How Special Relativity Makes Magnets Work

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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

    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
  • EducationEducation
  • Runtime: 4:19
  • veritasium  science  physics  magnet  magnets  magnetic  how do electromagnets work?  how do magnets work?  special relativity  length contraction  attract  repel  minutephysics  charged cat  

Comments: 4 320

  • tomblanco2001
    tomblanco2001 Hour ago

    The title should be "Special Relativity Theory Explains How Magnets Work." A theory does not make the world work, a theory provides a framework for understanding what we observe. On the other hand, it's hard to argue against over 1.5 million views.

  • Scarlet Speedster
    Scarlet Speedster 2 days ago

    Guys! I'm in a loophole.

  • Mathew Orman
    Mathew Orman 4 days ago

    That is a false picture as the electrons in wire move with accelerated motion and never in a straight line...

  • tom solomon
    tom solomon 5 days ago

    but the object feels the magnetic force without it moving at all. how is that happening? what is doing the movement of the positive cat?

  • VMEcycle6
    VMEcycle6 5 days ago

    So based on this explanation.
    A single proton or a single electron moving in a space doesn't generate magnet field around its path?

  • Lina Engineer
    Lina Engineer 7 days ago

    I believe contracting is not real it only is a time it take force going relative distance which act like contracting. This idea make people confused because maybe not true for real space. It is good in math because distance change at each time and force move same velocity. Charge moves in wire but I believe never proton only electron but real is force only which make electron from universal field. Is ok if we say electron moves. I ask how charge forces are different so charge create fighting and some change direction point toward opposite ones? Maybe a same reason for create gravity from atoms making general relativity behavior?

  • bhaskar sai
    bhaskar sai 8 days ago

    What about the direction of the field

    TANAY PATEL 9 days ago

    Can you explain about direction of the magnetic field..

  • Yuya fujikawa
    Yuya fujikawa 11 days ago

    Beautiful!! That's what special relativity is!

  • lvitch
    lvitch 12 days ago

    This couldn't be more incorrect.

  • Yuvi Bangarang
    Yuvi Bangarang 13 days ago

    but then, why the kitty cat doesn't experience a force when it is stationary and wire is havin a current, beacause then from cat's perspective, it's the electrons which are moving and they should contracted in space, must be creating a density imbalance which should cause a force to attract.!

  • Sean Fortescue
    Sean Fortescue 14 days ago

    I'm from India and in 10th standard your video helped me understand the magnetic effect of electric current really well so thank you and please keep up the good work..I have watched other videos on special relativity and E=mC2 and other high level even though I don't understand any of them completely yet they give me a good idea of how things work at the elementary level like I always wondered what actually causes magnetism in the first place why would an object attract another.. why does it produce a magnetic field while another material does not? Our 10th standard books don't explain that so once again thank you so much for these videos.

  • Gene Ruffalo
    Gene Ruffalo 15 days ago

    what a crock. relativity does NOT MAKE MAGNETS WORK. Magnetism existed before Einstein. May you should rephrase this to "magnetism make relativity available as ONE possible explanation for its existence". Keep eating what they are force feeding you. Come up with ZERO original ideas. Yes, that is the way to grow.

  • Impossible Physix
    Impossible Physix 18 days ago

    Can you explain induction with relativity

  • Akash Padhye
    Akash Padhye 18 days ago

    When cat is at rest and electrons are drifting then according to length contractions density of electrons should increase and cat should get attracted towards wire
    Why this thing doesn't happen????

  • Seth White
    Seth White 19 days ago

    from the first frame of reference, shouldn't the electric charges contract, creating a slightly negative charged wire?

    • Seth White
      Seth White 19 days ago

      actually wait, electrons have less mass than protons, so they would shrink less from the same relative speed

    • Seth White
      Seth White 19 days ago

      actually wait, electrons have less mass than protons, so they would shrink less from the same relative speed

  • Kyle MacDonald
    Kyle MacDonald 22 days ago

    Eric Mazur's webpage (linked in the description) is currently down. Do you have a source for this approach to EM? I only have Slater's book, which doesn't have this development in it.

