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A solar wind event from 2017 that hit Jupiter and compressed its magnetosphere created a hot region spanning half Jupiter's circumference.

The same dirt that clings to astronauts' boots may one day keep their lights on. Researchers created solar cells made out of simulated Moon dust. The cells convert sunlight into energy efficiently, withstand radiation damage, and mitigate the need for transporting heavy materials into space, offering a potential solution to one of space exploration's biggest challenges: reliable energy sources.

An innovative proposal would be a first for planetary exploration. Turns out, it’s as tough to drop inward into the inner solar system, as it is to head outward. The problem stems from losing momentum from a launch starting point on Earth. It can take missions several years and planetary flybys before capture and arrival in orbit around Mercury or Venus. Now, a new proposal would see a mission make the trip, using innovative and fuel efficient means.

The quantum double-slit experiment, in which objects are sent toward a wall with two slits and then recorded on a screen behind the wall, creates an interference pattern that builds up gradually, object by object. And yet, it’s crucial that the path of each object on its way to the screen remain unknown. If one measures which of the slits each object passes through, the interference pattern never appears.

Strange things are said about this. There are vague, weird slogans: “measurement causes the wave function to collapse“; “the particle interferes with itself“; “electrons are both particles and waves“; etc. One reads that the objects are particles when they reach the screen, but they are waves when they go through the slits, causing the interference — unless their passage through the slits is measured, in which case they remain particles.

But in fact the equations of 1920s quantum physics say something different and not vague in the slightest — though perhaps equally weird. As we’ll see today, the elimination of interference by measurement is no mystery at all, once you understand both measurement and interference. Those of you who’ve followed my recent posts on these two topics will find this surprisingly straightforward; I guarantee you’ll say, “Oh, is that all?” Other readers will probably want to read

• this post on measurement
  • (and perhaps this one too about how to make a measurement permanent)
• these posts on interference for one particle and for two particles
  • (and perhaps this one and this one for more examples)

The Interference Criterion

When do we expect quantum interference? As I’ll review in a moment, there’s a simple criterion:

• a system of objects (not the objects themselves!) will exhibit quantum interference if the system, initially in a superposition of possibilities, reaches a single possibility via two or more pathways.

To remind you what that means, let’s compare two contrasting cases (covered carefully in this post.) Figs. 1a and 1b show pre-quantum animations of different quantum systems, in which two balls (drawn blue and orange) are in a superposition of moving left OR moving right. I’ve chosen to stop each animation right at the moment when the blue ball in the top half of the superposition is at the same location as the blue ball in the bottom half, because if the orange ball weren’t there, this is when we’d expect it to see quantum interference.

But for interference to occur, the orange ball, too, must at that same moment be in the same place in both parts of the superposition. That does happen for the system in Fig. 1a — the top and bottom parts of the figure line up exactly, and so interference will occur. But the system in Fig. 1b, whose top and bottom parts never look the same, will not show quantum interference.

Fig. 1a: A system of two balls in a superposition, from a pre-quantum viewpoint. As the system evolves, a moment is reached when the two parts of the superposition are identical. As the system has then reached a single possibility via two routes, quantum interference may result. Figure 1b: Similar to Fig. 1a, except that when the blue ball is at the same location in both parts of the superposition, the orange ball is at two different locations. At no moment are the two possibilities in the superposition the same, so quantum interference cannot occur.

In other words, quantum interference requires that the two possibilities in the superposition become identical at some moment in time. Partial resemblance is not enough.

The Measurement

A measurement always involves an interaction of some sort between the object we want to measure and the device doing the measurement. We will typically

• use a small object to carry out the basic initial measurement, and then
• amplify the result so that it can be made permanent.

For today’s purposes, the details of the second step won’t matter, so I’ll focus on the first step.

Setting Up

We’ll call the object going through the slits a “particle”, and we’ll call the measurement device a “measuring ball” (or just “ball” for short.) The setup is depicted in Fig. 2, where the particle is approaching the slits and the measuring ball lies in wait.

