A Curious Observer’s Guide to Quantum Mechanics, pt. 4: Watching the stars

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A Curious Observer's Guide to Quantum Mechanics, pt. 4: Watching the stars

Aurich Lawson / Getty Images

One of the quietest revolutions In our current century, quantum mechanics has entered our everyday technology. Quantum effects used to be limited to physical laboratories and delicate experiments. But modern technology increasingly relies on quantum mechanics for its basic workings, and the importance of quantum effects will only increase in the coming decades. As such, physicist Miguel F. Morales has taken on the daunting task of explaining quantum mechanics to the rest of us laymen in this seven-part series (no math, we promise). Below is the fourth story in the series, but you can always find the starting story plus a landing page for the entire series so far on the spot.

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 movements of planets and galaxies follow the majestic clockwork of classical physics.

But there is no natural limit to the magnitude of quantum effects. If we look closely at the images from telescopes, we see the fingerprints of quantum mechanics. That’s because light particles have to travel in a wave-like fashion through the expanses of space to create the beautiful images that we enjoy.

This week we’ll focus on how photons travel through light years and how their inherent quantum waves enable modern telescopes, including Earth-sized interferometric telescopes.

Starlight

How should we feel about the light from a distant star? Last week, we used the analogy of dropping a pebble into a lake, where the ring of ripples on the water indicate the wave-like motion of photons. This analogy helped us understand the length of a particle ripple and how photons overlap and converge.

We can continue that analogy. Each star resembles the sun in the sense that it has one a lot of of photons. Unlike someone gently dropping some pebbles into a mirror smooth lake, it’s more like pouring them into a bucket of gravel. Each pebble makes a ring of ripples, and the ripples of each stone spread out as before. But now the ripples are constantly blending and overlapping. As we watch the waves crash against the far shore of the earth, we don’t see the ripples of every single pebble; instead, the combination of many individual ripples added up.

The chaotic waves of a gravel star crossing our pond. The ripples of many pebbles overlap, creating a complex series of waves.
Enlarge / The chaotic waves of a gravel star crossing our pond. The ripples of many pebbles overlap, creating a complex series of waves.

Miguel Morales. Placeholder image

So let’s imagine standing on the shore of a lake with the waves pouring in and looking at our gravel ‘star’ with a water wave telescope. The telescope’s lens focuses the waves of the star on one spot: the spot on the camera sensor where the light from that star lands.

If a second bucket of gravel is thrown into the lake further up the other shore, the ripples overlap on our shore, but are focused by the telescope in two different places on the detector. Likewise, a telescope can sort the light from the stars into two different groups: photons from star A and photons from star B.

But what if the stars are very close together? Most of the ‘stars’ we see at night are actually binary stars – two suns so close together that they appear as one bright pinprick of light. When located in distant galaxies, stars may be separated by light years, but look like a single spot in professional telescopes. We need a telescope that can sort the photons of the different stars in some way to solve them. Similar things apply when we want to depict objects such as sunspots or flares on the surface of a star.

To return to the lake, there is nothing special about the ripples created by different pebbles – the ripples of one pebble are indistinguishable from the ripples created by another. Our wave telescope doesn’t care if the ripples came from different pebbles in one bucket or from different buckets altogether – a ripple is a ripple. The question is how far apart do two pebbles have to be dropped for our telescope to distinguish that the ripples came from different locations?

If you’re dumbfounded, it’s best to take a slow walk along the beach. So we’ll have two friends sit across the shore to drop pebbles, as we walk our shore, watch the waves and think deep thoughts. As we walk along the beach, we see that our friends’ waves overlap everywhere, and the waves come in randomly. There does not seem to be a pattern.

  • The waves of two gravel “stars.” The waves of each star are circular (see next panels), but combine in an apparent jumble. However, we notice that while the wave train is chaotic at every location, in locations close to each other on the beach, the wave trains are very similar. At locations far below the beach, we see a completely different wave train.

    Miguel Morales. Placeholder image

  • The waves of just one star.

    Miguel Morales. Placeholder image

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

    Miguel Morales. Placeholder image

But on closer inspection, we see that spots on the beach very close to each other see almost identical waves. The waves to be random in time, but locations on the beach a few steps apart the same arbitrary series of waves. But when we look at waves hitting far on the beach, that wave train is completely different from the one close to us. Any two places on the beach that are close together will see nearly identical wave trains, but widely spaced locations on the beach will see different wave trains.

This makes sense if we consider the waves on the beach as the combination of tiny ripples from hundreds of pebbles. In nearby locations on the beach, the ripples from the pebbles fallen by both friends add up in the same way. But further down the beach, a friend’s ripples will have to travel farther, so the ripples add up in a different way, giving us another wave train.

While we can no longer see the ripples of individual pebbles once they combine into waves, we can slow down how far we have to walk to see a new wave train. And that tells us something about how the ripples come together.

We can confirm this by asking our two pebble friends to get closer to each other. When our friends are close together, we find we have to walk a long way along our beach to see the ripples add up in a different way. But if our friends are far apart, the wave trains will look different with a few steps on our beach. By counting down how far we have to walk before the waves look different, we can determine how far apart our pebble friends are.

Large and small telescopes look at the same two stars. Since the waves appear different at the extreme edges of the large telescope, 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 look at the same two stars. Since the waves appear different at the extreme edges of the large telescope, 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. Placeholder image

The same effect occurs with photon waves, which can help us understand the resolution of a telescope. If you look at a distant binary star and the light waves entering the opposite edges of the telescope look different, the telescope can sort the photons into two different groups: the photons from star A and the photons from star B. But if the light waves those entering opposite edges of the telescope look the same, then the telescope can no longer sort the photons into two groups and the binary star looks like one spot in front of our telescope.

If you want to solve nearby objects, it is obvious to increase the diameter of the telescope. The farther apart the edges of the telescope are, the closer the stars can be and still be distinguished. Larger telescopes have better resolution than small telescopes and can separate light from sources that are closer together. This is one of the driving ideas behind building truly enormous telescopes with a diameter of 30 or even 100 meters – the larger the telescope, the better the resolution. (This is always true in space, and true on the ground with adaptive optics to correct atmospheric distortions.)

For telescopes, bigger really is better.

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