"We are all in orbit around the center of the Milky Way galaxy. How big is this collection of stars? Somewhere between 200 and 400 billion suns in the Milky Way galaxy, about 100,000 light years across."

How do we measure the universe, and are we doing it wrong? Physicist Brian Cox uncovers the hidden assumptions behind our units of measurement, showing how human perspective distorts our understanding of space, time, and scale. Cox explores the fundamental constants—like the speed of light, Planck’s constant, and gravity—that underpin the very fabric of our universe.

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Biologically-based measurements
3 fundamental quantities
The speed of light
Strength of gravity
Planck’s constant
Observing a Planck length
Distance to the planets
Distance to other galaxies

The below is a true verbatim transcript taken directly from the video. It captures the conversation exactly as it happened.

When we think about the size of things, we tend to think of the size of things with reference to ourselves. So, you know, the foot, or the meter, or the inch or the centimeter – what are those things? Ultimately, historically, they're based on properties of the human body, so they're based on biology. And that's what we did historically 'cause why would you do anything else?

Is that fundamental? Well, the answer is no. It doesn't tell you anything profound or deep about the deep structure of the universe. But of course, the history of physics tells us – as we go into the 17th century, 18th century, 19th century, 20th century – we then begin to understand that there are things that are much bigger than us and much smaller than us. We are looking, I suppose, for units of measurement that you could imagine if we met some aliens from some different civilization. They might not even have arms, right? Or feet. But they might be very different in size and scale from us. So what would the common language be? Is there some unit of measurement that we could all agree on?

I'm Brian Cox. My full title is professor of particle physics at the University of Manchester, Royal Society professor for public engagement in science, and visiting scholar at the Creek Institute. Or you could just call me Brian.

What are the fundamental quantities, as far as we can tell, that really tell us something about the structure of nature?

So one would be the speed of light. Everything that is massless travels at the speed of light, at this speed, whatever it is. If you have any mass at all, you cannot travel, you cannot accelerate to this speed.

Another one would be the strength of the gravitational force. So what is the force between two objects of a particular mass? Or in Einstein's theory, how does a particular amount of matter or energy distort the fabric of the universe? The number that tells you about that is Newton's gravitational constant, which was first measured back in the 1780s, 1790s.

And then there's Planck's constant itself. In 1900, Max Planck made a revolutionary proposal. You could say, for example, that there's a fundamental limit on how accurately we can know the position of a particle and the momentum of a particle. You can't know them both with absolute precision. There's a fundamental limit, and it's a roundabout Planck's constant.

Planck first introduced it in the context of the frequency or the wavelength of light emitted from hot objects. Photons, what's the energy of a photon? A packet of light: It's Planck's constant multiplied by the frequency.

So those three things, speed of light, strength of gravity, and Planck's constant, allow you to define some distances, a particular distance called the Planck length. And it's a tiny length. It's about 10 to the minus 35 meters. Point naught, naught, naught, naught, naught, naught, with 35 naughts, one of a meter. So we have this fundamental, it appears, length scale in the universe. Unimaginably small. How could you picture that?

If you take a proton and expand it to the size of our solar system – so imagine that the nucleus of a hydrogen atom, and you imagine expanding that to the size of our solar system out, to the orbit of Neptune. Then something that's the Planck length would expand to let's say a virus or a living cell.

So the ratio in size between the Planck length and the cell, which we can just about see under a microscope, is the same ratio as a proton to the solar system.

It's unimaginably small.

So how do you observe something that's very small?

You have to shine light with a very small wavelength onto this thing to see the tiny thing. The wavelength can't be bigger than the tiny thing, otherwise you won't see it. But remember, quantum mechanics tells us that the smaller the wavelength of the light, the higher the energy of the photons.

So I have to start bombarding this thing with higher energy photons to see it. What happens if you try to approach something that's the Planck length? You get so much energy in there. What you do is you form a black hole. And then you put more energy in, you try to see what's going on, and the black hole grows. And so you get to a point, which is around the Planck length in size, where you can't, in principle, try to resolve the structure of this thing.

