The song of killer electrons

By Phil Plait | October 1, 2012 12:15 pm

Light and sound are two fairly different things. They’re both waves, but their similarity ends there. Sound is a compression wave: something happens (like a tree falling in a forest) that compresses air a little bit, and that wave travels outward at – shockingly – the speed of sound. Your ears detect it, and your brain translates it into sound.

Light is an electromagnetic wave. The whole story is fairly complicated, but it’s an oscillation of electric and magnetic fields, and doesn’t need a medium (like air or water) through which to travel.

But because they’re both waves, light and sound have a frequency and an amplitude. If you’re clever – and we humans are – you can convert one to the other… well, more like translate one to the other. It’s not like it’s a real conversion, with physical meaning (like converting feet to meters). But it can make for an interesting, and lovely, experience.

In August 2012, NASA launched a pair of satellites called Radiation Belt Storm Probes. They are designed to detect the electromagnetic radiation (also called EM for short) emitted as charged particles bip and bop around Earth’s magnetic field, high above the surface. Various phenomena in the geomagnetic field produce radio waves – which are a form of light, but too low energy for our eyes to detect. These waves can have the same frequency as sound waves we can hear – a few Hertz, or oscillations per second – so they can be directly translated into sound. Scientists did that using RBSP data, and it sounds eerie and beautiful.

Here’s the sound file of these waves. It’s… odd. Like birdsong, but unearthly. You can understand why scientists call these waves "chorus".

And they’re handy, too. Translating light to sound in this way can be helpful in understanding it, much like creating color pictures helps astronomers understand data better. The folks at NASA put together this video to explain all this:

This is actually pretty important stuff. Electrons blown from the Sun in the solar wind get trapped in Earth’s magnetic field, like bugs in a net. Usually these are low energy particles, which eventually follow the magnetic field lines to the Earths poles, where they are deposited safely into our atmosphere. But sometimes, something caffeinates these electrons, pumping them up to very high energies. They travel so quickly that if they hit one of our satellites, they can damage the electronics! The reasons benign particles can turn into "killer electrons" isn’t well understood, but it may have to do with these chorus waves. Understanding them better means we can protect our satellites better, and since we spend trillions of dollars on satellites, there’s some decent motivation to make sure they work well.

Also? It’s just cool. If you like the killer electron song, then you’ll get a kick out of other sounds created the same way, from the aurora to meteors burning up in our atmosphere! You’ll find links to those listed in Related Posts below.

Image of my good friend Dr. Nicole Gugliucci, aka Noisy Astronomer, used totally without her permission. Won’t she be surprised?


Related Posts:

- Hear the Sun’s roar
- Listen in on the Perseid meteor shower
- Saturn, the Forbidden Planet
- Phoenix sings
- Laying down the pulsar beat

Comments (49)

  1. To those of us of a certain age, it sounds like science fiction, the song of atomic rockets and 400-story skyscrapers, of Planet X, ion blasters, and the Space Patrol.

  2. Sindragosa

    I want to see gamma rays! I want to hear X-rays! And I want to smell dark matter! I want to reach out with something other than these prehensile paws, and feel the wind of a supernova flowing over me!

  3. Mike

    I think it’s the whale singing from that recent Dr. Who episode.

  4. Andrei

    “These waves can have the same frequency as sound waves we can hear – a few Hertz, or oscillations per second – so they can be directly translated into sound”
    Is this trully how they’re doing it? I always thought it involved some heterodyning to translate the radio frequencies to the audio range.
    If the EM waves have such low frequencies (well, more than a few Hertz like you said – maybe few hundreads or thousands Hertz) how can the satellites detect them? I mean for a 1kHz radio wave you’ll need an antenna in the order of tens of kilometers (75km for a quarter-wave antenna). Do they detect them as variations in the electric field (but then what is the reference potential?)

  5. MadScientist

    Years ago I wanted to install a Michelson interferometer in a building and ‘mix’ the building deformation (and wind blowing, and changes in air temperature and humidity) into the audio range – but someone just had to spoil that idea by telling me that it had already been done. Personally I see no great value – it’s a technique for demonstrating to people that there are these changes which are imperceptible to them, but for real science you’ve got to make measurements.

