No device could prove Einstein wrong... until now
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Combines an epic experimental setup on a mountain with a mind-bending correction to a common science museum myth (gravity is mostly time stretching, not space curving), ending on a concrete, shareable number.
Transcript
This might be the most ambitious long shot precision experiment that I've ever seen. Scientists have put one atomic clock down in Boulder, Colorado and one up in this observatory 4,000 m above sea level and they're going to try and measure the difference in time dilation according to general relativity. Because up here, time itself should be passing faster relative to down in Boulder. If you go into a science center, often they will explain gravity as the bending of space-time and what they will have is a little funnel here the particle goes around the funnel and that's meant to give you an impression of why the Earth goes around the Sun. But that's not actually what happens. It's not as much that space is curving, it's that time is stretching. So, the dominant effect of gravity that we experience is the slowing down of time as we get closer to a massive object. This is one of the predictions of Einstein's general theory of relativity and it actually predicts how much faster the mountain clock should be ticking. Put one clock in Boulder and another 2.7 km higher and the mountain clock should gain about 25 billionths of a second every day.
Why it works
Delivers the ultimate payoff of the entire video: seeing time itself run at two different speeds in real time on a graph, plus the blinded-study detail and the final verdict on whether Einstein was right.
Transcript
On the mountain, each clock signal is being sent over the air using pulses of a frequency comb. General relativity predicts that when you bring those pulses together, the clock in the lab should tick ever so slightly slower compared to the clock on the mountain. That is the theory, but what is the counter on the mountain actually say? So, that is light over the link. So, we locked in. We found in. We found it. So, the fact that this is moving, slowly ramping, Yeah. that's the time output changing because the clocks are running at different rates. That's time offset is slowly ramping and we can see it more or less in real time. Here it goes. If Einstein is right, these two clocks should differ by 2.885 * 10 ^ -13 seconds every second. This number has been calculated by another group based on measurements of the gravitational field in the lab and on the mountain. And while both teams are working to many more decimal places than this, they're actually keeping those final digits secret from each other until they're ready to compare findings. If the clocks measure a different angle on the slope, well, that's evidence that general relativity might stop working at this level. So, a few weeks later, we hopped on a call with the team to check out what the data is saying. Is Einstein still right? I'm going to answer that two ways. The first is a very simple answer, which is yes. Okay. We have no evidence that says no. The second answer is that we're still analyzing the data. You okay? So, you're down at sort of the 13th digit and a little change at sort of the a millionth of that slope is what you're looking for when you're looking for new physics.
Why it works
The host's genuine disbelief ('Wait, the light that you're trying to aim down is going into an 8-micron opening fiber optic cable? That's insane') paired with the humorous detail that wildlife would chew through a physical cable makes this peak infotainment.
Transcript
The mountain clock has to be compared against the one running down in the lab. The problem is how do you get the clocks to communicate with each other? Well, you could run a fiber optic cable all the way down to Boulder, but it turns out the animals in the region would probably chew through it, and the infrastructure just isn't there. So, instead, the clock signal is beamed to a rooftop just outside of Denver. But first, it has to travel from the clock in the tent through optical fiber to this box. Comes out of the fiber, it gets into a small beam, and then it gets into a beam expander, gets to a big beam. It's about 10 cm going out the door. But beams diverge, and turbulence makes it wider. So, it's about a meter by the time it gets to Denver. What what is actually the line of sight distance? It is about 80 km. So, you're aiming at 1-m spotlight 80 km? Yeah. So, it's a 8-micron core. It's a very small target. Um so Wait, the light that you're trying to aim down is going into an 8-micron opening fiber optic cable? Yeah. That's insane. The final part of the journey is made through optical fiber, and the same path is taken in reverse by the signal from the lab clock arriving to the terminal on the mountain. So, the two clocks can be compared directly. So, we get all the signals going, and then we come over to our computer. Right now, we're we're seeing no light over the link. We're measuring the essentially the detector noise. Our detection threshold is is this dash line here. And so, what we really want when we lock this up, so we want to see this guy hanging out up here. If we hit this fine receive button, and then you just see this pop up. Yeah? So, that is light over the link. So, we're locked in. We found it. We're locked in. We found it.
Why it works
Explains one of the biggest unsolved problems in modern physics—quantum gravity—in under 50 seconds using a crystal-clear thought experiment anyone can visualize.
Transcript
So, why are we still testing it? Because we know that at some quantum scale, this theory of gravity is going to stop working. The way we know there is some contradiction is that we know that particles can be in the superposition of two places here and there. That's what quantum mechanics tells us and what Einstein's theory of general relativity tells us is that if a particle is here, it bends space-time in a certain way and if it's there, it bends space-time in a certain way. So, what happens to space-time if a particle with mass is in two places at once? We don't have an answer to that question. Einstein's theory of general relativity is silent on the question of what happens to space-time when a particle is in superposition. And that leaves us with one of the biggest unanswered questions in physics. Does gravity itself obey the rules of quantum mechanics?
