Submitted by Ethan-Wakefield t3_1198h4o in askscience
I don't really understand this. Doesn't a magnet create excitations in the electromagnetic field? So, wouldn't any moving magnet spontaneously create light? So shouldn't all magnets glow if they're moving?
I know I'm misunderstanding this, but I don't know how or why. Can somebody help me?
Okay but fundamentally speaking, if I say rotate a magnet continually, it actually emits radio waves? That is… weird. I am tempted to ask why, but I know the answer is, because the math says it has to. But this makes no intuitive sense. At all.
So if I take a magnet, and I flip it into a charged black hole, it’s going to emit radio waves all the way until it gets to the event horizon? I don’t understand this at all.
If your wave your hand around and water does it not make waves in the water?
That's just an analogy, but why is the same concept so hard to believe when it's magnets in the EM field?
(It's just an analogy. Don't try to think of the EM field as little "molecules" of light that you are making waves in. It's a field and the photon is the discrete unit ("quantum") of energy in that field.)
I have a very basic question. What is a field? I mean I know what the definition is, but what is it really? What makes a field, a field? Do they even exist? How does light, which is an excitation in a field, interact with physical things?
All of our understanding of physics comes from making up models of reality and seeing how close to reality they match. The model of reality that treats all particles as excitation in fields is part of the single most accurate model humanity has ever come up with.
Is this model a true analogy of reality? Yes? Maybe?. At some level our 'real' understanding of realty turns into a version of "shut up and calculate" or "we don't know". It 'seems' reality is a bunch of rubber sheets stacked on top of each-other. Waves and ripples move through them and the energy from one sheet can transfer into other sheets like as if they touched eachother. Waves in one can 'bump' and create waves in other fields. "Physical things" are again just excitations in one or more of these rubber sheets.
The best answer to "what is a field?" is the definition of a field because that's what the model of reality assumes it is. True or not.
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> The model of reality that treats all particles as excitation in fields is part of the single most accurate model humanity has ever come up with.
Isn't that just "ether theory" with extra steps?
It’s “ether theory” that works. We adopt or discard models of the universe based on whether they make accurate predictions, and the ether theory of light didn’t.
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Superficially, kind of? There are many differences though. One is that the ether was proposed in order to provide a rest frame for light, whereas the fields upon which modern physics is based are fully relativistic. Another is that the ether was thought of as a physical thing thing with density, velocity, etc., and whereas fields can’t really be described in those terms, at least not as directly. It’s more that fields can give rise to them.
TL;DR an ether theory is similar to fields in that they permeate all of space, but they’re fundamentally different from each other in properties and mechanics.
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A field in quantum field theory (QFT), which is what this is about, is something that has a value at each point at space time. This value can be 0 but not null. More specifically every point in space is a quantum object that is a harmonic oscillator and according to QFT this is actually what everything is. Everything is emergent from these values, for example a particular wavelength of light is a particular value of these oscillators in the electromagnetic field of oscillators and its movement through space is just this value propagating through these oscillators like a wave. Objects can have values from multiple fields. For example a neutrino interacts with the Higgs field and the weak field but not the electromagnetic field so it is famously hard to detect. It also means that light literally does not exist to it.
In my head they are kind of loosely analogous to splines where one dimensional values can control the motion or path of an object through space.
What are these oscillators and do they actually exist? We don’t know. Maybe? Probably? I believe the current consensus is they may be fundamental as in they aren’t made of anything and are irreducible but in physics every time we have thought this we were shown to be wrong. The thing is it seems to be correct, very correct. This model has made predictions that turned out to be experimental verified later.
The problem is it’s essentially a purely mathematical construct and we’re getting into the realm of philosophy asking if it’s real or not. It depends on what math actually is/describes. It might be that at the very basic level of everything all there really is is math. All we can say for sure is QFT works very well.
Phisical things consist of atoms, which consist of subatomic particles, which are disturbances in their coresponding fields, so fields interact with other fields.
So I'm just a sentient culmination of quadrillions of field disturbances interacting with each other
Disturbing, isn’t it?