    @Andrei#5: Maybe the BA means the modulation frequency is in that range. As for radio waves in that range – of course you can detect them, just not as efficiently as you can with a monstrous antenna. The long-gone Omega Navigation System operated at ~10 to 15KHz and had enormous antennae, but the aircraft (and submarines) which relied on the signals didn’t have antennae anywhere near the size of the transmitters.

  6. Peter Davey

    “There’s not a sphere but that in its orbit sings, still choiring to the young eyed cherubim.”

    Or, to put it a slightly different way:

    “The man that hath no music in himself,
    Nor us not moved with concord of sweet sounds,
    Is fit for treasosn, stratagems, and spoils,
    The motions of his spirit are dark as night,
    And his affections dark as Erebus,
    Let no such man be trusted”

  7. I’m probably going to sound stupid here, but I’ve never been happy with the explanation of waves not needing a medium to travel in. Surely the whole concept of a wave is that there is something that is “waving”? Saying that electromagnetic waves “don’t need a medium in which to travel” sounds like a non-answer to me. Maybe it’s the fabric of space itself that is the medium? There must be some medium – a wave without a medium is like a facial expression without a face, if you get my drift. Illogical!

    OK, now everybody feel free to tear my argument to pieces, but please explain!

    (Oh – and while I’m here, what’s all this I’ve been reading about a comet next year that’s going to be “brighter than the full moon”? Is there any truth in this?)

  8. The band Meat Beat Manifesto uses a lot of weird samples, and one track in particular uses these sounds – ‘The Tweek’

    http://www.youtube.com/watch?v=GLPaFSVfcVQ
    (45 seconds in they start talking)

  9. DanM

    @ elwood herring:
    Not going to tear you apart at all. The ‘illogical’ conundrum you’ve pointed out is indeed one of nature’s great puzzles. The counter-intuitive nature of this phenomenon is why it took so long for humans to understand light.

    Before about 1900, most physicists believed that the universe was permeated by the ‘aether’, an invisible substance that was the thing doing the waving. You may have heard of the Michelson-Morley experiment (“the most famous failed experiment ever”), which disproved the existence of the aether. This result was part of what motivated Einstein to posit that electromagnetic waves were a unique sort of wave, one which does NOT require a medium in which to travel. Light is, instead, an oscillating ‘field’; that is to say, an electric field (also a magnetic field) which oscillates periodically in time, at any given point in space (or periodically in space at any instant of time). These fields are real things – they can exert forces on electric charges, and they transport energy. Their existence can therefore be verified by experiments, so they are not merely invented mathematical objects. And, they are described as waves (i.e., the mathematics of these waves are equivalent to the math describing more ‘ordinary’ waves like sound waves or water waves). Thus, it is not quite so illogical as it sounds to say that light is a wave that can travel in empty space. It is the electric field itself which is doing the ‘waving’ (and the magnetic field too). Counter-intuitive, but not illogical.

    However, the very fact that electromagnetic waves DON’T require a medium in which to move, the very fact that they can propagate through entirely empty space, leads to the quite shocking conclusion that their speed must be the same for all observers. This is Einstein’s theory of relativity (technically, the ‘special theory of relativity’). All of Einstein’s conclusions about the bizarre things that happen when you travel fast (e.g., your clocks run slower, your mass increases, etc.) follow from this initial observation. As do other strange effects, like the fact that two events which appear simultaneous to one observer are not going to appear to be simultaneous for other observers. Mind-bending, but not illogical. In fact the logic is quite compelling and beautiful, and I couldn’t possibly do it justice in this tiny little text box…

    So light is a wave, but it is a very unique wave indeed.

    Now, wrap your head around this one: Not only is light a wave (in empty space!), but it is ALSO a particle. At the same time. The particle is called a photon. This idea was also due to Einstein, by the way (although he was not the one who coined the word ‘photon’). This is even stranger. But also quite logical, when you see the whole picture.

