I made it to Charlottesville, and am all checked in to the Econo Lodge, which is a little more Econo than I was expecting. It’s an old-school motel, with rooms that open right into the parking lot, the sort of place where the towels are tiny and scratchy, the pillows and mattresses are thin, and the tap water tastes like cigarette smoke. Still, it’s not like I’m going to be doing much more than sleeping here.

I promised some entertainment in my absence, so this seems like a good time for a Dorky Poll. I’m going to be spending a lot of time over the next few days listening to talks about the interaction between light and matter, so here’s an esoteric question:

Which do you prefer: transverse waves, or longitudinal waves?

As always, leave your answers in the comments. If you’re confused about the question, I’ll put some explanation below the fold.

Waves are a disturbance in some medium (using a broad definition in which “the electromagnetic field” counts as a medium), and there are two main types of disturbance: the disturbance can be in the direction of motion of the wave (“longitudinal”), or perpendicular to it (“transverse”).

Sound waves are logitudinal. When you hit a wrong key, the speaker in your computer vibrates back and forth, pushing the air molecules next to it back and forth. These bump into air molecules a little further out, which bump into molecules further out, and so on until the wave reaches your ear. None of the molecules move all that far, but they shift back and forth a little bit along the line between the speaker and your ear.

Light waves are transverse waves. If a light wave is moving from left to right, it creates an electric field that oscillates up and down, and a magnetic field that oscillates in and out of the page (assuming, of course, that we’re in a classical limit where it makes sense to talk about light as a wave…). Particles set into motion by the wave will move in a direction perpendiculr to the line between the light source and your eye.

So, those are your choices. Which sort of wave do you like better?


  1. #1 gg
    May 20, 2009

    Personally, I like electromagnetic waveguide modes, which have both a transverse and longitudinal component. But, then again, some of my colleagues are regularly vexed by my refusal to take a stand on some issues…

  2. #2 Lethe
    May 20, 2009

    Transverse, of course. All the cool waves are transverse. Longitudinal waves are for, like, geologists or something.

  3. #3 Dr. Kate
    May 20, 2009

    For looking at (or drawing), I prefer transverse. For listening to, I prefer longitudinal…

  4. #4 Eric Lund
    May 20, 2009

    In my line of work, longitudinal waves are much easier to deal with. Transverse waves get you into messy sums over infinite cyclotron harmonics of terms that involve lots of special functions, some of them not covered in Abramowitz and Stegun. Not that you avoid the special functions with longitudinal waves, but at least you don’t have to compute an infinite series of them.

  5. #5 Uncle Al
    May 20, 2009

    Circular polarization or nothing. Put a little excitement into your life.

  6. #6 Odysseus
    May 20, 2009

    It took me quite a while until I had an idea of how longitudinal waves work at all, and still occasionally find them difficult to imagine. Transverse waves are much more appealing. And, as Lethe pointed out, they are just cooler. Ever seen a longitudinal wave propagate through intergalactic vacuum?

  7. #7 Mike Ackerson
    May 20, 2009

    As a geologist, I have to go with transverse. While longitudinal (P-waves) are a necessity for determining earthquake epicenters, transverse waves (S, Rayleigh and Love) are where the magic happens. Rayleigh and Love waves are the slow, high amplitude waves that are responsible for the real damage.

  8. #8 Eric
    May 20, 2009

    As a photons-from-space kind of guy, I’m gonna have to with transverse as well.
    Also, (@Odysseus) I always get confused when I hear geologists talk about S- and P-waves. I can’t imagine a connection between the transverseness/longitudinality of a wave in an earthquake and its angular momentum. Though maybe I’m insufficiently imaginative?

  9. #9 Alex R
    May 20, 2009

    When I taught lower-division physics, I always enjoyed doing the wave demos using a long slinky — the transverse waves were what you kind of expected to see, but for some reason I always found the longitudinal waves more unexpected and cooler. So consider this a vote for longitudinal.

    As an aside, one of the things that I noticed when doing these demos is that the velocities of the transverse and longitudinal waves in a slinky are approximately equal. I recall doing a calculation that showed that this was true for a massive stretched spring with equilibrium length zero, which a slinky is to a reasonable approximation… I suspect that in part it is this equality of velocities that allows a slinky to do interesting things like going down stairs.

