Can someone give me a geometrical description of the math in QM?

In summary, MikeyW is trying to visualize the time dependent wave equation in 3 dimensions. He is having difficulty because it is a function of four variables and he does not like the idea of picturing it in 5 dimensions. He has talked to his professor and found out that because he can still describe any point in the function with 4 coordinates, it is only 4 dimensional. He also understands that because the wave function is a real number, the square of the wave function is proportional to the probability of finding a particle in 4-space at position (x,y,z,t).
  • #1
superpaul3000
62
1
So I’m trying to visualize what is going on in QM geometrically. More specifically I would like to visualize the time dependent wave equation in 3 dimensions. So let’s start with dimensionality. Normally when I think of a function of some variables, I picture it in a space with the number of dimensions equal to the number of variables plus one for the function. For instance, y(x) is usually mapped in 2 dimensions. So then my first impression of the wave equation is that it is a function of four variables, psi(x,y,z,t), so I should picture it in 5 dimensional space right?


This seemed obvious to me but when I talked to my professor about it he didn’t like the idea but he didn’t really explain himself. So I thought about it some more and I guess his point was that because you can still describe any point in that function with 4 coordinates then it is only 4 dimensional (the definition of dimensionality). I think that does make sense. If we take y(x)=x^2, you could transform the coordinate system so that it is just a straight line with unique points or you could map it as a scalar field on the x axis. So then it really is only 1 dimensional. We then think of the wave function as a scalar field in 4 dimensions so I guess problem solved right?


I’m not so sure. Look back at y(x)=x^2 and add y(x)=x to that plot. Now there is no way to describe those two functions in one dimension because you need more than one coordinate to describe any point. Even if you transform the coordinate system or plot as a scalar field you will have overlap. This system is truly 2 dimensional. So what if we consider two electron guns firing together once every second. If you plot the scalar field of the probabilities of the two electrons then you need 5 coordinates to describe any point in that system (the x y z position, the time, and which particle). Trust me this sounds crazy to me too so I’m thinking I’m just missing something simple and obvious. Anyone care to explain?
 
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  • #2
superpaul3000 said:
y(x)=x^2 and add y(x)=x

Correct me if I misunderstood but adding these would produce a single value. The function would be f(x)=x+x^2.
 
  • #3
your wave function psi(x,y,z,t) maps R^4 onto C^1, four dimensions onto a single one in the complex plane (which does require 2 real numbers to specify).

How would you transform the coordinate system such that y=x^2 is a straight line? No linear transformation exists. Consider a cannon ball shot in the air; its path is parabolic. In whatever coordinate system you chose to apply this, it always reaches an intermediate hight twice during the path. The physical system dictates this, and it is necessary that physical phenomenon are independent of the coordinate system you chose to express them in.
 
  • #4
James Leighe said:
Correct me if I misunderstood but adding these would produce a single value. The function would be f(x)=x+x^2.

Ya but that is not what I'm talking about. That is still a function. I'm talking about the two functions combined in such a way that the result is not a function (2 y's for every x).
 
  • #5
MikeyW said:
your wave function psi(x,y,z,t) maps R^4 onto C^1, four dimensions onto a single one in the complex plane (which does require 2 real numbers to specify).

How would you transform the coordinate system such that y=x^2 is a straight line? No linear transformation exists. Consider a cannon ball shot in the air; its path is parabolic. In whatever coordinate system you chose to apply this, it always reaches an intermediate hight twice during the path. The physical system dictates this, and it is necessary that physical phenomenon are independent of the coordinate system you chose to express them in.

I guess coordinate transformation was a bad example. Can you elaborate on the first part you wrote?
 
  • #6
what MikeyW is saying is that your wave function is a mapping from Cartesian 4-space (R4) to the complex numbers (C1). That is each possible state in 4-space mapped to a single complex probability amplitude. Visualizing complex valued functions is not at all like real analysis.
The square of the wave function, however, is a real number, and is proportional to the probability of finding a particle in 4-space at position (x,y,z,t).
 
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1. What is the connection between geometry and quantum mechanics?

Quantum mechanics (QM) is a mathematical framework used to describe the behavior of particles on a microscopic level. It involves complex mathematical equations and concepts, including geometry. In QM, the position and momentum of particles are described using mathematical operators, which can be thought of as geometric transformations. Additionally, the wave function in QM is represented as a vector in a complex vector space, which has geometric properties.

2. How is geometry used in the mathematics of QM?

Geometry plays a crucial role in QM, as it helps describe the behavior of particles and their interactions. Specifically, geometric concepts such as vectors, matrices, and transformations are used to represent physical quantities in QM. For example, the position and momentum of a particle are represented by vectors, while operators such as the Hamiltonian are represented by matrices.

3. Can you explain the geometric interpretation of the wave function in QM?

The wave function in QM is a mathematical function that describes the probability of finding a particle in a certain state. It is often represented as a vector in a complex vector space, where the length of the vector represents the probability and the direction represents the phase of the wave function. This can be interpreted geometrically as the wave function having both magnitude and direction, similar to a vector in geometry.

4. How does the geometry of QM differ from classical mechanics?

In classical mechanics, particles are described using classical concepts such as position, velocity, and mass. These quantities are represented using geometric concepts such as points, lines, and vectors. In QM, however, these quantities are described using operators and complex numbers, which have a different geometric interpretation. Additionally, the laws of classical mechanics follow the principles of Newtonian physics, while the laws of QM follow the principles of probability and uncertainty.

5. Are there any limitations to using geometry in the math of QM?

While geometry is a powerful tool in understanding QM, it is not without its limitations. For example, the geometric representation of the wave function does not provide a complete picture of the quantum system, as it only describes the probability of finding a particle in a certain state. Additionally, the geometric interpretation of QM breaks down when dealing with more complex systems, such as those involving multiple particles and interactions.

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