \oddsidemargin 0.2in \parskip 2.8pt plus 1pt minus .5pt \pretolerance=2000 \renewcommand{\baselinestretch}{1.2} \newcommand{\lessapprox}{<\approx} % should use a special AMS char for this. % character 2F in msam \author{Steven A. Janowsky\thanks{Supported in part by National Science Foundation Mathematical Sciences Postdoctoral Research Fellowship DMS 90-07206}\hspace{.25em} and Joel L. Lebowitz\\ Departments of Mathematics and Physics\\ Rutgers University\\New Brunswick, NJ 08903} \title{Finite Size Effects and Shock Fluctuations in the Asymmetric Simple Exclusion Process\thanks{Supported in part by National Science Foundation grant DMR 89-18903}} \date{January, 1992} \begin{document}\maketitle \begin{abstract} We consider a system of particles on a lattice of $L$ sites, set on a circle, evolving according to the asymmetric simple exclusion process, {\em i.e.}\ particles jump independently to empty neighboring sites on the right (left) with rate $p$ (rate $1-p$), $1/20$. If we use the approximation (\ref{rhoapprox}), we must have $|\rho_{\rm avg} - 1/2| < (1-r)/(2(1+r))$. If this is not the case we no longer have the basic structure of two near-uniform regions meeting at a shock front, with local perturbations near the blockage; instead we have only one approximately uniform phase. Apparently, if the current in the uniform phase is sufficiently small, there is no need for phase segregation to reduce the bulk current to that at the blockage~\cite{Krug}. Similar behavior is observed when we examine an infinite system with one site blocked (see Section~\ref{infinite}). \subsection*{Tracking the Shock} While the average shock location is easy to determine, finding its microscopic position or even defining it precisely at a given time is nontrivial. Instead of determining the shock position directly, we track it through the use of a {\bf second class particle}~\cite{BCFG}. The second class particle is an extra particle added to the system, which is treated as a hole in exchanges with particles and as a particle in exchanges with holes. In other words, if we select site $i$ and it is occupied by an ordinary particle, it moves to site $i+1$ either if that site is empty or if it is occupied by the second class particle; in the latter case the second class particle simultaneously jumps (to the left) to site $i$, {\em i.e.}\ there is an exchange between the first and second class particles. If site $i$ is occupied by a second class particle, it moves to site $i+1$ only if site $i+1$ is unoccupied; if site $i+1$ is occupied it stays put. Therefore, if the second class particle is in a high density region (of ordinary particles), it will be forced to the left, while if it is in a low density region it will be able to move to the right. A shock consists of an abrupt change from a low density region to a high density region. To the left of a shock (a low density region) a second class particle moves predominantly to the right, while on the right side (a high density region) it moves predominantly to the left. The second class particle therefore executes a biased random walk with drift towards the shock position. Simple analysis shows and computer simulations confirm that the inherent fluctuations of this random walk are small compared with the movement of the shock---namely that if we fix the densities on either side of the shock (${\rho_{\rm low}}, {\rho_{\rm high}}$) and consider different values of the system size $L$, the motion of the second class particle about the shock position will have fixed variance while the variance of the shock position itself will grow with system size. Thus analyzing the motion of the second class particle will allow us to indirectly examine the shock motion. An alternative method of finding the shock is suggested by the surface growth model. Recalling equation~(\ref{surfaceheight}), we see that surface height increases in low (particle) density regions and decreases in high density regions, so that the shock location should correspond intuitively to the location of the maximum height. Unfortunately this position need