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Comment by pythonistic | Hacker News Reader

A very strong magnetic field has bad health (neurological and intracellular) consequences. I can't find a good citation, but this blog post should help explain the risks.

https://gravityandlevity.wordpress.com/2015/01/12/how-strong...

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That article is a completely different kind of "very strong". Our "very strong" is a 1-2 Tesla, the strength of a high end neodymium magnet. Levitation is about 10x higher than that but in practical terms, it's not really that different from holding a big magnet to your face. It does not affect any significant chemical reactions.

This article is talking about fields so high that chemistry fundamentally breaks down and matter breaks into individual atoms. This only happens in neutron stars. It's 1,000x stronger than any field made by humans and 10,000x stronger than levitation fields. At these fields the atoms literally change shape.

The frog levitates at 16 tesla. That's a long way from the fields the article talks about. Nevertheless, given that our brain reacts to magnetic stimulation, I'd be wary of using it as artificial gravity.

...Plus anything metal on/in you would be violently ripped out. This would make the fields used for a typical MRI look mild by comparison.

Inside a homogeneous magnetic field there is no force. It's kind of like being under very high pressure- humans can survive many hundreds of psi as long as the pressure is equal inside and outside the body.

The danger comes when there is a gradient to the field, as happens outside the coil. Then ferromagnetic material wants to move from the low to high field, like high pressure water will want to move to low pressure. As long as you're inside the high field area, you're okay.

This is part of the reason why people can be MRI'd even with shrapnel in their bodies. When the shrapnel is in the center of the volume being imaged, the field is relatively homogeneous except for the small induced gradient field. The shrapnel is already "touching the magnet", so it doesn't feel much more pull.

We're talking about using this in the context of "artificial gravity"... you're going to have a lot "outside of the coil" regions.

It would all occur inside the coil, as the field and therefor diamagnetic force drops off cubically outside the field.

Considering the original comment to which I'm ultimately referring, can you actually imagine such as situation as it relates to humans and "artificial gravity"? Presumably we're talking about the scale of a space craft or station...

Yeah, the entire station would be basically a giant solenoid. If you're outside the coil the artificial gravity doesn't work. You can have metal flying around without artificial gravity, but you can't get the artificial gravity in an environment that metal will accelerate inside.

Not necessarily be ripped out but it will heat up substantially.

Only in an oscillating field. In an MRI the gradient coils cycle on and off rapidly (rapid in human terms), which causes minor hysteresis heating. In a static field there would be no heating.

The human brain is almost completely unreactive to magnetic fields. The trick with TMS is that it creates an extremely short pulse- a time varying field, which induces a voltage at the focal point of the field. This voltage is on the order of 100 mV, which is well in excess of what is needed to trigger all of the nerves in an area to fire.

Static fields do not have the same effect unless you are moving through them at immense speed, eg passing from a region of 2 Tesla to 0 Tesla in a few milliseconds which would mean moving around the speed of sound through an MRI.

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