Thursday, June 04, 2009

This and That

Combining waves can make for quite spectacular effects. The June edition of Physics Today has two interesting articles available for free describing such situations:

  • In Probing stars with optical and near-IR interferometry, Theo ten Brummelaar, Michelle Creech-Eakman, and John Monnier explain how "new high-resolution data and images, derived from the light gathered by separated telescopes, are revealing that stars are not always as they seem." Now, the combination of radio telescopes to synthesize huge telescope apertures and gain the corresponding angular resolution is a well-established technique.
    But adding interferometrically the wave crests of visible light from a star collected in two separate telescopes is a more recent development. Performing some Fourier transforms results in images with an angular resolution of milli arcseconds - enough to see a human on the moon, or to resolve the size and shape of stars.

    Altair in the constellation Aquila, about twice as big as the Sun and 18 light years away, is rotating rapidly so that it is flattened. The image on the right has been created using interferometry with light waves. Credit: Ming Zhao (University of Michigan), from Imaging the Surface of Altair, John D. Monnier et al, arXiv:0706.0867v2.

  • Ocean waves are generated by fluctuating wind pressure on the water surface. Due to the dispersion of the phase velocity (longer waves run faster) and a weak interaction between different waves transferring energy to longer waves, dominant waves are the bigger the stronger and longer the wind blows. Together with the effect of opposing ocean currents, very big waves can emerge. In Rogue waves, Chris Garrett and Johannes Gemmrich look at the "rich and challenging physics [...] behind the gigantic ocean waves that seem to appear without warning to damage ships or sweep people off rocky shores."

    Rogue wave in the Bay of Biscay, France. Credit: NOAA Photo Library, via Wikipedia.


  1. Performing some Fourier transforms results in images with an angular resolution of milli arcseconds - enough to see a human on the moon, or to resolve the size and shape of stars.

    Now that's some fine work. Maybe these moon landing debunkers will be put to rest finally.

  2. Thanks for this post! It always amazes me to see how waves interact. You'd intuitively expect two waves colliding to destroy each other, but instead they add together briefly while they superimpose, then emerge from the interaction as if nothing has happened.

    Dr Dave S. Walton tried it with logical signals (TEM - trabsverse electromagnetic) waves carried by a power transmission line like a piece of flex. Logic signals were sent in opposite directions through the same transmission line.

    They behaved just like water surface waves. What's interesting is that when they overlapped, there was no electric drift current because there was (during the overlap) no gradient of electric field to cause electrons to drift. As a result, the average resistance decreased! (Resistance only occurs when you are having to do work by accelerating electrons against resistance from collisions with atoms.)

    Another example is the reflection of a weak shock wave when it hits a surface. The reflected pressure is double the incident pressure, because the leading edge of the shock wave collides with itself at the instant it begins to reflect, at doubling the pressure like the superposition of two similar waves travelling in opposite directions as they pass through one another. With strong shock waves, you get more than a doubling of pressure because there is significant dynamic or wind pressure in strong shocks (q = 0.5*Rho*u^2 where Rho is density and u is the particle velocity in the shock wave) and this gets stopped by a reflecting surface, and the energy is converted into additional reflected overpressure.

  3. Amazing! And from the Physics Today article "If issues associated with spacecraft stability and orientation perturbations can be adequately addressed, space-based interferometry has the potential to allow full-sky absolute astrometry at the microarcsecond level"

  4. Might be fun to look at this bookA Giant Step: From Milli- to Micro- Arcsecond Astrometry (IAU S248).

    Since the Moon is about a lightsecond away, and the Sun is 8 lightminutes ~ 500 lightseconds away, microsecond resolution means we could see a man on the Sun!!!!!!!

    If only sources were bright enough, we could make resolve the Earth-Sun system at a megaparsec, i.e., out in the Andromeda galaxy!

    Stefan, thanks, you really excited my imagination today!

