Live data from Hacker News

Security Analysis of a Full-Body Scanner

radsec.org

51–60 of 65 posts

Re: Security Analysis of a Full-Body Scanner

#51

Earlier quoted context omitted.

PS: so in electromagnetic radiation, shorter wavelengths carry more energy, whereas in sound, longer wavelengths (bass) carry more energy. (Right?) I'm not physics-savvy enough why these effects are opposite; is there an explanation for this?

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

I think you're right (audio frequency doesn't appear in the formulas I found for sound energy), but I don't think the analogy fully demonstrates why, since it's also possible to have bright or dim light of a particular color!

The issue about the energy of electromagnetic radiation depending on frequency is, if I understand correctly, a quantum issue.

https://en.wikipedia.org/wiki/Photoelectric_effect

That is, quantum theory explains why there is such a thing as nonionizing electromagnetic radiation.

Re: Security Analysis of a Full-Body Scanner

#52

Earlier quoted context omitted.

PS: so in electromagnetic radiation, shorter wavelengths carry more energy, whereas in sound, longer wavelengths (bass) carry more energy. (Right?) I'm not physics-savvy enough why these effects are opposite; is there an explanation for this?

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

It's also possible to have faint blue light, and bright red light.

Sound energy is quantized too, but for audible frequencies, hf is small compared to kT, so quantum effects are swamped by thermal effects.

Re: Security Analysis of a Full-Body Scanner

#53
post #31

Earlier quoted context omitted.

Theoretically, how many of these (or similar) techniques would work against the millimeter wave scanners currently deployed in US airports?

Don't the millimeter wave machines perform a 360 degree scan? That would at least eliminate the really facepalm-inducing "put your gun on the side of your body" attack.

Doesn't seem that way. The sensor revolves around the subject being scanned, but the images that have always been shown as coming from the machine are just front and back images.

From L3's sales site for the ProVision MMW machine, sample images:

http://storage.pardot.com/16582/23361/product_provision_L_3_...

Their "ATD" model that includes image analysis to automatically flag "anomalies" only seems to show front and back, too.

http://storage.pardot.com/16582/23381/products_ait_ProVision...

In 2012, the "TSA Out Of Our Pants" guy previously documented the "conceal a metal object on the side of your body" attack, and did so against the MMW machines with the ATD software on them:

http://tsaoutofourpants.wordpress.com/2012/03/06/1b-of-nude-...

Re: Security Analysis of a Full-Body Scanner

#54
post #32

Earlier quoted context omitted.

So you've got a bunch of responses, but as a guy with a Master's in applied physics I wanted to chime in with a bit more of a long explanation. The word "radiation" just means that something radiates -- it travels off to infinity. In this case we're talking about electromagnetic radiation, which spans a vast continuum from radio waves through microwaves to infrared and red, across the visible spectrum to deep blue, t…

I like this response a lot and am going to adapt it to explain this to my radiation-paranoid family. The concept of wavelength as the sole distinction between these types of radiation is probably abstract to most non-science-educated people. Can you (or anyone else) think of a good concrete real-world analogy of different types of X that vary only in Y and some X are harmless but those with Y are not? Maybe an analog…

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.

Re: Security Analysis of a Full-Body Scanner

#55
post #10

Earlier quoted context omitted.

Backscatter x-ray technology was removed from airports. [1] All airports now use millimeter wave technology. [2] [1] https://en.wikipedia.org/wiki/Backscatter_X-ray [2] https://en.wikipedia.org/wiki/Millimeter_wave_scanner

Millimeter wave scanners still subject the body to radiation, even though they don't have the work "X-Ray" in the name. Radiation damage to your body is cumulative throughout your life, so if you care about your longevity, you'd be wise to opt-out of subjecting your body to this radiation whenever possible.

If you're worried about this, you shouldn't fly at all. The amount of radiation your body is exposed to during the flight is several orders of magnitude larger than the amount it's exposed to during the security screening. Not only that, the radiation you're exposed to during the flight is higher energy (cosmic rays are the major component) and can do more damage to your body than the microwaves used in the security scanners.

Re: Security Analysis of a Full-Body Scanner

#56
post #32

Earlier quoted context omitted.

