Relativity and Magnetism - Did Veritasium Get it Right

If you watch carefully, you may find what looks like a mistake in the Veritasium video about electricity, magnetism, and special relativity. Here I explain that apparent mistake.
Here is the link to the Veritasium video:
• How Special Relativity...
Here's the link to my longer video:
• Electricity, Magnetism...
Here is the Vocademy textbook page on the same subject:
vocademy.net/textbooks/dcelect...

[@patricksaucier1203]

My father was an electronics engineer. I remember him explaining the moving holes interpretation of electrical current. Your video ties my interests in relativity to my childhood education in the 60's and 70's. Cool. Thank you!

[@PinkeySuavo]

Interesting, thanks for sharing!

[@revivalcycle]

Thank you! The surge of persons bringing science to youtube has caused a swiss cheesing of science. Thank you for eliminating the holes, and not just moving them around to fit the user's viewability.

[@Alberto-mq7gw]

So according to this explanation, you can never see only one type of charges moving. Either both are static or both are moving in opposite direction at the same speed. Now we have an even bigger problem than in the original video.

[@Alberto-mq7gw]

@@MikeDonaldson-eh2ru Because if both move in opposite directions at the same speed there never is a net charge, and therefore there can never be an electric field to attract or repel an outside charge. So the whole solution proposed in the original video wouldn't work anymore.

[@Alberto-mq7gw]

@@MikeDonaldson-eh2ru No, according to the theory in this video, when the outside charge is moving it won't see the charges in the cable moving at different speeds. They will both move at the same speed because of the supposed effect explained in the video. Think about it and you will see the problem.

[@Alberto-mq7gw]

I'm sorry if that sounded rude to you. It wasn't my intention to be rude. It's just that this is something easier to look at it and think about it than it is to explain it here in a comment. The part that we are talking about is the one that Veritasium and every other channel left out, so this channel is trying to address that problem. So I don't know why you say that everyone cannot be wrong and it's me who doesn't understand what this video is giving as a solution. No one has ever said what this video is claiming (starting at around 1:55). This video claims that when the cat is stationary, and the electrons are moving in the wire at 1 unit per second, the reason why the cat doesn't see a length contraction in the electrons and therefore a net negative charge is because of some strange effect he then goes on to explain. What he claims is that when the cat sees the electrons moving (because they are moving relative to him) he ALSO sees the protons moving in the opposite direction at the same speed (even if they are actually stationary relative to him, the cat). And because of that strange effect, he sees the same length contraction on the protons as on the electrons, so there is no net charge and no electric force. The problem he introduces with his own personal solution is that if when you see one set of particles moving in one direction, you will automatically see the other set of particles moving in the opposite direction at the same speed, even if they are not moving relative to you, then that would apply to the case described by Veritasium and any other channel as well. You either see all particles static or you see the different set of particles (electrons and protons) moving in opposite directions at the same speed (which would result in no net charge and no electric force). And that goes against the explanation of Veritasium and everyone else.

[@Kalumbatsch]

​@@Alberto-mq7gw The problem is trying to understand everything with pictures and prose, it just becomes vague intuition and freestyle babble. It has to be calculated. The starting point of the thought experiment is that we are at rest relative to the wire (the ions in the wire) and we don't measure an electric field, so the charge densities of the positive and negative charges cancel out, even though the negative charges are moving. Each of those charge densities changes under the Lorentz transformations between reference frames and that's where the additional electric field comes from. The stuff about holes is just nonsense.

[@PinkeySuavo]

Hmm I just took a look once again on Veritasium video and you're right... From Kitty's perspective the wire should be neutral. I hate my life... Im in rabbit hole. I just wanted to make antenna for my FPV drone, but I didn't get how EM waves are created... And I just keep asking questions and digging into more and more stuff.

[@AdamCiernicki]

Also worth noting - electron in a wire takes about 2000 s (over half an hour) to travel 20 cm. That's why we call it a drift, they're not moving much

[@DrDeuteron]

Not relevant.

[@divyanshumishra24136]

Thank you, sir, for your explanation. Also, I do recall him explaining the lack of electrons termed as a hole and its motion in his video of semiconductor, but I also think he left that part out for the sake of video's simplicity and its length.

[@robert-wr9xt]

I am going to have such great dreams tonight. Your ability to explain and share science should be celebrated. Thank you kind sir.

[@pissnotime1894]

Thank you so much for clearing this out !

[@claragabbert-fh1uu]

In a wire, positive charges (protonic nucleii) are locked in position to the solid matrix of the metal wire alloy.

[@robertoguerra5375]

Not just the protonic nuclei is locked in the metal crystal. All the neutrons and electrons, except for the one electron flying away with the electric field. Adding the charge of all those subatomic particles, you get a net +1 proton charge, locked in the crystal lattice.