  5. Wave "interference" is a tricky term and concept that misleads many, including physicists. First, amplitude addition (superposition principle) always applies regardless of phase difference. (Indeed, even waves "incapable of interference" such as with orthogonal polarization will combine to produce a wave with a new orientation.) This is one, fundamental meaning of "interference."

    But there is another use, implying a noticeable pattern from well-defined phase difference etc. Hence we hear, "incoherent waves don't interfere" or "waves of different frequency or incompatible polarization don't interfere" etc. Well, they still "interfere" in sense #1, but the result just isn't a clear "demonstration" of the wave character of what is nevertheless interacting in the same matter of principle as nicely, obviously "interfering" waves.

    I think this confuses those who put forth the post-modernish pseudo-explanation of "collapse of the wave function" called decoherence. Advocates like to say, that random fluctuations etc. ruin the phase relationships of the WFs involved so "they don't interfere [presumably they mean in sense #2] anymore." From this, they somehow get the idea that we can explain how "collapse" occurs, despite C being a matter of concentration of a WF into a small space and subsequent absence elsewhere.

    Their sloppy argument is that the statistical pattern of waves of incoherent phase is the same as that for classical particles (no fringes, etc.), and by post-modern double talk that shows how collapse "appears" [what does that suspicious word really mean?] to happen. Well, no. Even if the amplitudes of the waves were messy and not arranged for nice fringes, they are still *wave amplitudes* spread out over just as much space as before. Nothing even resembling an explanation is thereby given for how a detector one place clicks and then that can't happen anywhere else. Indeed, without a supervening collapse mechanism, there wouldn't be any "clicks" at all to have any patterns of any kind, whether regular or irregular.

    To top off the fallacy, the de-inco-herent are taking what happens in a multitude of detection runs (with varying phases) and pretending that somehow tells us what to expect in any one run. This whole thing really ticks me off. Similar complaints were made by Roger Penrose, and "The Quantum Pontiff" told me he doesn't accept deco' as an explanation either.

    See and maybe pitch in at the big, related discussion "Open forum: dish against or defend many-worlds and decoherence theories" at my blog.

  6. Hi Michael,

    Maybe these moon landing debunkers will be put to rest finally.

    Hm, but since the technique produces real images only after some mathematical transformations, that may not convince die-hard conspiracists ;-)

    Actually, Hubble has taken photos of the Apollo 15 and 17 landing regions, but unfortunately, resolution is too low by about a factor 100 to see any actual traces of the landing.

    Hi Arun,

    indeed, imaging with interferometry is fascinating, and space-based interferometry would be just jaw-dropping... Thanks that you like the post :-)

    BTW, I have not forgotten about Venus. I'll just have to get up early enough in the morning with a clear sky... Actually, it's now a good time to observe Venus, as this morning, it had its greatest elongation.

    Hi Nige,

    right, one should be careful with saying "don't interfere", when actually one just means that amplitude addition with random phases ruins the hallmark of "interference", stationary patches of darkness... But I hope this is not too confusing.

    Cheers, Stefan

  7. Hi Arun,

    oh, the proceedings volume "A giant step" you have mentioned is on google books - great, cool tip!

    Cheers, Stefan

  8. Stefan, I suppose you were replying to my comment below that of nige. In general, semantics underlies what people "see" and must be studied directly and worked around, not just taken as an obvious, natural background.

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  10. Hi Stefan,

    That was a very interesting article you pointed to on the use and progress of near IR interferometry. In reading about the problems with earth based setups the moon seems to be a better environment for such facilities. Perhaps when we go back this time we might be capable of doing more than collecting rocks, hitting golf balls and planting flags. It will indeed be interesting what such observations will have us learn and discover.

