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This might be the most ambitious long -shot precision experiment that I've ever seen.

Đây có lẽ là thí nghiệm chính xác, tầm xa và đầy tham vọng nhất mà tôi từng thấy.

Scientists have put one atomic clock down in Boulder, Colorado, and one up in this observatory 4 ,000 meters above sea level,

Các nhà khoa học đã đặt một chiếc đồng hồ nguyên tử ở Boulder, Colorado, và một chiếc khác tại đài quan sát này nằm ở độ cao 4.000 mét so với mực nước biển.

and they're going to try and measure the difference in time dilation according to general relativity.

và họ sẽ cố gắng đo sự khác biệt về sự giãn nở thời gian theo thuyết tương đối rộng.

Because up here, time itself should be passing faster relative to down in Boulder.

Bởi vì ở độ cao này, bản thân thời gian sẽ trôi qua nhanh hơn so với ở dưới mặt đất tại Boulder.

If you go into a science center, often they will explain gravity as the bending of space -time,

Nếu bạn đến một trung tâm khoa học, họ thường giải thích lực hấp dẫn là sự uốn cong của không-thời gian.

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 a 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 spacetime in a certain way.

And if it's there, it bends spacetime 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 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?

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 your understanding of the universe.

Which means three massive challenges.

1. Build a clock so accurate it can track time to 18 decimal places.

2. Take that clock out of the lab and into the sky.

3. 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 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 to clocks that are 400 kilometers off the Earth's surface on the International Space Station through this little dome.

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.

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 ,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 cesium -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 cesium -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 cesium 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 cesium 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 100 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 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 Iterbium.

I like to say it looks a little bit like iron pyrite, like fool's gold.

While cesium clocks operate in the microwave part of the spectrum, a terbium 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 1 part in 10 to the power of 18.

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So this clock won't gain or lose a second in more than the age of the universe.

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But when the ytterbium is solid, the atoms are so close together that neighboring atoms pull each other's electrons out of place.

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And this broadens and blurs the frequency at which the electrons make the transition.

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And that makes the natural resonant frequency pretty much impossible to find.

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So ytterbium first has to be turned into a gas.

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There's an oven where we heat this up to about 500 Celsius.

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We make a vapor of it

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and then at the end of the oven there's actually a hole where just a stream of this stuff comes shooting out.

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But now these atoms are traveling at around 300 meters per second, which causes a different problem.

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If an atom is moving towards or away from a photon as it's hit, it will absorb the light at a different frequency.

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This is called a Doppler shift, and it alters the reading of the natural resonant frequency by one part in a million.

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An error at the sixth digit, when you're aiming for 18 digits of accuracy, would end the experiment there.

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So the team has to find a way to slow down the atom's movement once they've reached the main vacuum chamber.

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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.

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So the blue laser operates first.

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It takes the hot beam of atoms that are at hundreds of Kelvin temperature

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and brings them all the way down to about a millikelvin or a thousandth of a degree above absolute zero.

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And then it trades off with the green laser, also propagating along the same axes.

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And that takes those millikelvin temperature atoms and brings them down to about a microkelvin or so.

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It might seem counterintuitive that you can cool atoms by firing lasers at them,

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but what the scientists are doing is using different electron transitions inside Ytterbium -171.

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They're now taking advantage of that Doppler shift.

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The lasers are tuned just below the frequency the atoms absorb, so an atom only absorbs light from a beam it's moving towards.

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And because light carries momentum, each photon it absorbs gives it a tiny kick slowing it down.

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The atom then emits a photon in a random direction,

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so over many cycles those random emissions average out while the absorbed photons keep slowing the atoms down.

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Overall, the lasers can cool the atoms down to just a millionth of a degree above absolute zero.

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Now, once the atoms are cold, the next challenge is keeping them in one place, and it turns out you need a laser for this too.

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We'll focus a beam into our vacuum system where the yaterbium atoms are located,

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and then we'll actually just put a mirror there to reflect it back onto itself.

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And if we could kind of zoom in enough,

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we'd see that there's this interference pattern between the two directions that leads to nodes and antinodes of laser intensity.

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The outgoing and reflected light waves overlap, and in some places they cancel each other out, and in others they reinforce each other.

