How Einstein saved magnet theory

Magnetism is one of the most bizarre of known classical physics phenomena, with many counter intuitive effects. Even weirder, when one uses Maxwell’s equations (the laws that describe electromagnetism) and traditional Galilean relativity, you can see that magnetism makes no sense at all. However, when one uses Einstein’s theory of relativity, it all makes perfect sense. In this video, Fermilab’s Dr. Don Lincoln helps sort it all out.

Magnetism for parallel wires:

Magnetism for parallel wires:

Purcell simplified:

Purcell E M and Morin D J 2013 Electricity and Magnetism Harvard University Mass. Third edition pp 259-264.

Length contraction video:

Fermilab physics 101:

Fermilab home page:

[jackieking]

I did know this.... was taught it 55 years ago and remember the feeling of amazement that relativity at such gentle speeds could so precisely explain electromagnetism. Thanks for bringing it all back.... it will help me fade away with a smile.

[mikecannon]

I heard this in passing about 45 years ago. It stuck with me but I never managed to chase down the details. Thank you.

[willi-fgdh]

there once was a fencer named Fisk
whose action was exceedingly brisk
so fast was his action
the Lorentz contraction
reduced his rapier to a disk.

[CarBENbased]

That was nuts... I was getting more confused but you set me up perfectly for it to click when you brought in the length contraction animation! I'd love to see a video on how this applies to permanent magnets and maybe even induced temporary magnetism.

[frederf]

I remember this explanation from school. I suppose given that E and M must function in all reference frames you could derive special (probably also general) relativity from the known results. The fact the E is M and M is E through relativistic transformations is so cool. Also it shows just how insanely charge dense normal matter is.

[Brindlebrother]

3:56 The physicist: "Everything I just told you is right"

Me who's not a physicist: "Welp, that's good enough for me"

[regth]

Always a pleasure to learn from these videos. Thank you to everyone involved with these presentations!

[muraliavarma]

This is actually insane. So relativistic effects of magnetism happen at such low speeds too? I should probably read the math behind it but one observer's magnetism is another one's electricity? Truly mind blowing!

[tensaisenshi]

I've heard of this before, but your way of explaining it really clears things up. Now I can say that I know, rather than having heard of this effect. Thanks.

[bramfran]

Thanks for sheding light on (mysterious) magnetism and for providing source with more info. I would absolutely love to see an explanation of why inductors and transformers work the way they do using only this phenomenon!

[Earwaxfire]

This is a good start. I would also note that the Coulomb force is so strong that even a small change in charge density produces a noticeable effect. Also the mass of proton charged nuclei is much greater than electrons and that makes them move slower giving rise to the greater special relativistic contraction of the flow of electrons in the frame of the outside charge observer. It would be fun to look at the difference between AC and DC currents.

[SiqueScarface]

Albert Einstein invented Special Relativity to explain Magnetism in his 1905 paper "On the Electrodynamics of moving bodies", where his starting point was the apparent asymmetry of a moving wire and a stationary magnet vs. a stationary wire and a moving magnet.

[himanshukhanna]

The concentration of charge due to length contraction causes electrical repulsion thus resulting in what is called the magnetic force. But how does that work for two point charged particles?

[philippk]

How does all of that work for freely moving charges in a vacuum without the presence of the opposite charge as in the wire?

I had to study the Berkley Physics Course books in the 70´s at the Groningen University so I got familiar then with the idea that magnetism comes from a special relativistic effect. As far as I can remember the explanation was set up differently in the book, but the idea is the same and at the time I was really excited by explanation.

[InssiAjaton]

Regarding the note about the polarity naming of current flow, I recall an old (older than me) book describing an electrolytic solution. It could and indeed had been used as a rectifier. And because the "material" - positive ions flow was chemically more evident than any flow of the electrons, that became the positive direction. So, the polarity was established by wet chemistry instead of either solid state or gaseous state observations. Such a funny thought!

[unduloid]

Remember: It's OK to be a little crazy!

[misterphmpg]

Hi Don, I imagine these very tiny relativistic effects on the moving electrons do matter in the macroscopic world because there are trillions and trillions of them and they add up to a macroscopically measurable force. But what happens if only two of them pass each other along their way? They interact by exchanging a virtual photon, so how does quantum mechanics fit into this picture? Do those exchanged virtual photons between the wires also add up to make that electric force? Should be, but for me that makes it even wilder to imagine... thanks for your amazing video!
And: if you look at many comments below it's amazing how easy it is to get dozens of nobel prize winners to pop up by just mentioning SRT. THAT is Einsteins most magical force it seems.

[corradoghinamo]

Phenomenally clear explanation. Congratulations

[spoddie]

If Insane Clown Posse don't understand magnets, then I have no chance