Comments of the Week #173: From quantum uncertainty to Earth's final total solar eclipse

“It will shine still brighter when night is about you. May it be a light to you in dark places, when all other lights go out.” ―Galadriel, LOTR, J.R.R. Tolkien

The scientific stories we've covered this week have been out-of-this-world here at Starts With A Bang! But the greatest show is still to come. Right now, I'm on my way down to the path of totality in Oregon, along with millions of others hoping to catch a glimpse and enjoy the experience of a sight unlike any others on Earth. When the sunlight goes completely out, some truly wonderful things will be revealed, and I hope to see them all! For everyone who's joining me, across the path of totality, I wish you clear and cloud-free skies, and a fabulous viewing experience!

And now, onto the scientific stories we covered this past week:

As the release date of Treknology approaches (less than two months now!), there will be a slew of talks and events occurring in Oregon and Washington to promote it and meet me, with more to come around the country as time goes on. Look for it! And now, onto the main event: our comments of the week!

From Steve Blackband on X-rays at the airport: "On the banana thing and airport x-ray scanners, an issue is not the total dose but the distribution. TSA seems to divide by the whole body, but the dose is concentrated at the skin so the dose there is many times higher."

This is actually not true of X-rays in general. Yes, they hit the skin first, but X-rays are of an energy such that the overwhelming majority penetrates the skin and goes into your body. There are a portion of the X-rays, however, that hit the skin and reflect, and that's how the backscattering X-ray imaging works. It's kind of the opposite of traditional X-rays, which measure what goes through your body. But as with all things, we've got to be quantitative. For the airport scanner, you'd need to go through it 200,000 times to equal the radiation of one CT scan.

By the way, there is radiation that primarily affects your skin: radioactive alpha-decay sources. They are the most harmless of all radiation, since the outer layer of your skin stops it. Only if you ingest or inhale an alpha-emitter are you in trouble.

Correctly calibrated satellite data, as well as the more recent temperature data up through 2016, shows that climate predictions and observations are perfectly in line with one another. Image credit: HadCRUT4.5, Cowtan & Way, NASA GISTEMP, NOAA GlobalTemp, BEST, via Ed Hawkins at Climate Lab Book.

From Denier on what his true beef was with my response to Heartland's climate article: "The boiled down core of what I’m driving at is I felt you wanted a win so bad that you decided hitting below the belt was justified. It wasn’t just Spencer making that observation about 2013. Schmidt(2014) noted it, and it is even reflected in the Climate Lab Book alteration of the IPCC AR5 graph you posted in the article. Heartland made a cherry-picked but accurate statement, and rather than calling it out for what it was, you straw-manned them and made your own counter-factual statement that was not supported by the best science we have on the subject.
A little lower in the article, Heartland did the same thing with a statistical decline in the strength of hurricanes making landfall in the US. 100% Accurate –and- 100% cherry picked. There again you failed to call it out for what it was and went with the cheap Ad Hominem about how the scientist citing the true statistic was biologically related to someone at Heartland.
There is so much good science to support your viewpoint that you don’t need to stoop to these tactics. You don’t need to Straw-man. You don’t need to deny good science. You don’t need to resort to Ad Hominem attacks. I was disappointed in your tactics and felt they were beneath you. That uncharacteristic behavior combined with your talk of de-platforming certain ideas made me think we were losing you to tribalism."

I've been thinking a lot about tribalism and appearances lately as well, and maybe I need to go easier on people who are politically much farther to one side than I am. I think reviewing Alex's latest book brought that to my attention as well, and after some reflection (and some investigation), I think I understand why it hits so many of us so hard. We aren't impartial or objective, no matter how hard we try to be. We view our work and our opinions on issues in terms of what we value as important in this world.

Imagine that politics is a left-right spectrum (I know that doesn't encapsulate it all, but we're oversimplifying for clarity), and you're somewhere on it. Let's assume you're near the center, but slightly to the left. Now there's someone you see who's also near the center, but slightly to the right. To the right of center, but also (and moreso) to the right of you. You both accept the same science facts about issues, but how you feel about and react to those issues are very different. How do you see the person to the right of you? Even if they write things that are both "anti-left" and "anti-right", you'll see the "anti-right" things they write as no-brainers, but then the "anti-left" things will appear biased to you. If you were instead far-right instead of left-of-center, you might see the converse: the "anti-left" things the author writes appear as no-brainers, but the "anti-right" things appear biased.

