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- 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 meters 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 where 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 kilometers higher, and the mountain clock should gain about 25 billionths of a second every day.
This effect of general relativity and time dilation has been proven in experiments time and time again. 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 spacetime 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 spacetime 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? So teams around the world are trying to pin down exactly where gravity might stop following Einstein's prediction.
And they want to find out if there's a correction term needed in the equations that might completely change our understanding of the universe. Which means three massive challenges. One, build a clock so accurate it can track time to 18 decimal places. Two, take that clock out of the lab and into the sky. And three, find a way to read a signal so fast that electronics can't even process it. 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 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 to clocks that are 400 kilometers off the earth's surface on the International Space Station through this little dome here.
Now, you might think that an atomic clock runs on radioactivity because after all, we use radioactive decay in 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. - So this is NIST F3. It's one of the Caesium 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 9,192,631,770. And after you've counted that high, you've counted exactly one 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 Caesium 133 oscillations to get past your detector has been the official definition of one second. Keeping clocks synchronized with each other is a centuries old problem, and that's because every oscillator has inconsistencies in the way that they're made. But for atoms, it doesn't matter where you get the atom from.
The natural resonant frequency is the same for all atoms of the same element across the universe. And it's the arrangement of these electrons in particular isotopes that make a very good atomic clock. So these atoms give us ticks that are incredibly consistent. How reliably one tick matches the next is the clock's stability. Some elements are more stable than others, which is why Caesium 133 is really popular. But there's another challenge. It's one thing for every tick to be consistent.
It's another to know that you're measuring the atom's true ticking rate. In other words, how close is our measurement to the true atomic frequency? That is accuracy. And unlike the atom itself, accuracy comes down to how well the clock is built. Now, the best caesium atomic clocks are over 50 billion times more accurate than the quartz crystal in your wristwatch. The overall performance of a clock can be described by how many seconds it will gain or lose in a given time expressed as an order of magnitude. So what order of magnitude is the caesium clock that we're now using to standardize time?
- Those realize fractionally about a part in 10 to the 16 of accuracy. So over the course of maybe tens of millions or a hundred million years, it would only lose of order a second. - But this is still not good enough to find where gravity might stray from the predictions of general relativity. 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 finally 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 ytterbium.
- I like to say it looks a little bit like, like iron pyrite, like fool's gold. - While caesium 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. But when the ytterbium is solid, the atoms are so close together that neighboring atoms pull each other's electrons out of place, and this broadens and blurs the frequency at which the electrons make the transition, and that makes the natural resonant frequency pretty much impossible to find.
So ytterbium first has to be turned into a gas. - There's an oven where we heat this up - Yeah. To about 500 Celsius. We make a vapor of it, and then at the end of the oven, there's actually a hole where just a, a stream of this stuff comes shooting out. - But now these atoms are traveling at around 300 meters per second, which causes a different problem. If an atom is moving towards or away from a photon as it's hit, it will absorb the light at a different frequency. This is called a Doppler shift, and it alters the reading of the natural resonant frequency by one part in a millionth. An error at the sixth digit when you're aiming for 18 digits of accuracy would end the experiment there. So the team has to find a way
to slow down the atom's movement once they've reached the main vacuum chamber. - You have to cool them down in three dimensions in free space, and so that requires having these laser beams in three-dimension 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 axes, 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. Now, once the atoms are cooled, the next challenge is keeping them in one place, and it turns out you need a laser for this too.
- We'll focus a beam into our vacuum system where the ytterbium atoms are located, and then we'll actually just put a mirror there to reflect it back onto itself. And if we could kind of zoom in enough, we'd see that there's this interference pattern between the two directions that leads to nodes and anti-nodes of laser intensity. - The outgoing and reflected light waves overlap, and in some places they cancel each other out and in others they reinforce each other. And the result is a repeating pattern called a standing wave. Now, the ytterbium 171 atom is neutral. The charges of its protons and electrons balance out, but light is an electromagnetic wave. So the laser beam carries an electric field,
and that electric field pulls those charges very slightly in opposite directions. This is called an induced dipole, and it gives the atom a potential energy so that now the electromagnetic force can act on it. The strength of the field varies along that same pattern as the standing wave. The electric field is weakest at the nodes of the wave and strongest at the antinodes. This gives the dipole a potential energy, causing the atom to be drawn closer and closer to the antinode until the field strength starts to level out.
