The Bohr model: The famous but flawed depiction of an atom

 The Bohr model is neat, but imperfect, depiction of atom structure.

A model of an atom according to Niels Bohr.

The Bohr model, introduced by Danish physicist Niels Bohr in 1913, was a key step on the journey to understand atoms.

Ancient Greek thinkers already believed that matter was composed of tiny basic particles that couldn't be divided further. It took more than 2,000 years for science to advance enough to prove this theory right. The journey to understanding atoms and their inner workings was long and complicated. 

It was British chemist John Dalton who in the early 19th century revived the ideas of ancient Greeks that matter was composed of tiny indivisible particles called atoms. Dalton believed that every chemical element consisted of atoms of distinct properties that could be combined into various compounds, according to Britannica.  

Dalton's theories were correct in many aspects, apart from that basic premise that atoms were the smallest component of matter that couldn't be broken down into anything smaller. About a hundred years after Dalton, physicists started discovering that the atom was, in fact, really quite complex inside. 

THE BOHR MODEL: JOURNEY TO FIND STRUCTURE OF ATOMS

British physicist Joseph John Thomson made the first major breakthrough in the understanding of atoms in 1897 when he discovered that atoms contained tiny negatively charged particles that he called electrons. Thomson thought that electrons floated in a positively charged "soup" inside the atomic sphere, according to Khan Academy.

14 years later, New Zealand-born Ernest Rutherford, Thomson's former student, challenged this depiction of the atom when he found in experiments that the atom must have a small positively charged nucleus sitting at its center. 

Based on this finding, Rutherford then developed a new atom model, the Rutherford model. According to this model, the atom no longer consisted of just electrons floating in a soup but had a tiny central nucleus, which contained most of the atom's mass. Around this nucleus, the electrons revolved similarly to planets orbiting the sun in our solar system, according to Britannica.

Some questions, however, remained unanswered. For example, how was it possible that the electrons didn't collapse onto the nucleus, since their opposite charge would mean they should be attracted to it? Several physicists tried to answer this question including Rutherford's student Niels Bohr.

NIELS BOHR AND QUANTUM THEORY

Bohr was the first physicist to look to the then-emerging  quantum theory to try to explain the behavior of the particles inside the simplest of all atoms; the atom of hydrogen. Hydrogen atoms consist of a heavy nucleus with one positively-charged proton around which a single, much smaller and lighter, negatively charged electron orbits. The whole system looks a little bit like the sun with only one planet orbiting it. 

Bohr tried to explain the connection between the distance of the electron from the nucleus, the electron's energy and the light absorbed by the hydrogen atom, using one great novelty of physics of that era: the Planck constant. 

The Planck constant was a result of the investigation of German physicist Max Planck into the properties of electromagnetic radiation of a hypothetical perfect object called the black body. 

Strangely, Planck discovered that this radiation, including light, is emitted not in a continuum but rather in discrete packets of energy that can only be multiples of a certain fixed value, according to Physics World.That fixed value became the Planck constant. Max Planck called these packets of energy quanta, providing a name to the completely new type of physics that was set to turn the scientists' understanding of our world upside down.

What role does the Planck constant play in the hydrogen atom? Despite the nice comparison, the hydrogen atom is not exactly like the solar system. The electron doesn't orbit its sun —the nucleus — at a fixed distance, but can skip between different orbits based on how much energy it carries, Bohr postulated. It may orbit at the distance of Mercury, then jump to Earth, then to Mars. 

The electron doesn't slide between the orbits gradually, but makes discrete jumps when it reaches the correct energy level, quite in line with Planck's theory, physicist Ali Hayek explains on his YouTube channel.

Bohr believed that there was a fixed number of orbits that the electron could travel in. When the electron absorbs energy, it jumps to a higher orbital shell. When it loses energy by radiating it out, it drops to a lower orbit. If the electron reaches the highest orbital shell and continues absorbing energy, it will fly out of the atom altogether.

The ratio between the energy of the electron and the frequency of the radiation it emits is equal to the Planck constant. The energy of the light emitted or absorbed is exactly equal to the difference between the energies of the orbits and is inversely proportional to the wavelength of the light absorbed by the electron, according to Ali Hayek.

