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A curious observer’s guide to quantum mechanics, pt. 2: The particle melting pot

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One of the quietest revolutions of our current century has been the entry of quantum mechanics into our everyday technology. It used to be that quantum effects were confined to physics laboratories and delicate experiments. But modern technology increasingly relies on quantum mechanics for its basic operation, and the importance of quantum effects will only grow in the decades to come. As such, physicist Miguel F. Morales has taken on the herculean task of explaining quantum mechanics to the rest of us laymen in this seven-part series (no math, we promise). Below is the second story in the series, but you can always find the starting story here.

Welcome back for our second guided walk into the quantum mechanical woods! Last week, we saw how particles move like waves and hit like particles and how a single particle takes multiple paths. While surprising, this is a well-explored area of quantum mechanics—it is on the paved nature path around the visitor’s center.

This week I’d like to get off the paved trail and go a bit deeper into the woods in order to talk about how particles meld and combine while in motion. This is a topic that is usually reserved for physics majors; it’s rarely discussed in popular articles. But the payoff is understanding how precision lidar works and getting to see one of the great inventions making it out of the lab, the optical comb. So let’s go get our (quantum) hiking boots a little dirty—it’ll be worth it.

Two particles

Let’s start with a question: if particles move like waves, what happens when I overlap the paths of two particles? Or said another way, do particle waves only interact with themselves, or do they mix together?

Enlarge / On the left is the interferometer from last week, where a single particle is split by the first mirror and takes two very different paths. On the right is our new setup where we start with particles from two different lasers and combine them.

Miguel Morales

We can test this in the lab by modifying the setup we used last week. Instead of splitting the light from one laser into two paths, we can use two separate lasers to create the light coming into the final half-silvered mirror.

We need to be careful about the lasers we use, and the quality of your laser pointer is no longer up to the task. If you carefully measure the light from a normal laser, the color of the light and the phase of the wave (when the wave peaks occur) wander around. This color wander is not discernible to our eyes—the laser still looks red—but it turns out that the exact shade of red varies. This is a problem money and modern technology can fix—if we shell out enough cash we can buy precision mode-locked lasers. Thanks to these, we can have two lasers both emitting photons of the same color with time-aligned wave crests.

When we combine the light from two high-quality lasers, we see exactly the same stripey pattern that we saw before. The waves of particles produced by two different lasers are interacting!

So what happens if we again go to the single photon limit? We can turn the intensity of the two lasers down so low that we see the photons appear one at a time on the screen, like little paintballs. If the rate is sufficiently low, only one photon will exist between the lasers and the screen at a time. When we perform this experiment we will see the photons arrive at the screen one at a time; but when we look at the accumulated pointillism painting, we will see the same stripes we saw last week. Once again, we’re seeing single particle interference.

It turns out that all the experiments we performed before give exactly the same answer. Nature does not care if one particle is interacting with itself or if two particles are interacting with each other—a wave is a wave, and particle waves act just like any other wave.

But now that we have two precision lasers, we have a number of new experiments we can try.

Two colors

First, let’s try interfering photons of different colors. Let’s take the color of one of the lasers and make it slightly more blue (shorter wavelength). When we look at the screen we again see stripes, but now the stripes walk slowly sideways. Both the appearance of stripes and their motion are interesting.

First, the fact that we see stripes indicates that particles of different energy still interact.

The second observation is that the striped pattern is now time dependent; the stripes walk to the side. As we make the difference in color between the lasers larger, the speed of stripes increases. The musicians in the audience will already recognize the beating pattern we are seeing, but, before we get to the explanation, let’s improve our experimental setup.

If we are content to use narrow laser beams, we can use a prism to combine the light streams. A prism is usually used to split a single light beam and send each color in a different direction, but we can use it backwards and with careful alignment use the prism to combine the light from two lasers into a single beam.

The light from two lasers with different color combined with a prism. After the prism the light ‘beats’ in intensity.
Enlarge / The light from two lasers with different color combined with a prism. After the prism the light ‘beats’ in intensity.

Miguel Morales

If we look at the intensity of the combined laser beam, we will see the intensity of the light ‘beat.’ While the light from each laser was steady, when their beams with slightly different colors are combined, the resulting beam oscillates from bright to dim. Musicians will recognize this from tuning their instruments. When the sound from a tuning fork is combined with the sound of a slightly out-of-tune string, one can hear the ‘beats’ as the sound oscillates between loud and soft. The speed of the beats is the difference in the frequencies, and the string is tuned by adjusting the beat speed to zero (zero difference in frequency). Here we are seeing the same thing with light—the beat frequency is the color difference between the lasers.