You can think of this like holding a marble on the lip of a bowl. Here, the marble has maximum gravitational potential energy, and when you let the marble go and it moves down the slope, the potential gets smaller and smaller until at the bottom of the bowl it reaches a minimum. The marble can't climb back out of the bowl, so it's trapped. Now, in the same way, the atom is drawn by the electric force away from the point of maximum potential energy to where the potential reaches a minimum. This formation is called an optical lattice. Now, the atoms are trapped in antinodes just 300 nanometers across, and their movement is so limited that it no longer impacts the measurement. And at last, the clock's frequency can be read out.
- So at this software, we are looking at the actual clock operation. So here we measure the number of atoms in the ground state, and then here we measure the number of atoms in the excited state. - That balance is how the NIST scientists know the laser is still locked onto the atom's natural frequency. So the laser becomes a stand-in for the atoms. Count its oscillations, about 518 trillion of them, and that's a second. - You can see that there's this large central chamber which has all of the windows on it.
Yeah. That's where the lasers are inserted into the system. - I mean, one question that is in my mind as you're explaining this is if this is the setup here and, you know, the lasers that you're using to treat it are on that bench and in the room across the hall, how do you bring this up a mountain? To measure the effect of gravity on time, the clock needs to run at different elevations, and that means climbing to 4,326 meters above sea level to the top of Mount Blue Sky. Oh, is that the observatory right there? - That is right up there. Yeah, that's where we're headed. - Oh, this is, this is a proper trail.
Kind of on the final bends now. - We're, we're nearly there. - So we just got to the observatory. I went on this little walk to video the view and I'm kind of out of breath going downhill from up there. - That looks pretty cold, Gregor. But this job gets you into some wild situations, whether you're at 14,000 feet or stranded out here in the desert. The question is, how do you protect yourself in such extreme environments? Got 60 seconds on the clock. Let's go. Scorching in the desert heat, wear plenty of sunscreen, drink a lot of water and find whatever shade you can.
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I wanted to thank NordVPN for sponsoring this video, and now back to you, Gregor. - This is the biggest vertical separation ever attempted using high precision clocks. An experiment at altitude means contending with lightning storms, hail and snow, even the local wildlife. And this short summer weather window means they only have a few weeks to do it. - So we'll go into the tent here. So we've got a couple of heaters that keep the temperature a little bit more stable, and it also keeps out this kind of invasive moth.
They get into the observatory. There can be tons of them. Anyways, the tent does a nice job of keeping them off of the, the lasers as well. - And is this, this is everything? - This is really the same as what we were looking at before, just on a little bit smaller scale. So we've got down on this end is that, that hot oven where we have the ytterbium atoms that form the beam that travels down to the main chamber, and then some of the lasers that introduce the, the different colors for atomic manipulation.
- But at the lab, you had a pretty extensive optical bench building up all these lasers. Is that all here? - Yep. That's all here. Some of it is tucked away below and on this adjacent rack. It's all here. - That's incredible. 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 centimeters 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 kilometers. - So you're aiming at one meter spotlight, 80 kilometers.
- Yeah. So it's an eight micron core. It's a very small target. So - - Wait, the light that you're trying to aim down is going into an eight 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, and we're seeing no light over the link.