You and I are always going to disagree about what's "good science" in this realm. I think if you're using the UAH data as it was before the calibration flaw found in 2014 was corrected, you're intentionally spreading falsehoods. That was my beef with what Heartland was doing on that particular issue. You and I may disagree about the egregiousness of cherry-picking data; we had an argument a year ago where you admitted that Newt Gingrich had done that with crime statistics, but you argued that his point was still valid based on the data he had selected. I think cherry-picking -- or "not looking at the full suite of evidence" as I often call it -- is just lying, usually with the intent to mislead.

In any case, you haven't lost me to tribalism, but on certain issues, you and I view one another's positions as inherently flawed. More on that when we get to the comments about Alex's book.

From Steve Blackband on speaking against Nazism: "I understand your frustration and feeling of powerlessness in the face of hatred and bigotry.
But this is not the forum. Keep this a science blog, please, so i don’t have to troll through all these folks e-shouting at each other.

Sorry, Steve, this is the only forum. This is the platform I have online where I get to go beyond my own science writing and get to talk about larger issues; I literally use ScienceBlogs as a Starts With A Bang forum. If I could write about science without people sending me death threats related to ovens, destroying my life, slurs against my ethnicity/religion/whatever-you-perceive-Jewishness-as, it might be a different story, but I hope not.

Some people will always e-shout about what their opinions are, and my options are to either ban them or not. I've chosen not for the people who are still around. You can scroll past the parts you don't like, but there will still be plenty of science, so long as people are still commenting about it.

A spinning neutron star, with its magnetic field lines illustrated. Image credit: ESO/L. Calçada.

From eric on neutron stars and magnetism: "if for atomic number you’re only going to count the core and not the surface, then when it comes to charge and magnetic field you should only count the core and not the surface too, right? Otherwise you’re arriving at your conclusion that a neutron star has a magnetic field but Z=0 only by flipping back and forth between two different definitions of “neutron star” – one that only counts the core, and one that includes the surface."

So I will say that this poses an interesting question. If you have a neutral object like a neutron, and you spin it, do you get a magnetic field? Your intuition would say "no," but now consider that a neutron is made up of charged particles itself. If there's a charge separation in there at all, and those charges move around, you could get a magnetic field, couldn't you? Here's the thing: we can take a single neutron and measure its magnetic moment. For electric charges, a proton is +1, a neutron is 0, and an electron is -1. For magnetic moments? Electrons are -1, protons are +2.79, and neutrons are -1.91. You would have a magnetic field, after all.

But it wouldn't be nearly as strong as the magnetic field if you include the neutron star's surface, which is no longer made up of neutrons, and which can no longer be treated as a single nucleus. How much stronger? We're not sure, but suffice it to say it's many orders of magnitude. Still, there's a big difference between a factor of 10,000 and a factor of 1,000,000,000, and I'm not sure where the core of the neutron star lies in this. An interesting consideration!

Front cover of the hard copy of the Little Black Book of Junk Science. Image credit: American Council on Science and Health.

From Alex Berezow, who dropped by to comment on a comment about the content of his book: "You wrote: “Ive been telling everyone that this book taught me that hexavalent chromium isn’t cancerous”
I explained in my book: “If inhaled, chromium-6 can cause lung cancer, but there is no reason it causes cancer when ingested.”"

When you've got a point to make, you're always going to appear biased to people who don't feel the same point is worth making when it comes to that particular issue. I think this is true for everyone; I get accused of my political bias in exactly that way every time I write about a science issue that's political also. And there are some issues with the Little Black Book of Junk Science that I have, but whether you think it's Alex's political bias or my political bias will depend on your politics. For example:

  • Natural gas is better than coal and oil for sure from a pollutant standpoint, but it still adds the same amount of CO2 for the energy you get out to the atmosphere.
  • Oil pipelines are much safer and cleaner than any other method of transporting crude, but it also represents a commitment to burning 100% of what's buried in the ground, and represents a commitment to doing it faster, regardless of how dirty it is.
  • Organic food is neither healthier nor does it deliver superior crop yields to conventional foods, but there are many problems inherent to our modern agricultural system that organic practices represent one small, incremental step towards improving, even though they've been co-opted by industrial agriculture.

