A Curious Observer Guide to Quantum Mechanics, pt. 4: Look at the stars

A Curious Observer Guide to Quantum Mechanics, pt. 4: Look at the stars

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A Curious Observer Guide to Quantum Mechanics, pt. 4: Look at the stars

Aurich Lawson / Getty Images

One of the quietest revolutions of our current century was the entry of quantum mechanics into our everyday technology. It was previously that quantum effects were limited to physics laboratories and delicate experiments. But modern technology is increasingly relying on quantum mechanics for its basic operation, and the importance of quantum effects will only grow in the coming decades. As such, physicist Miguel F. Morales has taken on the powerful task of explaining quantum mechanics to the rest of our laity in this series of seven parts (no math, we promise). Below is the fourth story in the series, but you can always find the start story plus a landing page for the entire series so far on the site.

Beautiful telescopic images of our Universe are often associated with the stately, classical physics of Newton. While quantum mechanics dominates the microscopic world of atoms and quarks, the motions of planets and galaxies follow the majestic clock of classical physics.

But there is no natural limit to the size of quantum effects. If we look closely at the images produced by telescopes, we see the fingerprints of quantum mechanics. This is because particles of light have to move in a wavy way through the large spaces to make the beautiful images we enjoy.

This week we will focus on how photons move over light years, and how their inherent quantum wavelength enables modern telescopes, including interferometric telescopes as large as Earth.

Sterlig

How should we think of the light of a distant star? Last week we used the analogy of dropping a pebble into a lake, with the ring of ripples on the water allowing the wavy motion of photons to enter. This analogy helped us understand the length of a particle of wrinkles and how photons overlap and contract.

We can continue with the analogy. Each star is similar to the Sun in that it is a very of photons. Unlike someone who carefully throws a few pebbles into a mirror-smooth lake, it looks more like they were thrown into a bucket of gravel. Each pebble makes a ring of wrinkles, and the wrinkles of each stone spread as before. But now the wrinkles are constantly mixing and overlapping. As we look around the waves against the far shores of the earth, we do not see the ripples of each individual pebble; instead, the combination of many individual wrinkles combined.

The chaotic waves of a gravel star crossing our dam. The ripple of many pebbles overlaps, creating a complex set of waves.
Enlarge / The chaotic waves of a gravel star crossing our dam. The ripples of many pebbles overlap, creating a complex set of waves.

Miguel Morales

So let’s imagine we are standing on the shore of a lake as the waves wash in and look at our ‘gravel star’ with a telescope for water waves. The lens of the telescope focuses the waves of the star on a place: the place on the camera sensor where the light of the star ends up.

If a second bucket of gravel falls further along the opposite shore into the lake, the ripples on our shore will overlap, but the telescope will focus on the detector in two different spots. Similarly, a telescope can sort the light of the stars into two different groups – photons of star A and photons of star B.

But what if the stars are very close to each other? Most of the ‘stars’ we see at night are actually double stars – two suns so close together that they look like one bright pin of light. If they are in distant galaxies, stars can be separated by light years, yet in professional telescopes look like a single place. We need a telescope that can somehow sort out the photons produced by the different stars to solve them. Similar things apply if we want to image features such as sunspots or torches on the surface of a star.

Returning to the lake, there is nothing special about the ripples made by different pebbles – the ripples of one pebble are indistinguishable from the ripples made by another. Our golf telescope does not care if the wrinkles come from different pebbles in one bucket or completely different buckets – a wrinkle is a wrinkle. The question is how far apart two pebbles must fall for our telescope to distinguish the ripples from different places?

Sometimes it is best to walk slowly along the beach when bumping. So we will sit two friends on the shore and drop pebbles as we walk along our shore, watching the waves and thinking deep thoughts. As we walk along the beach, we see that the waves of our friends are overlapping everywhere and that the waves are coming in randomly. There seems to be no pattern.

  • The waves of two “gravel” gravel. The waves of each star are circular (see following panels), but combine in an apparent tangle. However, we note that while the golf train is chaotic in every place, in places close to each other on the beach, the golf trains are very much the same. In places far down the beach we see a very different golf train.

    Miguel Morales

  • The waves of just one star.

    Miguel Morales

  • The waves of the other star. The waves can be combined to see the wave pattern seen in the first panel.

    Miguel Morales

But if we take a closer look, we see that spots on the beach see waves very close together. The waves is random time, but places on the beach a few steps apart see the same random wave train. But when we look at waves crashing far along the beach, the wave train is completely different from those hitting near us. Any two places on the beach that are close to each other will see almost identical golf trains, but different places on the beach see different golf trains.

It makes sense when we think of the waves on the beach as the combination of small ripples made of hundreds of pebbles. At nearby places on the beach, the ripples of the pebbles that are deposited by both friends combine in the same way. But further along the beach, the ripples of one friend will have to travel further, so that the ripples pick up in a different way and give us a new golf train.

Although we can no longer see the ripples of individual pebbles once they have been combined into waves, we can see how far we have to walk to see a new wave train. And that tells us something about how the ripples add up.

We can confirm this by asking our two pebble friends to move closer to each other. When our friends are close together, we see that we have to walk a long way along our beach to see how the wrinkles pick up in a different way. But if our friends are far apart, the golf trains will look just a few steps away on our beach. By determining how far we have to walk before the waves look different, we can determine how far our pebble friends are from each other.

Large and small telescopes looking at the same two stars. Because the waves at the corners of the large telescope look different, it can sort the waves into two sources. For the small telescope, the waves look the same across the lens, so it sees the two stars as a single unresolved source.
Enlarge / Large and small telescopes looking at the same two stars. Because the waves at the corners of the large telescope look different, it can sort the waves into two sources. For the small telescope, the waves look the same across the lens, so it sees the two stars as a single unresolved source.

Miguel Morales

The same effect happens with photon waves, which can help us understand the resolution of a telescope. When looking at a distant binary star, if the light waves entering opposite sides of the telescope look different, the telescope can sort the photons into two different groups – the photons of star A and the photons of star B. opposite sides of the telescope looks the same, then the telescope can no longer sort the photons into two groups, and the binary star will look like one dot to our telescope.

If you want to solve nearby objects, the obvious ones are to increase the diameter of the telescope. The farther apart the edges of the telescope are, the closer the stars can distinguish each other. Larger telescopes have better resolution than small telescopes, and can separate light from more closely placed sources. This is one of the driving forces behind the construction of a huge telescope of 30 or even 100 meters – the larger the telescope, the better the resolution. (This is always true in space and on the ground with customizable optics to correct legal distortions.)

For telescopes, bigger is really better.

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