So you've got a bunch of responses, but as a guy with a Master's in applied physics I wanted to chime in with a bit more of a long explanation. The word "radiation" just means that something radiates -- it travels off to infinity. In this case we're talking about electromagnetic radiation, which spans a vast continuum from radio waves through microwaves to infrared and red, across the visible spectrum to deep blue, t…

PS: so in electromagnetic radiation, shorter wavelengths carry more energy, whereas in sound, longer wavelengths (bass) carry more energy. (Right?) I'm not physics-savvy enough why these effects are opposite; is there an explanation for this?

"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.

Re: Security Analysis of a Full-Body Scanner

#57
post #32
post #10

Earlier quoted context omitted.

Millimeter wave scanners still subject the body to radiation, even though they don't have the work "X-Ray" in the name. Radiation damage to your body is cumulative throughout your life, so if you care about your longevity, you'd be wise to opt-out of subjecting your body to this radiation whenever possible.

So you've got a bunch of responses, but as a guy with a Master's in applied physics I wanted to chime in with a bit more of a long explanation. The word "radiation" just means that something radiates -- it travels off to infinity. In this case we're talking about electromagnetic radiation, which spans a vast continuum from radio waves through microwaves to infrared and red, across the visible spectrum to deep blue, t…

Thanks for the reply. I don't know if wikipedia is the best source, but talking about health risks, it states:

> A team led by Los Alamos National Labs found that although the forces that terahertz waves exert on double-stranded DNA are tiny, in certain circumstances resonant effects can unzip the DNA strands, tearing them apart. This creates bubbles in the strands that can significantly interfere with processes such as gene expression and DNA replication. [1]

Unzipping DNA strands in certain circumstances sounds pretty harmful.

I understand theoretically why millimeter waves are lower risk than backscatter x-rays, but it seems to me that there still are risks. In my opinion, before sending the population through these things, we need studies of their long-term effects from real-world exposure, not just theoretical reasoning about why the rays are most likely safe.

[1] https://en.wikipedia.org/wiki/Millimeter_wave_scanner#Possib...

Re: Security Analysis of a Full-Body Scanner

#58
post #53
post #31

Earlier quoted context omitted.

Don't the millimeter wave machines perform a 360 degree scan? That would at least eliminate the really facepalm-inducing "put your gun on the side of your body" attack.

Doesn't seem that way. The sensor revolves around the subject being scanned, but the images that have always been shown as coming from the machine are just front and back images. From L3's sales site for the ProVision MMW machine, sample images: http://storage.pardot.com/16582/23361/product_provision_L_3_... Their "ATD" model that includes image analysis to automatically flag "anomalies" only seems to show front and…

Interesting. I always assumed it was doing a 360 scan. The sample images do seem to hint at a 2D scan, though the ATD front & back cartoon should be enough to highlight anomalies anywhere on the traveler's body.

It's too bad the "TSA Out Of Our Pants" guy didn't have a hidden camera accomplice to film his entire trip through security.

Re: Security Analysis of a Full-Body Scanner

#59
post #32

Earlier quoted context omitted.

So you've got a bunch of responses, but as a guy with a Master's in applied physics I wanted to chime in with a bit more of a long explanation. The word "radiation" just means that something radiates -- it travels off to infinity. In this case we're talking about electromagnetic radiation, which spans a vast continuum from radio waves through microwaves to infrared and red, across the visible spectrum to deep blue, t…

PS: so in electromagnetic radiation, shorter wavelengths carry more energy, whereas in sound, longer wavelengths (bass) carry more energy. (Right?) I'm not physics-savvy enough why these effects are opposite; is there an explanation for this?

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.)

Re: Security Analysis of a Full-Body Scanner

#60
post #32

Earlier quoted context omitted.

So you've got a bunch of responses, but as a guy with a Master's in applied physics I wanted to chime in with a bit more of a long explanation. The word "radiation" just means that something radiates -- it travels off to infinity. In this case we're talking about electromagnetic radiation, which spans a vast continuum from radio waves through microwaves to infrared and red, across the visible spectrum to deep blue, t…

I like this response a lot and am going to adapt it to explain this to my radiation-paranoid family. The concept of wavelength as the sole distinction between these types of radiation is probably abstract to most non-science-educated people. Can you (or anyone else) think of a good concrete real-world analogy of different types of X that vary only in Y and some X are harmless but those with Y are not? Maybe an analog…

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.

Post reply on HN