> CFTs > how-special-relativity-makes-magnets-work-veritasium

How Special Relativity Makes Magnets Work

Veritasium - 2013-09-23

MinutePhysics on permanent magnets: http://www.youtube.com/watch?v=hFAOXdXZ5TM
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Magnetism seems like a pretty magical phenomenon. Rocks that attract or repel each other at a distance - that's really cool - and electric current in a wire interacts in the same way. What's even more amazing is how it works. We normally think of special relativity as having little bearing on our lives because everything happens at such low speeds that relativistic effects are negligible. But when you consider the large number of charges in a wire and the strength of the electric interaction, you can see that electromagnets function thanks to the special relativistic effect of length contraction. In a frame of reference moving with the charges, there is an electric field that creates a force on the charges. But in the lab frame, there is no electric field so it must be a magnetic field creating the force. Hence we see that a magnetic field is what an electric field becomes when an electrically charged object starts moving.

I was inspired to make this video by Prof. Eric Mazur http://mazur.harvard.edu/emdetails.php

Huge thank you to Ralph at the School of Physics, University of Sydney for helping us out with all this magnetic gear. Thanks also to geology for loaning the rocks.

This video was filmed in the studio at the University of New South Wales - thanks to all the staff there for their time and support.

Music: Firefly in a Fairytale, Nathaniel Schroeder, and Love Lost (Instrumental) by Temper Trap licensed from CueSongs.com

hobyrne - 2016-09-23

On my fourth viewing of this video, I realized I was looking at... a cat-ion.

chun yan gong - 2019-07-15

@Nine Eleven how?

y - 2019-12-07


Mark Mays - 2020-02-07

I was just watching this and thinking is this a cation pun? I need to see if anyone else thinks so. Thank you lol

Sean Kauder - 2020-02-12


casa home - 2020-02-25

now it makes sense

Destro Metro - 2019-07-12

Wife (While jogging): Honey, am I looking slim?
husband: Not even in my frame of reference?

Vish - 2019-07-17

So hows your single life going now

Bill A - 2019-07-27

Trust me, after 20 years in wifey prison, single life feels friggin' awesome! xD

ki kus - 2019-08-06

Vish woah no need to be so mean . Did u get offended by this joke? And personally i want to know why

Vish - 2019-08-06

@ki kus Is that supposed to be sarcasm or what? What led you to believe i got offended, I implied that his wife got offended and divorced him. Sorry Im confus

Hassan Akhtar - 2019-11-19

@ki kus idiot

Energy Man - 2019-07-07

In the case when cat is at rest and it sees electrons moving, the electrons are more densely accumulated and the positive charges are spread out. So according to this logic, even rest charge must interact with magnetic field.

Fuseteam - 2020-02-12

@Dr Deuteron 1. ions most definitely move
2. general relativity and quantum mechanics are fundamentally incompatible. Special relavitity is works just fine with quantum mechanics

Hashbrown - 2020-02-22

@Fuseteam I think what they're getting at is that the cations don't move overall. Electrons come out of the powersource in DC and replace those in the conductor, which are pushed off to the next part of the circuit. Positive ions DO NOT MOVE into the powersource to compensate. Or are you saying they do? This is a solid wire, not an electrolyte. The metal nuclei might move, but not in a net fashion to counteract the moving electrons, otherwise the copper would all melt and glob at one end, right? And if they move and "move back" then that wouldnt equalise anything. Heisenberg principle doesnt come into it since we're talking net.

I too would like to know why a stationary (with respect to the net shape of the conductor) positive charge wouldn't experience a force where electrons are moving through the conductor, yet this same positve charge if moving along with those electrons would.

I think maybe dr deuteron has helped by mentioning it's not a straight frame of reference. The wire is coiled.
But Im starting to think that, if the electron is a point particle we can ignore contraction on the electron itself, and maybe the spacing between electrons in this 1D representation doesnt actually contract either from the perspective of the wire and those stationary to it. Density is maintained even in flowing current, so no force. Then once a cation moves relative to the live conductor both charges in there have their density changed in opposite directions?

Fuseteam - 2020-02-23

@Hashbrown well in that context the electrons don't move in that way either. in fact what is really moving the photons that these electrons and "cations" emit and absorb to transfer energy, the power source isn't some type spring of electrons its a sourle of an electric field that excites charged particles.............

the point is from the perspective of the electrons the nuclei are moving and thus contracted producing a net positive charge, from the perspective of the nuclei the electrons are moving thus contracted producing a net negative charge and from the perspective of an outside observer both are moving cancelling each others electric fields producing a net neutral charge.

if the nuclei did not move then the wire would be negatively charged and things would be attracted to it making wires very dangerous :p

Roger AB - 2020-02-24

When there's no flow of current in the wire it stays neutral, then current is applied, electrons move so the length between them must contract increasing their density and negatively charging the wire contrary to what the video says. If the charge of the wire was neutral while electrons were moving, the wire should be positively charged when there's no current.