[@claragabbert-fh1uu]

Not exactly; the wire is not ionizing but it is conducting. The difference is "threshold", like a set of ball bearings hanging from a rail, knocking each other back and forth. For every electron that exits the wire, one is added. The threshold is the amount of energy required to make 1 electron move. If the electron balance in the wire was not constant, and the wire ionized by losing electrons, then it be chemically reacting (galvanizing), and the "lost" electron would have been captured by an element bonded to the wire (a "plating").@@robertoguerra5375

[@jamesblank2024]

You are correct in that Derek's derivation is wrong. Read Griffiths p489 to see derivation done properly. It starts with positive and negative line charges moving in opposite directions equally.

[@42laxer84]

In the 4th edition it starts page 550. This was very helpful to know though. Thanks.

[@DrDeuteron]

that's different, though. Those are two solid objects that contract equally to remain neutral. It is conceptually simpler than the thought-exp-wire, in which the proton lattice stays put and electron all move on independent but correlated trajectories so that the wire remains neutral (a la Bell's Spaceship Paradox). Since that paradox confused tf out of even experts, Griffith went with a simpler set up to show transformations of charge/current fields and maybe potentials? My edition is at work, covered in dust.

[@orparga140]

@@42laxer84 yes, page 550 thanks 😄

[@orparga140]

Derek's videos are creating confusion... another time. Maybe saying the same in a simplest way introducing the concep of the holes

[@PinkeySuavo]

2:15 oh my god I just watched veritasium video and asked exactly the same question! I was frustrated cause I felt like I'm missing something 4:25 Can we visualise the positive charge as a bubble in pipe where water flows? Like water (-) flows from left to right, but bubble (+) goes from right to left.

[@Bluesboy89]

Yes that was the question that crossed my mind too after seeing the original video! Thanks for the answer!!

[@user-nj5ec7ei3n]

I am so confused

[@moyanous]

The way I see it is that electrons and holes are not the same. Electrons can be injected into the wire or forced out of the wire by the external applied voltage. To a stationary observer, even though there is length contraction for the moving electrons and hence they should have higher density than the stationary holes, the externally applied voltage pushes enough of the electrons out of the wire to make the wire electrically neutral. For the moving cat on the other hand, the moving holes are fixed physical attributes of the wire which can not be added to or removed from by the external circuit.

[@mohamedshalaby8253]

“simplest but not simpler” said A.Einstein . I think Derek wanted to make things simpler. So he made such a simplification . I highly appreciate his and your attempts

[@DrDeuteron]

No, he didn’t make anything simpler. His video was complete.

[@diwakardas1807]

Great explanation! Thankyou soo much 😊

[@user-tq4lj4pn7x]

But with that in mind shouldnt the same thing occur when from the cats frame of reference the protons are moving? Its like the electrons are now the holes where a proton was meant to be. As the protons move, the holes (now in the place of the electrons) appear to be moving in the opposite direction, thus experiencing lenght contraction. So shouldnt the net charge remain neutral even if the cat is moving with the electrons?

[@DrDeuteron]

yes, this video is wrong. But don't forget: Lorentz contraction doesn't contract the object in its own rest frame, it's a property of the coordinates of the 'at rest' frame and the definition of simultaneity, since you measure a moving objects end points at the "same time", which aren't the same times in the 2 frames.

[@orparga140]

This video only attempts to explain why the cat does not experience forces when at rest. From what the video says, when the cat is at rest, both charges, free electrons and holes, contract equally. When the cat moves in the direction of the electrons, they contract slightly less than the holes, so the cat moves away. When the cat moves toward the holes, the holes contract slightly less than the electrons, so the cat moves closer. Anyway, it raises some questions for me. What happens in a solenoid or a coil? How are electrons and holes distributed when contracted? in a solenoid , do they gather around the cat? I don't think so. Is the radius of the solenoid reduced? From the cat's point of view, perhaps. But I don't think so, because the positive and negative charges would be equal the number of charges or holes in the solenoid increases depending on the direction of the cat? From the cat's point of view, perhaps.

[@nathanielkilmer5022]

Wouldn't each individual electron contract while the spacing between them remained the same? Or does the space contract as well? The space isn't moving, though, right? I'm going to dig out my Modern Physics book here shortly to review a little bit, but I can't remember if the space between objects also contracts, or if it's just the objects themselves. I think it must be space inclusive, since atoms are mostly empty space, but this principle still confuses me.

[@younesssahoui3913]

Because the electrons are close together, the space between them also contracts, because the faster an object is, the more space around it contracts, and this is what we call gravity, in addition to the curvature of time

[@TheEnigmaDreamer]

Why the wire is charged in relative frame?

[@eduardomeller6926]

You are correct. I think Veritasium made it more complicated than it actually is. There was no reason to make the cat move in the same direction to the electrons. As long as it has moving charges, the effect will be the same, independent of the reference. If you are stationary relative to the wire, you see the shrinkage due to the moving electrons. If you are traveling with Veritasium´s cat, you see the shrinkage of the protons. It is all fine.

