What Little-Known Physics Phenomenon Fascinates You?

[Opus_723]

What's your favorite bit of physics that most laypeople are unaware of? It could be something commonplace that doesn't get the attention it deserves, or something really exotic that doesn't tend to make it to the masses. For context, I've been on kind of an "outreach" kick with my friends and family, and having a lot of fun with it. I'm looking for some good subjects/phenomenon that are dramatic or quirky or simply underappreciated. I feel like lots of people see books about string theory and cosmology and frontier science, but it seems to me like there is so much stuff that we understand really well that people aren't aware of, and it needs to get more attention!

Some examples of my own to start things off:

For me, I remember when I first learned about Coriolis forces and how they produce the rotation of hurricanes. That was a really awesome connection for me freshman year, realizing that the same effect that makes it hard to play catch on a merry-go-round could cause the distinct spirals of these-planet-scale storms. This is a fun one to explain to people because it extends something relatable to something incredibly dramatic.

Another big moment for me was when I first noticed that the bubbles in my coffee were attracted to my stirring stick, and to each other. I still get probably an hour of entertainment out of this when I have some free time in the morning.

On the more exotic side, I'm going through my first Solids course now, and I'm constantly awed by the microscopic explanations for basic, taken-for-granted properties like the reflectivity of metals, although I haven't really come up with a good way to relate stuff at this level to family and friends. If anyone has any clever way to explain some of this more esoteric physics to laypeople, I'd love to hear it.

 

[jedishrfu]

The physics of mixing:

http://www.phschool.com/science/science_news/articles/mastering_mixer.html

 

[Seydlitz]

Transparent material or glass. I am just fascinated by how it works.

 

[Menaus]

Single-wire transmission lines... Not even most graduates know about it. :O

 

[Bobbywhy]

Bioacoustics! We can learn useful techniques from the study of how animals use sound to eat, survive, and reproduce. I spent over 25 years as an experimental sonar engineer, pinging in oceans all around our planet using low, mid, and high frequencies and then receiving and analyzing those echoes. Accordingly, I am admittedly biased towards acoustics. I can say with certainty that some of our military sonar systems have borrowed directly from a variety of sea animals that use mechanisms that have evolved naturally over eons of time. For instance, see this earlier post here on Physics Forums: https://www.physicsforums.com/showthread.php?t=636679&highlight=ultrasonics+infrasonics

Here are some examples:

Bioacoustics is the scientific study of biological sounds. It is concerned with the following topics: sound production in animals, including anatomy and neurophysiological processes,
sound propagation in water and air, vibrational communication of insects, biosonar or echolocation of bats and dolphins, ultrasonic signals (>20,000 Hz) of insects, rodents, bats and dolphins, infrasonic signals (<20 Hz) of large mammals, sound reception capabilities and mechanisms of animal hearing, ethology of animal acoustic communication, evolution, ontogeny and development of acoustic behaviour, relationships between animal sounds and their environment, effects of man-made sounds on animals, application of acoustic signals for taxonomic studies and for calibrating biodiversity, practical bioacoustic applications, in wildlife monitoring and in pest control. International Bioacoustics Council (IBAC)
http://www.ibac.info/

The research team recorded very low-pitched mating calls, which are inaudible to humans (below 20 hertz) produced by male peacocks vibrating their feather-train while shaped into a parabolic reflector. The findings were reported at an annual meeting of the Animal Behaviour Society in New Mexico.
http://scienceillustrated.com.au/blog/nature/male-peacocks-not-just-a-pretty-display/

http://www.birds.cornell.edu/brp/listen-to-project-sounds/listen-to-sounds

Listen to nature: explore wildlife sounds by animal group
http://www.bl.uk/listentonature/soundstax/groups.html

The Borror Laboratory of Bioacoustics (BLB) is a research and service unit of the Department of Evolution, Ecology and Organismal Biology at The Ohio State University. It is located in the OSU Museum of Biological Diversity. The BLB houses one of the largest collections of recorded animal sounds in the world. Founded by the late Dr. Donald Borror, Professor of Entomology and Zoology at The Ohio State University, the collection contains over 34,000 recordings of over 1500 species of animals.
http://blb.biosci.ohio-state.edu/

