# What is the difference between gravitation and magnetism?

If you compress a large mass, on the order of a star or the Earth, into a very small space, you get a black hole. Even for very large masses, it is possible in principle for it to occupy a very small size, like that of a golf ball.

I started to think, how would matter react around this golf ball sized Earth? If I let go of a coffee mug next to it, it would go tumbling down toward the "golf ball". Isn't that exactly how magnets work, with paperclips for example?

Magnets are cool because they seem to defy the laws of gravity, on a scale that we can casually see. Clearly, the force carrier particles that produce electromagnetic attraction are stronger than gravity on this scale (or are at least on par: gravity plays some role in the paperclips path, but so does electromagnetism).

My question is, why do we try to consider gravity as anything different than magnetism? Perhaps "great mass" equates to a positively (or negatively) charged object. Pull so much matter in close and somewhere you've crossed the line between what we call electromagnetic force and gravity force. They are one in the same, no?

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I'm going to leave this open for now, in the hope that you'll be satisfied with a decent explanation of why gravity and electromagnetism are completely different physical processes. But do be aware that this is dangerously close to the prohibition on unsubstantiated personal theories in the FAQ. –  David Z Oct 4 '11 at 9:07
I've proposed an edit for the original post which condenses it somewhat to focus on the core of the question; I hope this retains the sense of the question you would like to ask. –  Niel de Beaudrap Oct 4 '11 at 10:59

There are several qualitative and quantitative differences between gravity and magnetism.

1. When you attract 'neutral' bits of metal with a magnet, or attach it to something like a plate of metal, what's happening is that individual atoms of the metal react to the magnetic force. In a ferromagnetic metal, one with a similar electronic structure to Iron or Nickel, the individual atoms work like nanoscopic magnets; but they are very weak, and they are not lined up with one another, so that their fields cancel one another out over any macroscopic distance. But if you bring a "large" magnet (such as a fridge magnet) up to them, the field of the large magnet causes them to align with the field, so that they are pulled towards the magnet — and the magnet is pulled towards them. This is why some metal objects are attracted to magnets.

Other metals, such as aluminum or silver, also react to magnets, but much more weakly (and in some cases repulsively): the way that they react to magnetic fields is described as paramagnetism (for materials which align very weakly with magnetic fields) and diamagnetism (for materials which align very weakly against magnetic fields).

The very fact that different materials react differently to magnetic fields is something that sets magnetism apart from gravity. Gravitation works equally with masses of any sort, and is always attractive (as noted by Nic); magnetism can both attract and repel, and do so with different degrees of force, as between ferromagnetic, paramagnetic, and diamagnetic materials. But of course, quite famously, even a single object can be both attracted and repelled by magnetic forces: the north poles of two magnets repel each other, as do the south poles; only opposite poles attract each other. (This, of course, is the basis on which compasses work.)

2. The way that these forces operate over distance also varies. Gravity very famously (but only approximately) obeys an inverse-square law; the field far from a bar magnet, however, decreases like the inverse of the cube of the distance from the magnet.

3. Finally, moving electric charges produce magnetic forces; whereas they don't cause any gravitational forces which could not be accounted for just by the fact that the charged particles have mass (whether moving or at rest).

So, on both the macroscopic level and on the level of individual atoms, the forces of gravity and magnetism act quite differently.

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Gravity is only attractive, Magnetism is attractive and repulsive, hence not the same...

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There are profound physical differences between gravity and magnetism. The most important is that gravity affects all things passing through - charged matter, neutral matter, light, neutrinos, and even gravitational waves are deflected, all identically, by massive objects. It seems to be a property of the spacetime passed through, commonly explained these days as curved spacetime. Einstein and all that.

Magnetic fields, being one part of the electromagnetic field, affect only charged particles and things that have electric or magnetic dipoles (or more complex structures). Light, despite being an excitation of the electromagnetic field, passes by charged objects without deflection. Compared to electric fields, magnetic fields are interesting in that it acts perpendicularly to a charged particle's velocity, and increases in strength with higher speeds. (Knowing this is the secret to doing well playing Space Science Institute's Magnetic Golf game.)

Electromagnetism depends on "charge" whatever that ultimately really is, for quantifying both the strength of a field surrounding a charged object, and the response of a test particle in that field. Gravity is different. You could propose that mass is like charge - just compare Newton's inverse square law with Coulomb's - but no, there's something different. It really is all about spacetime geometry. Mass quantifies how much curvature you get from a given lump of mass/energy/momentum, but all test particles passing through a place are deflected the same regardless of their specific natures.

There are some interesting similarities between gravity and magnetism, however. If you are on a rotating platform and toss a ball, the ball will appear deflected by a "Coriolis force" appearing to push perpendicular to the ball's velocity, and also depends on the ball's speed. Interesting, but non-profound.

Also, it should be noted that massive spinning objects such as the Earth do produce a twist in spacetime that could be compared to a magnetic field. This gravito-magnetic field bears the same relation to the ordinary radially attractive gravity field due to the mass, as the magnetic field around a spinning charged object bears to the radial Coulomb field due to the charge. We are looking at the space or the time components of a holistic 4-dimensional tensor. This is profound, at the level of math and ways of representing physical fields with tensors and vectors, but still does not indicate profound physics.

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Just to be pedantic, have we actually measured the effect of gravity on neutrinos? To my knowledge, I don't think there has been such an experiment. –  Jerry Schirmer Oct 4 '11 at 23:41
Well, no, and there is that recent news story about the CERN neutrinos. So my web of lies unravels... –  DarenW Oct 5 '11 at 6:04

There are significant differences as many have pointed out. But, there are also many and great similarities. You stumbled on a similarity; how objects fall in an energy field.

Someone here said gravity and magnetism are completely different things. I disagree. And I believe most physicists would also disagree and tell you they are different forms of the same thing. But we don't know how that is. Yet!

If you can answer your own question completely, you will have found a significant part of the "Theory of Everything" (TOE) sometimes referred to as Unified Field Theory; and you will very likely be given a Nobel Prize for it.

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