Figure 2: A particle (blue) approaches a wall with two slits, behind which sits a screen where the particle’s arrival will be detected. Also present is a lightweight measuring ball (black), ready to fly in and measure the particle’s position by colliding with it as it passes through the wall. If No Measurement is Made at the Slits

Suppose we allow the particle to proceed and we make no measurement of its location as it passes through the slits. Then we can leave the ball where it is, at the position I’ve marked M in Fig. 3. If the particle makes it through the wall, it must pass through one slit or the other, leaving the system in a superposition of the form

• the particle is near the left slit [and the ball is at position M]
  OR
• the particle is near the right slit [and the ball is at position M]

as shown at the top of Fig. 3. (Note: because the ball and particle are independent [unentangled] in this superposition, it can be written in factored form as in Fig. 12 of this post.)

From here, the particle (whose motion is now quite uncertain as a result of passing through a narrow slit) can proceed unencumbered to the screen. Let’s say it arrives at the point marked P, as at the bottom of Fig. 3.

Figure 3: (Top) As the particle passes through the slits, the system is set into a superposition of two possibilities in which the particle passes through the left slit OR the right slit. (The particle’s future motion is quite uncertain, as indicated by the green arrows.) In both possibilities, the measuring ball is at point M. (Bottom) If the particle arrives at point P on the screen, then the two possibilties in the superposition become identical, as in Fig. 1a, so quantum interference can result. This will be true no matter what point P we choose, and so an interference pattern will be seen across the whole screen.

Crucially, both halves of the superposition now describe the same situation: particle at P, ball at M. The system has arrived here via two paths:

• The particle went through the left slit and arrived at the point P (with the ball always at M),
  OR
• The particle went through the right slit and arrived at the point P (with the ball always at M).

Therefore, since the system has reached a single possibility via two different routes, quantum interference may be observed.

Specifically, the system’s wave function, which gives the probability for the particle to arrive at any point on the screen, will display an interference pattern. We saw numerous similar examples in this post, this post and this post.

If the Measurement is Made at the Slits

But now let’s make the measurement. We’ll do it by throwing the ball rapidly toward the particle, timed carefully so that, as shown in Fig. 4, either

• the particle is at the left slit, in which case the ball passes behind it and travels onward,
  OR
• the particle is at the right slit, in which case the ball hits it and bounces back.

(Recall that I assumed the measuring ball is lightweight, so the collision doesn’t much affect the particle; for instance, the particle might be an heavy atom, while the measuring ball is a light atom.)

Figure 4: As the particle moves through the wall, the ball is sent rapidly in motion. If the particle passes through the right slit, the ball will hit it and bounce back; if the particle passes through the left slit, the ball will miss it and will continue to the left.

The ball’s late-time behavior reveals — and thus measures — the particle’s behavior as it passed through the wall:

• the ball moving to the left means the particle went through the left slit;
• the ball moving to the right means the particle went through the right slit.

[Said another way, the ball and particle, which were originally independent before the measurement, have been entangled by the measurement process. Because of the entanglement, knowledge concerning the ball tells us something about the particle too.]

To make this measurement complete and permanent requires a longer story with more details; for instance, we might choose to amplify the result with a Geiger counter. But the details don’t matter, and besides, that takes place later. Let’s keep our focus on what happens next.

The Effect of the Measurement

What happens next is that the particle reaches the point P on the screen. It can do this whether it traveled via the left slit or via the right slit, just as before, and so you might think there should still be an interference pattern. However, remembering Figs. 1a and 1b and the criterion for interference, take a look at Fig. 5.

Figure 5: Following the measurement made in Fig. 4, the arrival of the particle at the point P on the screen finds the ball in two possible locations, depending on which slit the particle went through. In contrast to Fig. 3, the two parts of the superposition are not identical, and so (as in Fig. 1b) no quantum interference pattern will be observed.

Even though the particle by itself could have taken two paths to the point P, the system as a whole is still in a superposition of two different possibilities, not one — more like Fig. 1b than like Fig. 1a. Specifically,

• the particle is at position P and the ball is at location ML (which happens if, in Fig. 4, the particle was near the left slit and the ball continued to the left);
  OR
• the particle is at position P and the ball is at location MR (which happens if, in Fig. 4, the particle was near the right slit and the ball bounced back to the right).