Using his model, Bohr was able to calculate the spectral lines — the lines in the continuous spectrum of light — that the hydrogen atoms would absorb. 

THE SHORTCOMINGS OF THE BOHR MODEL

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The Bohr model seemed to work pretty well for atoms with only one electron. But apart from hydrogen, all other atoms in the periodic table have more, some many more, electrons orbiting their nuclei. For example, the oxygen atom has eight electrons, the atom of iron has 26 electrons.

Once Bohr tried to use his model to predict the spectral lines of more complex atoms, the results became progressively skewed.

There are two reasons why Bohr's model doesn't work for atoms with more than one electron, according to the Chemistry Channel. First, the interaction of multiple atoms makes their energy structure more difficult to predict. 

Bohr's model also didn't take into account some of the key quantum physics principles, most importantly the odd and mind-boggling fact that particles are also waves, according to the educational website Khan Academy.

As a result of quantum mechanics, the motion of the electrons around the nucleus cannot be exactly predicted. It is impossible to pinpoint the velocity and position of an electron at any point in time. The shells in which these electrons orbit are therefore not simple lines but rather diffuse, less defined clouds. 

Only a few years after the model's publication, physicists started improving Bohr's work based on the newly discovered principles of particle behavior. Eventually, the much more complicated quantum mechanical model emerged, superseding the Bohr model. But because things get far  less neat when all the quantum principles are in place, the Bohr model is probably still the first thing most physics students discover in their quest to understand what governs matter in the microworld. 

PHYSICISTS ARE FINALLY READY TO TEST A KEY EINSTEIN THEORY

“Relativity has been tested over and over. It’s been holding true. Quantum mechanics has been tested over and over and has been holding true.”

EARTH’S GRAVITY DISTORTS

Spacetime. As far as we know, every point on Earth experiences time and space differently by a minuscule — but measurable — amount. Atomic clocks are one of the ways to detect that distortion. (In fact, that’s how GPS works.) Now two technological breakthroughs in next-generation atomic clocks are opening up new avenues for research into gravity and relativity at very small scales.

The studies were published back-to-back in Nature in early 2022. Both experiments use so-called “optical lattice clocks”, in which ultra-cooled strontium atoms floating in a vacuum are trapped in place by light and made to “tick” with a red laser. The laser then tells time by counting the ticks. These clocks keep time to an accuracy of a second in tens or even hundreds of billions of years, according to scientists.

Physicists from Jun Ye’s lab at JILA (a collaboration between NIST and the University of Colorado, Boulder) produced the most stable and precise optical lattice clock on record, precise enough to measure the difference in gravity at a separation of just 0.2 millimeters (200 microns) — far smaller than ever before.

“We haven’t done it between independent clocks yet,” says Tobias Bothwell, the study’s lead author. “But this is the first piece showing that this level of precision is attainable.”

Separately, physicists in Shimon Kokowitz’s lab at the University of Wisconsin produced a series of independent optical lattice clocks that could be activated and measured together.

“Now we’re really limited by the atoms and not by the laser anymore,” explains Kolkowitz. This provides a path to stable, precise, and portable clocks that might one day be developed into gravitational wave and dark matter detectors.

THE EINSTEIN CONNECTION

Einstein’s theories of general and special relativity still seem to hold true even at these very small distances, which are approaching the domain of quantum mechanics. But Einstein’s theories and quantum theory don’t play nicely together.

Relativity imagines the world and forces like gravity as smooth and continuous, while the fundamental tenet of quantum theory is that everything is quantized, that is, broken down into basic, discrete units. And that’s just the beginning of where they disagree. The problem for physicists is they both still seem to work.

“Relativity has been tested over and over. It’s been holding true. Quantum mechanics has been tested over and over and has been holding true. But we also know the two cannot be both true at the same time. At some scale, they have to break down,” explains Ye.

None of the researchers think this research can be used to detect a quantized version of gravity. but if they can investigate the effects of gravity on quantum systems, they might be able to see these two opposite kinds of physics interact.

ATOMIC CLOCKS AND QUANTUM THEORY

“This a funny time for physics, because it’s actually very similar to the time before Einstein came on the scene,” says Kolowitz.