[@MrMikeEdie]

I like this a lot. Very clear 😀

[@kiranchannayanamath3230]

Thanks for the explanation, but why do the magnetic fields encircle around the conductor, does this theory explain encircling too ?

[@rsm3t]

The resulting force is perpendicular to both the particle's direction and the direction of magnetic flux. The motion of the particle is parallel to the wire. The resulting force is radial with respect to the wire. The only way this can happen (for all positions around the wire) is for the flux to encircle the wire.

[@kiranchannayanamath3230]

@@rsm3t Thanks for your comment

[@simegnewtesfaye4455]

I swear this channel is special👍

[@shmish]

I had that exact question, thank you.

[@UNKNOWNPERSON-kk9kd]

Great video! But consider this: The electron drift is so slow, it is not significant for stationary external charges near the wire, whether those charges are positive or negative. The wire is electro-statically neutral (uncharged) for non-moving charged particles. When an external positive or negative charge (usually atomic or molecular in size) is moving - the speeds are going to be much higher than the observed electronic drift we see in electrical conductors so you will see electro-static repulsion or attraction depending on the charge's polarity.

[@cediv715]

The electron drift is moving negligibly slow compared to the speed of light (lets say ~10^9 times slower) unless you consider the contribution of all of the electrons in the wire. At a distance near the wire, there will be ~10^20 electrons in an area close enough to the charge to make any non-negligible difference. This seems like too much, but the distance from the wire means that if you do the math out, you will get the same exact force for the moving and stationary charges.

[@pteckhausen]

yeah but in this example of veritasium isnt the "cat" moving exactly with the electrons?

[@orparga140]

@@cediv715 the force for the moving and stationary charges are diferent. And also the force of charges moving in opposite directions. Consider that electrons moving in the same direction as the cat experience less contraction than holes moving in the opposite direction. The positive cat is then rejected from the wire when it moves in the same direction as the electrons, and is attracted when it moves in the opposite direction.

[@Kalumbatsch]

The assumption of the thought experiment is that in the reference frame where the wire (the ions) is at rest, the wire is electrically neutral. That's all there is to it. There's nothing wrong with it, it's a common textbook example. Look up the Feynman Lectures, chapter 13-6.

[@lukasrafajpps]

The fact that the electrons and protons were spaced equally in one frame of reference is Derreck's initial configuration which you can pick however you want. The holes don't solve anything since you could boost in the rest frame of the holes making the electrons to contract even more and holes stationary which is totally wrong since the protons would clearly be moving in that frame. You could use the hole argument in any frame of reference making the wire always neutral don't you think so?

[@randomcontrol]

There is no explanation why the positive charges should appear to be moving and be contracted. That doesn’t make sense to me. Unless they are really moving, which they probably aren’t in a solid metal.

[@James_Hello]

Yes, the delocalised electrons in the metal are free to move but the holes in the atom (positive ion or anion) does not move. Thank you.

[@RaddeFitnaat]

If you're moving in a car, you're in motion in the frame of reference of a person seeing you standing at the corner of the road, but for your frame of reference, that person is going in backwards motion.

[@PinkeySuavo]

I think it's like water going in a pipe to the right and there's a bubble at the end. |=========o| The water moves, but the bubble goes to the left |========o=| |=======o==| |======o===| |=====o====| ... |o=========| Now - did the negative charge (water) move to the right, or positive charge (bubble) moved to the left? Now with electrons: We apply voltage (+)(-) to the wire. - are electrons, + are the "holes". (+) | - + - + - + - + | (-) (+) | - - + - + - + + | (-) (+) | - - - + - + + + | (-) (+) | - - - - + + + + | (-) Did the electrons (negative charge) move to the left? Or did the positive charge move to the right?

[@DrDeuteron]

They don’t appear to move, and they don’t move. And nothing is Lorentz contracted in the lab frame.

[@davidhand9721]

They're moving relative to the cat. This is why they call it "relativity".

[@spulverable]

Still doesn't really explain to me the v or the cross product in the Lorentz force kaw

[@myvideosarefine]

That's awesome sir

[@samuelbelanger4646]

But if I go into the hole's frame of reference the electrons are the only ones moving and the net charge is negative. This doesnt solve anything???

[@TheYeeshkul]

I know this happens in semiconductors. But does it happen in conductors? I thought in conductors, the electrons flow towards the positive charge freely between the atoms. Do they really hop from atom to atom?

[@purplerpenguin]

Did not find this at all illuminating, possibly because of the speed of the presentation. But I believe you give Veritasium too much credit if you think he understood this and deliberately simplified.

[@user-yr2nb4vr3q]

Well relativity describes motion between two points of mass, I don't feel like relativity works by substituting charge

[@Henrysevenone]

I wonder if a single moving charge (like an electron in space) can then produce magnetic field??

[@Savethirdrock]

I think both of them totally ignore the spin.

[@DrDeuteron]

Of course it does.

[@annacastellvi6718]

And how do you explain DIAMAGNETISM with special relativity?

