Do negative numbers have any physical meaning? So, mindlessly wandering off into space, thinking about quantum and how cool physics is, I came to a realization that... well.. negative numbers to me make 0 sense.
You have either something, or not something.
It's always a yes or no answer, with everything
And everything always has an opposite; it's nonexistence.
I either have a red scarf, or not.
If there are 3 doors, labeled 1 2 3. I have to choose one of them. The choice isn't me choosing which door, the choice is what door i'll pick. I'll either pick door 1, or not pick door 1. The opposite of door 1 is no door 1.
Lets go really small. I have 2 grains of salt. I can't take 3 grains away.
I believe negative numbers were invented by man to create debt. Because it doesn't make any sense in physics. Can someone please explain this to me!!
 A: The first attempts at formulating a model of nature had no numbers in it. Numbers is the province of mathematics. Arithmetic was created really when bartering became necessary as communities became larger and larger and a number system was needed to keep a fair track of who owes what to whom, as you say. It is arithmetic that was the start of mathematics. Not physics.
When land possession started people used arithmetic to keep track of what was what, and finally geometry and then algebra were invented. Negative numbers started coming in solutions of algebraic equations

For a long time, negative solutions to problems were considered "false". In Hellenistic Egypt, the Greek mathematician Diophantus in the third century A.D. referred to an equation that was equivalent to 4x + 20 = 0 (which has a negative solution) in Arithmetica, saying that the equation was absurd.
Negative numbers appear for the first time in history in the Nine Chapters on the Mathematical Art (Jiu zhang suan-shu), which in its present form dates from the period of the Han Dynasty (202 BC – AD 220), but may well contain much older material. The Nine Chapters used red counting rods to denote positive coefficients and black rods for negative.

It is after all a useful way of keeping count of what is owed and what is gained in any human transaction, in two sets of numbers. Instead of using left hand fingers  for owed right hand fingers for gain, a symbol instead meaning "left hand".

Lets go really small. I have 2 grains of salt. I can't take 3 grains away.

No, you can count on your right hand "two in hand", and on your left "one in the bush" meaning expecting to get it from somewhere, or someone else's kitchen.
In any case negative numbers existed in mathematics before ever being used in physics.
Now for physics, mathematics is a tool. I have heard it said that "physics theory is a subset of mathematics" I suppose by mathematicians ? But it is not true. Mathematics allows for beautiful self consistent theories where positive, negative, complex and even worse definitions hang together and produce solutions .
Physics takes mathematical self consistent constructs and fits them, imposes them on physical observables by introducing physics postulates and laws on top of the mathematical axioms . These correlate physical situations, space and time changes, and negative numbers come with the package (also complex ones). Solutions of the mathematical equations predict data to be seen in the future. Physics cannot be blamed for negative numbers.

I believe negative numbers were invented by man to create debt.

You are correct, on the first part, but not to create debt, to keep count of debt in a better way than a left hand and a right  hand  give and take tally.
A: For certain questions you may pose about nature, a negative value makes no sense as an answer. Your question about subtracting grains of sand is an example of one such question.
For a vast range of other physical questions, negative numbers have an important role to play. Here's one way to start appreciating this:
We find it helpful to assign numbers to locations in space, called coordinates, in order to measure distances. For these coordinates to make sense, we need to choose a reference point from which distances will be measured in order to assign coordinates. This reference point can be assigned the value of zero in each of the three spatial dimensions. 
Now consider one direction, or axis, in relation to the reference point, like the north-south direction. We might decide to give a location a positive coordinate on the north-south axis if it lies north of the reference point. If it lies south of the reference point, we would then give it a negative value. Setting up a coordinate system like this allows to easily calculate distances between two locations using the ordinary laws of arithmetic applied to the coordinates, including the laws relating to negative numbers.
For example, if there is one location that is assigned a coordinate of 3 units north, and another that is assigned a coordinate of 2 units south, then the distance between them is 3 - (-2) = 5 units.  
Note that the location of the reference point, and the convention to assign positive coordinates to locations north of the reference point, were arbitrary decisions. That's okay. We still measure accurate distances even if we change these conventions by moving the reference point or making south the positive direction.
I've focused on distance coordinates here, but you'll find that negative numbers often enter into physics in similar ways for other quantities such as velocities, forces, electric currents, and generally speaking anything in physics that is a vector.
There are other ways that negative numbers can be useful aside from vectors, but learning about vectors is a great place to start.
A: Negative numbers might seem weird when you count things, as you've indicated but they do appear in many other contexts. Take a one-dimensional coordinate system, for example. Negative numbers in this context indicate position relative to some arbitrarily defined zero. To cite just one other example, more inline with your own, consider how negative numbers can indicate a decrease in a quantity. You have 3 grains, and 2 were taken away. The change is $-2$.
A: Ever noticed the difference between negative temperature and positive temperature?
You can feel the difference!
A: A negative number can be antimatter and complex number are a virtual world which is perpendicular to our world and interact with us!
the reason we have matter more than antimatter lies in cross and division if you cross or divide two negative number (antimatter) you have a positive number (matter)  