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It is irresponsible how incorrect this explanation of physics is. Rotating a magnet does not simply create EM waves. The changing magnetic field from the rotating magnet is not the same oscillating magnetic field induced by a photon.
Photons are not simply ripples that propagate outwards from some disturbance of a field.
Where did I say any of that?
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Rotating a magnetic does quite simply create EM waves, alongside other electromagnetic field changes. It just doesn’t only create EM waves. There is certainly nothing irresponsible or even really incorrect about their explanation. It’s a bit of a simplification, sure, but that’s appropriate in this context.
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A changing magnetic field induces an electric field. This is why moving a magnet through a coil of wire induces causes an electric current to flow in e.g. a bicycle dynamo.
Similarly, a changing electric field induces a magnetic field.
An electromagnetic wave propagates because a change in electric field causes the magnetic field to change, which causes the electric field to change, etc. in a self-sustaining wave travelling at the speed of light.
Yeah it’s difficult or impossible to have an intuitive sense of this. This might help though:
Electricity and magnetism are fundamentally linked to each other. They are not exactly the same but they work in similar ways.
A radio antenna works by accelerating the electrons in the metal back and forth. You could theoretically make a radio signal by doing the same with a magnet, but it would be much, much harder since you would have to move the whole mass of the magnet back and forth, not just some of the electrons.
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> Okay but fundamentally speaking, if I say rotate a magnet continually, it actually emits radio waves?
Yup! The connection between magnets and light is one of the most surprising parts of physics. If it were intuitive, it wouldn’t have taken us centuries to figure out!
A radio broadcast is made by waving around electric charges in a broadcast antenna. It is received by those radio waves themselves waving around electric charges in the receiver antenna.
So is the magnet here an antenna?
It isn't a permanent magnet... you can't use it to hang a picture on your fridge. But the antenna is the medium in which the moving electric field/moving magnetic field is created.
No I mean if I throw a permanent magnet into a black hole, is it an antenna? Because it’s radiating radio waves. Does that make it an antenna?
An antenna is one or more conductive elements electrically connected to a receiver or transmitter. What does a black hole have to do with anything?
Suppose the following:
I throw a permanent magnet (a chunk of iron or something) into a rotating black hole. The black hole has enormous mass, and it has to conserve angular momentum. So, as the chunk of iron falls into it, it should rotate. And if the magnet is very small, it should rotate very quickly (again, to conserve angular momentum).
My questions are:
As the magnet rotates, does it emits EM radiation?
If the magnet is emitting EM radiation, is it an antenna? Is it more-or-less a broadcast antenna that's powered by the black hole?
A stationary magnet has a static (fixed) "field" around it. In a perfect vaccuum, it doesnt emit any photons, because theres nothing interacting with it that would cause it to become "excited" and emit EM radiation.
A rotating magnet, regardless of being near a black hole, produces a dynamic (changing) magnetic field, and since those interactions contain information (quanta), we're all familiar with something heating up - which are photons in a spectrum we can't see with the human eye. Given enough time, in theory, the entire mass of the magnet would eventually be irradiated outward as light energy.
Lenz's law provides details on extracting energy from this very thing. You can spin the magnet around an electrically conductive material, like an iron nail. The nail becomes excited by the photons, and electrons start moving around (electricity). Alternatively, you can shake or spin the iron nail around a permanent magnet and produce the same thing via induction.
Your question in regards to a black hole is quite intriguing. Yes, a rotating magnet can act as a broadcast antenna, but its not very efficient to do so because magnets and all ferromagnetic materials have the uncanny ability of picking up and amplifying extra signal "noise" (ingress). What's fascinating to think about, is that a magnet in orbital motion around a black hole would indeed act as a "pickup" in exactly the same way a guitar pickup works. All these frequencies being thrown around together would be picked up and turned into the electrical signal version of the EMFs, then be induced back into the magnet in a Jimi Hendrix-esque feedback loop.
So when you're saying a "magnet" is spinning around a black hole, don't imagine a handheld rectangle magnet. Imagine instead that it's a magnetar... I mean... I can't even fathom the magnetic power of one of these alone, much less being involved in a stellar dance off with the energy source of a black hole. If something like a pulsar/magnetar were to be energetically involved with each other, theoretically you could place the magnetar in an energy jet stream of the black hole, and it would emit a "fucking huge" broadcast signal of their blended frequencies... may I say, "spacetime modulation?" Hmm...