Fuseteam - 2020-02-24

@Roger AB well no because nothing ever stands still on that scale, all "flow a current through the wire" actually does is give direction to movement of both the positive and negative charges

Pino K. K. - 2018-11-27

Trying to be flirty with nerds:
"You're looking slim"
"only in your frame of reference"

rent a shill - 2019-02-06

"Only in your frame of refrence"
gotta love that relativity jokes

zak ryals - 2020-02-12

Man you really get physics, you should be an astronaut why don't you make a clever joke about maths too. Theyll probably send you first to mars.

Martin Verrisin - 2015-10-29

"You are looking slim."
"Only in your frame of reference."

- how is this not a famous quote? XD

Dusara Gamidu - 2017-07-09

Because it happens only if there's a relative motion between the 2 ppl

Seth Bettwizilch - 2018-08-15

Person 2 calls person 1 fat.

theMetal973 - 2018-12-03

cringe quote xD

glitch gamer - 2019-01-26

The lol

musaire - 2019-10-04

@Võ Thái Sơn Tell them to not move perpendicular to you to the disc of accretion!

Brant Axt - 2019-07-02

If the electrons are moving relative to a stationary observer, won't they be contracted, and thus have a higher density of negative ions compared to positive ions, and thus be negatively charged to the observer?

mcfigu - 2019-10-17

@Mike Zelik you sure? Well in that case yeah it makes sense. Thanks for clearing this out man.

Anthony Norman - 2019-11-06

@Mike Zelik Hi. So it is a phenomenon? Then it is not explained, right? That would suggest that special relativity does not apply due to a phenomenon (magic)?

OneShotBruh - 2019-11-08

@Mike Zelik great explanation, thanks a lot!
The video would have been so much better if they explained this.

Gary Barbour II - 2020-02-13

It seems like if the electron density increases to maintain neutrality in our static frame of reference, then is should be proportionately magnified for the cation as well, leaving a neutralizing effect on the increase from proton contraction.

Atila Alp - 2020-03-01

Yes they already attracted.and experiments show no force between a stationary charge and a wire that has a current.

ChompChompNomNom - 2018-08-07

I've watched 5 videos today on what magnetism is and none of them actually explained what the "field" is made of or how it comes out of the material and back in again

David Schmidt - 2019-01-28

@kestutis nikolajevas He's an idiot, trying to fool people with made-up terms, and alluding to things he genuinely has no grasp of.

Peter Anoia - 2019-02-12

The 'force carrier' of a magnetic field is a 'virtual' photon. Like @PlanetBloopy says, the Standard Model is as close as we are to describing the fundamental particles that do the work, but it's an incomplete explanation. The virtual photon is a quantum of the field interaction.

Yep Yep - 2019-03-22

Could we say that those « virtual force carrier particles » moves at the speed of light. And if so, could their intensity be affected by dopplers effect?

` - 2019-04-18

It doesn't "go out and in". Its just that the force vectors in every point in space line up in such a way that makes it seem like it is going out and in.

Sharnu Nirgudi - 2019-10-22

Even scientists are trying to find it out

aligator381 - 2019-07-06

From Derek's frame of reference, don't the moving negative charges contract and therefore appear to be in higher density? What am I missing?

Mike Zelik - 2019-09-16

Your confusion stems from a little oversimplification in this video. The positive charges--or rather, the distribution of positive charge--is in reality not actually static but moves in the opposite direction as that of the negative charges.

Basically, you can loosely think of it as if an electron migrates away from a nucleus in some direction towards some other positive charge then it's former location because more positive, while the latter becomes more neutral. So, you have like a net negative charge movement or stream in one direction, together with a positive-like charge movement or stream in the opposite direction. So, from a rest charge perspective these opposing streams merely cancel one another out and the so-called "current-carrying" wire overall actually appears simply neutral. But, if that external charge happens to just so ever budge into the direction of one of these streams, then it will experience that whole associated effect (magnetic repulsion or attraction) due to the length contraction phenomenon presented in this video.

Freda Albl - 2019-11-19

Mike Zelik Finally, this is exactly the explanation I needed, thank you!