not be (microscopically) unique, so we prefer the use of the second class particle. It suggests, however, that in addition to the shock position we study other features from the growth model, such as the difference between the maximum and minimum surface heights, which is equivalent to studying the number of particles on either side of the interface in the ASEP. As a third possibility we could ignore the microscopic shock position entirely and simply study the width of the time-averaged shock density profile, such as that in figure~\ref{prof35}. However, this method obscures the distinction between the instantaneous microscopic width of the shock and the width due to fluctuations in the shock's position (particularly relevant in the extension to two-dimensional models), and the width is difficult to determine accurately because of lattice effects. In any case, we expect that as the system size goes to infinity that rescaling the shock profile by the standard deviation of the shock fluctuation (determined by any of the three methods) should yield a well-defined limiting shape. We sample the position of the second class particle after allowing the system to reach a steady state. For each system we obtain a distribution (histogram) of shock (second class particle) positions and compute the variance of the obtained distribution; a typical run consisted of approximately 25,000 samples where 15 sweeps ($15\times L$ monte carlo steps) were made between samples. We then compare the variances obtained from systems with different values of the parameters. We also perform the same set of computations for the number of particles found between site~1 and the second class particle position. In figure~\ref{var55} we plot the standard deviation of the interface position {\em vs.}\ system size for average density 0.55 and blocking rate $r=0.35$. \begin{figure}[hbt] \begin{center} % GNUPLOT: LaTeX picture \setlength{\unitlength}{0.33pt} \begin{picture}(1323,660)(167,40) \newcommand{\mD}{\makebox(0,0){$\diamond$}} \newcommand{\uP}{\usebox{\plotpoint}} \ifx\plotpoint\undefined\newsavebox{\plotpoint}\fi \put(264,113){\line(1,0){10}} \put(1436,113){\line(-1,0){10}} \put(264,145){\line(1,0){10}} \put(1436,145){\line(-1,0){10}} \put(264,173){\line(1,0){10}} \put(1436,173){\line(-1,0){10}} \put(264,197){\line(1,0){10}} \put(1436,197){\line(-1,0){10}} \put(264,219){\line(1,0){20}} \put(1436,219){\line(-1,0){20}} \put(242,219){\makebox(0,0)[r]{10}} \put(264,364){\line(1,0){10}} \put(1436,364){\line(-1,0){10}} \put(264,448){\line(1,0){10}} \put(1436,448){\line(-1,0){10}} \put(264,508){\line(1,0){10}} \put(1436,508){\line(-1,0){10}} \put(264,554){\line(1,0){10}} 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The solid line has slope 1/2.} \label{var55} \end{figure} The error bars (three symbols are plotted for each measurement: the actual value and that value shifted up or down by the error bound) represent statistical error based on the approximate number of independent samples selected from the steady state in each system. The line drawn through the data points has a slope of 1/2, so that fluctuations grow like the square root of the system size, indicating standard finite size behavior. In figure~\ref{var35w-1} we plot the same quantities as in figure~\ref{var55}, but the average particle density is 0.5. \begin{figure}[htb] \begin{center} % GNUPLOT: LaTeX picture \setlength{\unitlength}{0.32pt} \begin{picture}(1188,750)(212,44) \newcommand{\mD}{\makebox(0,0){$\diamond$}} \newcommand{\mC}{\makebox(0,0){$\circ$}} \newcommand{\uP}{\usebox{\plotpoint}} \ifx\plotpoint\undefined\newsavebox{\plotpoint}\fi \put(850,25){\makebox(0,0){System Size}} \put(264,133){\line(1,0){10}} \put(1436,133){\line(-1,0){10}} \put(264,168){\line(1,0){10}} \put(1436,168){\line(-1,0){10}} \put(264,197){\line(1,0){10}} \put(1436,197){\line(-1,0){10}} \put(264,222){\line(1,0){10}} \put(1436,222){\line(-1,0){10}} \put(264,245){\line(1,0){10}} \put(1436,245){\line(-1,0){10}} \put(264,265){\line(1,0){20}} \put(1436,265){\line(-1,0){20}} \put(245,265){\makebox(0,0)[r]{10}} \put(264,397){\line(1,0){10}} \put(1436,397){\line(-1,0){10}} \put(264,474){\line(1,0){10}} \put(1436,474){\line(-1,0){10}} 