  10. DanM

    @Andrei #5:
    Think of the size of the antenna on your AM radio: probably less than one meter. Now think of the wavelength of the AM radio signal that it is receiving: hundreds of meters. Deep subwavelength antennas can work just fine for signal detection. Especially if you have a good amplifier to boost the signal you measure. A good LC filter helps too (i.e., a radio tuner).

  11. DanM

    @MadScientist #7:
    I have a laser demo that I do routinely when teaching about interference, which involves taking the output of a slightly misaligned Michelson interferometer and directing it onto a photodiode which is plugged directly into the mic input of a speaker (with built-in audio amp). That way, when the interference pattern fluctuates, you can hear it directly. It is crazy sensitive to vibration, and makes the coolest sounds when you tap on one of the mirrors in the interferometer.

  12. There are links to recordings of “whistlers” and the plans for receivers here:

    http://www.auroralchorus.com/

  13. shunt1

    The video was rather lame, but the concept of using twin probes for stereo audio is very interesting.

    Sometimes things that are hard to figure out by viewing digital data alone, but when converted into sounds, they often expose a harmony that was never suspected before.

    I would give this one:

    Two thumbs up!

    @13. DanM:

    WOW.

    “…taking the output of a slightly misaligned Michelson interferometer and directing it onto a photodiode which is plugged directly into the mic input of a speaker …”

    Can you PLEASE provide that audio to me? But most of all, what gave you the idea in the first place?

    Was the basic concept to detect subtle gravity waves using audio?

  14. @#3. Sindragosa

    I can’t even express these things properly because I have to – I have to conceptualize complex ideas in this stupid limiting spoken language!

  15. Grand Lunar

    Didn’t Cassini make similar findings at Saturn, thus leading to the “spooky” sounds we’ve heard from there?

    Interesting mission, BTW.

  16. Nigel Depledge

    Andrei (5) said:

    If the EM waves have such low frequencies (well, more than a few Hertz like you said – maybe few hundreads or thousands Hertz) how can the satellites detect them? I mean for a 1kHz radio wave you’ll need an antenna in the order of tens of kilometers (75km for a quarter-wave antenna). Do they detect them as variations in the electric field (but then what is the reference potential?)

    Well, in principle, yes and no.

    For a narrow-band antenna, it is indeed preferable to have the antenna match some suitable factor of the wavelength. However, one of the best omnidirectional broad-band antennas is known as a “long wire” antenna (see if you can guess why!). To pick up radio emissions, all you need is a piece of wire and a tuned circuit. The tuned circuit is there not to enhance the detection of any specific frequency, but to block out frequencies you don’t currently wish to receive.

  17. Nigel Depledge

    Elwood Herring (9) said:

    There must be some medium – a wave without a medium is like a facial expression without a face, if you get my drift. Illogical!

    It’s OK, it’s all quantum.

    It’s all totally bona fido.

  18. Nigel Depledge

    Shoeshine boy (16) said:

    I have to conceptualize complex ideas in this stupid limiting spoken language!

    Technically, you are using a limited typewritten language, which is even less expressive than spoken language.

  19. DanM

    @ shunt1 #15:

    I’m afraid I do not have any audio recordings. Next time I set it up, I’ll see about making some for future posting. The idea actually came from a former colleague of mine who used it purely as an educational tool to ‘gee-whiz’-ify high school kids and persuade them to consider a career in science. As demos go, it is a pretty good one. I have been told that it is the most memorable demo of the entire semester…

    The interferometer that I build is a table-top setup with cheap(ish) components, so we won’t be detecting gravity waves (we are only off in the necessary sensitivity by about 13 orders of magnitude). But, the basic idea is similar to LIGO or other interferometric gravity-wave detection schemes.

    @ Nigel #19
    Actually, understanding how one can have waves in empty space does not require quantum at all. Bona fido, yes. But quantum, no. Now photons in empty space, that’s quantum.