Why it works
The paradox of using lasers (normally associated with heat) to freeze atoms to a millionth of a degree above absolute zero is a proven viral physics concept.
Transcript
We have to cool them down in three dimensions in free space, and so that requires having these laser beams in three dimensions as well. So, the blue laser operates first. It takes the hot beam of atoms that are at hundreds of Kelvin temperature and brings them all the way down to about a milliKelvin or a thousandth of a degree above absolute zero. And then it trades off with the green laser also propagating along the same axis and that takes those milliKelvin temperature atoms and brings them down to about a microKelvin or so. It might seem counterintuitive that you can cool atoms by firing lasers at them. But what the scientists are doing is using different electron transitions inside ytterbium 171. They're now taking advantage of that Doppler shift. The lasers are tuned just below the frequency the atoms absorb. So an atom only absorbs light from a beam it's moving towards. And because light carries momentum, each photon it absorbs gives it a tiny kick slowing it down. The atom then emits a photon in a random direction. So over many cycles, those random emissions average out while the absorbed photons keep slowing the atoms down. Overall, the lasers can cool the atoms down to just a millionth of a degree above absolute zero.
Why it works
Features a classic viral trivia detail—the brass altitude survey markers on the floor of every clock room because gravity changes time even across a few feet of elevation.
Transcript
This is the National Institute of Standards and Technology in Boulder, Colorado. NIST has an atomic clock that is so accurate that if it started counting when the universe began, it would not have lost or gained a second up until now. NIST has been home to pretty much every kind of atomic clock ever made from tiny chip-scale clocks and the most accurate clock on Earth to the first experimental atomic clock from way back in 1949. And it's that accuracy that makes atomic clocks powerful tools for testing fundamental physics. This little survey marker is actually in every room that has an atomic clock at NIST and it's the official telling of which altitude you're at above sea level so that you can take it into account for when you're aligning the atomic clocks. They even connect the clocks that are 400 km off the Earth's surface on the International Space Station through this little dome.
Why it works
Viewers love learning the hidden physical standard behind everyday things like how their phone knows the time, anchored by an absurdly specific number.
Transcript
So, this is NIST-F3. It's one of the cesium fountains we operate here at the National Institute of Standards and Technology. We use these machines in order to experimentally realize the definition of the SI second. What is the exact frequency? Count to 9 billion 192 million 631,770. And after you've counted that high, you've counted exactly 1 second. This clock feeds into a network that helps determine coordinated universal time, or UTC. It's the standard that keeps your phone, your computer, and much of the world synchronized. Now, since 1967, the time it takes for this exact number of cesium 133 oscillations to get past your detector has been the official definition of 1 second.
Why it works
Debunks the widespread misconception that 'atomic' clocks are radioactive and uses a clean progression from pendulum to quartz to electron quantum jumps.
Transcript
Now, you might think that an atomic clock runs on radioactivity because after all, we use radioactive decay and the half-life of elements to date things that happened billions of years ago and we can do that with an error of less than 1%. But that is not how an atomic clock works. For keeping time, we need something that will oscillate back and forth. For a grandfather clock, it's a pendulum, and for a wristwatch, it's a piece of quartz that is vibrating under an electric current. But, an atomic clock gets its frequency from a fundamental property of the atom itself. Electrons can only occupy specific energy levels, and they can jump from one level to another by absorbing a photon of light. But, this only happens when a photon's energy matches the energy difference between the two levels, and the energy of a photon depends on its frequency. So, you can use a laser, tune its frequency, and that will change the energy of its photons. Now, most frequencies are simply ignored, but when the frequency of such a laser is just right, you get this transition, and the electron makes the jump. And that tells us the atom's natural resonant frequency. Now, we can tune the laser to match that frequency exactly, and it's the oscillations of this laser light that act as the atomic clock's pendulum.
Why it works
Captures real field-test tension, a great car radio analogy for spectral alignment, and a funny dry-humor exchange about having 20 minutes of helicopter fuel left.
