Apparently San Francisco has just revoked an ordinance that would require cell-phone retailers to warn their customers about the 'potentially dangerous radiation levels' coming from their favorite portable gizmos. Last I'd heard about this subject, scientific consensus - well, to be honest, scientific evidence whatsoever - about a link between brain cancer and cell phone use was distinctly lacking. But as we've seen with GMOs, climate change, evolution, corn-based ethanol, the Copernican solar system, and pretty much any other subject where science and politics collide, that doesn't stop people from claiming that things they want to believe are solid, experimentally-established fact. Of course, nature itself isn't particularly swayed by these arguments, but nature doesn't have much to do with the operations of human society.
One of those operations, protecting our citizens from public health hazards, is I imagine something we can mostly get behind. To do this job well, our regulators (e.g. the FDA) need to have the best available understanding of what is actually damaging to people's health (e.g. well-funded scientific studies). Unfortunately, ideas that get repeated a lot start to sound true; plus, the first duty of an elected official is to her constituents, and if a loud subset of those constituents are deathly afraid of, say, windmills, the regulator might start to take that concern seriously, too. Good science is pricey and time-consuming: whereas good politics is fast and cheap. You can see the problem.*
But back to cell phones. This one caught my attention in particular because it involves the "R" word. There's a lot of woo about radiation, because of the very scary nature of the very real radiation disasters of the last hundred years or so. But there's a serious popular classification problem here, because the term 'radiation' covers a huge range of phenomena**:
- gamma rays (like gamma ray bursts, the most energetic events in the universe!)
- x-rays (like dentists)
- UV (like sunburns)
- visible light (like bulbs)
- infrared (like Predator)
- microwaves (like ovens)
- radio (like music)
The reason some kinds of radiation are dangerous is pretty interesting, actually. Everything in the list is the same kind of particle, a photon; they are differentiated just by how energetic the photons are.*** Physicists use a unit for energy called electron-Volts (eV)**** that's about the right size for individual particles. For instance, at room temperature, your average particle has about 1/40th of an eV of thermal energy. Another one: molecular bonds, such as the ones that connect the atoms in your DNA, take around 10 eV to split. That's important because cancer happens when something damages your DNA and it doesn't heal correctly. Let's look at the list again, this time with photon energy ranges included:
- gamma rays: > 200,000 eV
- x-rays: 200 to 200,000 eV
- UV: 3.2 to 200 eV
- visible light: 1.8 to 3.2 eV
- infrared: 0.001 to 1.7 eV
- microwaves: 0.000001 eV to 0.001 eV
- radio: < 0.000001 eV
You can imagine each photon as a little packages of energy, flying around looking for something to give it's juice to. For UV and up, each photon has enough kick to do some real damage: kick the electrons right out of some molecules and break up their structure. Whereas for visible light and down, the photons just don't have enough oomph to shove an electron out of its molecule.
So are cellphones dangerous? Different frequencies have been licensed for cell-phone use over their brief history, but 4G seems to use the highest frequencies, up to about 2700 MHz. You can play along at home if you like: just open up a new tab and google "Planck's constant * 2700 MHz in eV", to find out that each photon from your high-end cell-phone packs a measley 0.000011 eV, putting them at the low-end of the microwave range with about 10^5 less than ionization energies.
Since ionization is an all-or-nothing event*****, that means there is no way, no how that the radiation from your cell phone could damage your DNA and cause cancer like harder radiation can. Interestingly, it turns out that microwave ovens use a similar frequency for cooking your food. Of course, the absolute antenna power output of a cellphone is something like 1/200th of what a typical microwave oven produces; and the blood vessel system is designed to redistribute heat throughout the body efficiently; but there could potentially be some unexpected effects due to localized head heating. This sort of effect merits further study, and seems to be studied extensively, so far with no real demonstration of ill effects as far as I can tell.
But from a basic physics standpoint, a cellphone source for brain cancer seems outside the realm of possibility by many orders of magnitude.
* And you can see its solution: well-funded public science research.
** I left one out for clarity's sake: particles from nuclear decay (like alphas, electrons, neutrons; sorry, no simile here). These aren't photons, but they are called radiation because all these things were discovered at the same time. Plus, they 'radiate' from their source like spokes from a wheel, so the word fits.
*** Because of wave-particle duality, the energy of the photon is directly proportional to the radiation's frequency: E = hv, where v is 'nu', the frequency in Hz, and h is Planck's constant: 4.14 x 10^-15 eV/Hz. If you prefer wavelength, E = hc/λ, where c is the speed of light: 3x10^8 m/s, and λ is in meters.
**** Actually the amount of energy an electron gets when you put it through an electric potential gap of 1 volt! For once a unit is named conveniently.
***** An electron doesn't 'build up' energy by absorbing lots of photons over a long period of time: it exchanges energy in discrete packets, or 'quanta', called photons. That's the basic idea of quantum mechanics, and it's the idea that Einstein actually got his Nobel Prize for.