“We look out in the universe, and we see all these things that indicate that there’s stuff we don't understand yet. But every measurement we do, every test we do, every experiment we do is well described by the physics that we know about. We just can’t put it all together.”

Currently, Kolowitz’s group is trying to use their clocks to show for the first time in a lab that relativistic effects can be observed in any system that’s accelerating, not just those influenced by gravity. This has been shown indirectly in the past but never proven experimentally on its own.

“I THINK THAT MUST BE THE SPIRIT OF EINSTEIN.”

“We’re going to do the first direct test of this, and the first realization of this thought experiment that Einstein proposed 100 years ago,” says Kolkowitz.

“What is interesting and very exciting is we are starting to gain sensitivity to gravity at smaller and smaller scale,” says Bothwell of the future of their experiments.

“Now we’re at 200 microns, we think we can go to 20 microns. That’s not an impossible goal. That’s fairly soon.” At the moment, says Bothwell, that means improving lasers and engineering ways to put more atoms into the system.

Longterm, both groups want to use optical lattice clocks to probe deeper into unexplored areas of physics. Kolkolwitz is part of the Laser Interferometer Space Antenna (LISA) consortium, which is developing next-gen gravitational wave detectors. Ye’s group wants to push their experiment to a scale where the uncertainties of quantum physics bump up against the positional certainty of gravity. The hope is to find a regime where gravity plays a role in the way quantum systems behave.

“I’m not saying our experiment is going to be the judge,” says Ye. “But nevertheless, you have to keep exploring, So from that spirit, if nothing else, from that spirit of exploring deeper and deeper into nature, I think that must be the spirit of Einstein.”

NASA spacecraft snaps gorgeous new photo of Jupiter's moons Io and Europa

 

Taken during Juno's 39th close flyby of Jupiter on Jan. 12, 2022, this stunning view captures two of the planet's moons: Io (left) and Europa (right).

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NASA's Juno spacecraft beamed back stunning new photos of Jupiter's moons, Io and Europa. 

Juno's latest view of the two moons was captured during the spacecraft's 39th close flyby of Jupiter on Jan. 12. At the time, the spacecraft was about 38,000 miles (61,000 kilometers) above Jupiter's cloud tops, at a latitude of about 52 degrees south.

The new photo, which NASA shared on March 16, offers a stunning view of Jupiter's southern hemisphere, with two of its many moons to the right of the frame. A zoomed in view that the agency also shared brings the moons into clear view, with Io on the left and Europa on the right.

Jupiter's moon Io is the most volcanic body in the solar system. Hundreds of volcanoes dot its surface, some of them spewing sulfurous plumes hundreds of miles high.

Conversely, Europa, the smallest of Jupiter's four giant Galilean moons, has an icy surface, beneath which lies a global ocean of liquid water, scientists believe. Previous observations have found evidence of possible water plumes jetting from the Europa's south polar region, suggesting that there is water in the moon's subsurface ocean breaking out through cracks in the icy crust.

The Juno spacecraft is expected to make its closest fly-by of Europa later this year, in September. During this fly-by, the probe will use several of its scientific instruments to study Europa in greater detail and capture even more stunning views of the mysterious moon.

The Juno mission will also make close approaches to lo in late 2023 and early 2024, according to the NASA statement. The mission is currently expected to end in September 2025.

Two key spacecraft will soon follow in Juno's wake designed to focus exclusively on understanding the giant's moons: NASA's Europa Clipper mission and the European Space Agency's Jupiter Icy Moons Explorer (JUICE).

The new Jupiter photo was processed by citizen scientist Andrea Luck, using raw data from the JunoCam instrument. JunoCam's raw images are available online to the public; members of the community can also suggest features on Jupiter for the camera to photograph.

Nuclear fusion is one step closer with new AI breakthrough

 

The experimental TCV tokamak at Lausanne in Switzerland is used to test the behavior of hydrogen plasmas that will serve as fuel in future fusion reactors.

The green energy revolution promised by nuclear fusion is now a step closer, thanks to the first successful use of a cutting-edge artificial intelligence system to shape the superheated hydrogen plasmas inside a fusion reactor.