  • mg1s
    mg1s 22 days ago +1

    Relativity is a fairy tail connecting a few known points of science. It is dogma of new faith. Like religion’s dogma you must not question it. You must believe it. it is myth. Myth is mixture of fact & fiction. with fiction you can make anything work. it is Schizophrenic. One moment speed of light is fixed, next moment it slow down. Become red shift. Next it will increase and become blue shift. Any religion has special interest behind it. they make sure the believers get rewarded and non believers are not heard. There is no interest if it is correct or false. relativity dose not make magnet work. atom make magnet work. MG1

  • endless nameless
    endless nameless 22 days ago

    I don't think this has anything to do with the contraction/expansion effect that the cat perceives. Perception does not govern the universe. If the cat is stationary relative to the wire, it is attracted to the moving negative current. If the cat is moving at the same speed and direction as the negative charge, then moving positive charge of the cat cancels the negative current of the electrons, and the protons of the stationary wire will repel the cat until the cat reaches a neutral distance.This has everything to do with charges in motion, and nothing to do with the fictitious cat's perception.  If a pair of + & - charge are moving in the same direction at the same speed, they cancel. Its that simple.

  • Rönin Walid
    Rönin Walid 22 days ago

    First mistake at 0:14 second....

    AFG BOY 23 days ago


    DJFLLOUT 23 days ago

    However in your frame of reference (refer to approx. 3:15) are the ELECTRONS not moving (due to the current applied), and so the ELECTRONS will contract, making the wire more "electron-dense" and NEGATIVELY charged, theoretically attracting the Proton Cat?

  • santosh kumar KHADANGA

    then how can we explain the magnetic property of permanent magnets using spacial relativity?

  • Chandni Dixit
    Chandni Dixit 24 days ago

    But when the cat is static, standing next to the copper wire then the electrons according to the cat's frame of reference were moving, which is why according to the effects of length contraction, density of electrons would be obviously more, ergo it should be attracting the positively charged cat... does that happen and if not why?

  • fergal farrelly
    fergal farrelly 28 days ago

    I'm an electrician and I was taught that electricity flows at the speed of light. I challenge you on the last part of this video. this would give electrons mass and momentum. I was always taught electrons were almost massless.

    • Phoenix
      Phoenix 9 days ago

      The electric field travels at the speed of light, and since the field imparts a force on the electrons, the initiation of the electrons' flow down the wire happens at the speed of light. The drift velocity of the electrons, however, is extremely slow; the drift velocity being the electrons' velocity in the direction down the wire. Wikipedia has a nice little example of calculating drift velocity in a 2mm diameter copper wire:

      Electrons *do* have mass and momentum! Their mass is very small relative to a nucleon (proton or neutron)--about 4 orders of magnitude smaller--but it is not 0. Also, on a side note, massless particles can still have momentum. The photon, for example, is completely massless but has a momentum of p = E / c where p is its momentum, E its energy, and c is the speed of light in vacuum--a constant.

  • Freddie Vandyck
    Freddie Vandyck Month ago

    While I love your videos this is the first one to make my jaw physically drop for a full minute before I realised my mouth was open

  • nischay !
    nischay ! Month ago

    but why will magnetic field have no effect in the charge's frame of reference

  • Taylor Hedge
    Taylor Hedge Month ago

    But, as far as the cat and the wire, how can a physical tangible force be affected by perspective? Weather the positively charged cat and the negative electrons in the wire are moving and the positive electrons are not , or the positive electrons are moving and the negative electrons and the cat are not, the force applied is still the same regardless of perspective, correct?

  • LowRig Gamer
    LowRig Gamer Month ago

    What is magnetic field? I've learned that magnetic field can travel on matter and space. So what is that thing? alpha, beta or gama rays. A different lamda of particles of that 3 thingy? In class and these video you explain how they work and in relativity. What is that magnetic wave made of?

  • Radioactive Music 4 U

    So somehow gravity and the strong nuclear force are basically like this and technically the same thing? Isn't that what humans will get to in future science?

  • Suwarna Gummuluru
    Suwarna Gummuluru Month ago

    But if the electrons are moving in your frame of reference, then when the cat is not moving, the density of electrons is greater , so the cat should be attracted to the wire . I'm still confused.

  • Dale Anderson
    Dale Anderson Month ago

    Thanks Veritasim. Mind blown.

  • Kumar Sanu
    Kumar Sanu Month ago

    thanks a lot very much because u made it so simple to understand...... thank u.....☺

  • forestsoceansmusic
    forestsoceansmusic Month ago

    Fascinating. Just on that tiny fraction of the speed of light that electrons move through a wire, could you do a video on how long it takes from the time we flick the light switch to the time the light lights up?
    There was a huge debate in an Australian Electronics magazine in the early 1990's about this, with people of all qualifications (from PhDs down to trade electricians weighing in on ALL sides--)--> which ranged from below the speed of sound to close to the speed of light! After several issues (it was a monthly) the editors had to close the debate because of no conclusive result and too many letters on that topic.