[@orparga140]

The best of 21 comments I have read. Actually, all the explanations of paramagnetism and diamagnetism that I have read are limping... but I continue studying.

[@frizzarazz]

A hole is the absence of something, not itself something (except a concept). Special relativity, and therefore length contraction, does not apply to not-somethings.

[@orparga140]

think about all the free space inside matter... it also contracts when the matter is contracted

[@santiagobustamante6192]

This is incorrect in many ways. Although not explicitly stated in the original video, it is assumed that in the stationary wire frame of reference (lab frame) the positive and negative charge densities are equal in magnitude while the electrons are moving. In other words, the net charge density of the current carrying wire in the lab frame is zero. With this assumption, however, there will be a net charge density in the lab frame when no current is present, but this really doesn't matter as the electric/magnetic field equivalence can still be shown (which is the whole purpose of the problem and the video). Also in this picture we are only considering the motion of pairs of ions and electrons, meaning all of the electrons are effectively in the conduction band (although this concept here is not necessary at all). In this sense, you cannot just talk about holes and their motion as if they were real particles. I think you might have mixed the concept of ions and holes here.

[@elizabethreyna8354]

I do not understand veristasium in other video says electrons do not move at all

[@RaddeFitnaat]

5:23 so his cat repulsion explaination is correct, the only thing you wanna say is that holes moves opposite to the direction of flow of electrons due to the flow of electrons?

[@SnotFroth]

thanks for making this video. I watched Veritasium's video and immediately questioned why moving positive charges were affected by special relativity but moving negative charges were not

[@rsm3t]

Derek did not claim that moving electrons were not affected.

[@PinkeySuavo]

I asked this question myself and felt so good that I found this video! I thought I am dumb af and I dont understand something. But now I ask more questions... I feel like Kitty should feel the wire as neutral too... And that it should be actually attracted by pulling electrons to surface of wire lol

[@PinkeySuavo]

@@rsm3t He also didn't say they are and he didn't visually show it as he show it later in other scenarios

[@DrDeuteron]

They are both affected, he shows it in the video. There is a difference between a positive lattice on a wire and a bunch of free electrons, though.

[@EduardOganesian]

OK, what about a charge moving perpendicularly to a current wire? The charge will experience a force sideways, can this be predicted by Special Relativity as well?

[@DrDeuteron]

Yes, but it’s messy because in the test charge frame the currents are not parallel, and are time dependent, so it has no pedagogical purpose

[@EduardOganesian]

@@DrDeuteron Same question posted in stackexchange forum on physics, no satisfactory answer so far.

[@DrDeuteron]

@@EduardOganesian well then solve it. It’s a total mess because it’s no longer a static problem, and when you have moving currents and charges with non symmetric shapes, you have to use a computer with EM software, so no one is going to do it for you.

[@EduardOganesian]

@@DrDeuteron Why are you saying it's a mess, because the B field isn't uniform?

[@DrDeuteron]

@@EduardOganesian no, every thing is time dependent in the cat frame. In the cat frame the (scalar) charge density of the protons is: rho(x, y, z, t) = (rho_0)delta(y-b)delta(z-vt) for impact parameter b, moving at v=v_z. It also has vector current density that isn't static, even though it is zero in the wire rest frame. Then the current flowing electrons have both an x and z component in the cat's rest fame...so..go ahead and compute the fields analytically...and get back to me. Maybe next month? It's not easy.

[@seekingtruthfacts7743]

The drift velocity of electrons is tiny and doesn't come close to relativistic velocities. So how can the Theory of Special Relativity apply? The electric field should be included in the explanation.

[@marcossidoruk8033]

You don't get how ridiculously strong electric fields are. Even an absurdly tiny difference in charge density will have noticeable effects.

[@DrDeuteron]

In the thought experiment, the properties of real wires are totally irrelevant, other than they are neutral and carry current.

[@DrDeuteron]

@@marcossidoruk8033one gram of totally ionized hydrogen with no electrons in the center of the sun has enough electrostatic energy to gravitationally unbind the sun.

[@GabrielMirandaLima-hv7oe]

One should learn the covariant formulation of electrodynamics in order to fully understand it, Veritassium caused people to believe that magnetic fields are not real and are really just "hidden electric fields" which is false, in reality, electric and magnetic fields are just components of what is known as the electromagnetic tensor, and this tensor is what fully describes EM force in all frames of reference

[@drh255]

Dumb question: aren't electrons point particles, without size, and thus unable to experience length contraction? Since they're individual particles that are moving, the space between them shouldn't change and thus the charge density shouldn't change, right?

[@Deep4Phy]

its the length of wire thats changing, while no. of charges remain same. So charge density increases

[@drh255]

@@Deep4Phy no, I was referring to the lab reference frame, where the wire is at rest and the electrons are moving. People are saying that the electrons should be contracting like the positive charges are in the other reference frame. My point is that it doesn't seem like point particles should be able to length contract because they have no dimension.

