# Why doesn't light kill me?

I was attending my philosophy class and in the middle of student presentations, I found myself mentally wondering off and thinking about light. After a few minutes of trying to piece together how the sun works I came to ask a question I could not answer myself. Why does each individual photon have such a low amount of energy? I am hit by photons all day and I find it amazing that I am not vaporized. Am I simply too physically big for the photons to harm me much, or perhaps the Earth's magnetic field filters out enough harmful causes such as gammy rays?

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Slightly off topic, but in 'Profiles of the future', when talking about the possibility of invisibility, Arthur C Clarke says 'An invisible man wouldn't just be blind, he would be dead'. At first I misinterpreted this as saying any 'inivisibility potion' would kill you. But what it is saying is that normal light would disrupt the processes going on in your cells too much, so its a good thing you've got a fairly opaque skin to keep it out. –  quantropy May 22 at 7:38

Individual photons are very small and don't have much energy.
If you put a lot of them together in one place you can hurt somebody - by simply supplying enough power to melt an object (ask any spy on a table underneath a laser beam).

There is another very odd feature of photons. Although lots of them can provide a lot of energy and heat an object, it takes an individual photon of enough energy to break a chemical bond. So while a single high-energy ultraviolet photon can break a molecule in your skin and cause damage, a billion lower energy visible photons hitting the same point can't break that single bond. Even though they together carry much more energy, it is the energy that is delivered in a single photon that matters in chemistry.

Fortunately the Earth's atmosphere shields us from the photons with enough energy to break most chemical bonds.

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There are multiple absorption processes where several low energy photons can be absorbed to cause a relatively large energy transition. It's just that the probability of these events is exceedingly small (because, roughly, multiple photons need to be in the same place at the same time and the interaction with each individual photon is relatively weak). I think a powerful laser beam is necessary to begin to see these effects, but I'm not really the expert on it. –  Michael Brown May 17 at 0:46
@MichaelBrown - yes if you are made of KDP you should be careful of sunbathing even on a cloudy day. –  Martin Beckett May 17 at 1:17
This KDP? Like I said, not the expert... –  Michael Brown May 17 at 2:15
@Michael Brown: Multiphoton transitions can actually get to be more likely than single photon transitions if the intensity of the light is high enough. –  Dan May 17 at 6:33
It's not just good luck, life has evolved to adapt the environment, which includes a certain energy range fro photons. If the atmosphere shields allowed some other photons to pass, life would have evolved in some other way. –  Francisco Presencia May 17 at 10:21

I have a somewhat non-physics answer for you. If you allow me to broaden your question a bit to "why doesn't light kill or otherwise make all life on Earth impossible" the answer is that the Earth is in what we call "the habitable zone".

If the Sun produced so much light or light at such high energies that it would kill you, it also would heat the planet so much that liquid water would not be possible. In this case, it's probably reasonable to argue via the "anthropic principle" that we live on a planet in the habitable zone because otherwise we wouldn't exist to ask such questions. Note of course too that we've defined the habitable zone based on our own life parameters so there is a bit of a circular definition here.

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Yes indeed -- this question reminds me of others such as "why doesn't oxygen kill us" or "why do we like the colours blue and green"? –  David Aldridge May 17 at 7:23

This question is more interesting than I thought at first. I like it. There are several different parts to an answer to this question; I'll just contribute a couple that have something in common: our bodies (and everything else, it has nothing to do with bodies) also emit photons about as fast as they absorb them.

On the macroscopic/thermal scale, we have black-body radiation. Via black-body radiation, all matter emits a continuous spectrum of radiation. The distribution of this spectrum depends primarily on the temperature of the object. This is why objects placed in a fire appear to glow red, whether they be wood or metal or rocks. Our bodies also emit radiation this way, but at our temperature, this spectrum is in the infrared range, so it isn't visible (to humans—snakes can see body heat). Since everything absorbs and emits photons this way, there's an equilibrium where we receive as much thermal energy as we lose, although that's only in an environment where everything is at equilibrium. Hot things like the sun and incandescent lights can throw this off, which is why it feels hot to go outside... or to be under a heat lamp. Anyway, don't worry about filling up on too many photons, they leave you just as fast.

On the microscopic scale, we have the hard-to-spell phenomenon of fluorescence. When a high-energy photon is absorbed by an atom, some of its energy can be reemitted as a lower-energy photon. Of course, this doesn't happen every time, and I don't know if it happens much in our bodies. It depends on the material's properties. That's where we get fluorescent lights and pigments and laundry detergents—detergent manufacturers actually include fluorescent pigments in their products so that clothes emit more visible light than they physically should by absorbing UV light and reemitting it in the visible range. Anyway, while I'm not sure if that principle in particular saves you from atomization, it's worth remembering that not every photon that hits you stays there.

So in conclusion, even though the energy that light brings to our bodies (and the earth) is substantial (imagine if there were no sun—radiation is important!), we're not going to fill up on photons to bursting. We're at equilibrium.

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+1 for the equilibirum of incoming and outgoing energy related to different phenomenon. –  Stefan Bischof May 17 at 19:38

A general photon isn't too dangerous. Most photons that we encounter have the power to heat our bodies and not much else. The heat we absorb from photons daily isn't that much, so this is rarely a problem.

Now, an interesting thing about photons is that two photons of a lower energy do not make a single photon of higher energy (frequency). So a million visible photons still will not have the same effect as a single ultraviolet photon. For example, if a certain chemical bond requires a UV photon to be broken, shooting a lot of visible photons at it won't work.

Ultraviolet photons have the capabilities of mutating DNA and other essential molecules. Too much of it, and it's likely that you'll get skin cancer. Our bodies are tuned to be able to deal with a small amount of UV radiation (which e experience daily), so it's usually not a problem. If you're planning to be out in the sun, sunblock helps keep you doubly safe.

Gamma photons pass right through skin and affect other molecules inside our bodies. Again, our bodies can deal with a small amount of gamma radiation, but if it's a powerful beam then (aside from overheating), many essential chemicals in our body will be broken down into (possibly toxic) fragments.

The magnetic field isn't that effective to keep out photons1, but the Earth's atmosphere keeps out most UV/gamma radiation. Astronauts in space need special filters in their space suits (and in their shuttles) to avoid being burned by cosmic rays.

1 Charged components of cosmic rays like high-speed muons and various hadrons matter are prevented from entering by both the magnetic field and the atmosphere (the combined effect causes the aurorae). These (and their decay products) have the capability of doing quite a bit of harm if they came unhindered to the surface.

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+1 for mag field doesn't effect photons. –  jk. May 17 at 11:55
One visible photon has a ridiculous amount of energy to harm us: about $2\times10^{-19}$ Joules. That's about 50 000 000 000 000 000 times smaller than the energy of a raindrop falling on your head (0.01 Joules). But in one second, a sunlight beam of the size of a raindrop sends $10^{17}$ photons which makes it about as powerful as a raindrop.