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And the result is a repeating pattern called a standing wave.

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Now, the Iterbium -171 atom is neutral.

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The charges of its protons and electrons balance out but light is an electromagnetic wave.

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So the laser beam carries an electric field, and that electric field pulls those charges very slightly in opposite directions.

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This is called an induced dipole, and it gives the atom a potential energy so that now the electromagnetic force can act on it.

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The strength of the field varies along that same pattern as the standing wave.

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The electric field is weakest at the nodes of the wave and strongest at the antinodes.

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This gives the dipole a potential energy,

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causing the atom to be drawn closer and closer to the antinode until the field strength starts to level out.

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You can think of this like holding a marble on the lip of a bowl.

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Here, the marble has maximum gravitational potential energy, and when you let the marble go and it moves down the slope,

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the potential gets smaller and smaller until at the bottom of the bowl, it reaches a minimum.

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The marble can't climb back out of the bowl, so it's trapped.

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Now, in the same way, the atom is drawn by the electric force away from the

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point of maximum potential energy to where the potential reaches a minimum.

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This formation is called an optical lattice.

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Now the atoms are trapped in antinodes just 300 nanometers across

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and their movement is so limited that it no longer impacts the measurement.

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And at last the clock's frequency can be read out.

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So this software we're looking at the actual clock operation.

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So over here we measure the number of atoms in the ground state

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and then here we measure the number of atoms in the excited state.

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That balance is how the NIST scientists know the laser is still locked onto the atom's natural frequency.

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So the laser becomes a stand -in for the atoms.

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Count its oscillations, about 518 trillion of them, and that's a second.

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You can see that there's this large central chamber which has all of the windows on it.

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That's where the lasers are inserted into the system.

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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

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that you're using to treat it are on that bench and in the room across the hall.

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How do you bring this up a mountain?

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To measure the effect of gravity on time the clock needs to run at different elevations

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and that means climbing to 4 ,326 meters above sea level to the top of Mount Blue Sky.

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Oh is that the observatory right there?

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That is right up there yeah that's where we're headed.

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Well this is a This is a proper trail.

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Kind of on the final bends now.

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We're nearly there.

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So we just got to the observatory.

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I went on this little walk to video the view.

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And I'm kind of out of breath going downhill from up there.

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That looks pretty cold, Gregor.

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But this job gets you into some wild situations, whether you're at 14 ,000 feet or stranded out here in the desert.

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The question is, how do you protect yourself in such extreme environments.

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Got 60 seconds on the clock.

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Let's go.

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Scorching in the desert heat?

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Wear plenty of sunscreen, drink a lot of water, and find whatever shade you can.

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Risking it all to settle a physics debate?

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It feels makeshift, I'm not going to lie.

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Time to wear a crash helmet and trust in the engineering.

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Investigating a public health crisis?

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If it weren't so dangerous, it'd be a fun activity to do.

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I want to thank NordVPN for sponsoring this video, and now back to you, Gregor.

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This is the biggest vertical separation ever attempted using high -precision clocks.

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An experiment at altitude means contending with lightning storms, hail, and snow, even the local wildlife.

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And this short summer weather window means they only have a few weeks to do it.

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So we'll go into the tent here so we've got a couple of heaters

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that keep the temperature a little bit more stable and it also keeps out this kind of invasive moth.

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They get into the observatory there can be tons of them.

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Anyways the tent does a nice job of keeping them off of the the lasers as well.

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And is this this is everything?

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This is really the same as what we were looking at before just on a little bit smaller scale.

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So we've got down on this end is that

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that hot oven where we have the atterbium atoms

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that form the beam that travels down to the main chamber

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and then some of the lasers that introduce the different colors for atomic manipulation.

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But at the lab you had a pretty extensive optical bench building up all these lasers.

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Is that all here?

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Yep, that's all here.

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Some of it is tucked away below and on this adjacent rack.

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It's all here.

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That's incredible.

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The mountain clock has to be compared against the one running down in the lab.

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The problem is, how do you get the clocks to communicate with each other?

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Well, you could run a fiber optic cable all the way down to Boulder,

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but it turns out the animals in the region would probably chew through it, and the infrastructure just isn't there.