  20. @DanM: thanks for the response: yes, I do know about the Michelson-Morley experiment and the aether hypothesis, and I understand a fair bit about relativity too. I still don’t quite get the waves without a medium thing though, despite your excellent explanation! Maybe I’ll never wrap my head around it…

  21. Uncle Al

    it’s an oscillation of electric and magnetic fields, and doesn’t need a medium (like air or water) through which to travel.” Vacuum ballistic trajectory of photons as opposed to the EM propagation of waves, there is a medium – the quantum uncertainty of classical vacuum revealed as non-zero free space permittivity and permeability, and the Casimir effect giving the Scharnhorst effect.

    http://www.npl.washington.edu/AV/altvw43.html
    Vacuum engineering – but it gets really interesting only after it becomes non-empirical.
    http://arxiv.org/abs/1110.1919
    Huge Casimir effect at finite temperature in electromagnetic Rindler space

    The Scharnhorst effect allows the local speed of light to exceed c=1 only when the Casimir energy is negative. The finite-temperature-Casimir energy is positive in the published cases. Needs more studies.

  22. DanM

    @elwood:
    Here’s a long-winded answer which may provide additional clarity (or may just be boring, I dunno). One should start by asking the question “what is a wave?” Strictly speaking, a wave is anything that moves through space. Best illustrated with examples.

    When one talks about a wave on the surface of a lake, for example, the water is moving up and down, but the wave is moving across the lake (i.e., the direction of motion of the water molecules is perpendicular to the motion of the wave). So what is moving? Energy is moving across the surface of the lake, because the wave transports energy. That energy is transmitted by the motion of water, so you could not have a water wave without a body of water. Seems sensible enough.

    With EM waves, the situation is more subtle because you have to wrap your mind around the concept of a ‘field’. The electric field is a quantity that is defined at every point in space. It is, in essence, a force field, in the following sense: if we were to deposit a charged particle at a particular point in space, it would begin to accelerate. The electric field at the point where we put the charge exerts a force on it, according to F = qE where q is the magnitude of the charge and E is the strength of the electric field at that location. The electric field is equivalent to a voltage applied across a distance – in fact, the units of electric field are volts per meter. So you can use a battery to create a static (i.e., not changing in time) electric field.

    Notice that the existence of this electric field does NOT require any material – you can do this in air, or inside a block of glass, or even in a perfect vacuum. The force exerted may be different in these different situations, but the basic idea is the same: an electric field is the force field that pushes charged objects around. If we put an electric field across a wire, then current flows in the wire because the electric field pushes the electrons in the metal from one end to the other. If we put an electric field across a region of air, then any free charges will be pushed from one side of that region to the other. Of course, usually air doesn’t have any free charges floating around in it. But, if the electric field were very large, then we could rip the electrons off of the atoms and molecules in the air, and then they would be free to move. That’s called lightning. And if we put an electric field across a region of vacuum, once again nothing much will happen unless we arrange for some charged objects to be sitting in that region – in which case, those objects will feel a force, and will accelerate in response.

    Now let’s suppose that the electric field is not constant, but is spatially varying. That is, suppose that at one point in space, the electric field has a positive value (so that a charge will be pushed up, for example), but at a slightly different location in space (just a bit to the left, say), the field has a negative value (so that the same charge, placed at this new location, would be pushed down). Now, repeat this oscillation: as we move along a line in space from right to left, the electric field is alternately pointing up, then down, then up, etc. We now have a spatially varying electric field pattern, oscillating regularly along a line running from left to right.

    Now, let’s take the final step: suppose this spatially varying pattern is sliding to the left, rigidly without changing its shape, at a constant speed. If you were to sit at one point in space and watch this field pattern slide past you, you would experience an electric field that alternately pointed up, then down, then up, etc., at your particular location. Or, if you were to freeze time at any instant and take a snapshot of the situation, you would see an electric field pattern that was pointing alternately up, then down, then up, as you move along the line from right to left. This is an electric field which oscillates as a function of time (for any given point in space) and also oscillates in space (at any given instant of time).