Transcript
Oh yeah, look at that. That's it there. We can see it already. Now, watch your head. Slightly terrifying. Should be about it. Can you see anything yet? So, you've got to go up a a fair bit. Go go more to the right. Okay, that should be about it. Yeah, all right. You can see that little flash of light that's in through that door here. That'll be the the beacon laser coming in here. Oh, okay. So, that's amazing. But yeah, I guess it's not yet well aligned enough to get the uh the link fully locked. I think the problem is with uh it's a bit windy today and trying to have it like hovering, it's actually like the least smoothest kind of flight configuration of the helicopter. So, maybe we kind of get lucky and we get like a little a flash occasionally and we might get a little little bit of data, but that's not really any the ultimate configuration we want to get. Yeah, how's it looking? Um I mean, this thing is some sort of stuff moving around. The issue is um when we find ourselves spatially, we still need to find ourselves spectrally. Oh, yeah. uh the the frequency is can be gigahertz or or tens of gigahertz off pretty easily. How come? Uh at optical, there's so much extra extra uh bandwidth that's available, but we still need to be within I don't know. 5 megahertz of each other. So, pointing the beams each other isn't enough. The two lasers also have to be almost exactly the same frequency. To put that in perspective, a car radio only has to tune to within a fraction of a percent of a station to pick it up. But, these lasers have to match within a few parts in a hundred million. So, the terminal scans back and forth hunting for a window thousands of times narrower than the range it's searching. Yeah, we've got 20 more minutes of fuel left. Does that include going home time? Yeah, I don't know. And they proved the system on the ground could find its partner in the sky even if they only got a few brief locks. It's not enough data yet to properly compare the two clocks, but it's a big step towards future ground-to-satellite transfers.
Why it works
Showcases the jump from microwave to optical clocks with staggering numbers (56,000x higher frequency, 518 trillion ticks per second) and a clean payoff.
Transcript
If you want a more accurate clock, you can start with finding a higher resonant frequency. Because the more oscillations you can pack into a second, the more finely you can define it. And it turns out that elements have different natural frequencies. So, you just have to find one that fits. And the perfect candidate for the job is, it turns out, I like to say it looks a little bit like like iron pyrite, like fool's gold. While cesium clocks operate in the microwave part of the spectrum, ytterbium resonates in the optical band with a frequency 56,000 times higher. This equates to 518 trillion oscillations every second, and that allows for accuracies of one part in 10 to the power of 18. So, this clock won't gain or lose a second in more than the age of the universe.
Why it works
Short, punchy engineering spotlight showing how pockets of hot air from city roads ruin laser precision and how a 600Hz mirror cancels it out in real time.
Transcript
But, remember those fluctuations in the atmosphere? Well, those have to be accounted for, too. If you have lots of pockets of hot air rising from the road or over the city as well, that will take our laser beam that we're trying to send and move it around and introduce a lot of errors. So, what we do is we have a very special mirror, we call it a tip-tilt mirror, and what it does is it moves fast enough that it can account for all of those deviation angles that we're seeing. How fast does it move? So, it's moving at about 600 Hz, so 600 times a second is the rate that it can move. Um but yeah, so switching on the clock.
Why it works
Answers a smart engineering question ('Why not just use microwaves like GPS satellites do?') while showcasing a world-first helicopter experiment.
Transcript
At the University of Western Australia, one team is trying to solve exactly that. They've designed a system they hope will send optical clock signals from a moving satellite down to the ground in just a few years' time. And after trialing the link on pickup trucks and boats, now they're trying putting it on a helicopter. So, why start with a helicopter? Certainly no one's put a clock on a helicopter before. And for these sort of, I think scenarios where we're going to space, you know, people want to see that those stepping stones. But it's like a clock in a place that no one's ever put an optical clock before. Atomic clocks have been on board satellites for decades, but they're microwave clocks, lower in both frequency and accuracy. And their signals are beamed down using microwaves, too. Microwaves have a longer wavelength than visible light. So, where a cloudy sky might absorb or scatter an optical signal, microwaves can often still make it through. So, it might sound like the way forward is just to use microwave to transfer any signal. But that longer wavelength is actually why it doesn't work for optical clocks. See, to compare two clocks over a link, you need to know exactly when the signal arrives. And you work that out by measuring where each wave is in its cycle as it comes in. But fluctuations in the atmosphere distort the signal. Plus, there are all other kinds of noise, even from the equipment that detects it. How strong the signal is compared with that noise is called the signal-to-noise ratio. The more noise there is, the harder it is to work out exactly where each wave is in its cycle. You can only pin it down to within a small fraction of a wave. For a microwave, with its much longer waves, that small fraction of a wave lasts tens of thousands of times longer. So, your measurement could be out by tens of thousands of times more. So, to keep up with the precision of optical clocks, the signal has to stay at an optical frequency. And that means hitting a moving target with a laser beam.
Why it works
Opens with a massive '$72 billion dollars' hook and drops a highly shareable data point about Polymarket (only 0.1% of traders win) that sparks immediate debate in finance/crypto/tech circles.
Transcript
72 billion dollars. That's how much money changed hands on prediction markets in September 2026 alone. Their recent episode was about prediction markets and who actually wins on them. Shockingly, it turns out it's just the 0.1% at the top. The team scoured three years worth of past trades on Polymarket to find out what really happens on the platform and it's a really fascinating watch. So, if you're interested, I really recommend you check it out.