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So instead, the clock signal is beamed to a rooftop just outside of Denver.

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But first, it has to travel from the clock in the tent through optical fiber to this box.

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Comes out of the fiber, it gets into a small beam, and then it gets into a beam expander,

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gets to a big beam it's about 10 centimeters going out the door.

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But beams diverge and turbulence makes it wider.

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So it's about a meter by the time it gets to Denver.

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What is actually the line of side distance?

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It is about 80 kilometers.

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So you're aiming at one meter spotlight, 80 kilometers.

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So it's an 8 micron core.

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It's a very small target.

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Um, so...

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Wait, the light that you're trying to aim down is going into an 8 micron opening fiber optic cable?

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Yeah.

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That's insane.

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The final part of the journey is made through optical fiber,

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And the same path is taken in reverse by the signal from the lab clock arriving to the terminal on the mountain.

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So the two clocks can be compared directly.

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So we get all the signals going and then we come over to our computer.

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Right now, we're seeing no light over the link.

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We're measuring essentially the detector noise.

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Our detection threshold is this dashed line here.

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And so what we really want when we lock this up is we want to see this guy hanging out up here.

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If we hit this find receive button.

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And then you just see this pop up.

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Yeah.

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So that is light over the link.

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So we've locked in.

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We've locked in.

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We found it.

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With this link up and running, the experiment is finally underway.

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But there is actually another team who is already looking beyond the mountains.

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Because to test gravity to even finer scales, eventually a clock like this needs to get into space.

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Which creates a whole new problem.

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Because now, instead of hitting a stationary target, you have to hit one that is moving.

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At the University of Western Australia, one team is trying to solve exactly that.

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They've designed a system

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they hope will send optical clock signals from a moving satellite down to the ground in just a few years' time.

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And after trialling the link on pickup trucks and boats, now they're trying putting it on a helicopter.

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So why start with a helicopter?

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Certainly no one's put a clock on a helicopter before.

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And for these sort of, I think scenarios where we're going to space, people want to see those stepping stones.

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It's like a clock in a place that no one's ever put an optical clock before.

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Atomic clocks have been on board satellites for decades, but they're microwave clocks, lower in both frequency and accuracy.

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And their signals are beamed down using microwaves too.

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Microwaves have a longer wavelength than visible light, so where a cloudy sky might absorb or scatter an optical signal,

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microwaves can often still make it through.

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So it might sound like the way forward is just to use microwave to transfer any signal,

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but that longer wavelength is actually why it doesn't work for optical clocks.

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See, to compare two clocks over a link you need to know exactly when the signal arrives,

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and you work that out by measuring where each wave is in its cycle as it comes in.

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Fluctuations in the atmosphere distort the signal.

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Plus, there are all other kinds of noise, even from the equipment that detects it.

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How strong the signal is compared with that noise is called the signal -to -noise ratio.

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The more noise there is, the harder it is to work out exactly where each wave is in its cycle.

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You can only pin it down to within a small fraction of a wave.

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For a microwave, with its much longer waves, that small fraction of a wave lasts tens of thousands of times longer.

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So your measurement could be out by tens of thousands of times more.

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So to keep up with the precision of optical clocks, the signal has to stay at an optical frequency.

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And that means hitting a moving target with a laser beam.

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So this is the clock here and then we've got on top, this is our frequency transfer system and our beacon laser source.

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That beacon laser will run alongside the clock signal as a bigger target for the ground station to track.

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Everything that we have on the helicopter is completely reciprocal to what we have on the ground.

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So they also have their own beacon laser that we look for when we're scanning around with our terminal as well.

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But remember those fluctuations in the atmosphere?

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Well those have to be accounted for too.

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If you have lots of pockets of hot air rising from the road or over the city as well,

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that will take our laser beam that we're trying to send and move it around and introduce a lot of errors.

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So what we do is we have a very special mirror, we call it a tip -tilt mirror,

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and what it does is it moves fast enough that it can account for all of those deviation angles that we're seeing.

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How fast does it move?

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So it's moving at about 600 hertz, so 600 times a second is the rate that it can move.

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But yeah, so switching on the clock.

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When the technical checks are done, the experiment is ready for takeoff.

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Cocks up and running, let's go!