> Given enough time, in theory, the entire mass of the magnet would eventually be irradiated outward as light energy.
I don’t think this would happen. I think the spinning magnet would preferentially emit light with polarizations that would slow down the magnet’s rotation over time, until it’s no longer spinning. I think hardly any of the magnet’s mass would be converted into light in this process.
Thank you for that. I was thinking in terms of Newtons 1st law of motion in a perfect vacuum (and over eons of time) and didn't consider that light itself has inertia and would affect its angular momentum as its being radiated. But, isn't it still conceivable that even down to the last atom of the magnet, there's a mathematical improbability that the spin would be zero, considering entropy? Or maybe the opposite is true, that entropy was working toward bringing the very last atom down to the lowest energy state possible, essentially converting any remaining angular momentum into light so that it can achieve the lowest energy state possible. I could see it going both ways, maybe even being in an entangled state of both outcomes until an observation is made.
Wait long enough and every system will tend toward its highest entropy and typically lowest energy state. But then we’re not really talking about the effect of the magnet’s magnetic field anymore so that’s a whole different conversation that depends on things like the stability of atoms and protons.
Is radio long-distance crosstalk, then?
it emits not just radio (electromagnetic) waves, but also sound waves and gravitational waves.
A permanent magnet is a magnetic dipole. It doesn't create "excitations," a magnetic field forms surrounding the two poles, pointing out of the north end and into the south end.
Moving a magnet doesn't typically create light because the motion doesn't typically produce a sufficient amount of energy to excite electrons and cause the excitation-decay process that occurs in, say, LEDs. Said another way, the material physics for free electrons recombining with "holes" simply doesn't exist here. Theoretically it should be possible to move a magnet with the right frequency and in the correct plane near an LED to produce a bit of light, but this would be rather difficult because the direct interaction between this magnet and the electrons in the semiconductor material would be rather weak. Obviously we can use motors to rotate magnets in an electric generator to produce electricity in cables and wires and then create useful circuits that way, but the physics of photon production a bit more complex than just moving a magnet around.
Moving a magnet can induce an electrical current in a conductive material, i.e. a copper wire. This is because the magnet interacts strongly with the free electrons in the conductive material, and the electrons move to produce a current. Photons are released most commonly when an electron "decays" energy states (it loses energy) and changes from a high energy state to a lower energy state. The frequency of light emitted depends on the energy difference between these states, known as the "band gap." Semiconductors have many different medium sized band gaps. Conductors have band gaps with actually "overlap" and cross each other, indicating little to no energy change is necessary for the electrons to change states. This is why they can move so freely in the conducting band to create current. Insulators, meanwhile, have very large band gaps and the amount of energy required for an electron to change states in an insulator often while break down the material, i.e. burn it.
When you move a magent around, you simply cause that magnetic dipole to "wiggle," but this is not a photon. A photon is a continuously oscillating electrical and magnetic field moving through spacetime, with those fields oriented perpindicular to each other. Only if you understand higher-order quantum field theory should you try to make sense of the description of a photon being an "excitation of the electromagnetic field." I don't like that description. It is not good for lay people, and I don't even know if it is agreed upon in the physics community. It is its own electromagnetic fields, and they are oscillating constantly. It's not a ripple of water propagating outwards from some disturbance losing energy as it travels, like you threw a pebble into a pool. It is a much more complicated particle and wave than that.
Edit: I want to add something else here. Some people have stated that moving around a magnet does produce light waves, but that is not necessarily true. A changing magnetic field can induce a current, but only where free electrons are present, and a current does not necessarily produce light, or at least any useful light beyond the imperceptible statistical stray photon. A photon is a "packet" of energy which exhibits the characteristics of both a particle and a wave. It is a fundamental piece or building block of energy in the universe. Moving a magnet around does not inherently induce some quantized amount of energy to be flung about the universe in the form of a photon. A moving magnetic field, in a vacuum, is not itself any form of energy, even though the movement of this magnetic field superficially is similar to the changing magnetic field of a photon. A changing magnetic field can induce a force - the electromagnetic force - on free charges, but no energy exchange takes place until electrons begin moving.
