On short hops, a virtually weightless state is permissable, but if allowed on long journeys, the crew would suffer when re-subjected to gravity. That is why some form of on-board gravity is essential. Magnetic boots were ok for fifties "B" sci-fi movies but impractical for use in real life, and totally impossible if using non-ferrous metals in the construction of the ship. And magnetizing the deck would creat a magnetic field which would play havoc with instrumentation and render colour computer monitors useless.

So what about using rotating sections of craft as seen in some more recent sci-fi movies? Impractical. Newton also modified Kepler's Third Law which stated that "for every action there is an equal and opposite reaction" which basically means that the centre section of the ship would also rotate - in the opposite direction - thus making en-route stellar observations impossible.

There has been an extensive amount of research into gravity and how to manufacture it. But first we must know what it is. Well, Sir Isaac's description of an attraction is fair, but what causes or limits this attraction? On earth, gravity is measured as an accelleration factor of 985 centimetres per second every second. This acceleration due to gravity is approximately the product of the universal gravitational constant G and the mass of the Earth M, divided by the radius of the Earth, r, squared. But is this constant and reliable? NO. There is a limit to this accelleration. In early days, this was believed to be a result of friction from our atmosphere which slowed down the object as it neared the surface. But as man has acheived loftier altitudes, it has been found that the gravity accelleration is equally limited even in almost zero atmospheres. So perhaps we do not have all the story after all. So what causes gravity? Newton said it was the mass which was the controlling factor. But what is mass? It is not the same as weight. The earth's magnetic field is because of its molten iron core and it was once thought that gravity was somehow related to magnetic field. But look at Saturn which has no iron core (and thus no magnetic field to speak of) but has huge gravity.

So is it related to size? No. The moon has a sixth of our gravity but Luna is not a sixth of our size. Volume? Nope? Density? Think again. Saturn has 0.7792% of Earth's density but the escape velocity (speed needed to escape the gravity) is over three times as high (36.25 kilometres per second to the power of minus one compared to Earth's 11.18). So Comrade Newton's theory works fine on Earth but not very well anywhere else.

Bear in mind that Eistein's whole theory of relativity is based on the assumption that gravity is fixed. But it is not. Like so many other things, these theories are great in our own environment to which we have adapted them, but space travel changes all the rules. The goalposts, it seems, are on wheels (or thrusters).