  • Wendy Dravis
    Wendy Dravis Month ago +1

    If the iron in the middle of the world 🌎 is over the curry temperature then why does it make a magnetic field?

  • cody hedges
    cody hedges Month ago

    The speed of light in copper?

  • pinku kakati
    pinku kakati Month ago

    he's a genius.

  • Parth Suyal
    Parth Suyal Month ago

    wow superb thanks!!!!!!!!!!!!!!!!!!!!!!!!!!

  • jacob hughes
    jacob hughes Month ago

    why then in his real life example with the wire and the compass, isn't the compass moving? cus with the cat example the cat is moving which is causing the effect, but the compass isn't moving but we still see the effect.

  • TechyBen
    TechyBen Month ago

    So the question is... what is an "electric charge", in relation to special relativity with Quantum mechanics (the big question of how to combine the two).

  • Aryan Derania
    Aryan Derania Month ago

    Why on God's earth will the cat move along with the direction of electron flow?

  • Ross Petersen
    Ross Petersen Month ago

    Once you start mentioning the contraction of positive charges and separated negative charges, is this related to Coulomb's law or something?

  • Achint Rastogi
    Achint Rastogi Month ago +1

    its strange that ur video has less views than ur no. of subscribers

  • Willem Beltman
    Willem Beltman Month ago

    this doesnt seem right, and it is not the first video i get this feeling. are you sure the wave pattren of the electrons are not just creating a magnatic flux? and those are 2 different things, like water waves and wind.

  • Kuldeep Saini
    Kuldeep Saini Month ago

    why external stationary charge do not experience force when current is passing through wire? as e- are moving w.r.t. to it so acc. to special relativity -ve charge density should inc. than +ve and hence net attraction should precede?

  • Vince Noir
    Vince Noir Month ago

    0:53 University of Sydney :D

  • Buzz Werd
    Buzz Werd Month ago

    So where does the torque come in?

  • gottalikeit2010
    gottalikeit2010 Month ago

    i am reporting this as cat abuse

  • Nikša
    Nikša Month ago

    This is actually incorrect in electron-vitron universe model, only true model.

  • Larry B
    Larry B Month ago

    Wait! The positive charged particles are moving with respect to the cat, BUT the contraction due to relativity is for each particle, and THAT does not mean they are "ever so slightly contracted", they are NO closer together and the charge density is the same. And, additionally, the forces involved are units of charge, and there are no changes in the total charge of cat or wire. This sounds like a VERY bogus argument, at least as presented.

  • lolPantsProductions

    Can someone explain what is happening at 2:40 with the compass and the current? Thank you.

  • physics physics
    physics physics Month ago

    correction: the electrons are closer, the proton will be attracted

  • Alex Russett
    Alex Russett Month ago

    Do you have a say in who does your ads? Because the ad on this video was propaganda. I don't need a lecture from a retired president of a third world country.

  • Gonzalo Chumillas
    Gonzalo Chumillas Month ago

    Resulta curioso como algo tan subjetivo como es el cambio de sistema de referencia pueda tener consecuencias objetivas y medibles. Creo que la mecánica cuántica y la relatividad están íntimamente relacionadas, aunque no hayamos podido conciliarlas. Muy bueno el vídeo. Congratulations.

  • spelunkerd
    spelunkerd Month ago

    Finally, a popular cat video that does not drag viewers into the mindless abyss.

  • Hobby Metallurgist
    Hobby Metallurgist Month ago

    Why don't the moving negative charges contract in the original frame of reference?

  • FirstRisingSouI
    FirstRisingSouI Month ago

    Mind = blown.
    And I'm a physics graduate student teaching assistant.

  • NurchOK
    NurchOK Month ago

    Thank you for showing the electrons moving and not the protons and also for applying the right hand rule properly :)

  • Ethan Taylor
    Ethan Taylor Month ago

    but electron spin tho

  • Boss Hogg
    Boss Hogg Month ago +1

    Okay, so a (relative) concentration of positive charge will repel a moving cation, i get that part. However, I don't see how that explains an electromagnet. It just looks like a positive electric field. After all, an electromagnet attracts metal, yet a concentration of positive charge will not.
    One more question, if the cation were moving in the opposite direction of the current would it experience a negative charge since the electrons relative speed would now be greater than that of the wire.