The measurement process — by the very definition of “measurement” as a procedure that segregates left-slit cases from right-slit cases — has resulted in the two parts of the superposition being different even when they both have the particle reaching the same point P. Therefore, in contrast to Fig. 3, quantum interference between the two parts of the superposition cannot occur.

And that’s it. That’s all there is to it.

Looking Ahead.

The double-slit experiment is hard to understand if one relies on vague slogans. But if one relies on the math, one sees that many of the seemingly mysterious features of the experiment are in fact straightforward.

I’ll say more about this in future posts. In particular, to convince you today’s argument is really correct, I’ll look more closely at the quantum wave function corresponding to Figs. 3-5, and will reproduce the same phenomenon in simpler examples. Then we’ll apply the resulting insights to other cases, including

• measurements that do not destroy interference,
• measurements that only partly destroy interference,
• destruction of interference without measurement, and
• double-slit experiments whose interference can’t be located in physical space,
• etc.

It is generally accepted that the transition from hunter-gatherer communities to agriculture was the single most important event in human history, ultimately giving rise to all of civilization. The transition started to take place around 12,000 years ago in the Middle East, China, and Mesoamerica, leading to the domestication of plants and animals, a stable food supply, permanent settlements, and the ability to support people not engaged full time in food production. But why, exactly, did this transition occur when and where it did?

Existing theories focus on external factors. The changing climate lead to fertile areas of land with lots of rainfall, at the same time food sources for hunting and gathering were scarce. This occurred at the end of the last glacial period. This climate also favored the thriving of cereals, providing lots of raw material for domestication. There was therefore the opportunity and the drive to find another reliable food source. There also, however, needs to be the means. Humanity at that time had the requisite technology to begin farming, and agricultural technology advanced steadily.

A new study looks at another aspect of the rise of agriculture, demographic interactions. How were these new agricultural communities interacting with hunter-gather communities, and with each other? The study is mainly about developing and testing an inferential model to look at these questions. Here is a quick summary from the paper:

“We illustrate the opportunities offered by this approach by investigating three archaeological case studies on the diffusion of farming, shedding light on the role played by population growth rates, cultural assimilation, and competition in shaping the demographic trajectories during the transition to agriculture.”

In part the transition to agriculture occurred through increased population growth of agricultural communities, and cultural assimilation of hunter-gatherer groups who were competing for the same physical space. Mostly they were validating the model by looking at test cases to see if the model matched empirical data, which apparently it does.

I don’t think there is anything revolutionary about the findings. I have read many years ago that cultural exchange and assimilation was critical to the development of agriculture. I think the new bit here is a statistical approach to demographic changes. So basically the shift was even more complex than we thought, and we have to remember to consider all internal as well as external factors.

It does remain a fascinating part of human history, and it seems there is still a lot to learn about something that happened over a long period of time and space. There’s bound to be many moving parts. I always found it interesting to imagine the very early attempts at agriculture, before we had developed a catalogue of domesticated plants and animals. Most of the food we eat today has been cultivated beyond recognition from its wild counterparts. We took many plants that were barely edible and turned them into crops.

In addition, we had to learn how to combine different foods into a nutritionally adequate diet, without having any basic knowledge of nutrition and biochemistry. In fact, for thousands of years the shift to agriculture lead to a worse diet and negative health outcomes, due to a significant reduction in diet diversity. Each culture (at least the ones that survived) had to figure out a combination of staple crops that would lead to adequate nutrition. For example, many cultures have staple dishes that include a starch and a legume, like lentils and rice, or corn and beans. Little by little we plugged the nutritional holes, like adding carrots for vitamin A (even before we knew what vitamin A was).

Food preparation and storage technology also advanced. When you think about it, we have a few months to grow enough food to survive an entire year. We have to store the food and save enough seeds to plant the next season. We take for granted in many parts of the developed world that we can ship food around the world, and we can store food in refrigerated conditions, or sterile containers. Imagine living 5,000 years ago without any modern technology. One bad crop could mean mass starvation.

This made cultural exchange and trade critical. The more different communities could share knowledge the better everyone could deal with the challenges of subsistence farming. Also, trade allowed communities to spread out their risk. You could survive a bad year if a neighbor had a bumper crop, knowing eventually the roles will reverse. The ancient world had a far greater trading system than we previously knew or most people imagine. The bronze age, for example required bringing together tin and copper from distant mines around Eurasia. There was still a lot of fragility in this system (which is why the bronze age collapsed, and other civilizations often collapsed), but obviously in the aggregate civilization survived and thrived.

