From the text: "In the early 1950s, Heim began to rewrite the equations of general relativity in a quantum framework. He drew on Einstein's idea that the gravitational force emerges from the dimensions of space and time, but suggested that all fundamental forces, including electromagnetism, might emerge from a new, different set of dimensions. Originally he had four extra dimensions, but he discarded two of them believing that they did not produce any forces, and settled for adding a new two-dimensional "sub-space" onto Einstein's four-dimensional space-time.

In Heim's six-dimensional world, the forces of gravity and electromagnetism are coupled together. Even in our familiar four-dimensional world, we can see a link between the two forces through the behaviour of fundamental particles such as the electron. An electron has both mass and charge. When an electron falls under the pull of gravity its moving electric charge creates a magnetic field. And if you use an electromagnetic field to accelerate an electron you move the gravitational field associated with its mass. But in the four dimensions we know, you cannot change the strength of gravity simply by cranking up the electromagnetic field.

In Heim's view of space and time, this limitation disappears. He claimed it is possible to convert electromagnetic energy into gravitational and back again, and speculated that a rotating magnetic field could reduce the influence of gravity on a spacecraft enough for it to take off."

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