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\multiput(1143,426)(4,1.33){6}{\uP} \multiput(1163,432)(4,1.33){6}{\uP} \multiput(1183,439)(4,1.33){6}{\uP} \multiput(1202,446)(4,1.33){6}{\uP} \multiput(1222,453)(4,1.33){6}{\uP} \multiput(1241,459)(4,1.33){6}{\uP} \multiput(1261,467)(4,1.33){6}{\uP} \multiput(1280,473)(4,1.33){6}{\uP} \multiput(1300,480)(4,1.33){6}{\uP} \multiput(1320,487)(4,1.33){6}{\uP} \multiput(1339,494)(4,1.33){6}{\uP} \multiput(1359,501)(4,1.33){6}{\uP} \multiput(1378,507)(4,1.33){6}{\uP} \multiput(1398,514)(4,1.33){4}{\uP} \end{picture} \end{center} \caption{Interface fluctuations with average density 0.5, $r=0.35, 0.5$. The solid lines have slope 1/3.} \label{var35w-1} \end{figure} We examined two sets of systems, with blocking rates $r=0.35, 0.5$. Here the lines drawn through the data points have slope 1/3, so that fluctuations are suppressed compared with systems whose densities are different from 0.5, yielding growth only like the cube root of the system size. (Note that changing $r$ does not affect this conclusion; the relevant variable is whether or not the density is 0.5.) The reduction in the fluctuations of the interface when the average density is 1/2 is clearly caused by some type of cancellation since other fluctuations in the system still scale like $L^{1/2}$---in figure~\ref{var35h} we plot the standard deviation of the number of particles found to the left of the interface, {\em i.e.}\ between site~1 and the second class particle; \begin{figure}[hbt] \begin{center} % GNUPLOT: LaTeX picture \setlength{\unitlength}{0.30pt} \begin{picture}(1080,730)(250,36) \put(264,140){\line(1,0){10}} \put(1436,140){\line(-1,0){10}} \put(264,186){\line(1,0){10}} \put(1436,186){\line(-1,0){10}} \put(264,223){\line(1,0){10}} \put(1436,223){\line(-1,0){10}} \put(264,255){\line(1,0){10}} \put(1436,255){\line(-1,0){10}} \put(264,282){\line(1,0){10}} \put(1436,282){\line(-1,0){10}} \put(264,307){\line(1,0){10}} \put(1436,307){\line(-1,0){10}} \put(264,328){\line(1,0){20}} \put(1436,328){\line(-1,0){20}} \put(242,328){\makebox(0,0)[r]{10}} \put(264,470){\line(1,0){10}} 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\put(1026,517){\makebox(0,0){$\diamond$}} \put(1026,520){\makebox(0,0){$\diamond$}} \put(1026,523){\makebox(0,0){$\diamond$}} \put(1183,588){\makebox(0,0){$\diamond$}} \put(1183,592){\makebox(0,0){$\diamond$}} \put(1183,596){\makebox(0,0){$\diamond$}} \put(1316,620){\makebox(0,0){$\diamond$}} \put(1316,626){\makebox(0,0){$\diamond$}} \put(1316,631){\makebox(0,0){$\diamond$}} \put(1393,669){\makebox(0,0){$\diamond$}} \put(1393,675){\makebox(0,0){$\diamond$}} \put(1393,681){\makebox(0,0){$\diamond$}} \end{picture} \end{center} \caption{Fluctuations in the number of particles to the left of the interface: average density 0.5, $r=0.35$. The solid line, which is a best linear fit to the data, has slope 0.46.} \label{var35h} \end{figure} this data is consistent with scaling exponent 1/2 (and inconsistent with 1/3). An explanation of this behavior is given in the next section. \section{Analysis} To make sense of our results, it is convenient to decompose the stochastic nature of our system into two pieces: the random flow of particles through the blockage, and the random dynamics everywhere else. We hypothesize that the ``blockage randomness'' is responsible for the $L^{1/2}$ behavior and that the ``dynamical randomness'' is responsible for the $L^{1/3}$ behavior. A particle traversing the blockage can be thought of as two simultaneous events: a particle being released at site~$1$ while a hole is released at site~$L$. These two events are of course completely correlated, but the motions of the particle and hole {\em after} release are nearly independent. The position of the interface is determined by the {\em difference} between the number of particles which arrive from the left and the number of holes which come from the right. (It may be convenient for visualization purposes to consider the situation where the density to the right of the shock is very close to one and the density on the left is near zero.) If we neglect the dynamical randomness---{\em i.e.