[@maskedmarvyl4774]

But if the negative and positive charges cancel out because of special relativity, then doesn't that mean there should be No magnetic effect? I don't think the positive and negative charges actually cancel out. The positive charges may Appear to move, but they are actually fixed in the matrix, as others have pointed out. However, the electrons Are actually moving, which means they appear to be closer together and because of time dilation in specific relativity, create the magnetic effect. At least according to the poster of the other video.

[@DrDeuteron]

No, the electrons move in manner that keeps the wire neutral, by definition. Therefore it is neutral in the lab frame. By definition. Not so in the 🐈 frame.

[@DrakeLarson-js9px]

You gotta point...

[@MikeTooleK9S]

like music!

[@rekarrkr5109]

what still doesn't make sense is that if we look at the lab frame of reference we cant explain the cat being repelled ,since magnetism is just electric force in effect of special relativity, we shouldnt need different law in this frame of reference.

[@thesparetimephysicist9462]

The electrons will arrange them self in a way that the wire is neutral in the rest frame of the positive ions because this is the lowest energy state. Also we have to remember that it is a classical explanation that does not take into account that the electrons are in non-local quantum states. I don't think the shift of the holes lead to any contractional effects. Veritasium's explanation has a much bigger problem, since the electrostatic analysis only works in case of parallel motion next to an infinite wire. In reality you have to consider the dynamic field of the moving charges. I explain it in this video, and I later verified the result. kzbin.info/www/bejne/qIWoapqoYpWEZrcsi=tSumR9QO9JiNROrf Wikipedia now says the same thing en.wikipedia.org/wiki/Classical_electromagnetism_and_special_relativity

[@Kalumbatsch]

The same would apply to the rest frame of the electrons, so that's obviously wrong. The overall charge of the wire could be anything, but the assumption is that when we're at rest relative to the wire (the positive ions) we don't measure an electric field, so the charge densities cancel out (only in that frame) even though the electrons are moving.

[@thesparetimephysicist9462]

@@Kalumbatsch The same does not apply to the electrons because of the difference in weight between electrons and ions. In a similar way, it is the earth orbiting the sun, and not the sun orbiting earth (Ignoring the much smaller orbit of the sun around the center of mass). Try to watch the video i linked, it adds a nice detail.

[@Kalumbatsch]

@@thesparetimephysicist9462 So what if you have ions of approximately equal mass instead of ions and electrons? Let's take an infinitely long tube of molten potassium chloride.

[@thesparetimephysicist9462]

@@Kalumbatsch That is a good question. In this case the state of the charge carriers are more localized, so the classical model is more accurate. The system can only be electrically neutral in one single frame, and my best guess is that this will be the center of mass frame of the system. If it is a circuit then for sure the center of mass frame, but in case of the infinite wire, I do not know for sure.

[@user-bu2sc1bm7k]

Where is the return wire? Certain things just do not exist in natere and are simply abstracts. Forexample, there are no 2D obljects in nature, only 3D. By the same token, there is no current flowing in a wire segment in nature. Many electromagnetic induction illustrations do not show the flux return path and this is an abstract, resulting in misconceptions widely parroted inot only in the media but in literature. Thank you.

[@DDDelgado]

Electrons don't move at relativistic speeds to see an effect of it.

[@tictacX1]

This explanation of no net charge in the wire doesn't add up for me and I learned SR in my Bachelors.

[@NorthernWhiskyJack]

Veritasium got it right. He's not talking about a stationary charged particle outside the wire. He's talking about a moving external particle. If it's an electron moving in the same direction as the electrons in the wire, that electron will see positively charged nuclei in the wire moving in the opposite direction. From the external electron's point of view, this causes a tiny Lorenz contraction in the mass of positively charged nuclei, which results in an increased density of positive charges and hence an attractive electrostatic force. This force is only present when that electron is in motion and it is always at right angles to the direction of motion. When you do the math and compare the electrostatic force that would generated by the above mechanism to the magnetic force that would be calculated by conventional electromagnetic theory, the two forces are exactly equal. There are some videos here that go through the calculations, step-by-step.

[@DrDeuteron]

You forgot about the 🐈 also seeing a Lorentz dilation in the electron charge destiny

[@Mikelectric]

veritasium took the whole thing off a small paper written by an indian student 'EXPLAINING_MAGNETISM_BY_RELATIVITY' Nabin Timalsina

[@giovannicicerchia4726]

It's very obvious. Veritasium make me understand why an electric charge moving create à magnetic strength with relativity instead of maxwell équations came experiments, not à real state of theory. Am i wrong ? An other thing : The électromagnétism have got à status. Lorenz discovered the transform for magnetism. He was thinking To à motor in which the stator is moving and the rotor kip quiet. And the universe move with the stator... Later Einstein kéep the work of histoire teacher and apply his formula for everything. Thanks.

[@DDDelgado]

If you watch the Faraday paradox experiment, you'll see that's easy to have multiple reasonable explanations

[@DrDeuteron]

Every frame of reference has a reasonable expectation , but this video does not.