Whether the magnetic field itself uses photons - being the fundamental carrier particle for the EM force - to "communicate" with or "touch without touching" the electrons is something of a much higher level debate, to the best of my knowledge.
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Okay. So, fundamentally, I am correct to say that if I have a permanent magnet (an iron magnet for example), and I constantly accelerate it very, very quickly (for example, I throw it into a black hole), it will emit EM radiation all the way down? Is that correct? Could I (theoretically) detect a magnet falling into a black hole by observing the radio waves it emits, and infer that a magnet must be falling into the black hole?
What happens to a black hole that's rotating if it has charge? Does it emit EM radiation? Intuitively, I think the answer is "no" because a black hole can't emit anything. So I think I'm misunderstanding things. Where am I going wrong?
>I am correct to say that if I have a permanent magnet it will emit EM radiation all the way down?
No. Not at all. I thought that specific part was pretty clear. A magnet does not simply emit EM radiation, moving it doesn't change that either. Moving it in the vicinity of free charged particles can induce a current, but that is not the same as light either.
Light - i.e. a photon - is a quantized packet of energy. You're not just flinging around energy by waving a magnet, no matter how fast it moves.
>Could I (theoretically) detect a magnet falling into a black hole by observing the radio waves it emits, and infer that a magnet must be falling into the black hole?
No, because it isn't emitting radio waves this way.
>What happens to a black hole that's rotating if it has charge? Does it emit EM radiation? Intuitively, I think the answer is "no" because a black hole can't emit anything. So I think I'm misunderstanding things. Where am I going wrong?
You're trying to make sense of black holes before you make sense of the basic properties of the EM force. You need to slow down and try to get a better understanding of the basics before trying to understand what happens near a singularity.
>You're trying to make sense of black holes before you make sense of the basic properties of the EM force. You need to slow down and try to get a better understanding of the basics before trying to understand what happens near a singularity.
Yeah, I'm sorry. I'm trying to make sense of EM force, but I'm finding it really, really confusing.
That's because it is. What is your formal level of education on the topic? We have to begin with where you are correct. You seem to be confusing a magnetic field with photons themselves.
Honestly, zero. When I was in high school, I wanted to be a physics major in college, but I my math teacher told me I had no chance to ever learn the math necessary to do Physics, and it's just be a waste of my tuition money, so I abandoned that plan because I couldn't afford to fail my first year as a physics major and then need to change to a different program.
I've always kind of regretted my decision, and now I'm trying to self-study. I've been going through Taylor's Classical Mechanics, and basically picking up Calc and differential equations along the way as I go (I know, this is all a bad idea. Believe me, everybody has told me that I just have to take a bunch of math and make sure I'm 100% on that before I ever even look at a physics book, but I just can't do it. I hate math so much).
Taylor's Classical Mechanics is not exactly easy, but I felt like I was making some level of progress. I bought a used copy of Griffith's Electrodynamics because I found a used book store selling it for a really low price.
I'm confused now because I was trying to figure out what Maxwell's equations mean, and that took me to Stack Exchange, where somebody said that if we have a wave passing through the magnetic field, it induces an electrical field, and then that re-induces a magnetic field, which then self-propagates as the electrical wave makes a magnetic wave, and so on and so forth. We can this endless propagation a photon.
So then I thought, well wait that doesn't make sense. Because then moving a magnet through space would just make magnetic waves, and that would create photons? That makes no sense at all. But another comment said that photons are created by wave excitations in the EM field. Which sounds similar?
At that point I decided, okay I have no idea what is going on, I'd better ask.
And here we are.
That's all there is to it. I have no formal education at all. I'm just a humanities person who's in way over his head.
Let's start here:
>my math teacher told me I had no chance to ever learn the math necessary to do Physics, and it's just be a waste of my tuition money
What a horrible teacher! I don't care how hopeless it looks, a teacher should never, ever discourage our curiosities! I don't care if you were struggling with basic algebraic principles, you can learn the math necessary if you are truly curious about physics!