  10. #10 Jonathan Vos Post
    May 20, 2009

    Elliptical and circular polarization as part of a broader category called “helical polarization.”

  11. #11 Bob Hawkins
    May 20, 2009

    Transverse waves. Because that means polarization. And there are so many neat polarization effects.

  12. #12 Kevin Reid
    May 20, 2009

    Transverse waves. Because it’s easier to think about when the oscillation and propagation don’t occur on the same axis.

    (Just-past-1st-semester-of-physics comp sci student)

  13. #13 magista
    May 20, 2009

    (stupid misdirected mouse-click. stupid SB system that won’t let me edit or post an ‘oops’ message because I posted too recently… grr)

    Ahem… Transverse all the way, baby! Not only are they much easier to draw and identify amplitude, but they can also be used to model longitudinal waves in diagrams such as like for standing waves in air columns.

    Why yes, my two classes did just write their simple harmonic motion and waves final this morning, why do you ask? (And from the looks of things so far, aren’t doing all that well. Sigh)

  14. #14 GrayGaffer
    May 20, 2009

    My gut-level introduction to waves was also via a long Slinky. Very long – about 600 turns, would stretch the length of the 100′ corridor in the Physics building. Both forms are demonstrable. You do _not_ want to let go of your end.

    But then came optics (transverse), and then came music (longitudinal in air, transverse on vinyl). Tough to choose, to each its niche. But longitudinal feel better, transverse look better.

  15. #15 Peter Morgan
    May 20, 2009

    1 In the beginning God created the heaven and the earth.

    2 And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.

    3 And God said, Let there be light: and there was light.
    King James translation, Genesis I:1-3.

    I’m never happy with either/or. Can I go for the early stuff, fermions? That would be at least a little more explanatory than the Spirit of God. The transverse stuff is so new-fangled. I guess this comes out as spin-waves in Physics-speak.

  16. #16 Rogue Epidemiologist
    May 20, 2009

    It’s all about the transverse waves. I’m in favor of motion through vacuums.

  17. #17 Cherish
    May 20, 2009

    Emag geek = transverse.

  18. #18 Snoof
    May 20, 2009

    Transverse. Can’t surf on longitudinal.

  19. #19 Renleigh
    May 21, 2009

    As an optics major, I gotta go with transverse.

  20. #20 agm
    May 21, 2009

    Thank god Vos Post chimed in. Now I don’t have to agree with Uncle Al…

  21. #21 Chad Orzel
    May 21, 2009

    Thank god Vos Post chimed in. Now I don’t have to agree with Uncle Al…

    I wouldn’t suggest agreeing too closely with either of them– circular and elliptical polarizations are just sub-cases of transverse waves, not a third category as they seem to be implying.

  22. #22 Uncle Al
    May 21, 2009

    Polarization is obviously a vote for “transverse”. Uncle Al wearies of theorists disgorging faery tales protected from falsification by eldrich complexity. Theorists boast promiscuity while empiricists pay child support.

    The Woodard-Hoffmann rules earned a Nobel/Chemistry. Find just one legitimate reproducible exception and it’s an even bigger hoo-hah. Pyrex is the boojum, not paper.

  23. #23 CCPhysicist
    May 21, 2009

    Along the lines of what another poster mentioned, I’ve always been partial to longitudinal electromagnetic waves … that is, to virtual longitudinal photons.

    Otherwise, I will un-ask the question and say that I prefer water waves – which are circular, not transverse – because you can surf on them.

  24. #24 CCPhysicist
    May 21, 2009

    Eric @8:

    The easiest thing to remember is that S is for sideways (or shear), since they represent waves with sideways (transverse) movement. That is how I remember it when mentioning it in class. You can also associate P with momentum if you imagine the momentum of the fracture producing compression waves, but that is not as good a cognate as S for sideways waves.

    I like your idea though. I’ll have to ask a geologist if there are also d and f seismic waves, and see if they get it.