    So we’ve just described something which exerts a force on charged objects in the direction either up or down, and which transports energy to the left. How do we know that it transports energy? Well, because if this something continues to slide along to the left and eventually encounters a charged object, it will exert an alternating force on that object, first up then down then up etc., causing the object to oscillate up and down. In other words, our something, sliding along to the left, can cause stationary objects to begin moving, which means it has the capacity to do work. That must mean that it carries energy.

    Recall our description of a wave on the surface of water. Clearly there are some similarities. These similarities strongly suggest that the thing we’ve described should be called a ‘wave’. But our electric field wave can exist even in empty space, because a voltage can be applied across any region of space regardless of what fills that region. In fact, what we have described is nothing more than a light wave. Well, to be precise, that is the electric field component of an electromagnetic wave (of which visible light is one example). To really describe an EM wave, you also need to imagine a second field (the magnetic field), which is doing the same thing in the same way along the same line in space. Every EM wave, from radio to microwave to infrared to visible to ultraviolet to X-rays, is just a pair of fields (the electric field and the magnetic field), oscillating together in time and space.

    Don’t know if this is helpful, but it’s my best attempt.

  23. !AstralProjectile

    Slightly OTP question as to why higher frequency sound waves are attenuated more than low freq ones. Of course air in the the “peaks” are warmer than the “troughs” (PV=nkT). Any heat transferred from peak to trough would waste energy and attenuate the wave. My hypothesis is that the shorter the wavelength, the more heat is transferred, and that causes greater attenuation. Does anyone know if this is correct?

  24. Gavin Flower

    @DanM
    @elwood herring
    The conventional, and the most useful for practical purposes, is that (in a very simplified form, glossing over a lot of details!):
    (1) light is electromagetic, having both electric and magnetic fields
    (2) the 2 fields are at right angles to each other
    (3) the changing electric field creates a magnetic field in ‘front’ of it
    (4) the changing magnetic field creates an electric field in ‘front’ of it

    However, it appears that space time is far more complicated and subtle than the standard 3 space dimensions and one time dimension we normally deal with…

    Consider the physics of M-theory which invokes 6 extra space like dimensions wrapped up and an 11th dimension of some kind. One of the justifications of M-theory is that it provides a consistent explanation for for both Quantum Mechanics and Special Relativity, by postulating a minimum physical size, by virtue of strings of the order of the Plank Length (this is way smaller than the size of a proton). These strings vibrating in all these dimensions are responsible for all the fields we know about. String theorists don’t claim that M-theory is the ultimate Truth, merely that it is the best theory we have to date – even if it is fiendishly difficult to use and contradicts what many people (implicitly & explicitly) ‘believe’ is True (M-theory does not predict that the Universe must be the way we see it, it also suggests many other possible solutions)!

    There are arguments based on entropy, that suggest that space time has to be quantised – to avoid having infinite entropy. Note that this is consistent with M-theory.

    Some theoretical physicists think that time itself is a derived quantity!

    I went to a seminar on ‘causal sets’ which is an attempt to look at the nature of Space & Time in a much more fundamental ways than any of the above.

    So while we intuitively feel that ‘something’ must be waving in the ‘vacuum’ for light to travel through – our very notions of Space & Time are merely approximations to Reality!

  25. Peter Davey

    In response to Nigel Depledge’s comment, this condundrum has been compared to the Cheshire Cat, disappearing, but leaving its smile behind.

    “Curiouser and curiouser”.

  26. Loved the video! Just yersterday i was trying to convince my girlfriend of how amazing and mindblowing is to think that (spite the fact that they’re differente) both sound and light are formed by waves.
    I really appreciated this article, thanks Phil!

  27. James Evans
  28. DanM

    @Uncle Al #23 and Gavin #26:
    Sorry gentlemen, but I must correct you. The question at hand has nothing whatsoever to do with either quantum mechanics or string theory. This is a purely classical effect. The quantum fluctuations of the vacuum are in no way related to the non-zero permittivity or permeability of empty space, the Casimir effect (while fascinating) is entirely irrelevant to the discussion at hand, and one needs no understanding of strings or branes to understand light waves. Maxwell’s equations are actually quite adequate.