Bioacoustics - the International Journal of Animal Sound and its Recording
“Bioacoustics is the only international peer-reviewed journal devoted to the scientific study, recording and analysis of animal sounds. Bioacoustics primarily publishes high-quality original research papers and reviews on sound communication in birds, mammals, amphibians, reptiles, fish, insects and other invertebrates, on the following topics: communication and related behaviour; sound production, hearing, ontogeny and learning; bioacoustics in taxonomy and systematics; impacts of noise; bioacoustics in environmental monitoring; identification techniques and applications; recording and analysis equipment and techniques; ultrasound, infrasound, underwater sound; bioacoustical sound structures, patterns, variation and repertoires.”
http://www.bioacoustics.info/

“Stridulation is the act of producing sound by rubbing together certain body parts. This behavior is mostly associated with insects, but other animals are known to do this as well, such as a number of species of fish, snakes, and spiders.”
http://en.wikipedia.org/wiki/Stridulation

“Normally, a bat attack starts with relatively intermittent sounds. They then increase in frequency—up to 200 cries per second—as the bat gets closer to the moth "so it knows where the moth is at that critical moment," Corcoran explains. But his research showed that just as bats were increasing their click frequency, certain moths "turn on sound production full blast," clicking at a rate of up to 4,500 times a second. When the researchers played bat ultrasound to the hawk moths, they found that three species (Cechenena lineosa, Theretra boisduvalii and Theretra nessus) they had captured emitted ultrasound clicks in response. This furious clicking by the moths reversed the bats' pattern—the frequency of bat sonar decreased, rather than increased, as it approached its prey, suggesting that it lost its target. The biological mechanism behind the moth's defense strategy is still unclear to researchers. "Most likely, moth clicks are disrupting the bat's neural processing of when echoes return," Corcoran says. Bats judge how far away a moth is based on the time delay between making the cry and its audible return. This "blurring" of the bat's vision, he explains, "may be just enough to keep the moth safe."
http://www.scientificamerican.com/article.cfm?id=sonar-jamming-tiger-moths-bats-echolocation-defense

A number of animals use infrasound to communicate: elephants, (http://www.light-science.com/articles1003.html) giraffes, rhinoceroses, hippopotamuses, alligators, and whales all generate low frequency acoustic signals. The flightless cassowary birds, Rock Doves and pigeons emit infrasonic calls, and recently, the male peacock has been found to generate infrasonic energy to call for a mate by vibrating its feathered fan. See: http://www.birds.cornell.edu/brp/?lk=lpro/

“The Philippine tarsier (Tarsius syrichta), known locally as mawmag in Cebuano/Visayan and mamag in Luzon, is an endangered species of tarsier endemic to the Philippines. One of the world's smallest primates, which up to now had been mistakenly described as being "ordinarily silent", has all along been using inaudible ultrasound to communicate. No bigger than a man's hand, the Philippine tarsier can hear and emit sounds at a frequency that effectively gives it a private channel for issuing warnings or ferreting out crickets for a night-time snack, a study published on Wednesday found. Only a handful of mammals are known to be able to send and receive vocal signals in the ultrasound range, above 20 kilohertz (kHz), including some whales, domestic cats and a few of the many species of bats. And few of these can squeal, screech or squawk at the same sonic altitudes as the saucer-eyed tarsier, Tarsius syrichta, researchers found. Its finely-tuned ears are capable of picking up frequencies above 90 kHz, and it can vocalize in a range around 70 kHz. By comparison, humans generally cannot hear anything above 20 kHz, and a dog whistle is pitched to between 22 and 23 kHz.”
http://en.wikipedia.org/wiki/Philippine_tarsier

 

[jedishrfu]

Historical events and acoustics is pretty interesting too:

http://www.todayifoundout.com/index.php/2013/10/acoustic-shadow-american-civil-war/

The tides of battle changed because some general didn't hear the proper signal due to sound refraction.

 

[boneh3ad]

The field of boundary layer stability: the study of the growth of waves in fluids that lead to turbulence.

 

[mpresic]

I learned once a photon from the center of the Sun takes 10000 years to get to the surface of the Sun. (Not the 8 minutes to get to the Earth from the Sun). This is likely the result of multiple scattering. Several textbooks differ whether it is 10000 years or ~1000 years depending on assumptions made. I have talked to several solar scientists and they all say the point they make with students is it takes a "long time".

 

[mpresic]

Another interesting phenomena. We are all familiar with the idea the drag force on a baseball thrown in the air increases if the baseball is thrown faster. In a plasma, often the drag force on the electrical charge can diminish with increasing velocity (I learned this in my late forties).

I have often felt more students might become plasma physicists if they were exposed to this concept at as early an age as they are exposed to relativity or quantum mechanics (popular in the media).

 

[Meir Achuz]

"What's your favorite bit of physics that most laypeople are unaware of?"

