I am writing a play and need to know (based on the current world
population), assuming there is one soulmate for every person on the
planet, what is the probability that you will find that one person?
Please provide math completed to arrive at answer.

Just take the amount of people you think the average person will meet
in their lifetime and divide it by the world population (current
estimates put it at almount 6.5 billion).

Have a good day.

The only problem with that is that I have no clue how many people the
average person meets in their lifetime. I don't even know where to
find that information.

I would think that, for most people, their soul mate would share a
common language and culture.  So I don't think it would be equally
likely my soul mate would be in Yemen vs the United States.

That is actually along the lines of what sparked my thoughts and led
to this new work. I now have three friends of mine who met the person
they consider to be their "soulmate" while abroad. One friend actually
does not even speak the same language as their soulmate - yet when
they met, and could not communicate verbally, they just enjoyed each
other's company. They take care of each other and are very happy - but
do not communicate the same way that most couples do. So it started me
thinking about the probability of finding your soulmate on a planet
over over One Billion.

Assuming a Random Distribution of Soulmates, S (soulmate)=
1/6,591,078,000 (Pop. of Earth),for each encounter with another
person, x how many people a person encounters (e) in a lifetime...or S
= 1/6,591,078,000 x e

However, the probability of recognizing that soulmate must be
diminished by the cumulative factors that distract a person's
attention (ex. weather, other people, time, etc.)  If a person's
ability to pay attention to others is diminished by an average of only
20% during the times of day during which they interact with people,
then it's really S = (1/6,591,078,000 x e )x .8

or S = ((1/6,591,078,000 x e) x W/(365 x L) x 6,591,078,000/(P x L) x V x R)- B

where 
S=Probability of meeting Soulmate
e=People encounters in lifetime
W=weather factor
L=Life Expectancy
P=People within eyesight in a day
V=Vision accuity as % of normal
R=Random unknowable chaos variable
B=Preconceived notions which stimulate irrational aversion

It's a wonder anybody finds anybody...

WOW! I love that equation. Alex, you're a huge help. Now I just have
to figure out how many people the average person meets in their
lifetime. That might be a different question for the board. Thanks
everyone for your input. And Alex, I think I may actually put that
entire equation right into the production. I think it would be
fascinatingly mind boggling for the audience to actually see a
mathematical representation of their changes of finding their
soulmate. It is a wonder, isn't it....

It's absolutely inevitable!

But who knows where or when?

Do be aware that Alex's equation does not account for the fact that
most people have a HUGE preference for 1 gender over the other (unless
this gender bias is "irrational aversion" which I would have to argue
against).
So you may want to cut that total population in half for a normal
person... or to a reasonable percentage (perhaps 60%) to account for
the fact that a small portion of the population consider both genders
to contain a possible soul mate.

Good point, jack-of-few. I guess that technically there would be a
straight variation of the equation and a gay variation of the
equation. Which then would have to influence the "number of people who
see in a day" variable based on the % of gay or straight people who
you see who do not qualify for your preference.

Jeez - this question seemed so easy and straightforward when I thought of it. 

Though I appreciate all of your input as it is making the production
more and more fascinating as the days go by.

Keep the comments coming! And thank you!!!

Actually, the "gay" and "straight" equation would be the same, as both
only have a preference for one gender. The equation would only vary
for the "bi" population and only to the degree that they were
indifferent to the gender of the soul mate.

As the equation plainly shows, meeting your soul mate is a function of
the people you meet. This explains why gay men are attracted to a
career in the military; where else can you meet so many young single
men who are physically fit? Your incidence of meeting men is much
higher than in civilian life.

When I was thinking of a gay and straight variation - I was thinking
more that even though gay people are attracted the same NUMBER of
people as straight people, current statistics suggest that only 5-12%
of the population (depending on the study) is gay. Thus, they
immediately have a much smaller selection. Conversely, a straight guy
is attracted to women, but only maybe 90% of the women he meets in his
lifetime are straight.

Do those sound like reasonable assumptions to make?

Common, I think your % estimates are reasonable... but they don't
account for the fact that gay people will have a higher tendancy to
meet other gay people because:
1) they are more likely to seek out gay people
2) having gay friends will lead to introducing friends to gay friends

It's not the case for everyone of course, but society has a tendancy
to create homogeneous contacts.  That'll be a hard factor to
calculate.

You know what though - that's actually a good thing to mention. Right
now, I'm envisioning having a sort of hyper profession start
scribbling on a blackboard all of the equation - and as he starts
simply, it will grow more and more complicated as he adds variable
after variable. I think to have him frustratingly make that comment,
jack, it would be a good way to drive the message home.

Thanks!

Two things - firstly, you need to take into account 6 degrees of
separation as we mostly meet people through networks.  This
dramatically increases the likelihood of meeting the one.  It works on
the principle that people know each other in small social networks,
some members of which have further reaching connections.  It's been
proven for people within the States and people within the context of
the internet (both, obviously, subgroups of total world population). 
More information can be found on
http://www.deeperwants.com/cul1/homeworlds/journal/archives/000160.html.

Secondly, to run your equation you will need to assign mathematical
values to your variables.

e=People encounters in lifetime
At a given moment, most people know 250 people, though not necessarily
on a first-name basis.  (See
http://working.canada.com/calgary/resources/advisors/story.html?s_id=%2BTa664bn1Ffls0uj9Ify23rXv1L2M7180MpmVrFWgNnwbB9JULHnpA%3D%3D)
How fast these people change will depend on how much a person changes
their town, job, and lifestyle, which varies a great deal.

W=weather factor

Does this need to be included?  Hot summer days make for lazy
conversations, rain makes for sharing umbrellas and lingering in
coffee shops...

L=Life Expectancy

This varies for men and women, so depending on the sexuality of your
characters you will need to adjust accordingly.  It also varies
dramatically between countries.  As your soulmate could theoretically
be from any country, you need to take an average.  Averages for each
country are available at
http://geography.about.com/library/weekly/aa042000b.htm, and works out
to 65.

P=People within eyesight in a day

This is repeating the number of people one meets.

V=Vision accuity as % of normal

Blind people also meet people and soulmates - unless you want to
exclude them from the equation, it might be a good idea to exclude
this one.

R=Random unknowable chaos variable

B=Preconceived notions which stimulate irrational aversion

Impossible to set a mathematical value on this, but one might safely
assume that anything as powerful as a soulmate will balance out
'irrational aversion'.