    Pseudo-science does not clarify or educate. It confuses and obscures. Keep it real, folks.

  29. Gavin Flower

    @DanM

    What I said was not Pseudo-science.

    The point I was trying to make was that we wonder how light propagates in a vacuum, but take for granted the fundamental nature of Space & Time.

    The nature of the light propagating in a vacuum is not classical, as in classical mechanics information travels at infinite speed. Classical mechanics deals in rigid bodies, which requires that when the front of a rigid body stops, then the back of that rigid body stops instantaneously.

    You can build high performance fighter jets using approximations to classical mechanics, as the precision of engineering makes the corrections introduced by considering Special Relativity irrelevant. However, you do need to consider Special Relativity when building a GPS system.

    When people are working on the edge of what we know, they must extend our ideas into the unknown, and often the new approaches turn out to be wrong, sometimes completely. However, they still start with what we know.

    I remember a puzzle about dividing coconuts in various ways, the solution offered involved someone having a negative number of coconuts at one stage – but the starting and final states where physically valid. There are also problems in the mathematics of the Real number line that are best explained using Complex Numbers.

    It is easy to demonstrate that our intuitive understanding of Space & Time is deeply flawed without involving advanced physics.

    For example:

    ‘Everything is contained in something’ – so what contains the Universe? I don’t have a satisfactory answer, but the question is a useful starting point for considering the geometrical nature of space – as obviously space is non-Euclidean.

    People like to have a starting point for the Universe, then they ask what happened before that point! Yet people are often unhappy with the notion that the Universe is infinitely old. Postulating God creating the Universe simple converts the question into how/when/where did this god character come about – hence is a non-answer.

    So what really is the nature Space & Time?

  30. Charles Goodwin

    @DanM said
    “Sorry gentlemen, but I must correct you. The question at hand has nothing whatsoever to do with either quantum mechanics or string theory. This is a purely classical effect…
    Pseudo-science does not clarify or educate. It confuses and obscures. Keep it real, folks.”

    Whether or not Gavin’s comments were relevant to the nature of light, quantum mechanics and string theory are NOT pseudo-science. (OK, some people *may* consider string theory to be verging on pseudo-science due to the difficulty of getting testable predictions out of it (e.g. Feynman and Glashow) but it’s still a long, long way from things that can genuinely be described that way – astrology and so on).

    Making invidious comparisons does not clarify or educate…

  31. Nigel Depledge

    DanM (24) said:

    And if we put an electric field across a region of vacuum, once again nothing much will happen unless we arrange for some charged objects to be sitting in that region – in which case, those objects will feel a force, and will accelerate in response.

    In fact, this is how a cathode ray tube works – electrons “boil” off the heated cathode and are accelerated by the electric field between the anode and the cathode.

  32. Nigel Depledge

    Astral Projectile (25) said:

    Slightly OTP question as to why higher frequency sound waves are attenuated more than low freq ones. Of course air in the the “peaks” are warmer than the “troughs” (PV=nkT). Any heat transferred from peak to trough would waste energy and attenuate the wave. My hypothesis is that the shorter the wavelength, the more heat is transferred, and that causes greater attenuation. Does anyone know if this is correct?

    Because sound is a mechanical wave, higher frequency means that the molecules in the medium must move and be accelerated faster than for a lower frequency. And molecules of air (all kinds of molecules, in fact) have inertia. The faster you want to accelerate them, the more energy you need to do so.

    It seems reasonable to suppose that – in air at least – energy losses from the transferred wave will therefore be greater at higher frequencies. Not only are those higher-frequency waves accelerating the air molecules more vigorously, but (because with higher frequeny goes shorter wavelength) the peaks and troughs are closer together, so air can diffuse from peak to trough.

  33. Nigel Depledge

    DanM (31) said:

    Pseudo-science does not clarify or educate. It confuses and obscures. Keep it real, folks.

    For all we know, strings and ‘branes are real. They are certainly possible, so they ain’t pseudo-science.