It's the ordinary everyday things that people don't even know are physics.
My wife remarked "Why do the figure skaters keep their arms outstretched?"
I replied "so the Coriolis force (or conservation of angular momentum) will make them spin so fast when they pull them in."

 

[voko]

I learned once a photon from the center of the Sun takes 10000 years to get to the surface of the Sun. (Not the 8 minutes to get to the Earth from the Sun). This is likely the result of multiple scattering. Several textbooks differ whether it is 10000 years or ~1000 years depending on assumptions made. I have talked to several solar scientists and they all say the point they make with students is it takes a "long time".

Which is a misstatement. When a photon scatters, there is hardly any reason to call the resultant photon "the same photon", because everything about that photon is different and it does not have any memory.

 

[voko]

I find it very strange that most people I talk with have no idea what cardinal points really are. I mean, they know they exist, but when asked why they are where they are and not somewhere else, they rarely can explain that the rotation of the Earth gives us a physically observable reference frame for just about everything terrestrial and even orbital, and the cardinal points label particular features of the frame.

 

[D H]

I learned once a photon from the center of the Sun takes 10000 years to get to the surface of the Sun.

As voko said, that is a misstatement. That statement implies many things, all of which are wrong. A much better way to say this is that except for energy in the form of neutrinos (which escape rather quickly), it takes 10,000 years for the energy produced at the center of the Sun to make its way to the surface before it radiates out into the universe.

 

[dauto]

I teach introductory physics to freshmen students. Believe me, any bit of physics is little known by the layman.

 

[mpresic]

I do not think I agree. It is hard to see how a photon changes it's identity by scattering.
(I re-read voko and I am now in partial agreement. I grant you the photon changes it's momentum. Perhaps the photon concept is misleading. Nevertheless it is quoted in e.g. Kallenrode, Space Physics)

In any event certain things I stated were incorrect when I looked them up. Here is the upshot. Kallenrode, Space Physics 2nd edition states (page 106) it takes 100,000 years (not 10000) for a photon to diffuse from the Sun's core to the surface. Kivelson and Russell, Introduction to Space Physics, states radiation from the sun takes 10,000,000 years (100 times longer than Kallenrode) to reach the surface.

Clearly there is wide disagreement within the published sources. Anyway I am not here to take sides. I just find this fascinating that the radiation (except for the weakly interacting neutrinos, which contribute a small amount to the total radiation output of the Sun) takes so long to reach the surface.

 

[mpresic]

The entry by Bobbywhy on bioacoustics was eye-opening. Another bit of physics regards phase synchronization in oscillators as it applies to fireflies. Apparently they can all blink in unison under some environmental conditions. I learned this once when I was exposed (I confess I did not learn this as adequately as I should) to Landau damping. I do not say Landau damping is responsible to firefly chemistry, (perhaps similar mathematical equations apply). Anyway I submit it for your approval.

 

[voko]

I do not think I agree. It is hard to see how a photon changes it's identity by scattering.
(I re-read voko and I am now in partial agreement. I grant you the photon changes it's momentum. Perhaps the photon concept is misleading. Nevertheless it is quoted in e.g. Kallenrode, Space Physics)

I think it was Eddington who popularised the idea of a photon's taking eons to reach the surface of the Sun almost a century above. So it is hardly surprising the idea is now found in many books on astrophysics. I agree the story is fascinating, but, unfortunately, not very physical.

 

[mpresic]

Thank you. I did not know the idea may be popularized to Eddington. I had not realized it might be that old.

The thread regards favorite little-known physics. I should restate this tidbit as light takes thousands of years to reach the surface of the Sun from the deep interior. This has the advantage of accuracy, plain-speaking, and even more intriguing for the layman. Implicit is the layman's understanding (?) that when we say light, we may mean frequencies far above visible light.

I think this thread is quite valuable. I will have to examine cardinal points. I am unfamiliar with them.

 

[mpresic]

I have contributed a lot to this thread recently, but I would like to add one more. I visited the Ontario Science Centre in the late 1970's and I never forgot this experiment. They rolled two balls simultaneous; one down an "straight-line" incline plane, another down a cycloid (or it may have been a circular arc, I do not remember). At the bottom was some electronics recording the time of the roll. The straight-line path took longer. Refer to the brachistochrone for more detail. I think the layman should find this intriguing.

 

[DrZoidberg]

The "levitating" slinky is also quite interesting.

 

[Habeebe]

I teach introductory physics to freshmen students. Believe me, any bit of physics is little known by the layman.