    As a mildly related note it appears Canada is about to have its first space tourist. To be honest I’m not certain what to make of all this space tourism stuff, for my idea of the intent, utility and purpose of the international space station doesn’t include it serving as a hotel for the rich and frivolous. While real scientists stand in line for dwindling research moneys we have this guy reciting poems about issues that will not be solved by wishful thinking reflective of half baked visions. It is fitting however that as all this is looking more like a circus one of the creators of the best known should be involved. What was that famous line spoken by P.T. Barnum?

    Sorry about the brief outburst , it’s just with all the discussion about waves I thought I’d make a few of my own:-)



  11. Just thought you might be interested since talking about Moon and it's elemental distribution and all.

    Dvali adds that the impact of modified gravity is able to be tested by experiments other than the large distance cosmological observations. One example is the Lunar Laser Ranging experiment that monitors the lunar orbit with an extraordinary precision by shooting the lasers to the moon and detecting the reflected beam. The beam is reflected by retro-reflecting mirrors originally placed on the lunar surface by the astronauts of the Apollo 11 mission.NYU’S Dvali Says Change in Laws of Gravity, Not ‘Dark Energy,’ Source of Cosmic Acceleration Article has been pulled, yet remnants remain as proof of reported experimental hypothesis.

    Not only Mercury's measure in geological data transmission then, but the Moon's as well. Why not use Hubble if not to gain in future mining prospects?

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  13. Hi Plato,

    Dvali’s proposal as to what accounts as being dark energy attributable to an extra dimension in which only gravity resides and leaches out to I find as intriguing yet hardly original. That is it's similar to a model which Lisa Randal has in the past proposed yet in this case only within the multi-verse perspective.

    I’ve often wondered if gravity is in effect a dimension of its own, rather than a consequence of the existence of one, This of course is more in line with GR where gravity is simply the architectural reality of space/time being affected by the matter/energy concentration within and not a force at all in the conventional sense. It’s almost as if space itself reacts to matter/energy as an intruder of sorts and thus countering with two different strategies to rid itself of it. That is one in its concentration with the limit being a black hole and the other with its dilution through disbursement, recognized as an accelerated expansion of space.

    I realize this is just groundless conjecture, yet it gives me a way to picture how both attraction and repulsion could coexist as undeniable prime elements of nature. It would be then interesting if such measurements could tell us more as to what this expansion relates to being.



  14. Hi Stefan,

    The earthly wave form that I find most fascinating and yet must admit being the most destructive is the Tsunami. It shows clearly how natural waves are in transmitting energy efficiently with little loss over great distances and leaves one less to wonder why when energy transfer is to be explained that the wave appears so much as being part of that description.

    What I find many people are confused about in wave forms such as this is that no actual material is moving forward as the wave propagates, yet rather only oscillates up and down. For instance the energy of a Tsunami in the deep ocean can travel (be transmitted) at speeds exceeding that of sound in air and yet nothing of substance is moving in that direction, yet rather only the energy.

    It’s also a great example of how amplitude and wave length change can have such dramatic effect where waves are considered. That is in the deep ocean the wave may have a amplitude of only one metre, yet a wave length of as much as 200 kilometres. Then when it approaches the shallow of the coastline being forced to compress to wave lengths of 20 kilometres or less, yet amplitudes (heights) exceeding 30 meters at times. The interesting thing is as the wave finally makes land fall the energy leaves the medium of the water to only again be transferred in other ways and medias as a wave form.

    So even though the Tsunami may be considered as gone the energy lives on and continues to be propagated as waves. So I find it difficult to imagine how some can consider the wave as being not a undeniably fundamental and thereby telltale characteristic of nature.



  15. The ocean wave thing, at first hearing, sounds like Landau damping*, or some similar mechanism in an appropriate phase space.

    *Been a few years, but as I recall, basically dumping energy into plasma by driving the particles near the appropriate cyclotron frequency.

  16. With these ultrahigh angular resolutions, a problem that remains is the brightness of the sources. I was wondering whether a nova or supernova might illuminate the neighborhood enough, to e.g., be able to see planetary systems withing 25-50 light years of the nova.


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