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So we've just left Lilani, she's going to get into helicopter now and they're going to take off really soon so we're rushing back to UWA.

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If it works the two beacons will lock onto each other

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and the signal from the clock on the helicopter can be sent down and compared with a clock on the ground.

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So this is the the roof of the physics building here at the University of Western Australia

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and then Sabrina over here is looking at our local optical terminal so that's just in here.

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You've been able to recognise a very similar one that was on the helicopter before.

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Oh yeah look that's it there we can see it already.

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Watch your heads.

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Slightly terrifying.

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Should be about it can you see anything yet?

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So you've got to go up a fair bit.

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Can we go more to the right?

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Okay that should be about it.

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Yeah got it.

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You can see that little flash of light that's in through the door here.

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That'll be the the beacon laser coming in here.

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Oh okay so that's amazing.

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But yeah I guess it's not yet well aligned enough to get the the link fully locked.

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I think the problem is with that it's a bit windy today

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and trying to have it like hovering it's actually like the least smoothest kind of flight configuration of the helicopter

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so maybe we kind of get lucky

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and we get like a little a flash occasionally

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and we might get a little bit of data but that's not really the ultimate configuration we want to get.

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How's it looking?

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I mean we're seeing some sort of stuff moving around.

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The issue is when we find ourselves spatially we still need to find ourselves spectrally.

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The lasers can be gigahertz or tens of gigahertz off pretty easily.

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How come?

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At optical there's so much extra bandwidth that's available

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but we still need to be within I don't know five megahertz of each other.

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So pointing the beams at each other isn't enough.

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The two lasers also have to be at almost exactly the same frequency.

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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.

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But these lasers have to match within a few parts in a hundred million.

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So the terminal scans back and forth, hunting for a window thousands of times narrower than the range it's searching.

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Okay, we've got 20 more minutes of fuel left.

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Does that include going home time?

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I don't know.

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And they proved the system on the ground could find its partner in the sky, even if they only got a few brief locks.

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It's not enough data yet to properly compare the two clocks, but it's a big step towards future ground satellite transfers.

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In the end it sounds easy,

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but all we have to do is make the mounts just track a bit better and just a bit faster and stuff.

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And that's one thing that we can easily do, you know, with a bit more time and a bit more engineering.

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Since then they've been up

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Mount Fuji, testing their transfer system through two back -to -back typhoons.

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I think our target is just down here.

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Here's our terminal set up for the day.

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Beautiful views all around.

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Absolutely amazing.

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Their link will help Hidetoshi Kotori's group at Riken

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and the University of Tokyo do their own test of general relativity in the spring.

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But there's one final problem, and that's because even if the link is established successfully,

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there are still 518 trillion oscillations coming through those electronics every second.

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And it turns out that there is no electronic device in the world that can read out signals this fast.

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Even some of the fastest transistors can only switch on and off about 10 billion times a second.

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And that's still 50 ,000 times too slow.

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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.

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What you'd need is some sort of reference to tie it to, at a frequency that you already know exactly.

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For example, you could start with a signal in the microwave spectrum that was easy and familiar to create,

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and then multiply its frequency up in stages until it reached the optical range.

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And then you could find the exact frequency of the light

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that you want to measure from the optical atomic clock by comparing it against the signal that you generated.

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Okay, so I have two tuning forks here

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and they're both at 440 hertz and when I hit them you can hear that they're both at the same frequency.

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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.

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You hear that sound?

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So what you're hearing is a new waveform that is pulsing at the frequency difference between these two forks.

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It's called a beat note.

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And the same thing can be done with light.

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You overlap two lasers of different frequencies and you get a beat note too.

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And the thing is, Since the frequency difference is so much smaller than the signal that we're trying to find,

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we can actually read it out using transistors and we can do a simple calculation.

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The reference frequency that you know plus the beat note would equal the exact frequency of the unknown signal.

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But there's a problem.

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The visible spectrum alone spans around 400 ,000 GHz.

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It's extremely broad and a beat note can only be measured

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if the two signals are within a few tens of gigahertz of each other.

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So the chances of your reference signal being close enough to the unknown signal that you're trying to find are really small.

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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.

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So a reference signal is down here in the microwave part of the spectrum.