We're measuring essentially the detector noise. Our detection threshold is, is this dash line here. And so what we really want when we lock this up is we want to see this guy hanging out up here. If we hit this find receive button, and then you just see this pop up. - Yeah. - So that is light over the link. - So we locked in, we found. - We're locked in. We found it. - With this link up and running, the experiment is finally underway, but there's actually another team who is already looking beyond the mountains because to test gravity to even finer scales, eventually a clock like this needs to get into space,
which creates a whole new problem because now instead of hitting a stationary target, you have to hit one that is moving. - 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 where 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. - So this is the clock here, and then we've got on top, this is our frequency transfer system and our beacon laser - Source. That beacon laser will run alongside the clock signal as a bigger target for the ground station to track.
- Everything that we have on the helicopter is completely reciprocal to what we have on the ground. So they also have their own beacon laser that we look for when we're scanning around with our terminal as well. - But remember those fluctuations in the atmosphere? Well, those have to be accounted for too. - And 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 hertz, so 600 times a second is the rate that it can move. But yeah, so switching on the clock. - When the technical checks are done, the experiment is ready for takeoff. - Clock's up and running. Let's go. - So we've just left Leilani. She's going to get into the helicopter now and they're going to take off really soon. So we're rushing back to UWA. If it works, the two beacons will lock onto each other and the signal from the clock on the helicopter can be sent
down and compared with a clock on the ground. - So this is the roof of the physics building here at the University of Western Australia, and then Sabrina over here is looking at our local optical terminal. So that's just in here. You've been able to recognize a very similar one that was on the helicopter before. Oh yeah, look, 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're going up a fair bit. Go more to the right.
Okay, that should be about it. Yeah, got it. You can see that little flash of light that's in for the door here. That'd be 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 link fully locked. I think the problem is with that, 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 flash occasionally and we might get a little bit of data, but that's not really the ultimate configuration we want to get.
How's it looking? We're seeing some sort of stuff moving around. - The issue is when we find ourselves spatially, we still need to find ourselves spectrally. Oh yeah. The - Frequency - Changes. The lasers can be gigahertz or tens of gigahertz off pretty easily. - How come? - At optical, there's so much extra, extra bandwidth that's available, but we still need to be within, I don't know, five megahertz of each other. - So pointing the beams at each other isn't enough. The two lasers also have to be at 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? 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 satellite transfers.
- In the end, it sounds easy, but all we have to do is make the mounts just track a bit better and just a bit faster and stuff. And that's one thing that we can easily do, you know, with a bit more time and a bit more engineering. - Since then, they've been up Mount Fuji, testing their transfer system through two back-to-back typhoons. - I think our target is just down here. Here's our terminal set up for the day. Beautiful views all around. Absolutely amazing. - Their link will help Hidatoshi Katori's group at Recon and the University of Tokyo do their own test of general relativity in the spring.
- But there's one final problem, and that's because even if the link is established successfully, there are still 518 trillion oscillations coming through those electronics every second. And it turns out that there is no electronic device in the world that can read out signals this fast. Even some of the fastest transistors can only switch on and off about 10 billion times a second. And that's still 50,000 times too slow. So for decades, scientists had a strange problem because they could build an optical atomic clock, they just had no way to accurately measure its signal. What you'd need is some sort of reference to tie it to at a frequency that you already know exactly. For example, you could start
with a signal in the microwave spectrum that was easy and familiar to create and then multiply its frequency up in stages until it reached the optical range, and then you could find the exact frequency of the light that you want to measure from the optical atomic clock by comparing it against the signal that you generated. Okay, so I have two tuning forks here and they're both at 440 hertz. And when I hit them, you can hear that they're both at the same frequency. Now I've put a small weight on one of the forks and that is going to change the frequency slightly, so listen to what happens now.