When one writes about a topic and doesn't address what you believe the core, or most important, issue on that topic is, their writing is going to appear severely biased (or missing-the-point) to you. That doesn't mean it's wrong, but it may mean it's misleading, depending on how you feel. I don't know that there's a solution to this, other than to acknowledge that most of these issues -- yes, even climate science -- are multi-faceted. Someone who disagrees with you may not be wrong as much as they possess different values and focus on different conclusions that the facts may also support.

Visualization of a quantum field theory calculation showing virtual particles in the quantum vacuum. Image credit: Derek Leinweber.

From Elle H.C. on what particle/antiparticle pairs look like: "Cool to see the QCD animation of how particle/antiparticle pairs pop up, how connected ‘holes’ show up and how the Vacuum starts to be shake up. Curious if these tremblings differ very much from Gravity waves?!"

It's vital to remember, when you see either the animation above (representing quantum fields) or the one from the original article (representing individual pairs), that this is a visualization only. This is not what's actually, physically happening. Quantum field theory is a calculational tool, an extremely useful calculational tool, but it is not literally what's going on with the Universe. You can't grab these particles that "pop into existence" and scatter off of them. You can't bend through empty space because of their electric or magnetic fields. They would need to be "real" particles (like the valence quarks, gluons, or sea quarks inside a proton) for that to happen.

But gravity waves are very different, and are produced by accelerating masses in a non-uniform spacetime. They're real. They do affect everything they pass through. They interact. They are more than just a calculational tool. That's the major difference.

An illustration between the inherent uncertainty between position and momentum at the quantum level. Image credit: E. Siegel / Wikimedia Commons user Maschen.

From D.C. Sessions on quantum uncertainty: "Voltage and charge have a different product from the others?
This is news to me."

There are all sorts of quantum commutation relations that go beyond "position and momentum" or "energy and time". I talked about the angular momentum one (for the Stern-Gerlach experiment) but there are others. Voltage and free electric charge, magnetic vector potential and electric current, and so on. If you obey the canonical commutation relation, i.e., your commutator is non-zero, you're in for a world of uncertainty.

The Moon and Sun each take up approximately half a degree on the sky as viewed from Earth. When the Moon is slightly larger in angular size than the Sun is and all three bodies perfectly align, a total solar eclipse is the result, but only if you're in the path of totality. Image credit: Romeo Durscher / NASA / Goddard Space Flight Center.

From eric on eclipse safety during totality: "You can take your glasses off and look at the sun at totality only if you’re in the narrow region of the country where the eclipse is total."

Of course! If you're not in that region, you don't get totality. But this is worth saying: do not take off your eye protection and look at the Sun if any part of the solar disk is visible.

If you make the wrong decisions as far as what you do and look at during the moments of totality, you risk squandering the experience of a lifetime. Image credit: Luc Viatour / If you make the wrong decisions as far as what you do and look at during the moments of totality, you risk squandering the experience of a lifetime. Image credit: Luc Viatour /

From Brian Bassett on the 5 things you mustn't do during totality: "Vague, regurgitated hash off other sites. Yawn!"

I know, right? It's almost like there's this vast body of knowledge that some group of people have been gathering and developing for centuries, distilling it down to its most important rules and essences, and then disseminating that knowledge and those conclusions worldwide. So boring, right? ;-)

The Newtonian and Einsteinian predictions for gravitational deflection of a distant radio source during the Earth's orbital period (1 year) due to the Sun. The black dots are 2015 data. Image credit: The deflection of light induced by the Sun's gravitational field and measured with geodetic VLBI; O. Titov, A. Girdiuk (2015).

From Anonymous Coward on eclipse science since Eddington: "The bending of light experiment has been repeated many times since Eddington’s day. Astronomers from Lick Observatory went to Australia for the 1922 eclipse and repeated the observations. It was done again during an eclipse in 1952 by Yerkes Observatory astronomers who travelled all the way to Khartoum, Sudan to see it. In 1973, astronomers from the University of Texas went to the Chinguetti Oasis in Mauritania to do the same thing. Each time they found results reasonably consistent with General Relativity. It seems people are going to try to do the same thing with this upcoming eclipse. NASA has even given instructions on how to do it:"

All of this is true, but I'll do you one better. Do you see the image above? That's a radio source that exists far beyond the Solar System. And the x-and-y-axes? That's how much its position deviates over the course of a year. The cause of that deviation? That's the gravitational influence of the Sun! If we could see stars during the day, we never would have needed solar eclipse's or Eddington's work to do the confirmation. As it stands, radio astronomy gives us that ability (not with every star, but with some bright-enough radio sources), and it agrees tremendously with General Relativity.