[@claragabbert-fh1uu]

Positive charges do not move in a wire; negative electrons move. In a galvanic cell, positive charges can move being in an electrolyte solution. Junk begins with your misunderstanding of reality.

[@abcdef2069]

at 2:20 veritasium needs to delete that video. if you put a + charge outside the metal wire, it will create a powerful negative image charge inside the wire and it WILL attract the + charge no matter what. now if you put current through the wire, the charge outside will also be pushed to the right, then move circularly till it collides with the wire. and one more, when the fermi electrons move in a wire, it will create E-flux change that creates the circular B-field on the outside charge, this is also an attractive fore. therefore you cant do the next experiment, the charge moving with the same speed of e-s

[@DrDeuteron]

Ok then. It’s not a wire. Its a positively charged rod, and a bunch of of negative charges moving? No image charges. You can’t let practical concerns ruin an SR though experiment.

[@abcdef2069]

@@DrDeuteron both exist in a same material? i always fail to do this, i dont know even how they can coexist. if but the rod contains only one charge distribution, this one is easy, this answer is everywhere in text books i think it is not a SP problem, its a general relativity problem, einstein's idea please check this video and Unruh effect kzbin.info/www/bejne/qHypq4luqLt3qaM if anyone can answer this 1st, a naturally falling charged body in a curved space and time will emit bremsstrahlung radiation observed by stationary observers

[@abcdef2069]

​@@DrDeuteron both coexist? i always fail to do this, i dont even know how both can coexist but if the rod has only one charge distribution, this is easy, the answer is everywhere in textbooks, and they dont even use a test charge. i think it is not a SP problem, i think it is a general relativity problem please check these terms, i cant give you the links, youtube will delete this post Einstein's Gravito-Electromagnetism, by eugene unruh effect 1st if one can answer if a naturally falling charged body in a curved space time can emit bremsstrahlung radiation observed by stationary observers.

[@DrDeuteron]

@@abcdef2069 trust me, Veritasium's video is correct, though it is lean on the explanation of the electron density (but not wrong). Here's the deal: I have been doing SR for 40 years. I have a PhD. I studied under Kip Thorne, who is the world's greatest. I am not him, but he had no complains with my understanding. This video is garbage. It is hard to explain in a yt comment, so do what I always say: If you are confused, do the Lorentz transform yourself. it is a linear transform. y = mx +b, a slope and an offset. It is easy af. Don't be intimidated. when comparing frames, look at events, not long prose about "dilation" and "contracting" ...LT's are like hips: they don't lie.

[@abcdef2069]

@@DrDeuteron please explain at 2:27 veritasium's video saying no force on a stationary charged cat...,but electrons are moving and they are NOT contracted conveniently this time. if the charged cat is moving, + charges will be contracted too so it is NOT just magnetic field alone.

[@rudiwiedemann8173]

Excuse me, but this theory DOES NOT explain the magnetic field surrounding a moving charge in free, empty space WITHOUT a conductor! I call BS.

[@sahilsheikh5651]

Ah, no

[@whymikey]

What bothers me is the cat. If the cat is moving in the direction of the electrons, then it is repelled. But if the cat was moving in the direction of the protons, then it should be attracted. Am I wrong?

[@rsm3t]

No, you aren't wrong, because reversing the direction of motion reverses the direction of force (because it's a cross product).

[@PinkeySuavo]

what bothers me is according to this video, Kitty should also see neutral wire... And assuming wire has free electrons they could go to the wire surface and get attracted with the kitty

[@rsm3t]

@@PinkeySuavo the wire *is* neutral -- in the lab frame. But, considering the segment closest to Kitty, the segment end that lies in the direction of her travel is approaching, while the end behind her is receding. Because of the relativity of simultaneity, Kitty samples these two ends at different lab-frame times. Hence, the protons are compressed in her frame.

[@PinkeySuavo]

@@rsm3t But can't we say exactly the same to the human standing next to a wire? In our reference frame it's the electrons that move. In kitty reference frame it's the protons that move. Why would she feel something different than us?

[@rsm3t]

@@PinkeySuavo No. Just like the wire itself, the human is in the lab frame. Sorry, I can't draw you a Minkowski diagram in this medium. This is one of those things that is better illustrated graphically than verbally.

