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[♪ INTRO]
Lasers are an incredible human invention.
They’re basically a way to hack light and turn it into this narrow beam that can do things like cut through metal
and also perform super-precise surgery on people’s eyeballs.
But even though we came up with the idea on our own,
humans didn’t actually make the first lasers.
Soon after these tools were invented in the 1950s and ’60s,
scientists discovered natural lasers beaming through space.
They’re actually pretty common, and they’ve let us explore space in ways
that wouldn’t otherwise be possible.
Like many stories in physics, this one starts with Einstein,
who came up with a theory back in 1917 called the quantum theory of radiation.
In it, he predicted that it was possible to make certain waves of light line up and travel in sync,
creating a very narrow, powerful beam.
A few decades later, in the 1950s, scientists started bringing his prediction to life in the lab.
They’d start with a specific material, like a cloud of gas or a ruby.
Left alone, the electrons in these materials would mostly hang out in their lowest energy level.
But atoms, and this is a very basic version of this,
have different levels separated by fixed amounts of energy.
And if you shine a light into them with just the right frequency, they can absorb this energy,
and their electrons will bump up to a higher energy level.
So when scientists did this in the lab,
they ended up with materials whose electrons were all in this excited state,
except they were really unstable up there.
If another photon of light came along with the right energy,
it could easily bump an excited electron back down to its original energy level.
That set off a chain reaction where the excited electrons dropped back down all at once.
And as they did, they would each release a packet of energy as light.
Scientists called their new invention a laser,
which stands for light amplification by stimulated emission of radiation.
“Stimulated emission” because you have to pump energy into a material to make it emit this light.
And it’s “amplified” because all of those waves of light that get released have the exact same amount of energy,
and they travel perfectly in sync, with all their peaks and troughs lined up.
Unlike light from a flashlight or the Sun,
which is made of all different wavelengths that are not lined up at all,
this light is extremely focused, and incredibly bright.
Lasers were a big hit, and the inventor, Charles Townes, won a Nobel Prize for his work.
But soon after lasers were invented, astronomers discovered that
they were not as new a thing as everyone thought.
In 1963, researchers at U.C. Berkeley were studying light coming from the Orion Nebula,
a bright, star-forming region in the Milky Way.
They were trying to detect radio waves emitted by hydroxyl groups,
molecules where oxygen is bonded to hydrogen,
but the signal they picked up was way stronger than they expected.
Like, it implied that the gas was several thousand trillion degrees Celsius,
which is way hotter than even stars.
This result was so mind-boggling, they thought
that they might be looking at a molecule they didn’t even know about,
and they called it mysterium.
Eventually, though, they confirmed that they had been looking at hydroxyl molecules all along,
but the hydroxyl wasn’t emitting regular radio waves, it was emitting laser light.
What they figured out was that clouds of hydroxyl molecules
were getting pumped up by radiation from the gas,
so their electrons were all excited into a higher energy level.
As electrons fell back down to a lower energy level in a chain reaction,
they were emitting synchronized waves of light.
And that was why the signal they were detecting was so bright,
not because it was some strange, unidentified mysterium,
and not because it was trillions of degrees Celsius.
Now, people had predicted that it would be possible for natural processes to create lasers,
even before it had been done in the lab,
but no one was really looking because they thought natural lasers would be really rare.
Turns out, though, they are not at all.
Since the ’60s, lasers have been found all over the universe,
in places like clouds of gas, red giant stars, even in the atmospheres of Mars and Venus.
These lasers aren’t the narrow beams of light that shoot out of a laser pointer or a barcode scanner,
in fact, they can be light-years wide,
but the phenomenon making that light works exactly like our laser technology.
It starts with a population of atoms or molecules with electrons in an excited state,
which you can get from chemical reactions or collisions between particles,
or when particles absorb specific frequencies of light.
In nature, as electrons drop out of that excited state,
the light that gets released is often at microwave frequencies,
so it’s invisible to our eyes, but some lasers are visible, too.
And now that we know they’re out there, they can all help us explore the universe.
Like, the fact that lasers are so bright and focused means that we can detect them really deep into space,
where we wouldn’t normally expect to detect things like gas emissions,
which are usually pretty faint.
For instance, in regions where stars are being born, water vapor and methanol often emit laser light,
which can help us see what’s going on in star-forming regions before that gas ignites into stars.
Researchers have even proposed that, under the right conditions,
we could detect laser light from water in the atmospheres of exoplanets,
which could help us figure out what environments might be suitable for life.
It turns out that as useful as modern lasers are,
the old-school lasers that have always been out there still have a lot to show us.
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