  • Will Stanks
    Will Stanks Month ago

    Water fire air and dirt

  • Olsen Olsen
    Olsen Olsen Month ago

    Good morning ! Magnets at speakers are very heavly damaging our magnetic field. Only. I am not will talk twice !!! Only tesla speakers are not damaging environment. Very bad souls of humans WAS TRYING TO TAKE CONTROL under speaker production. Speakers got magnets who is need to be magneted. That magneted force is scoming back to earth with time apperance. But now it is coming from earth to speaker side. Please kill every speakers !!! OR YOU WILL KILL US WITH TOO MUCH OF MAGNETIC FIELD "REAL ABSORPTION"

    Some of you have an 3 or more frequencies of telephone communication, antenas with Wifi or something(3 or more always), antenas everywhere for cable TV, (probably new types of TV LED !(holographic but not really visible by user)(you can be programmed by one smart arab with antena and drone)(your brain)(because it is now that technology)(we must kill electricity)(ad/dc///////////-)(change)(your brain is programmed by arabians

  • mdiem
    mdiem Month ago

    Three years later, and I'm back to review this.

  • Bartosz Olszewski
    Bartosz Olszewski Month ago

    his logo stands for V(ap)e 4:20 lol
    im so dumb

  • Andrew Combs
    Andrew Combs Month ago

    My head hurts in the most wonderful way possible...

  • StarlightVisual
    StarlightVisual Month ago


  • George K
    George K Month ago

    no force on a kitty. <-- this should be the leading comment

  • Marco Discendenti
    Marco Discendenti Month ago

    Are really wires electrically neutral in general?

  • bhargaw gaikwad
    bhargaw gaikwad Month ago

    if special relativity is the reason then charge must attract in your frame of reference as the moving electrons suffer from length contraction.My friend I read many books but no one give explanation of electromagnetism through special theory of relativity.
    please check the hypothesis.

    • DiabloDBS
      DiabloDBS Month ago

      So what kind of books have that been? Any titles you remember on EM or SR?

  • Rajat soni
    Rajat soni 2 months ago

    love it

  • Sander Wildeman
    Sander Wildeman 2 months ago

    The video is also quite confusing to me. Maybe the dilution of the electrons (from the point of view of the cat) can also be viewed as an effect of time dilation.

    If we imagine the full circuit, i.e. including the battery which pushes out the electrons (for simplicity assume that the electrons are injected from the left and absorbed on the right), then in the frame of the cat the "battery" is moving and hence its "clock" runs slow. In other words, as far as the cat is concerned the battery pushes out electrons at a lower rate, and hence the density is less.

    The distance between ions is contracted (along with the battery and the rest of the circuit, and including the space between the electrons) as explained in the video.

    Also note that in the example the magnetic field according to the cat is not zero, as there is still a current of ions. So it is not true that the magnetic field simply gets "converted" into an electric field and so it is not true that a magnetic field is "explained" by special relativity. It just means that "the" electric and magnetic field are relative concepts (depending on your motion), just like space and time.

    Just my ten cents to add to the confusion :)

  • Josh Richter
    Josh Richter 2 months ago

    why is it that electromagnets can act upon stationary objects as well? (not just moving ones)

  • Hyperflip
    Hyperflip 2 months ago

    Woah, after learning about both elecromagnetism and special/general relativity for my final exam, this is such a mindblowing connection!

  • Saw Teck Rul
    Saw Teck Rul 2 months ago

    thanks Henry and Derek. :D

  • David Zare
    David Zare 2 months ago

    sorry the explanation is sooooo wrong from beginning to the end. not to mention that regardless of protons/electrons charge neutrality every moving charge is a source of magnetic field.

  • Paul Anderson
    Paul Anderson 2 months ago

    I still can't wrap my head around this. - Why do things change? I can't get past the fact that it seems that we're only talking about perception. Not reality. Just perception.

  • Paul Anderson
    Paul Anderson 2 months ago

    watched this twice...head still hurts...

  • Professional Insultant
    Professional Insultant 2 months ago

    are you trolling us?

  • charles joshi
    charles joshi 2 months ago

    awesome video sir, thank u so much

  • Siddarth Arcot
    Siddarth Arcot 2 months ago +1

    The "Only in your frame of reference" made me laugh idk why

  • Sean Munro
    Sean Munro 2 months ago

    So, in the real world, what are the cats? E.g. Are the protons in the compass the cats? How and why do they move when he sends current through the wire? :\

  • Nikhil Joshi
    Nikhil Joshi 2 months ago

    Question and a possible experiment : If the magnet and or the cat is itself shaped like a wire and of the same length as wire, will it still experience the repulsive force ?