}\ if we assume that particles in the low density region move at their average velocity $1-{\rho_{\rm low}}$, and holes in the high density region move at their average velocity $-{\rho_{\rm high}} = -(1-{\rho_{\rm low}})$, and consider a shock in the center of the system, then no matter what the nature of the creation of excitations at the blockage, particles and holes will always impinge upon the shock in pairs: when a pair of excitations is created, the hole and particle will travel with equal speeds and opposite directions, meeting (and annihilating) when they have each traveled a distance $L/2$ and reach the shock position. Since the ``blockage randomness'' plays no role here, a system with density~0.5, and therefore particle-hole symmetry (figure~\ref{var35w-1}), has fluctuations determined only by the ``dynamical randomness''---the deviation of the particle velocities from their average. On the other hand, if the shock is not in the center of the system, the nature of ``blockage randomness'' becomes relevant as the times for particles and holes to reach the interface are no longer the same. For example, if the average density is greater than 1/2 the (average) shock position will be less than $L/2$, and particles which start at the blockage and travel to the right will reach the shock before holes (starting at the same time and place) traveling to the left---the arrival times of the particles no longer match those of the holes because the distances they need traverse are different, and the resulting motion of the shock position will depend upon exactly how the excitations are created. So a system with density different from 0.5 (figure~\ref{var55}) and thus with an average shock position shifted from the center will have fluctuations from the ``blockage randomness'' proportional to the amount of the shift. The $L^{1/2}$ scaling of the ``blockage randomness'' results from the rapid decay of correlations in both time and space in the stationary state of the ASEP, as well as the use of an independent random choice for each attempt to cross the blockage. Thus the creation of one excitation is very nearly independent from the creation of another. Therefore the fluctuation in the number of excitations will scale as the square root of the average number. There is a finite density of excitations, so their number is $O(L)$, yielding fluctuations that scale like $O(L^{1/2})$. In fact, if we neglect the dynamical randomness and treat particle flow through the blockage as a Poisson process, we can determine through straightforward analysis the variance of the shock position. Each particle or hole added to the shock moves it by $(\rho_{\rm high} -\rho_{\rm low})^{-1}$. Thus the shock is shifted by an amount $|\rho_{\rm avg} - 1/2| L/(\rho_{\rm high} -\rho_{\rm low})$, which is half the difference in path length between particles and holes. Therefore the time differential is $2|\rho_{\rm avg} - 1/2| L\big/ \left(\rho_{\rm high}(\rho_{\rm high} - \rho_{\rm low})\right)$. The probability of the formation of an excitation is $\rho_{\rm high}\rho_{\rm low}$ so that the standard deviation of the shock position should be \begin{equation}\label{STDSP} (\rho_{\rm high} -\rho_{\rm low})^{-1}\left( \frac{2|\rho_{\rm avg} - 1/2| L}{\rho_{\rm high}(\rho_{\rm high} - \rho_{\rm low})} \rho_{\rm high}\rho_{\rm low}\right)^{1/2} = \left(\frac{2{\rho_{\rm low}} |\rho_{\rm avg} - 1/2| L}{({\rho_{\rm high}} - {\rho_{\rm low}})^3}\right)^{1/2}. \end{equation} This calculation should be accurate for $r$ near zero but will be too large otherwise, as the actual variance of the particle flow is less than that of a Poisson process since we do have (small) memory effects. For the system of figure~\ref{var55}, the measured behavior is $0.50L^{1/2}$ while eq.