  34. Nigel Depledge

    Gavin Flower (32) said:

    . . . as obviously space is non-Euclidean.

    Wait, what?

    “Obviously”?

    Why “obviously”?

  35. Gavin Flower

    Euclidean space is infinite

    @Nigel Depledge
    Hmm… my comment “as obviously space is non-Euclidean” in the paragraph in which it appeared, are not consistent. I think at the back of my mind I was thinking that space is finite yet unbounded – but, simply not being contained in anything _IS_ consistent with Euclidean Geometry! Sloppy thinking on my part.

    Looks like I have to invoke Relativity to support the idea space is non-Euclidean – too tired to investigate possibly simpler arguments at the moment (11:30pm Wednesday here in NZ).

  36. Gavin Flower

    @Nigel Depledge

    I would not say that “strings and ‘branes are real”, but currently M-theory is the best explanation we have to resolve the conflict between Relativity and Quantum Mechanics. Certainly, though, M-theory is Real Science – but that does not mean it has to fit what people feel is ‘Right & Proper’!

  37. Nigel Depledge

    Gavin Flower (38) said:

    Certainly, though, M-theory is Real Science – but that does not mean it has to fit what people feel is ‘Right & Proper’!

    Agreed!

    Classical physics was based on what people felt was right and proper, and it ended up in all sorts of trouble.

  38. Messier Tidy Upper

    The Van Allan belts I take it, right?

    Strange to have deadly radiation sound so peaceful and sweet. So much like birdsong.

    Fascinating clip. Surreal to hear our planet sing. Cheers! :-)

  39. DanM

    @ Gavin #32:
    “The nature of the light propagating in a vacuum is not classical, as in classical mechanics information travels at infinite speed.”

    Actually not quite true. The speed of light is a direct consequence of Maxwell’s equations. In fact Maxwell himself calculated the speed of light and was correct within a few percent, if I am remembering my history correctly. This is firmly in the realm of classical physics. Now, admittedly, Maxwell and his cohorts were not aware that this speed constituted an upper limit on the speed of information – but that is not a quantum effect, it is a relativistic effect. And it certainly is not relevant to the discussion at hand, which was “How do waves travel in a vacuum when there is nothing present to do any waving?” The answer to THAT question does not require an understanding of relativity (although that answer does IMPLY that special relativity must be true).

    “Classical mechanics deals in rigid bodies”

    My apologies for the use of jargon. Most physicists use the phrase “classical physics” when they mean “physics prior to the emergence of relativity and quantum mechanics”. What I (and most other physicists) mean by “classical physics” includes Maxwell’s equations. Thus, the nature of light propagating in a vacuum IS classical, according to this use of the term “classical”.

    “However, you do need to consider Special Relativity when building a GPS system.”

    In fact, if I am remembering correctly, it is GENERAL relativity that is relevant here. The speed of the satellites is not the key consideration. It is the fact that they are farther from the gravity well, so their clocks run at different rates. That is a general relativistic effect, not a consequence of special relativity. Quite different topics, despite the very similar names.

    @Nigel #35 AND Gavin too:

    My apologies, I should not have called string theory or M-theory ‘pseudo-science’. There is an active debate in the field as to whether it is science at all, seeing as many experts believe that it is fundamentally untestable by experiment (and therefore firmly in the realm of philosophy rather than science). Personally, I am agnostic on this question, since it’s not my field of expertise – my understanding of string theory comes from popularizations (e.g. Brian Greene), not from any actual technical source. However, I do tend to support the notion that predictions must be testable, at least in principle if not in practice, in order for them to be worth spending time (and research funding) on. If a prediction is fundamentally impossible to test no matter how big a machine you build, then it teaches us nothing about the world.

    On the other hand, I do know a lot about E&M and about quantum mechanics, since I teach both subjects regularly. String theory, the fundamental nature of the vacuum at the quantum level, and the ultimate consequences of these facts for our perception of space-time may be interesting subjects, but they are all irrelevant to the question I was answering initially. That question can be answered fully and completely with no reference whatsoever to quantum mechanics.