[@monkerud2108]

length contraction of space is an optical effect, it relates to a change of basis of what you consider the rest frame in an equivalent stationary medium model of the Lorentz invariant physics. the changing of charge densities have nothing to do with relative velocities, you can change the charge density or you can change the optical effects, i prefer the normal even charge densities and the effects that are optical with respect to relative motion personally. but most people butcher these ideas like crazy. you made a mistake when you say that if the charges are moving in your frame the distance between them will be length contracted, that isn't true. space never length contracts at all in reality, that is an artifact of changing basis the way that is done in special relativity, in special relativity, which is just one form of representation of the coordinate independent physics, you change basis in the speed of light and in the densities in the state when you shift reference frames, that is entirely arbitrary and nonphysical as a choice of representation. if you have a loop of wire in your reference frame at rest with respect to you and you accelerate the charges inside the wire into a current, the individual charged particles length contract in their direction of motion but the distances between them on average do not, space never length contracts, not physically anyway. special relativity is a misleading representation of the physics. so what you explained about the holes moving the other way is not necessary at all, and Derek's explanation is superfluous and not quite right. to avoid the pitfalls of using the Einstein's formalism just start with a stationary background with the same physics, equivalent to doing all the work in one frame of reference in normal relativity. we then instead of mixing representations by changing frames of reference in this weird way to mix physical and optical effects, instead we start with a set of wires that are moving at different velocities with respect to the background, so some are more length contracted than others as a whole, they are all neutral and they will stay that way current or no. as we change the current from zero in all those wires that are moving with respect to the background they all get the analog of a magnetic field around them, due to the current and the motion. they will still remain neutral when they are in motion, no boost of a loop of wire with a current in it in this point of view will gain or loose any charges by being accelerated, that straight up breaks conservation of charge, so the answer given here cant be even close to true. just work it out using only one frame for a moving setup and a non moving setup, don't worry about changing basis it is a bunch of nonsense.

[@TonyDalSanto]

Sorry, but I don’t think this helps. Your golf analogy shows it isn’t holes that move but the material. You defeat your argument by appealing to “holes moving”. The motion of “positive holes” might be a convenient mental image, but isn’t what’s happening. Can you explain it without appealing to such holes? If not, then it seems suspect.

[@russchadwell]

And, so the cat wondered off because "food"

[@maskedmarvyl4774]

I wonder where the cat wondered off to.

[@digitalsiler]

yet the wire still produces a magnetic field that no one can explain

[@johnpettit6886]

Charged particles are moving (i.e. the electrons), hence the magnetic field around the wire. The protons stay in the atomic nucleus' that forms the wire and besides jostling around with the atom - they don't 'move'. You can reverse this to view it from the classic current direction by imagining that the 'holes' are moving - instead of electrons.

[@digitalsiler]

@@johnpettit6886 you've seen this? sure

[@PinkeySuavo]

@@johnpettit6886 still i dont get why magnetic lines would be circular... From Veritasium video it looks like they shold spread outwards of the wire. And applying this video here to Veritasium's video, from Kitty's perspective the wire would be neutral too...

[@br3nto]

@@PinkeySuavothe electric field spreads out radially from the wire from the charged particles moving in the wire. The magnetic field is perpendicular to this and so wraps around the wire.

[@PinkeySuavo]

@@br3nto why its perpendicular though

[@SusiloAdi]

Relativity-ly speaking.. earth is the center of the universe.. 😅

[@user-mq1zo7xe8l]

This is incorrect. The length contraction between the (moving) negative charges is perfectly compensated (in the rest frame of the wire) by a time dilation effect increasing the space between charges.

[@MrNibiru2112]

I dont see any positively charged particles moving.Holes are not moving, a hole is not a material.A much better expaination is required.but good try

[@Jono4174]

‘The “hole” appears to have been moved.’ Kinda kicks the can down the road. Is it (in principle) possible to explain this (do the math) with the actual particles (quantum thingys) in the wire?

[@Jono4174]

Don’t worry I am no Vertitasium sycophant. 1. He does a whole 10 minute video about aerogel and its mass and never once mentions the buoyancy of air (which he is measuring the aerogel in) 2. Video called “the Baysean trap” he actually uses non-independent evidence to narrow the odds. New evidence has to be independent of the old evidence to use the base theorem twice. You can’t do 100 mammograms to be sure whether you have breast cancer or not.

[@5ty717]

Space itself doesn’t and spacetime does have a unique FoR for a traveling light source. Special Relativity means the source produces photons with speed of causality no faster no slower . We are neither dealing with photons nor special relativity when we measure an arbitrary drift speed in a wire. Read again if necessary. Veratasium Derek has hijacked Special Relativity along a dubious Feynman referral. Recall, charge though the wire does have a unique frame and it’s protons certainly dont move relative to the wire FoR. Recall conducting band electrons in surface of metals may be described as a unique FoR equivalent to an arbitrary potential drift velocity relative to the atomic structure of the non neutral wire. Hence what might be relative in spacetime falls on its head wrt presuming a moving FoR of protons relative to the wire lattices. No. The length contraction is ONLY relevant to relativistic velocities where time dilation offsets. No this is 10s mm/sec velocity not 3x 10^8mps . Sorry… sorry the protons are not squished the way conduction band electrons may be and the drift velocity is ten orders of magnitude too slow for the speed of causality (light-vacc) to be relevant. The problem here is of your own making trying to fit special relativity in for both + and -. Am I wrong…? Happy to be corrected. But make it watertight… cause i might plunge in otherwise

[@mikeroman5208]

Actually, the positive charges Veritasium was referring to are not holes but the "stationary" atomic nuclei that balance out the negative charge from the electrons (ie, one proton for each electron). Not disagreeing with your explanation but I think you misunderstood Veritasium's model.

