In this series, I’m discussing how ideas from calculus and precalculus (with a touch of differential equations) can predict the precession in Mercury’s orbit and thus confirm Einstein’s theory of general relativity. The origins of this series came from a class project that I assigned to my Differential Equations students maybe 20 years ago.
One technique that will be necessary for this confirmation is the method of successive approximations. This will be needed in the context of a differential equation; however, we can illustrate the concept by finding the roots of a polynomial. Consider the quadratic equation
.
(Naturally, we can solve for using the quadratic formula; more on that later.) To apply the method of successive approximation, we will rewrite this so that appears on the left side and some function of appears on the right side. I will choose
, or
.
Here’s the idea of the method of successive approximations to obtain a recursively defined sequence that (hopefully) convergence to a solution of this equation:
- Start with an initial guess .
- Plug into the right-hand side to get a new guess, .
- Plug into the right-hand side to get a new guess, .
- And repeat.
For example, suppose that we choose . Then
This sequence can be computed by entering into a calculator, then entering , and then repeatedly hitting the button.
We see that the sequence appears to be converging to something, and that something is a root of the equation , which we now find via the quadratic formula:
.
So it looks like the above sequence is converging to the positive root .
(Parenthetically, you might notice that the Fibonacci sequence appears in the numerators and denominators of this sequence. As you might guess, that’s not a coincidence.)
Like most numerical techniques, this method doesn’t always work like we think it would. Another solution is the negative root . Unfortunately, if we start with a guess near this root, like , the sequence unexpectedly diverges from but eventually converges to the positive root :
I should note that the method of successive approximations generally converges at a slower pace than Newton’s method. However, this method will be good enough when we use it to predict the precession in Mercury’s orbit.