Regarding gradients: The gradients used in MRI vary in *position*. Yes in time, as well, but only because they are pulsed. We can ignore ramping issues to first order. Since the field varies as a function of position, when you move around, indeed the flux is changing which can induce currents in looped conductors so as to oppose the change. This is called induction. Many people, my self included, notice a strange sensation when first entering an MRI magnet. This is because the field is only homogeneous over a relatively small volume, outside of which there are once again field gradients (these are different than the intentional field gradients used to obtain an MRI image). It is probably not axons but something in the ear that is picking this up, I am not sure. Also, field strength has *nothing* to do with this effect. It's how fast the field changes as a function of position, i.e. the gradient, combined with the velocity of the pickup object.
Regarding repulsion: Water is diamagnetic. That means that the little spins (i.e. electrons) orbiting the atoms of a water molecule tend to align *against* the applied field direction. These spins will experience a repulsive force, hence the levitation.
Large and costly magnets? Why don't we just start levitating things at the LHC?
magnets aren't nearly strong enough, that's why
I am a scientist who believes strongly that government funding of R&D needs to be increased. Often times, I hear the argument that it is not the government's role to do this. Most of our basic R&D now occurs in the universities and the national labs. But it wasn't always so.
Several years ago, I was an intern at Bell Labs, in Murray Hil, NJ, the main research engine of AT&T before the 1984 breakup. Some of the greatest inventions of the 20th century were created there, including the transistor and the laser. The cosmic microwave background was discovered at Murray Hill as well, an example of a pure scientific discovery, serendipitous but yet made more likely by the concentration and dynamic of the brilliant minds working there. As time went on, the research became more and more applied, less basic, less fundamental.
By the time I got there, Bell Labs was part of Lucent, which was a slave to its stock price. All kinds of financial shenanigans were going on in the background, and the business had become focused almost solely on fiber optics and other communications media/equipment. Some of the leftovers from the glory days of basic R&D were retiring, but there were still quite a few more recent hires. These people were let go during my summer. It was sad. It was the death of Bell Labs. All that were left were the old fogies and the people doing work related to the core business. Lucent's stock tanked, and the whole company became a shell of what it once was, and Bell Labs became special only in the history books.
Bell Labs was the greatest death of the old industrial research powerhouses. Few are left, most notably IBM. But even these are more application-oriented than in the past. They depend on the government to fund basic R&D in its labs and universities to keep the technology engine revving. Should that process stop, perhaps industry will revert to its old way, but that will not be a quick process. For almost a generation, we would be left with our pants down while our global competitors assert the lead in the technology race. This will put us at not just an economic disadvantage, but in poor strategic positioning politically. It is paramount that we fund basic R&D via government funds now. If we desire a different system where private industry does the brunt of basic R&D, then we must redesign the system via proper incentives to allow for a smooth transition to such a paradigm. Maintaining science funding at the levels they are at right now is not sustainable in the short term- the quicker we enhance funding, the better off we will be.
After an instrument has been assembled, extra components will be found on the bench.