You no longer need an eclipse to confirm relativity, even in the exact same fashion that it was first confirmed!

On this semilog plot, the complexity of organisms, as measured by the length of functional non-redundant DNA per genome counted by nucleotide base pairs (bp), increases linearly with time. Time is counted backwards in billions of years before the present (time 0). Image credit: Shirov & Gordon (2013), via

From John on life coming to Earth from space: "Drs. Hoyle and Wickramasinghe were serious proponents of Panspermia."

Yes, but.

Before I go any further, panspermia, you must realize, can take many different forms. See that graph, above? At what stage do you think this life came to Earth? According to Hoyle and Wickramasinghe, it was "the advanced prokaryotes that gave rise to modern cell, like diatoms, were what came to Earth." Also, they argued, that life couldn't have begun on Earth at all, that the conditions were all wrong.

Neither of these statements is likely to be correct. That they are part of a larger "panspermia" story, some of which may be true, does not translate into anything they said having any validity. Wickramasinghe continues to make the same claims he made in the 1970s... no matter what the modern evidence shows.

No matter how you choose to experience the eclipse, I hope it's a spectacular one for you. Image credit: Beawiharta/Reuters.

From PJ on eclipse wishes that we all share: "May good viewing fall upon those who venture out for Monday’s grand view."

May the entire path of totality be cloud-free. If you do have clouds, may they not lessen the spectacular nature of the show for you. May there be no hazes or wildfires affecting the attendees. May traffic move smoothly. May everyone be safe, and bring enough food, water, blankets, and comfort.

Good luck out there; these wishes apply to me, too!

The gold-plated aluminum cover (L) of the Voyager golden record (R) both protects it from micrometeorite bombardment and also provides a key to playing it and deciphering Earth's location. Image credit: NASA. The gold-plated aluminum cover (L) of the Voyager golden record (R) both protects it from micrometeorite bombardment and also provides a key to playing it and deciphering Earth's location. Image credit: NASA.

From Candice H. Brown Elliott on whether we should announce our presence to aliens: "Not that thought that creating even a perfect map was a good idea… frankly a truly sapient species would have understood that using Bayesian logic, that even if only a tiny handful of other species were dangerous, the risks aren’t worth taking and it would be better to keep one’s head down and NOT announce one’s presence to other sentients in the universe. (This is my favorite solutions to the Fermi Paradox.) But we are too foolish and too disunited to follow such a course."

A lot of people feel the way you do, Candice. Sometimes, Stephen Hawking expresses similar fears. So does Elon Musk, for example. In any great endeavor into the unknown, there are naysayers. There is the sentiment, "Beware! Here be dragons!"

But the alternative goes against everything it means to be human. To remain here, alone, isolated, and "safe." Yes, sometimes curiosity kills the cat, but you cannot stop us from being curious. We want to know, we want to explore, and we want to find out. If that is how we'll meet our demise -- no matter how unlikely that possibility is -- we'll meet it exactly the way we should: by aiming for the best possible options humanity could ever aspire to. To shoot for the planets, the stars, and the Universe beyond.

In short, I do not agree with your recommendation.

The evolution of some of the Sun’s properties over time. Luminosity is what impacts the temperature here on Earth. Note how slightly the radius changes over the next billion years. Image credit: Wikimedia Commons user RJHall, based on Ribas, Ignasi (2010), “The Sun and stars as the primary energy input in planetary atmospheres”.

And finally, from Omega Centauri on the possibility of solar eclipses going away entirely: "The sun is also swelling due to evolution. As I understand it at this epoch, the surface temperature stays nearly constant, but the radius must increase to accommodate the increasing luminosity. Both the sun growing fatter, as well as the moon looking smaller push towards annular eclipse. Maybe it will happen faster than your calculation has it (assuming you only used one factor)?"