The successful trial indicates that the use of AI could be a breakthrough in the long-running search for electricity generated from nuclear fusion — bringing its introduction to replace fossil fuels and nuclear fission on modern power grids tantalizingly closer.

"I think AI will play a very big role in the future control of tokamaks and in fusion science in general,"  Federico Felici, a physicist at the Swiss Federal Institute of Technology in Lausanne (EPFL) and one of the leaders on the project, told Live Science. "There's a huge potential to unleash AI to get better control and to figure out how to operate such devices in a more effective way."

Felici is a lead author of a new study describing the project published in the journal Nature. He said future experiments at the Variable Configuration Tokamak (TCV) in Lausanne will look for further ways to integrate AI into the control of fusion reactors. "What we did was really a kind of proof of principle," he said. "We are very happy with this first step."

Felici and his colleagues at the EPFL's Swiss Plasma Center (SPC) collaborated with scientists and engineers at the British company DeepMind — a subsidiary of Google owners Alphabet — to test the artificial intelligence system on the TCV.

The doughnut-shaped fusion reactor is the type that seems most promising for controlling nuclear fusion; a tokamak design is being used for the massive international ITER ("the way" in Latin) project being built in France, and some proponents think they'll have a tokamak in commercial operation as soon as 2030.

Artificial intelligence

The tokamak is principally controlled by 19 magnetic coils that can be used to shape and position the hydrogen plasma inside the fusion chamber, while directing an electric current through it, Felici explained.

The coils are usually governed by a set of independent computerized controllers — one for each aspect of the plasma that features in an experiment — that are programmed according to complex control engineering calculations, depending on the particular conditions being tested. But the new AI system was able to manipulate the plasma with a single controller, he said.

The AI – a "deep reinforcement learning" (RL) system developed by DeepMind – was first trained on simulations of the tokamak — a cheaper and much safer alternative to the real thing. 

But the computer simulations are slow: It takes several hours to simulate just a few seconds of real-time tokamak operation. In addition, the experimental condition of the TCV can change from day to day, and so the AI developers needed to take those changes into account in the simulations.

When the simulated training process was complete, however, the AI was coupled to the actual tokamak. 

The TCV can sustain a superheated hydrogen plasma, typically at more than 216 million degrees Fahrenheit (120 million degrees Celsius), for a maximum of 3 seconds. After that, it needs 15 minutes to cool down and reset, and between 30 and 35 such "shots" are usually done each day, Felici said. 

A total of about 100 shots were done with the TCV under AI control over several days, he said: "We wanted some kind of variety in the different plasma shapes we could get, and to try it under various conditions." 

Although the TCV wasn't using plasmas of neutron-heavy hydrogen that would yield high levels of nuclear fusion, the AI experiments resulted in new ways of shaping plasmas inside the tokamak that could lead to much greater control of the entire fusion process, he said.

Shaping plasma

The AI proved adept at positioning and shaping the plasma inside the tokamak's fusion chamber in the most common configurations, including the so-called snowflake shape thought to be the most efficient configuration for fusion, Felici said.

In addition, it was able to shape the plasma into "droplets" — separate upper and lower rings of plasma within the chamber — which had never been attempted before, although standard control engineering techniques could also have worked, he said.

Creating the droplet shape "was very easy to do with the machine learning," Felici said. "We could just ask the controller to make the plasma like that, and the AI figured out how to do it." 

The researchers also saw that the AI was using the magnetic coils to control the plasmas inside the chamber in a different way than would have resulted from the standard control system, he said.

"We can now try to apply the same concepts to much more complicated problems," he said. "Because we are getting much better models of how the tokamak behaves, we can apply these kinds of tools to more advanced problems."

The plasma experiments at the TCV will support the ITER project, a massive tokamak that's projected to achieve full-scale fusion in about 2035. Proponents hope ITER will pioneer new ways of using nuclear fusion to generate usable electricity without carbon emissions and with only low levels of radioactivity.  

The TCV experiments will also inform designs for DEMO fusion reactors, which are seen as successors to ITER that will supply electricity to power grids – something that ITER is not designed to do. Several countries are working on designs for DEMO reactors; one of the most advanced, Europe's EUROfusion reactor, is projected to begin operations in 2051.