You hear that? Sound. So what you're hearing is a new waveform that is pulsing at the frequency difference between these two forks. It's called a beat node, and the same thing can be done with light. You overlap two lasers of different frequencies and you get a beat node too. And the thing is, since the frequency difference is so much smaller than the signal that we're trying to find, we can actually read it out using transistors and we can do a simple calculation. The reference frequency that you know plus the beat note would equal the exact frequency of the unknown signal. But there's a problem. The visible spectrum alone spans
around 400,000 gigahertz. It's extremely broad, and a beat node can only be measured if the two signals are within a few tens of gigahertz of each other. So the chances of your reference signal being close enough to the unknown signal that you're trying to find are really small. What scientists needed wasn't one frequency, but many spread out so that the unknown signal would always sit close to at least one of them. So a reference signal is down here in the microwave part of the spectrum, so let's call that F of R, and then our unknown signal is somewhere up here in the optical, so F of optical. So what I need is some sort of ruler that is going to be able to quantify this difference in frequency between my reference signal
and my unknown signal of the clock that I'm trying to find. In theory, you could set up a number of lasers to act as the notches of the ruler, but this frequency in microwave and this frequency in optical are very far apart. So if I want to bridge this gap, I would need, well, a lot of lasers. So instead what would be useful is if I could manipulate the light of just a single laser to produce all of these different frequencies at the same time, something that would span all the way from the microwaves that electronics could count up to the light scientists wanted to measure. In 1994, a German physicist called Theodor Hänsch was, had a trade show when a demonstration of a new type of laser struck him with an idea.
See, for decades, he'd been coming up with different ways to try and measure optical signals, but this particular titanium sapphire laser could generate incredibly short pulses of light, each packed with a vast range of frequencies. Now on a frequency graph, that light looked like the teeth of a comb. This could finally be the ruler that everyone was searching for. But to make the wide range of frequencies an effective ruler, each frequency needs to be related to the other frequencies, otherwise you can't really predict the pattern. I mean, a ruler is only useful if these notches are equally spaced. So Hench and his colleagues put the laser to the test.
They measured its light and found that the frequencies contained inside were perfectly evenly spaced. This is an optical frequency comb. By 1999, a team in Boulder led by John Hall had engineered this comb into a ruler so wide that it could measure almost any color of light. So now I have a comb with, well, hundreds of thousands of teeth, and some of them are going to be close enough to that unknown signal that I'm trying to find. The comb is emitting its own frequency. So there's going to be a beat note that rings between this comb tooth and the unknown signal. So by observing that beat note with a photo diode,
we have a formula to calculate the frequency of our unknown optical signal. That's the tooth position number multiplied by the repetition rate plus the frequency of the beat note. And because the repetition rate is locked to our reference signal, every number in that formula is one that electronics can measure. Within a year, frequency combs were reading the first optical atomic clocks. Hänsch and Hall alongside Roy Glauber were awarded the 2005 Nobel Prize for this work. On the mountain, each clock signal is being sent over the air using pulses of a frequency cone.
General relativity predicts that when you bring those pulses together, the clock in the lab should take ever so slightly slower compared to the clock on the mountain. That is the theory. But what does the counter on the mountain actually say? - So that is light over the link. - So we locked in we found. - We locked 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. So the 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 times 10 to the power of negative 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. 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. So it's a very small change. - I mean, it's incredible that you have the sensitivity to look at a millionth of, of this slope that is already a trillionth of a second. So wow. I mean that, that's crazy. And are you guys excited? Are you nervous to, to know that the data is there, you just have to crunch the numbers? Are you sleeping well?
- You got to kind of, whatever it is, put that number out there. And whether it agrees with the classical prediction or not, that's a little bit of a nerve-wracking moment. - Yeah, I mean it's huge. - The best terrestrial test that's been done here on Earth is about at the 100 part per million level. Our hope is that we'll be able to do approaching the 10 part per million level with testing general relativity in a, in a new and deeper way.
- Have you got any more of these tests planned in? - I think the big next step would be to try for a link to space. - It'll be a few months before Laura and Andrew's team are ready to share their final results. So we've linked their group's page in the description if you want to follow their progress. I wanted to say a huge thank you to them and the rest of the NIST team and to Sasha Leilani and the team in Perth for all their help in this video. And one final thing, you might have noticed that we launched a second channel called InGenius.
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. I want to thank you for your support and of course, as ever, thank you for watching.