This. When I tutored freshman level physics, I was surprised at how much stuff I take for granted that people don't have any intuitive feeling for, for instance the idea that position and velocity are relative.

I think my favorite is the amount of empty space inside of solids, and the implications that follow, including transparency/opacity of materials.

 

[Filip Larsen]

Orbital mechanics and maneuvers that in some situations work in reverse of what you'd think, like fire your thrusters towards a space station when you are lagging behind it in the same orbit in order to close in on it.

In fact, pretty much any spacecraft maneuver in space will baffle laypersons thanks to Hollywood's almost universal use of Star Wars Physics.

 

[MikeGomez]

fire your thrusters towards a space station when you are lagging behind it in the same orbit in order to close in on it.

I don't get it.

 

[Filip Larsen]

I don't get it.

Not sure which part you are referring to, but most laypersons find it counter-intuitive that you have to accelerate away from something in order to get closer to it.

 

[MikeGomez]

Not sure which part you are referring to, but most laypersons find it counter-intuitive that you have to accelerate away from something in order to get closer to it.

That's interesting. Please direct me to some literature about accelerating away from something in order to get closer to it.

 

[dipole]

The "levitating" slinky is also quite interesting.

That's awesome.

 

[Filip Larsen]

That's interesting. Please direct me to some literature about accelerating away from something in order to get closer to it.

I will keep this short in order not to go too much off-topic. If you think my reply here is inadequate please ask your question again in a new post.

Even though it is fairly simple to explain I have not been able to find a good online description that is not also too technical for most laypersons. Some of the keywords that are relevant are "impulsive transfer orbit" or, more technical, "Clohessy Wiltshire Hill equations". Regarding textbooks there are really many that explains orbital mechanics. If I had to pick a few then Fehse [1] gives a very detailed description of rendezvous and docking dynamics (and it has has a nice trajectory plot on page 51 that illustrates my claim. Unfortunately, I have not been able to locate an online preview of this book that shows this page). However, anyone new to orbital mechanics would probably not want to start with this. For a more general introduction that also contains a bit on rendezvous I strongly recommend Wiesel [2]. It very well written and touches on a ton of interesting topics with regards to the dynamics of launch, orbital maneuvering and reentry of spacecraft s.

[1] Automated Rendezvous and Docking of Spacecraft by Wigbert Fehse from Cambridge University Press.
[2] Spaceflight Dynamics by William E. Wiesel from McGraw-Hill.

 

[MikeGomez]

So you’re really talking about rendezvous and docking. In that case you don’t accelerate away from something in order to get closer to it, you accelerate away from something in order not to crash into it (because you are already approaching it).

 

[Filip Larsen]

So you’re really talking about rendezvous and docking. In that case you don’t accelerate away from something in order to get closer to it, you accelerate away from something in order not to crash into it (because you are already approaching it).

I think you misunderstand. A spacecraft is in the same (circular) orbit around the Earth as a space station, but trailing, say, 100 km behind it. As long as neither of the two maneuvers (i.e. changes orbit) the distance between them stays the same. The spacecraft then wants to perform a single impulsive maneuver that will bring it on a new (transfer) orbit that goes right by the station. The surprise for most people now is, that the spacecraft actually has to accelerate away from the station in order enter this transfer orbit. Of course, later, at just the right time when it passes the station, the spacecraft has to make another maneuver (opposite in relative direction of the first maneuver) if it wishes to stay close to the station.

 

[mpresic]

The elementary discussion I remember can be referenced in the second (extended) edition of the popular selling physics textbook, Fundamentals of Physics by Resnick and Halliday (c1981). This is a big pinkish textbook.

His example 16.7 tells the story of Sally and Igor. I like this example and I wish it was maintained in the later editions. It mentions to catch up the follower should aim the thrusters (not necessarily accelerate forward) in a forward direction (to speed up by getting closer to the Earth). Later to adjust the Speed to catch the leader, the astronaut should aim the thrusters backward, (to slow down by moving outward). All this is oversimplified and Kepler's equation should be solved for a complete understanding of how the circular orbits become ellipses and vice-versa. But this is the main story.

 

[mpresic]

I forgot to say, that In the book Space by James Michener, the astronauts are warned that this counter-intuitive idea will make rendezvous and docking difficult. Presumably, that is why the astronauts conduct simulations.

 

[CWatters]

What's your favorite bit of physics that most laypeople are unaware of? It could be something commonplace that doesn't get the attention it deserves,

The fact that no energy is required to hold something up against gravity. Take the humble fridge magnet. A significant percentage of the population would probably argue that a fridge magnet must use energy and will eventually wear out because in their experience it takes energy to carry shopping or hold a book out at arms length.