~(\ref{STDSP}) yields $0.63L^{1/2}$. What is the nature of the randomness in the dynamics? Recent work by van Beijeren~\cite{vB} indicates that density fluctuations grow in time like $t^{1/3}$, while since the distance is of order $L$ and the speed is $O(1)$ the time required for an excitation to reach the interface from the blockage is of order $L$. Thus an initially nonrandom arrangement (or a random arrangement with number particles = number holes) of excitations will have fluctuations of order $L^{1/3}$ by the time it reaches the interface. We confirm this by examining a system where we alter the behavior near the blockage so that particles pass through the blockage in a near-uniform manner. Of course we can not have a completely uniform current since the occupation numbers of each site are limited to 0 and 1, but we try to make the flow as uniform and non-random as possible: if $n(t)$ is the number of particles already having passed through the blockage at time $t$, we specify the current exactly by opening the blockage if $n(t) < Jt$ and closing it otherwise. If a particle is immediately to the left of the blockage while it is open and the site to the right is vacant, the particle jumps through the blockage. Thus we attempt to schedule particle jumps at intervals of $1/J$; if a jump is not possible because the appropriate occupancy conditions are not satisfied we ``make up'' that jump as soon as possible. By altering the nature of the blockage, we alter the behavior of the shock fluctuations. For this uniform-block model, the lack of ``blockage randomness'' means that even without particle-hole symmetry, {\em i.e.}\ even when the shock is displaced from the center we have fluctuations that scale like $L^{1/3}$ for {\em both} the interface position {\em and} the number of particles to the left of the interface---see figure~\ref{varu}. Thus we have (partial) confirmation of our hypothesis. \begin{figure}[htb] \begin{center} % GNUPLOT: LaTeX picture \setlength{\unitlength}{0.32pt} \begin{picture}(1290,800)(165,70) \newcommand{\mD}{\makebox(0,0){$\diamond$}} \newcommand{\mC}{\makebox(0,0){$\circ$}} \newcommand{\uP}{\usebox{\plotpoint}} \ifx\plotpoint\undefined\newsavebox{\plotpoint}\fi \put(264,228){\line(1,0){10}} \put(1436,228){\line(-1,0){10}} \put(264,310){\line(1,0){10}} \put(1436,310){\line(-1,0){10}} \put(264,374){\line(1,0){10}} \put(1436,374){\line(-1,0){10}} \put(242,374){\makebox(0,0)[r]{5}} \put(264,426){\line(1,0){10}} \put(1436,426){\line(-1,0){10}} \put(264,470){\line(1,0){10}} \put(1436,470){\line(-1,0){10}} \put(264,508){\line(1,0){10}} \put(1436,508){\line(-1,0){10}} \put(264,541){\line(1,0){10}} \put(1436,541){\line(-1,0){10}} \put(264,571){\line(1,0){20}} \put(1436,571){\line(-1,0){20}} \put(242,571){\makebox(0,0)[r]{10}} \put(264,768){\line(1,0){10}} \put(1436,768){\line(-1,0){10}} 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\put(1403,717){\uP} \put(1407,718){\uP} \put(1410,719){\uP} \put(1414,720){\uP} \end{picture} \end{center} \caption{Fluctuations with uniform (non-random) flow through the block. Average density=0.55. Solid lines have slope 1/3.}\label{varu} \end{figure} \section{Infinite System with Block}\label{infinite} We have been considering particles evolving according to the ($p=1$) ASEP dynamics on a circle of $L$ sites with a blockage parameterized by $r$. For each such system with a fixed number of particles $N$ (or average density $\rho_{\rm avg} \equiv N/L$) the stationary state is unique. The question naturally arises, ``What happens to these states in the infinite volume limit $L\rightarrow\infty$, $N\rightarrow\infty$, $N/L\rightarrow\rho$ fixed?'' To make this question precise we have to specify the position of our domain of observation relative to the blocked site as $L\rightarrow\infty$. This is unlike the case in the finite system where except for labeling the state is independent of the position of the blocked bond, since the domain of observation corresponds to all $L$ sites. Let us label the sites of our system (relative to the observation domain) from $-L/2$ to $L/2$, and put the left end of the blocked bond at $K_L$. Then if $K_L\rightarrow\pm\infty$ as $L\rightarrow\infty$ we are in the situation of an infinite uniform system, and the stationary