While this makes sense when thinking about instrument strings, it is rather surprising when thinking of photons. We started with two steady streams of light, but now the light is bunched into times when it is bright and times when it is faint. As the difference between the colors of the lasers is made larger (they’re de-tuned), the faster the pulsing becomes.

Paintballs in time

So what happens if we again turn down the lasers really low? Again we see the photons hit our detector one at a time like little paintballs. But if we look carefully at the timing of when the photons arrive, we see that it is not random—they arrive in time with the beats. It does not matter how low we turn the lasers—the photons can be so rare that they only show up one every 100 beats—but they will always arrive in time with the beats.

This pattern is even more interesting if we compare the arrival time of the photons in this experiment with the stripes we saw with our laser pointer last week. One way of understanding what is happening in the two-slit experiment is to picture the wave nature of quantum mechanics directing where the photons can land side to side: the paintballs can hit in the bright regions and not in the dark regions. We see a similar pattern in the paintball arrival in the two-color beam, but now the paintballs are being directed forward and back in time and can only hit in time with the beats. The beats can be thought of as stripes in time.

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NASA selects SpaceX as its sole provider for a lunar lander

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Enlarge / Rendering of SpaceX’s Starship vehicle on the surface of the Moon.

NASA

In an extraordinary announcement on Friday, NASA said that it has selected SpaceX and its Starship vehicle to serve as the lunar lander for its Artemis Program. This is NASA’s plan to return humans to the Moon later this decade.

About a year ago, NASA gave initial study and preliminary development contracts for Moon landers to SpaceX, Dynetics, and a team of aerospace heavyweights led by Blue Origin. The cost of SpaceX’s bid was about half that of Dynetics, and one-fourth the amount received by Blue Origin. That frugality, at least in part, led NASA on Friday to choose SpaceX as the sole provider of landing services during the down-select phase.

“We looked at what’s the best value to the government,” said Kathy Lueders, chief of the human exploration program for NASA, during a teleconference with reporters on Friday.

NASA said it will award SpaceX $2.89 billion for development of the Starship vehicle and two flights. One of these missions will be an uncrewed flight test of Starship down to the lunar surface and back. The second mission will be a crewed flight—the first one of the Artemis program—down to the Moon.

Starship offered several advantages, NASA officials said. It has a spacious cabin for astronauts, two airlocks, and ample payload capability to bring large numbers of experiments to the Moon and return samples to Earth. Significantly, the NASA engineers also praised the vehicle’s innovative design and future-looking technology that might also one day be used on Mars.

Ultimately, the selection criteria were based on a company’s technical proficiency, management, and cost. SpaceX scored well in all three. But budget appears to have been the biggest factor. The space agency has had difficulty securing funding from Congress for the lunar lander aspect of the program. For the current fiscal year, NASA said it needed $3.3 billion in funding to meet the goal of landing humans on the Moon by 2024. Congress provided just $850 million, and as a result, NASA acknowledged that 2024 was no longer a realistic target.

Making Artemis affordable

At the direction of the Trump administration, NASA formally created the Artemis Program about two years ago to send humans back to the Moon in a sustainable way and establish a base there. The goal was to move beyond the flags-and-footprints forays of the Apollo Program and gain the knowledge needed to eventually send humans to Mars. The Biden administration has endorsed this basic goal, and it’s working to update the Artemis Program with a more realistic timeline given the budget predilections of Congress.

Friday’s announcement is part of that process of making Artemis more affordable. A sole-source award to SpaceX for the Human Landing System will certainly not be particularly popular in Congress, where traditional space companies such as Lockheed Martin and newer entrants like Blue Origin have more established lobbying power. But it sends a clear message from NASA and the White House to budget writers in the House and Senate.

This award effectively says that NASA is serious about getting to the Moon with the funding it has. And if Congress were to fully fund the Human Landing System program, NASA could bring on a competitor. Ideally, of course, there should be competition. This approach has worked well for NASA’s commercial cargo and crew programs. But NASA is getting a small fraction of what it needs to run a lunar lander competition.

In addition to this development award, NASA said it would soon move to procure “recurring landing services” from industry. This contract will be for operational missions to the lunar surface, and it seems like SpaceX would have a significant advantage in winning the award. However, there may be an opening here, if Congress provides more funding for the Human Landing System, for either Dynetics or the Blue Origin-led team to play a role in human landings.

Self-funding Starship

SpaceX has largely self-funded development of the large Starship vehicle for about five years, with the intent of using it to settle humans on Mars one day. Starship is a fully reusable upper stage that will launch atop the Super Heavy rocket. SpaceX is in various states of testing and developing both of these vehicles at its facility in South Texas.