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So let's call that F of R and then our unknown signal is somewhere up here in the optical.

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So F of optical.

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So what I need is some sort of ruler

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that is going to be able to quantify this difference in frequency between my reference signal

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and my unknown signal of the clock that I'm trying to find.

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In theory, you could set up a number of lasers to act as the notches of the ruler,

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but this frequency in microwave and this frequency in optical are very far apart.

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So if I want to bridge this gap, I would need a lot of lasers.

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So instead, what would be useful is

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if I could manipulate the light of just a single laser to produce all of these different frequencies at the same time,

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something that would span all the way from the microwaves that electronics could count up to the light scientists wanted to measure. In 1994,

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a German physicist called Theodor Hensch was at a trade show

389

when a demonstration of a new type of laser struck him with an idea.

390

See, for decades, he'd been coming up with different ways to try and measure optical signals.

391

But this particular titanium sapphire laser could generate incredibly short pulses of light, each packed with a vast range of frequencies.

392

Now, on a frequency graph, that light looked like the teeth of a comb.

393

This could finally be the ruler that everyone was searching for.

394

But to make the wide range of frequencies an effective ruler, each frequency needs to be related to the other frequencies,

395

otherwise you can't really predict the pattern.

396

I mean, a ruler is only useful if these notches are equally spaced.

397

So Hensch and his colleagues put the laser to the test.

398

They measured its light and found that the frequencies contained inside were perfectly evenly spaced.

399

This is an optical frequency cone. By 1999, a team in Boulder led by John Hall had engineered this comb into a ruler

400

so wide that it could measure almost any color of light.

401

So now I have a comb with, well, hundreds of thousands of teeth,

402

and some of them are going to be close enough to that unknown signal that I'm trying to find.

403

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.

404

So by observing that beat note with a photodiode, we have a formula to calculate the frequency of our unknown optical signal.

405

That's the tooth position number multiplied by the repetition rate plus the frequency of the beat note.

406

And because the repetition rate is locked to our reference signal, every number in that formula is one that electronics can measure.

407

Within a year, frequency combs were reading the first optical atomic clocks.

408

Henschen Hall, alongside Roy Glauber, were awarded the 2005

409

Nobel Prize for this work.

410

On the mountain, each clock signal is being sent over the air using pulses of a frequency cone.

411

General relativity predicts that when you bring those pulses together,

412

the clock in the lab should take ever so slightly slower compared to the clock on the mountain.

413

That is the theory.

414

But what does the counter on the mountain actually say?

415

So that is light over the link.

416

So we've locked in.

417

We're locked in.

418

We found it.

419

So the fact that this is moving, slowly ramping, that's the time output changing because the clocks are running at different rates.

420

So the time offset is slowly ramping and we can see it more or less in real time.

421

Here it goes. If Einstein is right,

422

these two clocks should differ by 2 .885 times 10 to the power of negative 13 seconds every second.

423

This number has been calculated by another group based on measurements of the gravitational field in the lab and on the mountain.

424

And while both teams are working to many more decimal places than this,

425

they're actually keeping those final digits secret from each other until they're ready to compare findings.

426

If the clocks measure a different angle on the slope, well, that's evidence that general relativity might stop working at this level.

427

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?

428

I'm going to answer that two ways.

429

The first is a very simple answer, which is yes.

430

Okay.

431

We have no evidence that says no. The second answer is that we're still analyzing the data.

432

So you're down at sort of the 13th digit

433

and a little change at sort of the a millionth of that slope

434

is what you're looking for when you're looking for new physics.

435

So it's a very small change.

436

I mean, it's incredible

437

that you have the sensitivity to look at a millionth of of this slope that is already a trillionth of a second. So, wow.

438

I mean, that's crazy.

439

And are you guys excited?

440

Are you nervous to know that the data is there?

441

You just have to crunch the numbers.

442

Are you sleeping well?

443

You got to kind of whatever it is, put that number out there and whether it agrees with the classical prediction or not,

444

that's a little bit of a nerve -wracking moment.

445

Yeah, I mean it's huge.

446

The best terrestrial test that's been done here on Earth is about at the 100 part per million level.