 

[A.T.]

Here is a list of "Interesting factoids in science and technology":
http://www.pabr.org/scifactoids/scifactoids.en.pdf

 

[mpresic]

This maybe more about math than science.

In the random walk in two dimensions (drunkards walk), it is certain that the "drunk" returns to his/her starting point given sufficient time. However,

In the random walk in three dimensions (I call it the random robin), the "drunk robin" does not necessarily return (i.e. fly) to his/her starting point (maybe the robin should walk after drinking).

 

[Lsos]

I think you misunderstand. A spacecraft is in the same (circular) orbit around the Earth as a space station, but trailing, say, 100 km behind it. As long as neither of the two maneuvers (i.e. changes orbit) the distance between them stays the same. The spacecraft then wants to perform a single impulsive maneuver that will bring it on a new (transfer) orbit that goes right by the station. The surprise for most people now is, that the spacecraft actually has to accelerate away from the station in order enter this transfer orbit. Of course, later, at just the right time when it passes the station, the spacecraft has to make another maneuver (opposite in relative direction of the first maneuver) if it wishes to stay close to the station.

I think I see. However, this only makes sense if the distance between the spacecraft are large enough...such as the 100 km you specified.

Orbital mechanics is weird, because unituitively by decreasing your speed, you enter a new orbit which is actually FASTER than the old one. So, by accelerating away from the space station, the spacecraft ends up falling towards the Earth and actually GAINS speed in the process, in a new orbit which is overall smaller AND faster than that of the space station. Therefore, it will start to catch up to the space station...but it will also be going underneath it until it completes a full orbit.

Is this the general idea? All those hours spent playing Kerbal Space Program were not for nothing !

 

[Filip Larsen]

I think I see. However, this only makes sense if the distance between the spacecraft are large enough...such as the 100 km you specified.

The situation is the same even if there is only 10 m to the station. However, when short distances are involved, like in the final phases of docking, it becomes increasingly more viable for the spaceship to make a faster approach using pulsed or continuous thrust instead of just two small maneuvers separated by around 45 minutes of coasting. The fuel expenditure will be greater though.

Orbital mechanics is weird, because unituitively by decreasing your speed, you enter a new orbit which is actually FASTER than the old one. So, by accelerating away from the space station, the spacecraft ends up falling towards the Earth and actually GAINS speed in the process, in a new orbit which is overall smaller AND faster than that of the space station. Therefore, it will start to catch up to the space station...but it will also be going underneath it until it completes a full orbit.

That is correct.

Is this the general idea? All those hours spent playing Kerbal Space Program were not for nothing !

One can hope that with a new generation growing up with KSP the old days where people believe Star Wars physics is correct will be gone (or at least put somewhat in doubt)

 

[CWatters]

Any day now someone is going to publish a long list of all the errors in the film Gravity :-)

 

[Opus_723]

These are all great. I had never even heard of some of them before.

Another simple one that I think most people never notice is how you can see the difference between phase velocity and group velocity in water ripples. Throw a rock in a lake and you can see that the waves that you watch cross the lake are really made up of smaller ripples that travel from the back of the wave to the front at twice the speed of the overall wave.

It was neat because I read about it a few times before I really had the chance to try it out and almost didn't believe it until I could see it for myself. It's a cool connection between sort of abstract math to a nice bit of physics that you can see with your eyes.

As a side note, Frank Crawford's book on waves from the Berkeley Physics Series is just about my favorite introductory text on anything ever.

 

[mpresic]

No one has contributed for about a month so I thought I might kick things off. Take a book with a tight rubber band around it so that the pages and cover does not flop open(for example). A rigid rectangular block is better. The Block should have three unequal moments of inertia. (For a homogeneous block, you want length, not equal width, not equal thickness). Throw the block in the air gently at the same time rotating the block around the short axis. The book rotates predictably. Now do the same rotating about the longest axis. Again, the book rotates predictably. Finally repeat, rotating about the intermediate (not long, nor short) axis. This time, the block flops around in the air.

This is called the intermediate axis instability in rigid body force-free motion. This is treated in (e.g. Goldstein Classical Mechanics). This is so puzzling I have seen graduate students try this with Goldstein throwing it in the air when they got to this part.

Actually all three dimensional (space) rigid body motion (e.g, the general motion for the heavy symmetric top), as well as force free motion has puzzling aspects.