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Last week we lost legendary scientist Stephen Hawking. To honor of one of the greatest legacies in cosmology, we wanted to celebrate and unpack some of his most famous findings.

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[♪ INTRO].

Last week, the world lost one of the few science celebrities of the modern era: Stephen Hawking. He was a man who gave lectures to sold-out auditoriums, and he has the honor of being the only person to ever play himself on an episode of Star Trek.

And Hawking’s fame was not unfounded. He published papers for over 50 years! He worked on a lot of problems in modern physics, from proposing what could have existed before the Big Bang, to hunting for an elusive theory of everything.

But his most notable work was on black holes. In honor of one of the greatest legacies in cosmology, we wanted to celebrate and unpack some of his most famous findings. First, in the 1960s, theoretical astrophysicists finally settled on black holes being a ‘real’ thing.

At the time, they believed that once a particle passed an event horizon, what we call the ‘surface’ of a black hole, it could never come back. These things were a one-way trip to ultimate obliteration. But in the 1970s, a handful of physicists started to question that, including Hawking.

After building on previous ideas and doing lots of math, he eventually generated a startling conclusion:. Black holes are not truly black. Surprise!

Just when you think you know what’s going on in the universe,. Stephen Hawking comes along! It turns out black holes emit a faint but steady stream of particles now called Hawking radiation.

And by proposing it, Hawking set the stage for some of the most important work currently happening in physics. This radiation happens because quantum mechanics requires an intrinsic uncertainty to the universe. The short explanation is that, even in so-called ‘empty’ space, there are actually pairs of particles constantly popping into and out of existence over very small time spans.

We call them virtual particles. As long as these particles collide and annihilate one another, the laws of physics don’t care. The energy is returned to space super duper quickly, so there’s no net change in the universe.

Now, Hawking realized that if one of those pairs happens to form at the event horizon of a black hole, the object’s extreme gravity could force them apart. That would give them enough energy to become real particles. Then, one could fall into the event horizon, while the other could fly away.

And those escaping particles are what we’d detect as Hawking radiation. As they escaped, these particles would also carry away the energy the black hole gave them. And as a result, the hole would lose a tiny bit of mass.

In other words, Hawking radiation causes black holes to evaporate. We haven’t actually detected this radiation yet, mostly because our technology just isn’t good enough. But it is required to work out the math, so we’re pretty confident it’s real.

As a bonus, this phenomenon also sits at a critical juncture in astrophysics. Because it involves both huge black holes and tiny particles, it connects the laws of general relativity and quantum physics. These are the laws that respectively govern the very large and the very small.

These principles don’t tend to play well together, and scientists have been trying to unite them for a while. And if they can do that, they could create a theory of everything, a single model that could explain how the whole universe works. So by proposing this radiation’s existence, Stephen Hawking laid the foundation for the progress that’s been made on that theory in recent years.

Of course, black hole evaporation also presents us with a bit of a physics problem, which we’ve mentioned on SciShow Space before. Separately, both general relativity and quantum mechanics predict that the present time period preserves information about the past. And that makes sense.

After all, we usually think about causes happening before effects. But no matter what sorts of particles fall into a black hole, the outgoing Hawking radiation is always the same. The black hole seems to lack all information of what you threw in there, which doesn’t make sense.

This is called the Black Hole Information Paradox. And Hawking and other physicists have attempted to resolve it over the decades. Back in the 1990s, Hawking and fellow physicist friend Kip Thorne even made a wager about the answer, which was pretty common for Hawking.

They bet Thorne’s colleague John Preskill that the math would eventually prove that the information that falls into a black hole is indeed lost forever. Unfortunately, despite lots of work, there’s no definitive answer yet. Hawking did concede in 2004, but many physicists argue the puzzle isn’t really solved.

Still, this doesn’t make the discovery of Hawking radiation any less significant, it just means there’s always more to know. Another one of Hawking’s major ideas, which he proposed in 1971, was the existence of black holes that did not form from collapsing stars. Instead, he suggested that they formed within the very first second of the universe, thanks to the density of energy fluctuating as space expanded and cooled.

Because they didn’t come from stars, these so-called primordial black holes would supposedly have a wide range of possible masses. They could be anywhere from one ten-thousandth of a gram, the smallest they could possibly be according to physics as we know it, to thousands of times the mass of the Sun. Hawking said the really tiny ones would have evaporated a long time ago, but those with a mass around 10 trillion kilograms would just be ending their lives now.

As the mass of these black holes approaches zero, they should experience runaway evaporation. That would create a massive burst of radiation equivalent to millions of one-megaton hydrogen bombs. And those explosions could, hypothetically, be detected by instruments like NASA’s Fermi Telescope.

Hawking suggested that several primordial black holes could even lie hiding in the Milky Way’s halo. And other astrophysicists have identified them as a possible candidate for the mysterious stuff we currently call dark matter. So it could turn out that Hawking was responsible for proposing what dark matter really is.

But only time will tell. As we keep building off Hawking’s work, there’s a chance he could still get his name somewhere else in the astrophysics textbooks. But no matter if that happens or not, it’s clear that the thoughts Stephen Hawking had and the work he contributed will resonate with people, and not just scientists, for generations to come.

Besides being a brilliant physicist, he was also a powerful advocate for science, and a lot more people are in love with the universe because of what he did. So thanks, Stephen Hawking. Thanks to you, we’ll remember to look up at the stars and not down at our feet.

And thank you, for watching. If you’re looking for a tangible way to celebrate your love of the universe, please check out This is a little corner store of the internet that Hank and the team set up so that we could share physical things that remind us how much we love science and care about the universe.

One of the items that I picked out is the PocketLab which lets you gather data and do experiments wherever you are. And I just got my bee heart pin, so I’m hoping to run into people out in the world with the same pin and we can talk about mason bees, and pollination, and colony collapse! Another of my favorite finds too, are the Climb Mars socks with Olympus Mons on them.

My plan is to wear them when I’m not feeling as adventurous as I want to, so if I do forget and start looking down at my feet, they’ll remind me to look up at the stars and remember Stephen Hawking. [♪ OUTRO].