Ybis Zedex - 2019-12-19

@Mike Zelik But fisically there are static positive charged nucleus and moving free electrons. Why we must think of it as an opossing streams of abstract charges?

unknown - 2019-12-28

You all are making a wrong assumption when electrons or to be precise "negative charges" is moving in right direction "positive charges" is moving in left (opposite) direction, with same magnitude of velocity (speed) so when cat it is at rest length contraction does in fact happens with negative charges but it also happens with positive charges with equal magnitude, but when the cat starts moving in one direction (say in direction of of negative charge) the relative speed of flow of negative charge decreases and date of positive charge increases, making length contraction differ in magnitude !! And that explains everything.
P.S- when negative charge moves in one direction positive charge doesn't remain stationary it always moves in opposite direction.

Fuseteam - 2020-01-09

@Ybis Zedex because nothing is static on the subatomic scale everything is always moving

Dave Mateo - 2017-12-11

This was amazing. Took a whole bunch of stuff I thought I knew and showed how it actually worked. Mind = Blown

Sarthak Girdhar - 2019-10-01

0:52 that contracted car looks hilarious lmaaao

deadlyvirus - 2019-07-31

Please send help, I have been trapped in a loop of clicking on the video from the alternating channels, at each end I just end up at another beginning, which was my previous end, but also my previous previous beginning, but also my previous previous previous end, but aslo...

How do I become FREE!

Tamás Mile - 2020-02-13

I feel the more I understand it the more confused I am. So how does the electromagnet attract a not moving metal object then?

Wesley Ashley - 2019-09-22

Need a cat for relativity since quantum mechanics has a cat. Then maybe they will get along.

richard meagher - 2016-05-13

A positively charged cat is a cation.

Electromorphous - 2018-01-21

richard meagher oh goood noooo. Why. Why???????

Chasing A Murderer - 2018-03-18

richard meagher if he wanted you be a idiot, he say, "all know it all's comment Bellow"

FGV Cosmic - 2019-05-06

Pawsitively charged

PandemoniumMeltDown - 2019-06-18

@FGV Cosmic Oh this went way fur ther than I thought it wood. Joy!

P Ä - 2019-06-27

That’s your takeaway from this video???
You should win the Nobel prize

Arull Murugan S - 2017-08-30

Thank you . you helped me to fight with my friend about magnet.

Shirsh Zibbu - 2019-05-23

1:43 "the cat is moving in the same DIRECTION as the electrons with the same VELOCITY".

vladislav vasilev - 2019-09-19

Hold on, if magnetism is a relativistic effect then why are string theorist looking for a mono-pole particle?

TheDJHoller - 2019-08-24

I was about to go onto minutephysics' video when I realised that was where I came from in the first place

lk K - 2016-01-29

but from my frame of reference the jedi are evil.

Andy Arb - 2019-05-01

-then you are lost!

P Ä - 2019-06-27

Lord Vader is that you?

John Ortiz - 2017-10-05

God this is beautiful explain, on all my physics class i just assumed the magnetic force as a fact, and you just proved it with relativity

Jim C - 2018-07-05

This vid is brilliant! I've never heard such a thing between relativity and magnetic field! (is it a new theory? being a EE engineer I feel embarrassed not knowing that before)

slavarus - 2018-02-03

"magnetic field is an electric field seen from a different frame of reference" - BEST explanation ever!!

tom kop - 2017-12-11

Whene were magnets discovered and by whom?

Michael Englo - 2015-11-29

But what if the cat is not moving? Aren't the electrons moving relative to the cat, which means more density of electron and thus pulling the cat?

Dr Deuteron - 2020-02-11

@Steve Greenfield No. The electrons are not a rigid object, and their spacing increases in their rest frame and remains at 1 lattice spacing in the lattice frame.

Steve Greenfield - 2020-02-13

@Dr Deuteron What is a "rigid object"? Does a bag of rocks fall at a different speed than one solid rocks, or a bunch of rocks not in a bag? How about if a group of rocks flies by at a significant fraction of the speed of light? Do you think the rocks will look foreshortened, but the distance between them will not? Why not, when if they are touching, you would not expect to see the relative spacing increase?

Dr Deuteron - 2020-02-14

@Steve Greenfield why complicate with gravity, it's harder than SR? You can read all about Born Rigidity on the web--it's a large subject. In this abstraction, all electrons begin linear coordinate motion at the same time in the wire frame, and that would violate causality for a rigid object. Because their spacing doesn't change in the lab, they must be spread out in their conducting rest frame.

Atila Alp - 2020-03-01

Yes they already attracted to each other and experiments show no force between a stationary charge and a wire that has a current.