447

Our hope is

448

that we'll be able to do approaching the 10 part per million level with testing general relativity in a new

449

and deeper way.

450

Have you got any more of these tests planned in?

451

I think the big next step would be to try to for Link to Spice.

452

It'll be a few months before

453

Laura and Andrew's team are ready to share their final results.

454

So we've linked their groups page in the description if you want to follow their progress.

455

I wanted to say a huge thank you to them and the rest of the NIST team and to Sasha,

456

Lilani, and the team in Perth for all their help on this video.

457

And one final thing, you might have noticed that we launched a second channel called InGenius.

458

$72 billion.

459

That's how much money changed hands on prediction markets in September 2026 alone.

460

Their recent episode was about prediction markets and who actually wins on them.

461

Shockingly, it turns out it's just the 0 .1 % at the top.

462

The team scoured three years worth of past trades on Polymarket to find out what really happens on the platform.

463

And it's a really fascinating watch.

464

So if you're interested, I really recommend you check it out.

465

I want to thank you for your support.

466

And of course, as ever, thank you for watching.

467

Thank you.

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Từ vựng và ghi chú luyện nói cho bài học này

Bài luyện nói này dựa trên video “No device could prove Einstein wrong... until now”. Những từ được nhắc lại nhiều nhất trong bài: clock, frequency, laser, signal, atom. Video này có 344 câu và 6029 từ để luyện shadowing. Phần lời nói dài 34:23. Người nói nói với tốc độ tự nhiên, khoảng 175 từ mỗi phút, gần với hội thoại hằng ngày. Chỉ 80% số từ nằm trong 3.000 từ tiếng Anh thông dụng nhất, nên từ vựng khá khó.

Từ vựng quan trọng trong video

15 từ đáng học trong video, kèm phiên âm và nghĩa:

Từ	Phiên âm	Nghĩa
frequency danh từ	/ˈfɹiːkwənsi/	tính thường xuyên
laser danh từ	/ˈleɪzɚ/	la-de, kích quang
atom danh từ	/ˈætəm/	nguyên tử
microwave danh từ	/ˈmaɪkɹoʊˌweɪv/	vi ba, vi sóng
helicopter danh từ	/ˈheliːˌkɔptə(ɹ)/	máy bay lên thẳng, máy bay trực thăng
node danh từ	/noʊd/	gióng
ruler danh từ	/ˈɹulɚ/	thước kẻ, thước
comb danh từ	/koʊm/	lược
physics danh từ	/ˈfɪz.ɪks/	vật lý học
experiment danh từ	/ɪkˈspɛɹ.ə.mənt/	thí nghiệm
terminal danh từ	/ˈtɚmɪnəl/	nhà ga
absorb động từ	/æbˈsɔɹb/	hút
spectrum danh từ	/ˈspɛktɹəm/	quang phổ
electronics danh từ	/ɪˌlɛkˈtɹɑ.nɪks/	điện tử học
marble danh từ	/ˈmɑɹ.bəl/	đá hoa, cẩm thạch

Cụm động từ bạn sẽ nghe

Từ	Nghĩa
check out động từ	tính tiền, trả phòng
go out động từ	ra, chết
go up động từ	lên
take off động từ	bỏ

Những câu đáng luyện lại

Các câu ngắn, trọn ý trong video mà bạn có thể dùng lại khi giao tiếp hằng ngày:

That's what quantum mechanics tells us.
The question is, how do you protect yourself in such extreme environments?
We're measuring essentially the detector noise.
Can you see anything yet?
We're seeing some sort of stuff moving around.

Ngữ pháp trong video

Những cấu trúc người nói dùng nhiều nhất, kèm đúng cụm từ trong video:

Cấu trúc	Trong video
Câu điều kiện if + mệnh đề, will/would + động từ — điều kiện và kết quả	If it weren't so dangerous, it'd be · If it works the two beacons will lock
Thì hiện tại hoàn thành have/has + quá khứ phân từ — việc đã xảy ra nhưng còn liên quan đến hiện tại	I've ever seen · have put · has been proven
Câu bị động be + quá khứ phân từ — nhấn vào việc xảy ra, không phải người làm	has been proven · are simply ignored · is built
Mệnh đề quan hệ who / which + mệnh đề — thêm thông tin về người hoặc vật	others, which is · chamber which has · answer, which is

Phát âm cần chú ý

Người nói dùng 82 dạng rút gọn, ví dụ you're, we're, they're. Hãy nói theo dạng ngắn đúng như bạn nghe.