So I think it is legitimate to make the argument that – given what we know about the universe, given the measurement we make of the strength of gravity, the measurement we make of Planck's constant, and the measurement we make at the speed of light – then there is something fundamental about this very tiny length, 10 to minus 35 meters.

Of course, the things we can really get a feel for are things that are around, let's say, a few inches, a few centimeters to a few meters. But then when you start to talk about the distance to the planets or the distance to the Sun – the so-called astronomical unit – 93 million miles, what does 93 million miles mean?

So look at the moon. The moon is the same radius on the sky as the Sun. You know that because of total solar eclipses. It is a coincidence based on the way that our solar system has evolved, but it's a nice coincidence. So we can perhaps conceive of what the Sun looks like on the sky 93 million miles away.

You can fit a million Earths inside the Sun. So how do we conceive of that? The radius is something like 100 times the radius of the Earth. That means that if you've got in a passenger aircraft, well, it'll take something like a year to fly around the Sun in a passenger aircraft. It becomes inconceivable. And the Sun's quite a small star. So then we start to think of bigger distances.

So the most distant object that we created, we built, the Voyager 1 spacecraft is now well over 150 astronomical units from the Earth, 150 times the distance from the Earth to the Sun.

What does that mean? It takes light over 22 hours to reach it. So a signal at the speed of light to go to the most distant objects we are in communication with, which has been flying since the 1970s. And then we have 365 times that a light year, which is to the frozen edge of the Sun's influence, the edge of the Oort cloud. Four times further than that, you get to the nearest star, the Proxima Centauri, the Alpha Centauri system, that star's about four light years away or so.

So that's inconceivable. And then you start to talk about the, well, a galaxy then. The Milky Way galaxy – we are all in orbit around the center of the Milky Way galaxy – how big is this collection of stars? Somewhere between 200 and 400 billion suns in the Milky Way galaxy, about 100,000 light years across.

And then you say, "Well, what about the nearest galaxy?" So you go outside on a clear night where there's no moon, and it's dark, away from the city lights. And if you know where to look, you can just about see our nearest neighboring large galaxy. It's called the Andromeda galaxy.

That galaxy is two and a half million light years away. It means the light entering your eye began its journey before we had evolved on Earth. And then you start to say, "Well, what about the other galaxies?"

So we've measured galaxies now out to close to the edge of the observable universe with instruments like the James Webb Space Telescope from which the light has journeyed for over 13 billion years to reach us. 13,000 million years to reach us. And the universe has been expanding in that time.

So the most distant thing you can see in the universe that we can detect light from is called the cosmic microwave background radiation. So the cosmic microwave background radiation is light that was emitted 380,000 years after the Big Bang. So that's been traveling for 13.8 billion years or so across the universe to reach us.

But then if you ask the question, where is that place now? The place that it emitted that photon from the cosmic microwave background radiation that came across the universe for 13.8 billion years into our detectors? Where is it now? 'Cause the universe has been expanding.

You get an answer, which is something like 46 billion light years away now. So you might say, "Well, the radius of the universe is 92 billion light years or so." It isn't, because we know, we know that there's more universe beyond that. That's just as far as we can see.

The universe, for all we know, and given the accuracy of our measurements at the moment, might be infinite in extent. And that genuinely is inconceivable.

When we contemplate the size and the scale of the universe and our place within it, which you're forced to do when you think about the distance scales and the sheer size and age of the universe, then I think it's very natural for us to tend to come to the conclusion that we don't matter at all.

But one of the great joys about essentially being a scientist is that you can come across a point of view and you think, "I hadn't thought of that." And I found it happened to me recently. I was reading a book by David Deutsch, who's one of the greats, one of the founders of quantum computing. And he made a point, which I had heard before actually in a book called "The Anthropic Cosmological Principle" by John Barrow and Frank Tipler, which is a huge influence on me. But David Deutsch and Barrow and Tipler pointed out that it's not necessarily the case that life will always be a speck – that life remains insignificant on a cosmic scale.

You shouldn't assume that because if life persists sufficiently long and becomes sufficiently knowledgeable and powerful, then it may be able to influence larger structures, not just planets and not just solar systems, perhaps not just even galaxies. You almost say that life manipulates the universe such that it becomes immortal, and it's a very beautiful idea.