>everybody has told me that I just have to take a bunch of math and make sure I'm 100% on that before I ever even look at a physics book, but I just can't do it. I hate math so much).
Also not the best advice for everybody. We all have areas of strengths and natural curiosities and other things are just work. This advice amounts to "you can never play an instrument if you don't learn nusic theory and how to read sheet music." It's just not true. You should try to learn the formal rules and principles, and respect the wisdom and truth they contain, but if you love to play music and can do it without reading sheet music, then do it! This is kind of the same thing, although you can't completely disregard the math in science, whereas you can have a successful career playing music even if you never learn to read a musical note.
>I was trying to figure out what Maxwell's equations mean, and that took me to Stack Exchange, where somebody said that if we have a wave passing through the magnetic field, it induces an electrical field, and then that re-induces a magnetic field, which then self-propagates as the electrical wave makes a magnetic wave, and so on and so forth. We can this endless propagation a photon.
Oof. I don't like that explanation at all. They might have some truth in some of it, but it is safe to just ignore this explanation. Also, Maxwell's equations are very difficult to understand intuitively. I'm an electrical engineer by profession, I have an undergrad degree in that and one in business, and I am deeply interested in the physics, I should have double majored or at least minored in physics. I also may later pursue a PhD in physics, but for now my career is to be an engineer, which I do enjoy.
Maxwell's equations describe basically the totality of the electromagnetic force. It helped einstein to come up with special relativity and it also provided clues to quantum theory. These equations described the electromagnetic force better than Newton described classical mechanics. Quantum theory doesn't blow up Maxwell's equations the way quantum mechanics blow up Newtonian physics. It's amazing. But it's also not very intuitive and I have no idea how I would even go about summarizing Maxwell's equations to a lay person. So don't sweat it if you don't "understand" these equations!
Now, here's a story.
Me, an EE who graduated with a 3.3 GPA from a top 50 school, loved math in high school but didn't love science. I struggled badly with my second semester of physics in college for my business degree. Then, after graduation, I read this book because I thought it would just be good general knowledge:
This book completely changed my perspective on physics principles that I never could grasp before. You can read the entirety while ignoring some of the math he presents. It's a textbook but reads very conversationally most of the time, and is meant to be very approachable for people who aren't necessarily STEM majors.
Now, I ask you before we go farther, what exactly do you want? Do you want to just gain a better layman's understanding of physics, or are you exploring going to college or a career change?
Because how we talk about this I think depends on your goals.
>Then, after graduation, I read this book because I thought it would just be good general knowledge: > >https://press.princeton.edu/books/hardcover/9780691135045/physics-and-technology-for-future-presidents
Just ordered this, thanks for the recommendation. FYI: coupon code PUP30 applies a 30% discount.
I am very excited for you. If you were interested enough in the description to order that book I have no doubt you'll find it intriguing from cover to cover. Enjoy, and stay curious!
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Light is a collection of photons. Photons are the carrier particle of electromagnetism.
Another way of thinking of carrier particles is thst they are excitations of a pre-existing field. So photons (light) are an excitation of the electromagnetic field.
This is why such a big deal was made of the discovery of the Higgs boson, as it provided evidence for the Higgs field.
To your other question, any moving magnet does perturb the electromagnetic field and creates electricity in an electrical conductor; this is essentially known as Faraday's law.
A photon is the quantised amount of energy transferred between an electron and a the electromagnetic field when it moves between shells. Electromagnetic radiation always travels as waves, it is the transfer of energy between the field and atoms that is quantised. The concept of a photon being a light particle is incorrect.
So, can we see photons as the "highest point" of a wave travelling across the EM field? Or does the wave analogy stop being relevant at this point?
The analogy doesn't hold well at this point. The previous comment saying "Electromagnetic radiation always travels as waves" is misleading because neither classical waves or particles can describe all the behaviours of EMR.
If you're asking "Where in the wave is the photon", the answer is that it's in all the places at the same time until you check. You can consider the height of the wave to be the probability that the photon would be in that position if you were to measure it. The position and path a photon travels is literally not concretely defined until it is measured, at which one of the possible positions is randomly selected. When we have a lot of photons we can sweep a lot of the probabilistic stuff under the rug by summing them all together into something that resembles our intuitive understanding of waves, but it does not mean that a single photon is a tiny wave.