Agricultural technology was so successful it now supports a human population of over 8 billion people, and it’s likely our population will peak at about 10 billion.

The post The Transition to Agriculture first appeared on NeuroLogica Blog.

The Torino scale assess’ the risk of a near-Earth object impacting Earth. The list has just had a new addition, asteroid 2024 YR4 which poses a risk to Earth in 2032. The risk has been downgraded to 0% but there’s still value in studying asteroids that are going to come close to Earth. The James Webb Space Telescope just joined in the study by observing the asteroid to provide a new estimate of its size and showed that it’s spinning rapidly.

The news is always full of images from the Hubble Space Telescope and more recently the James Webb Space telescope but there is a new kid on the block. NASA’s SPHEREx space telescope was launched back in early March and we can already see its first image. The telescope has six detectors and together they can capture a region of sky 20 times wider than the Moon. The first images are uncalibrated but they give a hint as to the capabilities of the instrument.

People seem to be less impressed when others lose weight with the drug Ozempic than when they achieve it via lifestyle changes

As China’s vast electrical grid relies more on wind, solar and hydropower, it faces a growing risk of power shortages due to bad weather – and that could encourage the use of coal plants

How can Uranus be used to indirectly study its moons and identify if they possess subsurface oceans? This is what a recent study presented at the 56th Lunar and Planetary Science Conference hopes to address as a team of scientists investigated using passive radar sounding methods from Uranus to study its five largest moons: Miranda, Ariel, Umbriel, Titania, and Oberon. This study has the potential to help researchers better understand the formation and evolution of Uranus and its largest moons despite a spacecraft not currently visiting Uranus.

When galaxies run out of primordial hydrogen and helium, they cease star formation, shifting to primarily long-lived red stars. These galaxies are considered "red and dead." It usually takes billions of years for galaxies to run out of hydrogen, but now astronomers using JWST have found examples of galaxies that have already stopped forming stars just 700 million years after the Big Bang, much earlier than predicted by cosmological models.

An international team has merged two lab-synthesized materials into a synthetic quantum structure once thought impossible to exist and produced an exotic structure expected to provide insights that could lead to new materials at the core of quantum computing.

Nostalgia for video games seems to be strongest for those played during childhood – at least for Nintendo Switch players

Sometimes, the best way to learn how to do something is just to do it. That is especially true if you're learning to do something using a specific methodology. And in some cases, the outcome of your efforts is something that's interesting to other people. A team from across the European Union, led by PhD candidate Domenico D'Auria, spent a few days last September performing just such an exercise - and their work resulted in a mission architecture known as the Planetary Exploration Deployment and Research Operation - Venus, or PEDRO-V.

NASA's Perseverance Rover is an ambitious mission. Along with its day-to-day exploration, the rover carried an experimental rotorcraft and is also caching samples for eventual return to Earth. But there's another aspect to its mission that's hidden in the glare of its ambitions. The rover is busy testing five different spacesuit materials.

When we move, it's harder for existing wearable devices to accurately track our heart activity. But researchers found that a starfish's five-arm shape helps solve this problem. Inspired by how a starfish flips itself over -- shrinking one of its arms and using the others in a coordinated motion to right itself -- scientists have created a starfish-shaped wearable device that tracks heart health in real time.

When we move, it's harder for existing wearable devices to accurately track our heart activity. But researchers found that a starfish's five-arm shape helps solve this problem. Inspired by how a starfish flips itself over -- shrinking one of its arms and using the others in a coordinated motion to right itself -- scientists have created a starfish-shaped wearable device that tracks heart health in real time.

High-speed drones will be put to the test in the extreme Arctic environment as part of a project to assess how quickly glaciers in Greenland are retreating

Feedback is baffled – baffled! – as to why Facebook owner Meta's attempts to suppress a previous employee's memoir sent the book rocketing to the top of the book charts

A cost-cutting initiative in the world of passenger aviation could see flight-deck staff reduced to just a captain, with their co-pilot replaced by AI. It may save money, but it's a risk too far, argues Paul Marks