The swelling is only a few percent, however. Do keep this in mind; as the Moon spirals outward, the Sun grows, but only by about 1% every 250 million years. I thought this was taken into account in the 600-700 Myr calculation I did, but then Michael Richmond showed me that I was in error. As you can see from his graph, below, that tiny rate-of-growth makes a big difference!

The angular diameter of the Sun and Moon as seen from Earth, over time, with the top lines representing perigee/perihelion and the bottom representing apogee/aphelion. Image credit: Michael Richmond.

As Michael Richmond noted: "The increase of the Sun’s radius due to solar evolution has a significant effect, too. Using the evolutionary models of from the Dartmouth Stellar Evolution website, one can show that the last total eclipse will occur around 450 million years in the future."

However, if, as Denier says, the Earth-Moon system spirals away from the Sun during the red giant phase, then perhaps billions of years into the future, when the Sun quiets down to a white dwarf, the Moon's shadow will once again fall on the Earth. If it does, it will be approximately the size of the Moon, instead of just tens-to-hundreds of kilometers across, and solar eclipses will be truly spectacular once again on the charred remnant of our world.

And on that note, have a great rest-of-your-weekend and enjoy tomorrow's eclipse!


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And may good viewing be yours!

If's you want a great souvenir to pass on, the US postal service is selling some cool eclipse stamps:…

" The Total Eclipse of the Sun stamp is the first U.S. stamp to use thermochromic ink, which reacts to the heat of your touch. Placing your finger over the black disc on the stamp causes the ink to change from black to clear to reveal an underlying image of the moon. "

Mail it out tomorrow to a family or friend for a lasting treasured memory or just keep as a keep sake.

By Ragtag Media (not verified) on 20 Aug 2017 #permalink

OK, I should have been more specific and said hexavalent chromium 'isn't cancerous WHEN INGESTED'.
The book implies it only causes cancer when inhaled.
But there was a paper linking HC to cancer from China, from drinking water– a few years later a follow-up paper said it didn’t, but was retracted. This is what the Brockovich case rested on.
Check out wiki
“In July 2014 California became the first state to acknowledge that ingested chromium-6 is linked to cancer and as a result has established a maximum Chromium-6 contaminant level (MCL) of 10 parts per billion (ppb).[30] [31] ”

On the x-ray scanner - the report by four UCSF professors turns out to wrong. I stand corrected. Although the radiation does indeed have a limited depth penetration, they used the right numbers, when they assumed they had it wrong. "....the imaging penetration experiment shows that 4.8 and 10 mm (0.19 and 0.39 in) plastic samples reduce the image darkness by 23% and 50% respectively. Dr. Smith states that those who calculate high skin dosage have incorrectly used the shallow imaging penetration value of a few millimeters (~0.16 in), whereas the actual dosage is calculated by the deeper dose penetration.[50]" So they did account for the area of dose and not the whole body correctly.
Thanks! Very informative!!

In Georgia right now waiting to be eclipserated. Thrilling. All enjoy!

By Steve Blackband (not verified) on 20 Aug 2017 #permalink

@ Ragtag
"Mail it out tomorrow to a family or friend for a lasting treasured memory or just keep as a keep sake."

would be cool if anyone still actually mailed anything anymore. Other than my bills, I haven't received or sent any mail for almost a decade

By Sinisa Lazarek (not verified) on 20 Aug 2017 #permalink

@ Sinisa
I was in London 3 months ago and the post cards I mailed JUST arrived last week. Now that is true snail mail.
Still kinda cool to have special thermo ink stamp of the eclipse post marked on the day of it.

By Ragtag Media (not verified) on 20 Aug 2017 #permalink

Thanks Ethan, for the indirect link. That picture and its caption was a big enough clue for me to find the paper by Titov and Girdiuk: "The deflection of light induced by the Sun's gravitational field and measured with geodetic VLBI." I'd heard about the radio measurements of light deflection from the sun but didn't know of any primary sources.

By Anonymous Coward (not verified) on 20 Aug 2017 #permalink


"this is a visualization only"

Sure, but it's about particles popping into existence, and they can let real Black Hole vaporize, so there's some real vibe going on when these fluctuations happen in reality. Perhaps not in this exact form, but it was the frequency into relation with gravity that I was curious about.

