In “Family WhatsApp Group Confirms 5G Causes Colds”, a nephew from Bengaluru tried to explain why radio waves can’t build a virus, and was removed from the group for his trouble. Before he went, he made a point that deserves more than three exclamation marks. Light, he said, comes in packets. A stronger signal doesn’t make the packets stronger, only more numerous. And a million feathers thrown at a window will not break it the way one stone will. That sounds like a nice comparison. It is actually one of the most important discoveries of twentieth-century physics, and it explains why sunlight can damage your skin while a phone tower can’t.
It begins with a puzzle that bothered physicists around 1900. If you shine light on a clean metal surface, it can knock electrons out of it. This is called the photoelectric effect. If light were simply a wave, spreading energy smoothly, you’d expect that brighter light, carrying more energy, would knock electrons out more easily, whatever its colour. But that’s not what happens. Shine dim ultraviolet light on zinc, and electrons fly out at once. Shine red light on it, as bright as you like, for as long as you like, and not a single electron comes out.
In 1905, a young patent clerk named Albert Einstein proposed the answer. Light, he said, is absorbed in tiny, indivisible packets, now called photons. And the energy of each packet depends only on the frequency of the light: Planck’s constant, a very small number, multiplied by the frequency. Ultraviolet light has a high frequency, so each of its photons carries a lot of energy. Red light has a lower frequency, so each of its photons carries less. Crucially, an electron in the metal absorbs one photon at a time. If one photon has enough energy to free it, it flies out. If not, it doesn’t, and making the light brighter just means sending more photons, each of which is still too weak. A million feathers. Einstein won his Nobel Prize for this idea, not for relativity.
Now apply it to your body. Your skin and your DNA are held together by chemical bonds, and breaking a typical bond takes a certain amount of energy, a few units on the tiny scale that physicists use for single molecules. Ultraviolet photons from the Sun carry about that much energy, sometimes more. When one is absorbed by a DNA molecule, it can make two neighbouring thymine letters on the same strand bond to each other, a kink in the code that the cell has to cut out and repair. Most of the time the repair works. Occasionally it doesn’t, and errors build up. That is why ultraviolet light causes sunburn and, over many years, skin cancer, and why you can burn badly on a cool, cloudy day: it isn’t heat that burns you, it’s individual high-energy photons.
Visible light photons carry a little less energy, enough to trigger the pigment in your eye, which is why you can see, but mostly not enough to break the sturdier bonds in DNA. And radio waves? A 5G signal at 3.5 gigahertz has a frequency roughly three hundred thousand times lower than ultraviolet light. Each of its photons carries hundreds of thousands of times too little energy to break a single chemical bond. No number of them can add up to one stone, because a molecule absorbs them one by one, and each one is just a feather.
That doesn’t mean radio waves do nothing at all. When enormous numbers of low-energy photons are absorbed, their energy ends up as heat: they make molecules wiggle and twist a little faster. A microwave oven does exactly this, pumping hundreds of watts of microwaves into a sealed metal box so that water molecules in the food twist back and forth billions of times a second and warm up. The only established effect of radio waves on the body is warming, and the safety limits for phones and towers are set far below any level where that warming would matter: much less than you get from sitting in the sun or going for a walk.
So the line between dangerous and harmless light isn’t brightness. It’s the energy of each photon, and that is set by frequency alone. Physicists call the high-energy end, from ultraviolet onwards to X-rays and gamma rays, ionising radiation, because its photons can knock electrons off atoms and break molecules apart. Everything from visible light downwards, through infrared, microwaves and radio, is non-ionising. It can warm you. It can’t rewrite you.
The nephew, by all accounts, has not been readded to the group. But he has the satisfaction of knowing that he was standing on the shoulders of Einstein, which is more than the forward could say.