states are superpositions of product states. The product states are of two kinds: for $00$ will then depend on $r$ (for $J=0$ we have again the fully blocked states and $r$ is irrelevant). We believe that it can be shown that for any $J>0$ the asymptotic distribution to the left and right of the origin is a product measure with densities $\rho_{\rm left}$ and $\rho_{\rm right}$ where either \begin{equation}\label{infequal} \rho_{\rm left} = \rho_{\rm right} = \left(1\pm\sqrt{1-4J}\right)/2 \end{equation} or \begin{equation}\label{infunequal} \rho_{\rm right} = \left(1-\sqrt{1-4J}\right)/2 < \rho_{\rm left} = \left(1+\sqrt{1-4J}\right)/2. \end{equation} The problem is then to find the phase diagram in the $J$--$r$ plane. We have used the results from our finite system studies as well as numerical simulations intended to imitate the infinite situation (we use boundary conditions of fixed density) to obtain the following results: Away from the blockage the infinite stationary state resembles a product measure, so that the need for the current to be the same on both sides requires (as in equations~(\ref{lh})--(\ref{lhdens})) that the densities on either side of the block be equal or be symmetric about $1/2$, in full correspondence with (\ref{infequal})--(\ref{infunequal}). Let $\rho_{\rm crit} = \rho_{\rm low} = 1 - \rho_{\rm high}$ refer to the lower density of the segregated periodic system with the same blockage transmission $r$ as the infinite system. Then there are three nontrivial classes of extremal stationary states for the infinite system in addition to the zero-current measures (we can also have superposition measures as in (\ref{superposition})): \begin{enumerate} \item $\rho_{\rm left} = \rho_{\rm right} < \rho_{\rm crit}$,\label{classlow} \item $\rho_{\rm left} = \rho_{\rm right} > 1 - \rho_{\rm crit}$, \label{classhigh} \item $1 - \rho_{\rm left} = \rho_{\rm right} = \rho_{\rm crit}$. \label{classhighlow} \end{enumerate} We see that we are justified in referring to $\rho_{\rm crit}$ as a critical density; the system cannot be stationary at densities between $\rho_{\rm crit}$ and $1 - \rho_{\rm crit}$ because the required current is too high. We can illustrate this in figure~\ref{J-r} where the critical line is approximated by the no-correlation formulae (\ref{Japprox})--(\ref{rhoapprox}). \begin{figure}[hbt] \begin{center} % GNUPLOT: LaTeX picture \setlength{\unitlength}{0.240900pt} \begin{picture}(1200,800)(250,80) \ifx\plotpoint\undefined\newsavebox{\plotpoint}\fi \newcommand{\ubpp}{\usebox{\plotpoint}} \put(264,113){\line(1,0){1172}} \put(264,113){\line(0,1){719}} \put(264,113){\line(1,0){20}} \put(1436,113){\line(-1,0){20}} \put(242,113){\makebox(0,0)[r]{0}} \put(264,832){\line(1,0){20}} \put(1436,832){\line(-1,0){20}} \put(242,832){\makebox(0,0)[r]{0.25}} \put(242,472){\makebox(0,0)[r]{$J$}} \put(264,113){\line(0,1){20}} \put(264,832){\line(0,-1){20}} \put(264,68){\makebox(0,0){0}} \put(1436,113){\line(0,1){20}} \put(1436,832){\line(0,-1){20}} \put(1436,68){\makebox(0,0){1}} \put(850,68){\makebox(0,0){$r$}} \put(264,113){\line(1,0){1172}} \put(1436,113){\line(0,1){719}} \put(1436,832){\line(-1,0){1172}} 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\put(750,330){\makebox(0,0)[l]{(type 1,2)}} \end{picture} \end{center} \caption{Phase diagram for infinite system with block; current {\em vs.}\ blockage rate.}\label{J-r} \end{figure} The first two families of (near) uniform states correspond to the (near) uniform states in the periodic system, when $\rho_{\rm avg} < \rho_{\rm crit}$ or $\rho_{\rm avg} > 1 - \rho_{\rm crit}$. The nonuniform state corresponds to segregated systems with $\rho_{\rm crit} < \rho_{\rm avg} < 1 - \rho_{\rm crit}$. An apparent fourth type of state with $1 - \rho_{\rm right} = \rho_{\rm left} < \rho_{\rm crit}$ is not stationary since the interface is not bound to the block and thus will eventually drift to infinity, even though its average velocity is zero. Nearly as interesting as the stationary states themselves are their basins of attraction. Choosing as initial conditions $\rho= \rho_{\rm