  • Math
    Math 2 months ago

    @ 3:23- Therefore, electrons drift through wires at about 5.9 or 5.86 miles per month

  • Noel Ruppenthal
    Noel Ruppenthal 2 months ago

    The compass needle is neither moving with the direction of current in the wire, nor is positively charged, so I'm not seeing how length contraction is responsible for the wire deflecting the compass needle

  • Akshay Kumar
    Akshay Kumar 2 months ago

    In our reference frame, charged cat is stationary and some current is in wire, current means electron is moving and positive charge is stationary. If electron is moving, distance between electron will contract. and it will produce some electric field that will attract the positive charged cat. Why this is not happening?

  • Istasis Mishra
    Istasis Mishra 2 months ago

    In Derek's frame of reference, since the electrons are moving, the electrons should seem like they have contracted and the wire should seem positively charged and this accounts for the repulsion of cat. Why isn't this the case?

  • AstralPotatoe
    AstralPotatoe 2 months ago

    my mind is blowned, magnetic fields are a lie, they are just electric fields

  • Saad Selwan
    Saad Selwan 2 months ago

    why is a stationary magnet attracted to a copper wire with a current flowing in it ?

  • Orion Michael Guy
    Orion Michael Guy 2 months ago

    Its the Motion of the Earth that gives the Magnet its energy

  • Aaron Forbes
    Aaron Forbes 2 months ago

    there were way too many different metaphors/similes in this to make coherent sense for me

  • Simon Barry
    Simon Barry 2 months ago

    well said

  • Chandan Bhuyan
    Chandan Bhuyan 2 months ago

    I am bit confused can anyone help.?
    1)when cat is stationary & electrons were moving then from cats reference due to length contraction electron density increases and proton spaced out making overall - ve charge which should attract the cat.. But didn't
    2) In this video a positive object which is
    Moving at speed of electron is repelled by electromagnet but what about stationary object or - ve charge object

  • Nosknut
    Nosknut 2 months ago

    Almost quote 1: "No force on the stationary cat next to a wire conducting electricity."

    Almost quote 2: "Force on a stationary cat next to a wire conducting electricity."


  • Science Asylum Fan
    Science Asylum Fan 2 months ago

    Why moving magnets move the electrons in a conductor? A moving magnet induces current in a conductor.Why?

    Changing magnetic field creates a current in a conductor.What is the reason at the microscopic scale. Explain without any technical jargon?

  • יאיר ערד
    יאיר ערד 2 months ago

    now I will pretend that I understood this video

  • Atif Wahab
    Atif Wahab 2 months ago

    Why don't electrons contract in the first case? Because their motion in reality is random?

  • Jon Do
    Jon Do 2 months ago

    well i think quantum Physicians would expect the universe to split. for the cat the wire is positive so in its universe it gets pushed away. And you should enter a universe there the cat does not get pushed away.

  • sanmveg saini
    sanmveg saini 2 months ago +1

    To all those who are asking that why that cat wasn't attracted when it was at rest (or more specifically, moving with the velocity of the positive ion in the wire, if any ), then just remember that in a neutral wire (a wire with no current in it) has electrons moving with random velocity, yeh! the velocity (speed+direction). and the number density is so high that you will always find two electrons moving with velocity in opposite direction in a particular volume of wire, and as you have heard that current is the flow of electrons(very basic definition ), so over all there is no current, hence neutral implies no force.

    although i do not know special theory of relativity in detail, but from the given explanation we figure out that, in the video he said that the length got contracted IN DIRECTION OF MOTION (not shouting just highlighting), so two electron will move in opposite direction and hence length will contract in opposite direction hence no net change in the length hence there is no change in the number density of the electrons hence no net force.

    hope you understand, have a grate day!

  • abc def
    abc def 2 months ago

    you need to come back with the correct answer, at 2:26 . moving electrons didnt get the length contraction, this is an inconsistancy, illiogical.

  • Landon Fitzgerald
    Landon Fitzgerald 2 months ago

    DEREK CAN YOU MAKE ANOTHER VIDEO CLARIFYING THIS PLEASE?? We're confused here because the car analogy doesn't pair perfectly with the stationary electromagnet. What is the car representing? You had no moving objects other than a current that generated an attractive force in your electromagnet. Additionally, why don't the electrons contract? Can you explain this using BOTH electrical motors and generators? Any help is much appreciated, thanks!

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