    Let’s remind ourselves that Maxwell’s equations are a perfectly accurate description of every phenomenon involving electromagnetism at any energy scale accessible to human experimentation (even the LHC). Relativity does not modify Maxwell’s equations at all, and neither does quantum theory. When light is acting like a wave (i.e., when you have enough field amplitude so that the classical treatment is valid), its behavior is perfectly predicted by this entirely classical theory. Of course, if the field is very small, so that you are working at the level of a single photon, you have to think differently – then quantum effects become important. But the question at hand was about fields, not photons. We weren’t talking about single photons. Yet :)

  40. Andrei

    @DanM
    Please don’t confuse the equations (Maxwell’s in this case) with the physical phenomenon. The equations describe only the relations between various parameters of the electromagnetic field but they don’t tell you why those specific parameters are involved.
    Think a bit from a mathematical point of view, it’s just like when you have a group / ring / filed and so on. You can define an arbitrary set of functions with specific properties that behave much like Maxwell’s equations but without describeing a physical system (well, maybe it’s isomorphous with the physical system but only from a pure math point of view).
    You can’t say that the gravity exists because you have the relation between the masses and the distance sepparating them (Newton’s equation), in fact, it’s the other way around.

  41. DanM

    Who is confusing equations with phenomena? I don’t think I did that.

    I would not claim that electric fields can propagate in vacuum because of Maxwell’s equations (which I think is what you are implying that I said). However I would claim (and I did claim) that Maxwell’s equations embody all of the physics necessary to understand why waves can propagate in vacuum. And further, that no other physics (which implies: no other equations) are required.

    That’s all I said. I think. And I stand by those statements, although I completely agree with your assertion that equations before phenomena is horse before cart.

  42. DanM

    or, er, cart before horse. Or whatever.

  43. Andrei

    @DanM
    If it’s evident that Maxwell’s equations describe the electromagnetic wave propagation without the need of a support, then what was the point of Michaelson-Morley experiment (some 20 years after Maxwell first published his equations)?
    All I want to say is that there is a net distinction between real phenomena (the intrinsic nature of them – the WHY) and the model (equations) that are describeing them (the HOW). Again, comparing to gravity (let’s stick to Newton’s frame), the apple falls down not because it and the earth have mass, but beacause (in classic physics) mass has an intrinsic property – in Newton’s frame, that intrinsic property has no explanation, his law merely describes the net effects of that intrinsic property when you deal with massive objects.
    The same is with Maxwell’s equations – they describe HOW the wave propagates but make no claim on the subjacent nature of electromagnetic waves – i.e. WHY they propagate in vacuum or at all.

  44. Gavin Flower

    @DanM #42

    How did you get your post the number ’42′ – you do know that is the answer to ‘the Answer to Life, the Universe, and Everything’? According to the ‘Hitch Hikers Guide to the Universe’, by Douglas Adams.

    The speed of light in a vacuum in Maxwell’s equations, is derived from 2 ‘constants’ εo (the dielectric permittivity of a vacuum) and μo (the magnetic permeability of a vacuum). Apart from the fact that no explanation is given as to why a vacuum has to have these particular values for those constants, there is also no explanation for why a changing electric field creates a changing magnetic field – AFAIK.

    I said “However, you do need to consider Special Relativity when building a GPS system.” this is simply carelessness on my part – you need General Relativity to cope with the acceleration and differences in gravitational potential, associated with the orbit of a GPS satellite.

    See
    http://www.youtube.com/watch?v=NZ-ElsvYKyo&feature=related
    for a series of lectures on String Theory by Leonard Susskind (he used to be plumber!).

    The facts that light propagates through a vacuum and has a constant speed are found through observation. However, the interesting problem for me is what is the fundamental nature of Space & Time that makes this so. Hence, Maxwell’s equations are insufficient to explain why light travels through a vacuum.

  45. It’s the Ether, Stupid! ;-)
    Actually, it kinda is.
    But it exists in not-yet-directly-observable form.
    Intuitive models break at low scales.

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