Security Analysis of a Full-Body Scanner

Freedom of movement is considered a constitutional right. http://en.wikipedia.org/wiki/Freedom_of_movement_under_Unite... If you live in a remote area where the only option is to fly commercial you may have a strong case.

However, my understand is these are only added to major airports to avoid encountering such a situation. As you don't need to fly to get from Chicago or NY to just about anywhere.

Which creates another issue. Your not re-screened after traveling from a minor to a major airport which renders these precautions practically useless vs an intelligent advisory.

"The right to travel is a part of the 'liberty' of which the citizen cannot be deprived without due process of law under the Fifth Amendment. If that "liberty" is to be regulated, it must be pursuant to the law-making functions of the Congress. . . . . Freedom of movement across frontiers in either direction, and inside frontiers as well, was a part of our heritage. Travel abroad, like travel within the country, . . . may be as close to the heart of the individual as the choice of what he eats, or wears, or reads. Freedom of movement is basic in our scheme of values." - Justice William O. Douglas

Like everything there is a balancing act between the interests of the individual, group, and state. One can argue that commercial air travel is so important to modern travel and movement of people (are you going to take a boat to Hawaii?) that going through needlessly invasive screening without any suspicion or probable cause as a prerequisite might be an undue limit on freedom of movement.

No, "the court ruled 8 to 1 that such an intrusive search without the threat of a clear danger to other students violated the Constitution's protections against unreasonable search or seizure".

How would her having illegal drugs be a clear danger to other students?

That's not how evolution works...

Umm...glass is a "solid object" the last time I checked. And no, it's electromagnetic radiation, period -- it's a different wavelength (just like blue is a different wavelength than red).

Look for the reply from the physicist for greater detail as to why you're completely confused, though. I opted out of the Backscatter X-Ray machines due to what seemed like poor science; the MM wave devices may violate privacy, but that's their worst offense.

Maybe you could relate it to temperature? A simple quantity, but intuitively safe at some magnitudes and dangerous at others.

Ocean waves crashing against a retaining wall. It doesn't matter how many small waves come, it's only waves greater than a certain height that are dangerous.

The sand/bullet one is probably apt, if indirect. The issue is that different "kicks" get absorbed in different ways by the stuff which makes up your cells. One gets absorbed by the chemical bonds and tears them apart; one gets absorbed by the molecule as a whole.

I think maybe the best way to give the idea is to contrast a car crash with a bullet at the same energy. According to Wikipedia, a really strong rifle cartridge (in this case, the .220 Swift) fires its bullet with at most 2.4 kJ of kinetic energy, which is about the energy of me moving at 15 mph. So you can imagine me, getting shot, flying backward at ~10 mph. But also there's the exact same energy and momentum transfer as is in me falling asleep and veering into a tree while driving at 10 mph. So why do I think that the 10mph crash is survivable as long as I don't get thrown from the vehicle, while the bullet would likely be fatal?

The difference is that the seat belt distributes the force over most of my upper torso, whereas the rifle distributes all of its kick in a particular place. Similarly, the "smaller" photons distribute all of their kick on particular atoms, while the "bigger" photons tend to distribute their kick on the protein as a whole. One "rips the stuff apart", the other causes it to bump into its surroundings more.

Sound energy is unrelated to audio frequency. Intuitively, it's possible to have quiet, deep sounds and loud, high-pitched sounds.

"More energy" can mean two different things. In the case of sound, only one of those meanings--the amplitude of the wave--is relevant: soft sounds have lower amplitude and loud sounds have higher amplitude. The frequency of the wave, as jessaustin pointed out, is independent of how soft or loud it is, and for sound, there isn't any useful sense in which any frequency carries "more energy" than any other frequency, given that the amplitudes are the same.

With regard to light, however, there is a second possible meaning for "more energy": not a higher amplitude of the classical wave, but more energy contained in a single quantum of the light (i.e., a single photon). Light of higher frequency (toward the blue/ultraviolet end of the spectrum) has more energy per quantum; light of lower frequency (toward the red/infrared/microwave end of the spectrum) has less. This sense of "more energy" is still independent of wave amplitude: wave amplitude corresponds to number of photons (more precisely, the number of photons is proportional to the amplitude squared). So you can have faint light (few photons) of high frequency (more energy per photon); or you can have bright light (lots of photons) of low frequency (less energy per photon). The total energy contained in the light depends on both of these factors; but the total energy in the light is not the crucial factor involved (see below).

It turns out that, when you are looking at the possible effects of radiation on the body, the energy per photon is the most important factor, because that is what determines what kind of interactions the radiation can have with the molecules that make up your body. UV, X-rays, and gamma rays have enough energy per photon to break apart the chemical bonds that hold together things like proteins in your cells; that's what makes them potentially cancer-causing. Microwaves, OTOH, don't have enough energy per photon to break chemical bonds; all they can do is make the molecules, like proteins, vibrate more rapidly, which just means heating them up.

It's worth understanding that the light which you see is the result of many photons -- though it's usually estimated that maybe, if our rod/cone cells were a little more sensitive, they might have been able to perceive individuals. So actual energy/mass transfer comes from a lot of particles each offering their own individual kicks. So the overall intensity is not just (energy per kick) but really (number of kicks) * (energy per kick). The number of kicks counts.

Phonons, for sound, actually are quantized in the exact same way as for light -- just replacing the speed of light c with the speed of sound in the medium v. (Both arguments are based on analogies to a harmonic oscillator problem which every quantum mechanics course works out; basically any vibration in the bottom of a potential well X can be dealt with by approximating that potential well with a parabola, which leads to particles, we could call them X-ons, of one form or another. Quantum mechanics makes all particles act like waves but also makes all waves quantize into particles.)

So, long-story-short, the physics is the same, and long wavelength phonons also carry less energy per kick than shorter ones, just like photons. That's not the difference, and the E = h f equation is still the same. But for phonons it's more about numbers.

The biggest sound particles you can hear correspond to 20,000 Hz frequencies. The smallest light particles you can see correspond to 700 nm wavelengths or 10^15 Hz frequencies. So photons are more than 50 billion times more energetic than phonons, as a rule.

This changes a lot of features about them. For example, every degree of freedom in a system has some average energy due to just random energy-sharing, which is known colloquially as the "temperature" of the system. If you work out how many 20kHz phonons there are simply due to a room being at room temperature, it works out to about 300 million. Any detector which registers 20kHz phonons, including the hairs in your ear, have 300 million of those phonons in them just from random thermal excitation. On the other hand, the red light receptors in your eye have on average (300 million) / (50 billion) = 0.006 photons in them on average, meaning that if you look at a thousand of those photon-absorbers you'll see on average only six of them which are excited randomly just by thermal excitations. So we can, at room temperature, detect individual photons, where we can't detect even ten thousand phonons at once without it getting hopelessly buried in thermal noise. (But our eyes simply do not fire the neurons when a light-sensitive cell only sees one photon.)