By Elle H.C. (not verified) on 20 Aug 2017 #permalink

@ ragtag
"I was in London 3 months ago and the post cards I mailed JUST arrived last week"

wow... I thought waiting for electronics from alixpress for 45 days was bad :) I won't complain ever again.

By Sinisa Lazarek (not verified) on 20 Aug 2017 #permalink

Let's apply Ethan's 'feeling' that alien contact would be a 'good' idea... just because he's curious, and has watched too much Star Trek as he segues into bombastic Captain Picard like drivel when he says:
". If that is how we’ll meet our demise — no matter how unlikely that possibility is — we’ll meet it exactly the way we should: by aiming for the best possible options humanity could ever aspire to. To shoot for the planets, the stars, and the Universe beyond..."
If you wish to speak in euphemisms, fine.
First, don't drag humanity into your incredibly expensive suicide. When you wish to use 'we' for stupidity like hailing down a dragon to chat for tea, you should very much stick to it being just 'you', and be quite certain it doesn't know where your little village is, as most of them probably don't want to be involved in being dragon-chow alongside you for your suicidal science experiment and 'I'm curious' ennui.
Second, inject your naïve curiosity into some historical context and watch what happens to people who scoff at dragons.
Long version:
Let's pretend the Aztecs decided, 'What the hell, we're curious, let's send some boats out over the ocean with directions on how to get back to us, let's also send a message of how much gold we have, how vast our lands are, and make a show of how invincible our spears and arrows are, and how we'd just love to have house guests come over and play' and see what happens.'
What happens: The Aztecs (and the rest of America) would have been conquered much sooner than they were, and if those delightful Europeans they contacted didn't have the technology to build the boats to make the trip before, they certainly did soon after. It really doesn't pay to advertise about being the land of gold.
Short version (for Ethan's more visceral understanding of what he is scoffing at):
Let's pretend we are a Jewish family hiding in Warsaw trying not to get dead in 1943. The husband brings his wife some literature his good 'friend' had just covertly passed to him.
"Yaffa my dear, let us take our family and leave our hiding place and go to this nice Hotel Polski. My friend in the Zagiew passed me this brochure for the place and it looks lovely, it say's they will help us escape to South America!"
What happens: Yaffa and her beloved family end up in the maw of an oven in Auschwitz. Evil exists. There be dragons closer than you think, whether you believe in them or not, and they would very much like to entice you with sweet promises before they meet you.
What-ifs aside, The Earth is the most pleasant place humanity actually knows about. The vacuum of space and toxic worlds are not anywhere near as hospitable. If our species can not learn to at least live together on this nurturing world, there really is no point outside of brief entertainment value of going anywhere else far more inhospitable, as our problems will just follow us into the heavens the same way they followed us everywhere in this world, only to be played out with ever more powerful weapons. You can't run away from your problems if they are yourself.
Humanity has not yet discovered how to live with itself. Until it does, it should not even try to play with others.


If no man is an island, then neither is any species.

I saw V.
I also read up on the history of european arrival in the New World.

I vote we discover them before they discover us.

By Craig Thomas (not verified) on 21 Aug 2017 #permalink

@Craig Thomas #11,
I vote we not poke the hornets nest and think we can outrun the consequences.

CFT: I'm skeptical this is anything but an academic question, but I don't think a few naive messages would set us up for being attacked anyway. Why not? Because if you're a nasty domination-bent civilization and you want to entice 'lesser' civilizations to show you where they are without giving away your intent, what would be a good way to do it? Send a naive lower-tech message out. Make them think (a) you're peaceful and (b) not much of a threat even if you aren't.

So I doubt very much any alien civilization that would want to go all ID4 on us would accept a voyager-like invitation at face value. And similarly, if we discovered an unsophisticated alien message, I'm sure there would be many many humans arguing that we shouldn't give away our position in response, in case it's a honey pot/trap.

Hey Ethan,
I continue to take issue with your airport scanner comments. Here is my comment and your reply.

From Steve Blackband on X-rays at the airport: "On the banana thing and airport x-ray scanners, an issue is not the total dose but the distribution. TSA seems to divide by the whole body, but the dose is concentrated at the skin so the dose there is many times higher."