Atila Alp - 2020-03-01

@AP PPP consider this :we have twe parallel wire, left wire and right wire and they have same current and in same direction :in right wire positive charges see no difference between positive and negative densities in left wire so they neither feel pulling or pushing(because they are stationary) but the negative charges in the right wire see some difference between negative and positive charges densities so they feel some forces.

Kevin Player - 2017-11-11

I saw this in my dad's physics book many years ago. It is beautiful. Thank you for the video.

Maybe try the Aharanov Bohm effect next so we can all grok gauge theory.

Edwin John - 2019-09-16

1:49 the cat just touches his hair's tip 😂

pappu - 2020-02-10

I finally got a answer to one of my life long questions.

obada dukhan - 2019-07-22

@Veritasium So you're saying that the cat will feel the magnetic force only if it was moving?
If so, Then why does a compass feel the magnetic force of the wire even if they both are stationary relative to each other?

Gambeir Bay - 2019-11-26

@Paul Jenkinson Yea, Paul is right, and Paul....Just look at all these poor people who are being told all this crap about Einsteian Physics. Good God it's like being a survivor of an alien pod people invasion.

Gambeir Bay - 2019-11-26

​@DANG JOS Electron Theory works fairly well for electrical engineering. It doesn't work when trying to explain a magnet. Jenkinson is right on this. Einsteinian Physics is well managed and overseen by an army of gate keepers. Maybe your'e one, but it's fundamentally a prop used to cloak the real physics which predated the 1930's overthrow of the Ether Theory, and that theory became part of the National Security State classified higher than the H-Bomb.

DANG JOS - 2019-11-26

@Gambeir Bay The electron isn't just a theory, it's an observed phenomenon. Denying it is almost like denying the existence of atoms at this point. We have electron microscopy and have even done the double slit experiment with single electrons. There already exists theories for the electron and its relation to electromagnetism. Do you really think physicists haven't thought of this? If you really think you have some revolutionary discovery that overturns these theories, then I challenge you to go to your local University, and bring these ideas to a theoretical physicist, or some other physicist in a related field.

Mike Zelik - 2019-11-29

Lol to answer the OP's question (obada dukhan's) ---> the generally accepted answer is that static magnetism requires quantum mechanics not explained here.

Dr Deuteron - 2020-02-11


Grzegorz Baran - 2016-08-03

At primary school I never understood why magnets work... Now I see why it never gets explained to kids;)

Manikiran Reddy - 2018-12-27


NUKE - 2019-02-01


HuKu TON - 2019-02-19

even adult cant understand

Jan Kowalski - 2019-07-23

I never understood too... And now I do

sedative chunk - 2017-10-26

"If you can't explain it simply, you don't understand it well enough." - Richard Feynman

t0caia - 2019-08-24


am trapped in a loop between MinutePhysics and Veritasium >.<"

michael steven - 2019-08-13

I love how interesting these videos are

Calisto Hüttich - 2020-01-19

I don't get it. Since when do magnets require a moving positive charge?

Volbla - 2014-12-01

I got it! I finally got it! I was wondering, from the stationary point of view, why the electrons don't contract and attract the positive cat. It is because from the moving cat's point of view the entire world - the positive nuclei and the wire along with it - is moving. That is why the space between the positive charges contract. Because that space itself (i.e. the wire they're in) is moving in this frame of reference.

From the stationary point of view, however, only the electrons themselves are moving. This only means that the electrons are squished down a bit and get flatter, but the space between them (again, the conducting wire) is stationary. Therefore the distance between charges remains the same and there is no change in the charge density.

I feel so happy right now.

Kenr Nimmo - 2019-09-07

@Carina https://www.youtube.com/watch?v=QL-VevqUgMc&t=277s it is explianed better in this video. at 4:00 he says the electrons are more spaced out from the external charge perspective. they say it in this video also at 2:08. it isnt that they get spaced out when the cat ion moves with them. this is just when the cat ion can see them more spaced out. from the electrons reference frame they become more spaced out when they start moving. this is only the case because this video says there is the same density when the wire has no current or when it has current. not a phenomenon you can learn about. it is just plain special relativity.
if you understand relativity and logically think about it then this must be the case. in order for the density to remain equal when the electrons start moving (from cat ion at rest reference frame) they must have become more spaced out in their reference frame. if this was not the case they would have had length contraction associated with their movement (in cat ion at rest reference frame). that is really all there is to it. how else could electrons have kept the same spacing and had length contraction. from their perspective they became more spaced out when they started moving. then the cat ion sees the same when it moves with them.
i think that the reason you dont get what im saying is because you are thinking about the cat ion moving with electrons and seeing them in their proper frame. this means seeing them at rest without any contraction and therefore the same spacing they had. what your not thinking about is that before the cat ion started moving with them they had contraction yet equal spacing as the protons. then when the cat ion moves with them this contraction goes away and they become more spaced out.