The previous comment said "The concept of a photon being a light particle is incorrect", which is true, but the concept that it is a wave is also equally incorrect. They're both interpretations that aim to simplify the probabilistic nature of quantum objects into something intuitive to us, and which interpretation you use will depend on which behaviour you want to describe. Can you model electromagnetic radiation travelling as particles? Yes absolutely, but you'd be describing it in terms of the sum of those infinite probabilistic potential paths the particles could travel.
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Say I had a beam of photons with a very specific wavelength and I was able to check the position of a particle, would that position be somewhere along a very well defined sine curve? Or is that just a simplification like the nebulous clouds of atomic electron shells were dumbed down to be circular orbits that look cool as sciency logos in the 50s and 60s?
What does highest point mean?
>the wave analogy stop being relevant at this point?
It doesn't stop being relevant, you just have an incorrect picture of what is happening. You likely are basing this image on your intuition of waves of water.
Actually, a useful image is more like sound waves. A sound wave consists of oscillating pressure in some medium, such as air or even water. When you hear sound, do you just hear the "high" pressure peaks? Or the low pressure valleys? Or, do you hear the full range of changes over some period of time?
It's the latter.
In fact, if you were only hearing the "peaks" or the top half or the bottom half of a sound wave, you would hear something that is distorted. This is actually how sound distortion in music works, such as for guitar amps or synthesizers. When we speak, or a piano hammers a tuned string, the sound created is relatively smooth, like a sine wave (speaking has more complicated wave patterns but the patterns consist of a combination of relatively smooth waves). A distorted sound appears more like a square wave or something with more corners on it, when plotted visually. So instead of your ear sensing the smooth undulations of pressure changes, it experiences sustained pressure (such as the top of a square wave) followed by (or preceding) a much more abrupt and instantaneous change (the vertical part of a square wave). This is more jarring and unexpected, which is why it sounds "unnatural."
Vision and light share some of these characteristics of experience, in that when your eyes see, they are typically experiencing a smooth range of changes in the Electromagnetic spectrum over time as the photon passes the receptors in your eye. It's not simply the peaks or valleys of this wave, it is the frequency and the total energy ( simply: how many photons in the frequency range) that your eyes sense. You can't really experience an instant of a photon. You need the changes of the wave over time for your body and brain to sense and interpret these signals.
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well Einstein and other physicists don't seem to agree with you. and i'd rather go with them.
>excitations of a pre-existing field
What "pre-existing field" are you refering to? What sources exist to create such a field?
In this case, the electromagnetic field. As I understand field theory, it states that forces exist as fields everywhere through spacetime. Perturbations in that field, where it rises to a non zero value, are interpreted as force carrying particles or bosons. So we could state that the photon is an excitation in the electromagnetic field just as the W and Z bosons are for the weak field.
>In this case, the electromagnetic field
I asked which one?
>Perturbations in that field,
How does this field differ from the fields surrounding charged particles such as electrons and protons? Is this the same field? Are electrons and protons then not also "perturbations?"
If this is an explanation from quantum field theory, how does this description of one single, continuous electromagnetic field differ from the luminiferous aether? And, again, how do we understand the fields surrounding stationary or moving charged particles?
I don't know what an "excitation" is. When I try to think about it, the first thing is a la ola wave at a soccer game - excited people.
When you hit an atom with a burst of energy, it causes electrons to move from an orbit close to the nucleus, to an orbit slightly further from the nucleus (excitation). When that induced energy is removed, the electron "sinks" back to the original orbit and emits a "particle" of light.
At least that's how I learned it quite a few years back!
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JohnnyCanuck t1_j9nak5p wrote
An accelerating magnet does, just not at a wavelength that you can see. Keep in mind that the electromagnetic spectrum (light) includes radio waves, microwaves, infrared, visible light, UV, X-rays, and gamma rays. If you wave a magnet around, the emissions are going to be at the sub-radio end of the spectrum.