As part of the Artemis Program, SpaceX has proposed launching a modified version of its Starship vehicle to lunar orbit. Shortly afterward, a crew of NASA astronauts would launch inside an Orion spacecraft on top of a Space Launch System rocket, both of which were developed by NASA. Orion would rendezvous with Starship in lunar orbit, board the vehicle, and go down to the surface. Starship would then lift off from the lunar surface and link back up with Orion, and the crew would come back to Earth in the smaller capsule.

Left unsaid is the reality that SpaceX is also planning to launch humans on Starship from Earth. It does not seem like all that much of a stretch to question the need for the much more costly Orion and Space Launch System rocket, when lunar crews could simply launch in a Starship into low-Earth orbit, undergo refueling there from another Starship, and then go to the Moon and back. But NASA knows that Congress—which is heavily invested in Orion and the SLS rocket, and their jobs across all 50 states—would not support a SpaceX-only program.

The choice of SpaceX was applauded by some industry officials on Friday. “The selection of SpaceX as the sole-source developer of the Human Lander System is a sign of how far both the company and their relationship with NASA has come over the last ten-years,” said Lori Garver, a deputy administrator for NASA under President Obama. “SpaceX’s involvement in Artemis is sure to elevate public interest and will hopefully lead to our soonest possible return to the Moon.”

For years, space industry leaders like Garver have advocated for NASA to increase support for commercial space companies that have sought to drive down the costs of spaceflight. After all, SpaceX’s bid for the entirety of its Human Landing System, $2.9 billion, is about what NASA spends each year on the Space Launch System and associated ground systems development. Now, the space agency appears to be boldly embracing such a future.

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Arkansas representatives pass bill to allow creationism in schools

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Enlarge / The Arkansas state capitol.

Last week, the Arkansas state House of Representatives passed a bill that would amend state education law to allow teachers in public schools to teach creationism as “a theory of how the earth came to exist.” As it stands, the act promotes blatantly unconstitutional behavior as made clear by a precedent set in a 1982 case involving the Arkansas Board of Education. Despite that, the bill passed 72-21, and it already has a sponsor in the state Senate.

The body of the bill is mercifully short, consisting of two sentence-long amendments to the existing Arkansas code:

A teacher of a kindergarten through grade twelve (K-12) science class at a public school or open-enrollment public charter school may teach creationism as a theory of how the earth came to exist.

This section is permissive and does not require a teacher to teach creationism as a theory of the earth came to exist.

But those two sentences are enough to land teachers and their local school system in a world of trouble, in that the permission given runs afoul of a lot of legal precedent. In a key case that involved Arkansas itself, McLean V. Arkansas Board of Education, a group of plaintiff’s banded together to challenge a state law that mandated the teaching of “creation science” in public schools. The judge in that case correctly recognized that creation science was actually religious in nature, and therefore it violated the constitution’s prohibition against the establishment of state religion.

That ruling wasn’t appealed, meaning the legal precedent only applied to Arkansas. But later in that same decade, a similar case from Louisiana made it to the Supreme Court, and it reached the same conclusion. The prohibition against creation science has applied nationally since.

These precedents only apply to the teaching of creationism as science; there are other contexts, like a comparative religion class, where it might be appropriate to teach this idea. But the bill’s use of “theory” clearly indicates that it’s intended to insert the concept into science classes.

While the state might end up being sued if this law passes, it’s just as likely that a teacher in Arkansas will exercise this permission and the suit will end up targeting the teacher and the school board they work for. If the local school board loses (which it would), there is a good probability it will end up liable for the legal fees of whoever sues. Thus, the legislation serves as an invitation for local school districts throughout the state to rack up enormous legal bills.

Although the legal history of creationism is available to anyone with a working Internet connection, the bill passed with 72 representatives, all Republicans, voting in favor. Of the chamber’s 22 Democrats, 21 voted against it, and one other didn’t vote.

It’s not clear whether these legislators are simply unaware of the legal precedents or if they are simply using this bill as an opportunity to signal their cultural affiliations. We’ve contacted its two sponsors to find out. As of the time of publication, neither had responded.

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SARS-CoV-2 variant found in Brazil: More infectious, may limit immunity

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Enlarge / COVID-19 has taken a terrible toll on Brazil.

Almost from the moment it made the jump to humans, the SARS-CoV-2 virus has been picking up mutations and creating new lineages as it expands into different populations. In practical terms, the vast majority of these mutations make absolutely no difference; the resulting virus has the same properties as the unmutated form it’s derived from.