  25. #25 onkel bob
    May 21, 2009

    The real inquiry should be whether the stadium wave should be tolerated :^)

    Circular polarization is wild stuff. I worked on microwave radios in the USAF and had a wonderful time working on some the steampunk radio sets. (Think SHF vintage 1948, no transistors!) When I got out and worked with some SRI guys, I was exposed to satellite communications for the first time.

  26. #26 Compositionalist
    May 21, 2009

    As a music dork, my vote is for both longitudinal and transverse waves. We need both to hear music, speech or anything else, since our brains have to translate sound waves into electrical signals.

  27. #27 DrYak
    May 21, 2009

    Why is there any argument over this? Can you surf on a logitudinal wave? No! – case closed dudes…

  28. #28 agm
    May 22, 2009

    Chad, circular polarization is not automatically a vote for transverse though, it can also be a vote for interesting things with mixed modes. Radio waves in magnetized space plasma. Crystal birefringence. Seismic shear wave splitting due to anisotropic stress fields. And so on.

    Longitudinal and transverse in 3D just tend to be the most useful decompositions of oscillatory motion in many cases because of the beauty and (relative) simplicity of the theoretical description in a lot of situations.

  29. #29 Jonathan Vos Post
    May 22, 2009

    The more subtle question: I can have waves where the oscillation is along the x-axis, waves where the oscillation is along the y-axis, waves where the oscillation is along the z-axis, and various combinations. How can I have (or why can I not have) waves where the oscillation is along the t-axis? You need not limit yourself to electromagnetism, nor earthquakes, nor sound. Show your work.

  30. #30 Paul
    May 22, 2009

    Oceanographer here, so transverse waves for me.

  31. #31 Pieter Kok
    May 22, 2009

    JVP: if by waves you mean solutions to the wave equation, all waves propagate both in space and time.

  32. I wouldn’t suggest agreeing too closely with either of them– circular and elliptical polarizations are just sub-cases of transverse waves, not a third category as they seem to be implying.

    Well there’s certainly the ‘both’ category (which water people will like). More interestingly though: could there be a third category, or more? Gravitational waves, perhaps?

    all waves propagate both in space and time.

    And some propagate space-time?

  33. #33 Nantwich dentist
    May 23, 2009

    Waves propagation and its 3d structure for both the longitudinal and transverse are good.

  34. #34 eddie
    May 24, 2009

    My fave waves are both longitudinal and transverse, but they oscillate transversely in (some would say more than a dozen) imaginary directions. The longitudinal aspect comes because of compression/stretching of the spacetime medium they move through, so their wavelength changes along their path.

    In short; a complex probability wave through spacetime with gravity waves.

    Now that’s slinky!

  35. #35 Roger Mason
    May 24, 2009

    Transverse, of course. And fewer sarcastic comments about geologists.

  36. #36 John
    May 24, 2009

    Transverse. I mean what good are longitudinal waves in the vacuum of space?

  37. #37 William
    May 25, 2009

    Hi Nerds(benevolent)-
    What would cause different noises or sounds to get trapped on metal, wood, and other materials. I can release sounds from my home and car by shining a light or laser in fixed positions…
    Yes….I need help, big time!1.5 years ago I was making $500k a year selling packaging as a economics grad. Something changed, as if Im being guided…or used… My brain cells need to come home. now!
    Perhaps yall could help. Anyone know Steve Jobbs? I eyeyaeyae my eye phone

  38. #38 Dolores
    May 25, 2009

    This blog is too complicated! I’d say, scientific- – – – But in my humble opinion, “I” like MY

    waves in the Atlantic! The Pacific is way too cold for ME. The coastline tends to be rough

    with rocks, and is dangerous to navigate, as opposed to the easy access of the Atlantic.

  39. #39 Dolores
    May 25, 2009

    This blog is too complicated! I’d say, scientific- – – – But in my humble opinion, “I” like MY

    waves in the Atlantic! The Pacific is way too cold for ME. The coastline tends to be rough

    with rocks, and is dangerous to navigate, as opposed to the easy access of the Atlantic.

  40. #40 katherine
    May 25, 2009

    Longitudinal definitely. And my introduction to waves at school involved a slinky at well :D

  41. #41 Wirral teeth whitening
    June 24, 2009

    Creative way to present and share interesting information.

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