Humans and most other animals are only able to see light in the visible spectrum. But what about infrared, which the alien in Predator famously uses to see prey approaching just by their heat signature. Why can't we do that?
Until now, nobody was entirely sure, and the basic answer really just boiled down to that simply not being the way we evolved. But new research has revealed just why animal vision need to stay within the bounds of visible light - if we saw in the infrared spectrum, it would completely overwhelm our eyes.
The reason infrared is synonymous with heat vision is because of thermal radiation, which is the heat energy given off by the motion of charged particles. Everything in the universe gives off thermal radiation, and at various different wavelengths - for instance, the thermal radiation of an incandescent light bulb falls within the visible spectrum, which is why they work as a light source in the first place. That's also why light bulbs get hot, because they need to reach a high temperature in order to emit thermal radiation in the visible spectrum.
For objects at around room temperature, however, thermal radiation is given off pretty much exclusively in the infrared. Indeed, for the sake of convenience, the term "thermal radiation" is often just restricted to infrared light. That's why the Predator is always able to see humans no matter the visible light levels - we're constantly emitting thermal radiation in the infrared, which gives us nowhere to hide (unless we're willing to cake ourselves in mud, as Nobel Prize winning physicist Arnold Schwarzenegger demonstrated in the original movie).
So then, infrared vision seems like a fairly useful ability for any predator to have, and we humans are at the top of the food chain. Why didn't we or any other animals on Earth evolve this ability? One place to start with this problem, according to Johns Hopkins researcher King-Wai Yau, is to look at cases where humans do see into the infrared, if only by accident:
"A photon, the unit of light, is just energy, which, when captured by the pigment rhodopsin, most of the time causes the molecule to change shape, then triggering the cell to send an electrical signal to the brain to inform about light absorption. If rhodopsin can be triggered by light energy, it may also be occasionally triggered by other types of energy, such as heat, producing false alarms. These fake signals compromise our ability to see objects on a moonless night. So we tried to figure it out; namely, how the pigment is tripped by accident."
The researchers wanted to understand how much thermal energy would be required to trigger the eye's pigment molecules, even in the absence of any visible light. Scientists had long speculated that the mechanisms by which visible light and thermal radiation are detected in the eye were somehow fundamentally different, because otherwise it seemed as though heat energy should pretty much constantly be triggering false alarms.
But that's not what their experiments revealed. It turns out that, as far as the eye's pigment molecules are concerned, light energy and heat energy are pretty much the same thing. The only reason we're not overwhelmed by heat signatures being misinterpreted as light sources is that our pigment molecules stay far away from the infrared spectrum. The closer we get to infrared light - in other words, the closer to the red end of the visible spectrum - the more our eyes lose any ability to differentiate between light and heat.
The researchers speculate that animals never evolved infrared-sensing pigments because it was the only way to keep these false alarms to a bare, manageable minimum. It might be possible to see in visible light or infrared light, but not both, and there must have been a greater evolutionary advantage in seeing light than seeing heat.