But there have been a number of cases where variants surge in frequency. Early on in the pandemic, this was often the product of the variant moving into a previously unexposed population—a matter of chance rather than a feature of the virus. Separating out these cases from instances where mutations make the virus more dangerous is a serious challenge. But this week, an international team of researchers has published evidence showing that a variant first characterized in Brazil is likely to represent a significant additional threat.

There’s a lot of uncertainty about the details, but the virus appears to be more infectious and more likely to infect those who have immunity to other viral strains, and it might even be more lethal. And as of when the paper was written, the lineage had been detected in over 35 countries.

The second wave

Earlier this year, we described the situation in the Brazilian city of Manaus, which the first wave of coronavirus infections had hit hard. But that was followed by a long period of low infections, despite an indifferent response to the pandemic by the Brazilian government, leading some to suggest that the city might have reached a level of infection sufficient to provide herd immunity.

That hopeful thought was brought to an end in December, when a second wave of infections started up in the city, straining its health care systems and causing another surge in deaths. The infection rates were so high that it raised the suspicion that there might be a new strain of virus that could evade the immune response generated by infections that occurred during the first wave.

Brazilian healthcare workers responded to the rise in cases by sequencing the genomes of some of the viruses causing the second wave of infections. Prior to this second wave, only seven viral genomes had been obtained from Amazonas, the state where Manaus is located. The new effort increased that number by 184, although not all of these were complete genomes.

The genomes revealed the presence of a lineage researchers call P.1, which is an offshoot of a strain that had been present during the first wave. Since then, P.1 had picked up a large number of mutations, including 17 individual mutations that altered the amino acid sequences of the proteins it encodes, one insertion of new bases, and three deletions of bases. That’s a substantial number of changes and suggests a high level of mutations picked up since March. Timing estimates suggest that P.1 originated in November, just before the start of the large second wave in Manaus.

Over the course of the second wave, the P.1 variant went from not being detectable in the samples taken to accounting for 87 percent of viruses sampled just seven weeks later. Viral genomes from elsewhere in Brazil indicated it was also spreading rapidly within the country, showing up in cities that were on popular flight routes from Manaus first. This indicates that P.1 likely originated in the city.

What is this thing?

Tests for the virus that use polymerase chain reaction (PCR) involve a cyclical amplification of the virus’s genome. As a result, if you start with more viral genomes, you’ll reach a detectable level of signal in fewer cycles. This is thought to mean that the cycle count needed for detecting the virus provides a rough measure of the viral load carried by the person the sample came from. In the case of the P.1 strain, tests showed a fairly consistent, if small, indication of increased viral load.

Because the samples came at different times after infection, however, the researchers can’t tell whether this is indicative of higher maximum levels of the virus or a longer infection duration. Neither is especially good.

To try to understand how P.1 might have influenced the second wave of infections in Manaus, the researchers developed an epidemiological model that allowed them to track two different strains of the virus. The first strain was set up with the typical properties of SARS-CoV-2. For the second, they were able to adjust the properties of the virus, such as the immunity provided by prior infections and its transmissibility. This let them determine which properties were consistent with the dynamics of the second wave in Manaus.

Overall, the model suggests that P.1 is very likely to be more transmissible than prior strains of SARS-CoV-2, and it’s likely to be roughly about twice as infectious. There’s also an indication that it can evade the immune response generated by past infections to some extent. The model suggests there’s at least a 10 percent chance that the variant can evade immunity, but it’s unlikely to be more than a 50 percent chance.

There was some evidence of enhanced lethality due to infection by the P.1 strain. But the timing of the strain’s rise was such that the evidence came from a period where the hospitals were on the verge of being overwhelmed. So the authors are treating this possibility cautiously.

What might be causing these changes? At least 10 of the mutations seen in the P1 strain affect the virus’s spike protein, which the virus uses to latch on to cells it infects. At least eight of those mutations seem to have been selected for over the course of the strain’s evolution, suggesting they assist in making it more infectious. Three of the specific changes have also been seen in another lineage of virus that has caused concerns, and at least one of them has been shown to interfere with antibodies that attack the virus.

So while this data isn’t really a decisive indication that P.1 poses a distinct threat to us, it’s all certainly consistent with that concern. And it would help explain why Manaus had two distinct waves of infection that seem to have hit a substantial fraction of the city’s population. Still, as the authors of the new paper point out, we don’t fully understand the consequences of mutations that alter proteins targeted by antibodies. Until we get a grip on that, we won’t really know how worried we need to be about P.1 and other variants.

Science, 2021. DOI: 10.1126/science.abh2644  (About DOIs).

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