חגי שפירא - 2019-11-30

nice, i was getting disappointed from all the previous incorrect "solutions". this seems right finally. thanks

Kenr Nimmo - 2019-11-30

if that reply was too the original comment it is not right. when the electrons start moving they spread out in their proper frame but stay the same spacing from the cat ions reference frame. this is just so the video can make the point it is trying to make. it needs the wire to stay neutral. whether this would actually happen in practice is not the point of the video.

unknown - 2019-12-28

You all are making a wrong assumption when electrons or to be precise "negative charges" is moving in right direction "positive charges" is moving in left (opposite) direction, with same magnitude of velocity (speed) so when cat it is at rest length contraction does in fact happens with negative charges but it also happens with positive charges with equal magnitude, but when the cat starts moving in one direction (say in direction of of negative charge) the relative speed of flow of negative charge decreases and date of positive charge increases, making length contraction differ in magnitude !! And that explains everything.
P.S- when negative charge moves in one direction positive charge doesn't remain stationary it always moves in opposite direction.

Atila Alp - 2020-03-01

No that's is not correct. Consider the distance between two electron when they are stationary and when they are moving at same speed. When they are moving the distance between them contracts.

Luke Palmer - 2019-11-27

After saying all those zeros at 3:20 you can hear his nerd accent coming through

ej felipe - 2019-07-16


Arull Murugan S - 2017-08-30

I am a great fan of you and also special relativity. I always wanted to talk to you and ask a lot of questions which is infinite to my frame of reference

Keith Reynolds - 2019-07-02

2:30 How dare you push that cat over. 🤣

F FRANKLIN - 2018-09-03

///Welcome to Babylon!!!:)
The land of confusion...

TheRecycledToys - 2017-09-30

I love your 999999...thing .That was funny.
laughed my head off!

Kornelije Kovac - 2018-04-11

2:09 "Remember, objects take up more space when they're not moving , than when they are."
Why doesn't that happen with protons at 1:30?

Life of Phy,rap,run!!! - 2019-08-24

0:46,wear headphones and listen to the effect.

Sharnu Nirgudi - 2019-10-22

Yo bro u have nice observation skills

djscurge - 2015-09-06

6 years of Electrical Engineering curriculum which included extensive study in EM and I was never taught this...... I'm somewhat disappointed in my university. I actually think I asked this specifically: "I understand all the effects of a magnetic field, but what IS it fundamentally?" and after some discussion of permeability and Maxwell's equations I lamented that no one in that class, professor included, actually knew. We could all describe a magnetic field by its effects and influences and even the qualities and characteristics of materials that can support a magnetic field and the methods of inducing one, but not what it actually is. Thank you so much for this video. I can now (at least more fully) answer that question "What IS a magnetic field".

Paul Freda - 2019-01-02

@NukeML What do you mean by "energy carrier" ?

Paul Freda - 2019-01-02

djscurge No one knows what it is ! What is like Why, an unanswerable question. ..... We only know what we see with detectors [ eyes, ears, or instrumentation] in macroscopic physical reality and experiment. And we then make a theory that explains what we see. That theory is then expressed in the language of Mathematics which is far more articulate than any spoken language like English. For EM it is Maxwell's Equations.

NukeML - 2019-01-03

@Paul Freda what is the theoretical particle that mediates a magnetic field

Paul Freda - 2019-01-03

@NukeML Maxwell's 2nd Law [Gauss] says there is none. The concept of a force field is the explanation for what we see with experimentation. "What" is what we see and "How" is our explanation for it. "Why" is the province of religion.

Kanhaiya Yadav - 2019-01-04

This is why there is pure field of pure science. Engineering is not about 'why things work'.
They are about 'how things work'

이승준 - 2019-05-11

한국어 변역 미쳤나ㅋㅋㅋㅋ

Titanic - 2018-11-12

1:55 he assumed the kitty's gender

Aditya Kumar - 2019-06-29

I would love to see an extension of this video on the origin of Lorentz force and how he himself was unaware that it is based on a new concept of space and time even before Einstein showed up with his relativity theory

Arjekay - 2017-12-06

Ohmg, thank you so much for this video!

Vince Noir - 2017-07-05

0:53 University of Sydney :D

Rubayat Hassan - 2017-09-18

how come no one told me this when they were teaching me about magnets!