This is actually not true of X-rays in general. Yes, they hit the skin first, but X-rays are of an energy such that the overwhelming majority penetrates the skin and goes into your body. There are a portion of the X-rays, however, that hit the skin and reflect, and that's how the backscattering X-ray imaging works. It's kind of the opposite of traditional X-rays, which measure what goes through your body. But as with all things, we've got to be quantitative. For the airport scanner, you'd need to go through it 200,000 times to equal the radiation of one CT scan.

I agree the overwhelming majority enters the skin - thats is EXACTLY what we are worried about. So we need to know the dose where the X-rays go. The X-rays don't all pass through your body - some are absorbed by tissues to different amounts depending on the tissue (otherwise their would be no contrast in the image). The dose is low in an airport scanner, so it only penetrates a short distance into the tissue. Thus to get dose correctly you must divide by the volume of the penetration depth, not the whole body volume. If the depth were 1cm, the dose would be 35 x more in that region than if you divided by the whole body. If its 1mm, 350x more - numbers to be worried about. This is countered by the applied x-ray power being being lower, but you need to be concerned about tissues that may be more susceptible to x-ray damage than others, like skin. Or maybe the eye surface (cataracts anyone)?
The UCSF guys were concerned but had the penetration depth too small. In the end its OK when you get quantitative as you say and they stood corrected - the dose is nothing to worry about. Science working as it should - a voiced concern, in depth study, and finally a clear answer.

The same issue occurs in MRI - when we use rf magnetic fields to image they cause heating. If you calculate by just dividing by total body mass, you miss potential 'hot spots' caused by heterogeneity in the tissues susceptibility to heating, or hot spots caused by the rf coil design. For example we worry about the interior of the eye which is avascular and cannot carry heat away as well. Thus complex models have been made of the heterogeneity of rf heating in tissues.

Hope that clear that topic up.

By Steve Blackband (not verified) on 23 Aug 2017 #permalink

BTW is the guy in the grey beard and crown you?

By Steve Blackband (not verified) on 23 Aug 2017 #permalink

The dose is low in an airport scanner, so it only penetrates a short distance into the tissue.

The standard x-ray scanners used for luggage use transmission and probably aren't something you should regularly be exposed to.

The more recent whole-body scanners use the Compton effect and x-ray backscattering. Reflectance (rather than transmission or absorbance) off of different types of surface, basically. Compton scattering is intentionally low intensity.

Personally, I'm more worried about the technology being a waste of my taxpayer dollars (both in purchase and in operation) than I am it's safety.

You imply that you can tailor the X-rays in some way so that they either transmit or reflect, or are absorbed. You cannot do that. When you irradiate with x-rays, most trnansmit into the sample, some are then absorbed, and if the power is high enough some get through. A small fraction reflect. The power you use determines how far into the sample the X-rays go. At higher powers they go all the way through, used for luggage and Xray imaging and CT. For scanning just the surface using backscattering the sensitivity is there so that you can use low power, and so what is transmitted is also relatively small. Back scattering isn't 'intentionally low intensity'. BUT you only need relatively low intensity to scan the surface and detect, say, guns and knives and stuff i.e. you only need relatively low spatial resolution. If you wanted to increase the resolution to detect smaller objects, like 1mm diameter ball bearings, you would have to increase the power of the applied x-rays to get the resolution. In that case the the back scattering would be 'intentionally high intensity'. Back scattering is not 'intentionally low intensity' - it is by its nature a small fraction of the applied power. Much like the incident, reflected and transmitted waves of light are determined by the properties of the material you shine the light onto and the angle of the applied light.
The power, or density of the x-rays determines what you can see, the spatial resolution. It is possible to do x-ray microscopy, with microscopic (below 100 microns) spatial resolution - but the power of the applied x-rays is so high I would not recommend it for live people - think toast!! (but we do scan for example excised bone samples at microscopic resolution to look at trabecular structure, or small live samples that i am sure won't live long after they have been nuked).

Personally, I am worried about wasting tax payer dollars AND safety. The present governments dismantling of the EPA and head in the sand view of climate change depresses the hell out of me - I want my tax payer dollars spent there, not on a stupid useless multi-billion dollar wall that cannot work.

By Steve Blackband (not verified) on 23 Aug 2017 #permalink