left,0}$ to the left of the blockage and $\rho = \rho_{\rm right,0}$ on the right we found that \begin{itemize} \item For $\rho_{\rm left,0} < \rho_{\rm crit}$, $\rho_{\rm left,0} < 1-\rho_{\rm right}$, the system approaches a state from class~\ref{classlow} above with $\rho_{\rm left} = \rho_{\rm right} = \rho_{\rm left,0}$. \item For $\rho_{\rm right,0} > 1-\rho_{\rm crit}$, $\rho_{\rm right,0} > 1-\rho_{\rm left}$, the system approaches a state from class~\ref{classhigh} above with $\rho_{\rm left} = \rho_{\rm right} = \rho_{\rm right,0}$. \item For $\rho_{\rm left,0} \ge \rho_{\rm crit}$, $\rho_{\rm right,0} \leq 1-\rho_{\rm crit}$, the system approaches a state from class~\ref{classhighlow} with $1-\rho_{\rm left} = \rho_{\rm right} = \rho_{\rm crit}$. \end{itemize} This behavior is presented graphically in figure~\ref{phase}. \begin{figure}[hbt] \begin{center} \setlength{\unitlength}{0.24pt} \begin{picture}(1200,1100)(-50,-50) \put(0,0){\line(1,0){1000}} \put(0,1000){\line(1,0){1000}} \put(0,0){\line(0,1){1000}} \put(1000,0){\line(0,1){1000}} \put(-12,0){\makebox(0,0)[r]{0}} \put(-12,1000){\makebox(0,0)[r]{1}} \put(-12,300){\makebox(0,0)[r]{$\rho_{\rm crit}$}} \put(-85,500){\makebox(0,0){$\rho_{\rm left,0}$}} \put(0,-27){\makebox(0,0){0}} \put(1000,-27){\makebox(0,0){1}} \put(700,-27){\makebox(0,0){$1-\rho_{\rm crit}$}} \put(700,0){\line(0,1){20}} \put(500,-65){\makebox(0,0){$\rho_{\rm right,0}$}} \put(0,300){\line(1,0){700}} \put(700,300){\line(0,1){700}} \put(700,300){\line(1,-1){300}} \put(145,800){\makebox(0,0)[l]{$1-\rho_{\rm left} = \rho_{\rm right} = \rho_{\rm crit}$}} \put(145,750){\makebox(0,0)[l]{(type 3)}} \put(195,150){\makebox(0,0)[l]{$\rho_{\rm left} = \rho_{\rm right} = \rho_{\rm left,0}$}} \put(195,100){\makebox(0,0)[l]{(type 1)}} \put(725,700){\makebox(0,0)[l]{$\rho_{\rm left} = \rho_{\rm right}$}} \put(760,650){\makebox(0,0)[l]{$= \rho_{\rm right,0}$}} \put(725,580){\makebox(0,0)[l]{(type 2)}} \end{picture} \end{center} \caption{Phase diagram for infinite system with block; the axes represent the initial densities on each side of the block.}\label{phase} \end{figure} In the first two cases the system chooses whichever initial state has the lowest current. In the last case the system forces itself critical whenever the initial current on both sides of the block is greater than that which can be sustained through the block. \section{Conclusion} We have examined a version of the asymmetric simple exclusion process on a finite periodic lattice. Our model has many of the features of the time dependent system but none of the sensitivity to initial conditions, and one can use the presence or absence of particle-hole symmetry to indicate the model's behavior. It thus provides a more convenient platform for the study of the asymmetric simple exclusion process and similar models. We have used the presence of a ``slow bond'' as a technique to introduce a shock into the finite system, but the study of such an impurity is of intrinsic interest as well. One can examine the effects of inserting one blockage, or a periodic or random array of blockages, into the infinite system. We have identified some of the gross features of such systems, but much of the determination of the local and global properties remains an interesting open problem. Our investigations can also be extended to dimension greater than one. Preliminary results~\cite{ACJL} indicate the importance of particle-hole symmetry in the two-dimensional system and illustrate the utility of studying interface behavior by forcing the formation of a shock in a stationary state. \subsection*{Acknowledgments} We thank Maury Bramson, Zheming Cheng, Herbert Spohn and particularly Henk van Beijeren for their contributions to this work. \begin{thebibliography}{99} \bibitem{Lig} T. M. Ligget: {\em Interacting Particle Systems}, New York: Springer-Verlag, 1985. \bibitem{LPS} J. L. Lebowitz, E. Presutti and H. Spohn: ``Microscopic Models of Hydrodynamic Behavior'', {\em J. Stat. Phys.} {\bf 51}, 841--862 (1988). \bibitem{Dit} P. 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