Âm “th”: tooth /tuθ/, sixth /sɪksθ/, millionth /ˈmɪljənθ/, wavelength /ˈweɪvˌlɛŋ(k)θ/
Âm “sh” và “zh”: prediction /pɹɪˈdɪkʃən/, measurement /ˈmɛʒ.ə.mənt/, precision /pɹɪˈsɪʒ.ən/, fraction /ˈfɹæk.ʃən/, transition /tɹænˈzɪʃ.ən/
Từ dài — đặt trọng âm cho đúng: helicopter /ˈheliːˌkɔptə(ɹ)/, accuracy /ˈæk.jɚ.ə.si/, experiment /ɪkˈspɛɹ.ə.mənt/, electronics /ɪˌlɛkˈtɹɑ.nɪks/, observatory /əbˈzɜːvət(ə)ɹi/

Những âm người Việt thường gặp khó:

Cụm phụ âm cuối — đọc đủ từng âm, đừng nuốt âm cuối: physics /ˈfɪz.ɪks/, experiment /ɪkˈspɛɹ.ə.mənt/, absorb /æbˈsɔɹb/, electronics /ɪˌlɛkˈtɹɑ.nɪks/, bend /bɛnd/
Âm /tʃ/, /dʒ/ và /ʒ/ — tiếng Việt không có, đừng đọc thành “ch” hay “gi”: digit /ˈdɪd͡ʒɪt/, chamber /ˈt͡ʃeɪ̯mbɚ/, measurement /ˈmɛʒ.ə.mənt/, precision /pɹɪˈsɪʒ.ən/, bench /ˈbɛnt͡ʃ/
Âm /æ/ — mở miệng rộng hơn âm “e”: atom /ˈætəm/, gravity /ˈɡɹævɪti/, accuracy /ˈæk.jɚ.ə.si/, absorb /æbˈsɔɹb/, fraction /ˈfɹæk.ʃən/

Cách luyện với video này

Nghe hết video một lần, chưa cần nói, và ghi lại những từ bạn chưa biết.
Bắt đầu ở tốc độ 0,75×, nói đuổi từng câu, rồi quay lại tốc độ bình thường khi đã quen.
Ghi âm giọng mình rồi so với bản gốc, chú ý các từ như frequency, laser, atom.

Phương Pháp Shadowing Là Gì?

Shadowing là kỹ thuật học ngôn ngữ có cơ sở khoa học, ban đầu được phát triển cho chương trình đào tạo phiên dịch viên chuyên nghiệp và được phổ biến rộng rãi bởi nhà đa ngôn ngữ học Dr. Alexander Arguelles. Nguyên lý cốt lõi đơn giản nhưng cực kỳ hiệu quả: bạn nghe tiếng Anh của người bản xứ và lặp lại to ngay lập tức — như một "cái bóng" (shadow) đuổi theo người nói với độ trễ chỉ 1–2 giây. Khác với luyện ngữ pháp hay học từ vựng bị động, Shadowing buộc não bộ và cơ miệng phải đồng thời xử lý và tái tạo ngôn ngữ thực tế. Các nghiên cứu khoa học xác nhận phương pháp này cải thiện đáng kể phát âm, ngữ điệu, nhịp điệu, nối âm, kỹ năng nghe và độ lưu loát khi nói — đặc biệt hiệu quả cho người luyện IELTS Speaking và muốn giao tiếp tiếng Anh tự nhiên như người bản ngữ.

Phương pháp shadowing: đọc hướng dẫn từng bước đầy đủ →

Luyện nói tiếng Anh bằng Shadowing qua video: No device could prove Einstein wrong... until now

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Từ vựng và ghi chú luyện nói cho bài học này

Từ vựng quan trọng trong video

Cụm động từ bạn sẽ nghe

Những câu đáng luyện lại

Ngữ pháp trong video

Phát âm cần chú ý

Cách luyện với video này

Phương Pháp Shadowing Là Gì?