[@aniksamiurrahman6365]

Yeah, that's what called hole. Specially if u study semiconductor.

[@DrDeuteron]

This is totally wrong. Nothing is length contracted in wire frame. Moreover, the thought experiment is a thought experiment, and the nature of conduction in a wire is totally irrelevant in the analysis. That real wires have a tiny drift velocity is not relevant. In the thought experiment, they can move at any speed less than c. You also left out the part where the moving cat sees a lower electron density than in the wire rest frame.

[@petervals4624]

Huh??

[@rsm3t]

Easier for me to just consider the effect of the Lorentz transformation on the electromagnetic field tensor. The transform intermixes electric and magnetic components of the EM tensor. So, in a moving frame of reference, the electrical field becomes a magnetic field.

[@atheistaetherist2747]

No. The protons are not moving at any time.

[@SURok695]

1) That's not true. Protons show some movement 2) Holes are not protons. It's a quasiparticle used for describing the absence of an electron. It makes the maths much easier

[@spulverable]

He never said they do

[@randomcontrol]

@@SURok695but in the end the positive charges have to be protons, or what are they?

[@SURok695]

@@randomcontrol lack of electrons. Imagine a bunch of electrons that can be only in fixed positions but can move between them. Some of these positions are free, but mostly filled with electrons. Then we apply the electric field. If the electron have the free space to move, it jumps, filling the empty space and leaving a new one behind. So we have a huge amount of electrons that either don't move or jump sometimes. Or a few "bubbles" that move constantly. Electrons move against the electric field like all negative charged particles. Those "bubbles" move along the field like positive charged ones. And it happened to be easier to describe mathematically the movement of those empty spaces, or holes, than electrons.

[@dominiqueubersfeld2282]

This guy is clueless about electromagnetism. It seems he never learned Maxwell's equations (published in 1861-62) and has no idea of Lorentz Group that is the frame of Special Relativity. What he thoroughly explains is totally irrelevant

[@hqs9585]

The mistakes the guy made were very apparent for a HS senior!

[@dominiqueubersfeld2282]

One more error: conduction by holes happens only in semiconductors like silicon and is explained by the existence of a band gap in electrons energy distribution. Copper is not a semiconductor, therefore conduction by holes doe not exits in copper. Go back to Electromagnetism 101, Sir.

[@xiangcheng9107]

The explanation of this video is problematic.

[@MetalMakeover]

There's still a glitch though, the fact that Special relativity is not involved at all, because it's an erroneous theory. You seem like smart guy, do all objects fall at the same rate when dropped from the same height, excluding air resistance? Well if a train was moving at such a velocity that an observer at the station would see clocks inside it going at half the rate that his own clocks go but objects fall the same distance in unison inside and outside the train then obviously the velocity of the object in the train would calculate to be double the velocity of the object outside the train, because distance/time= velocity and less time passed in the train while the object fell the same distance as the one outside the train, because height contraction is not a feature of Special Relativity. The laws of gravity prevent Special Relativity from being a valid theory, simple as that, no way around it.

[@rsm3t]

You are neglecting the relativity of simultaneity. If I'm on the train, and you watch me drop a book from a 5-foot height just as I pass you, and you drop a book from 5 feet also at that moment, when the books hit the floor/ground, it will not be at the same time, because by then, we will be separated by some distance. So they are not falling at the same rate in either of our reference frames. Events can only be simultaneous in both frames when the separation in the direction of movement is zero.

[@MetalMakeover]

No, they both hit bottom together, well established fact of physics.@@rsm3t

[@rsm3t]

@@MetalMakeover "Together" implies simultaneity. Simultaneity is relative. Sure, you can find *a* frame of reference in which what you are saying holds, but that frame is neither the train frame nor the station frame. If you want to try to disprove a theory, first you need to learn what the theory says. The theory says simultaneity is relative. You claim it is not, in order to defend your claim that special relativity is inconsistent. But you can't assume the consequences of a theory are false in order to refute the theory -- that's a fallacy (petitio principii). You have to *demonstrate* that at least one consequence is false. Yet every experiment designed to test special relativity has upheld its predictions. Sorry to disappoint you, but these are the facts.

[@MetalMakeover]

Why doesn't Encyclopedia Britannica, in their "muons" entry, say anything about Relativity then? It's not proved at all. If objects did not hit ground at the same time regardless of whether or not they are also moving sideways then wouldn't at least one physics book mention it? I'm afraid you can't just pull out "relativity of simultaneity" to explain away every obvious flaw in Special Relativity.@@rsm3t

[@MetalMakeover]

The speed of gravity is constant in all inertial frames. The speed of light needs to vary as dictated by gravity, which is also the source of time. Light is not the source of time, gravity is.@@rsm3t