1. Why Do Atoms Give Off Only Certain Colors of Light?
By the late 1800s, scientists had found the electron and radioactivity, and they were proposing different models of what an atom looks like inside. [1] One puzzle was that an atom does not glow in a smooth rainbow. It gives off light of fixed wavelengths, which is why each element has its own pattern of colored lines.
In 1913 Bohr proposed a theory for the hydrogen atom, using the quantum idea that some physical quantities can take only certain separate values. Electrons move around the nucleus, but only in prescribed orbits. When an electron jumps to an orbit with less energy, the difference is sent out as radiation. The Nobel Foundation says this explained why atoms emit light of only fixed wavelengths. [1]
A simple way to picture it is a staircase with no ramp: you can stand on step two or step three, but never between them. The electron's "steps" are the allowed orbits, and each jump down gives off a flash of light with a matching color. This picture is only a comparison, not Bohr's own words.
2. How Did a Boy From Copenhagen End Up Explaining the Atom?
Bohr was born in Copenhagen on October 7, 1885. His father was a professor of physiology, and the Nobel Foundation says he was largely responsible for awakening Niels's interest in physics while he was still at school. His younger brother Harald became a professor of mathematics. [2]
Bohr finished his doctorate in 1911. His thesis was a purely theoretical study of metals, and it was in this work that he first ran into the implications of Max Planck's quantum theory of radiation. That autumn he spent time at Cambridge in England, then went on to Manchester, where he worked in the laboratory of Ernest Rutherford. [2] In 1913 came his model of the atom.
Later he returned to Copenhagen, where he became head of the university's Institute for Theoretical Physics, now called the Niels Bohr Institute. The Nobel Foundation calls it a legendary research environment and a center for the development of quantum physics. [1] Bohr won the 1922 Nobel Prize in Physics "for his services in the investigation of the structure of atoms and of the radiation emanating from them." [2]
3. What Did Einstein Doubt About the New Quantum Physics?
The atom was only the beginning. During the 1920s, physicists built a fuller quantum theory, and its rules were strange. It could often tell you only the odds of what you would measure, and some pairs of properties, such as an exact position and an exact motion, could not both be pinned down at once. This is the "uncertainty" in the uncertainty principle.
In the debates at the Solvay conferences, meetings of leading physicists held in Brussels, Einstein challenged Bohr's view of how to understand this theory. One of his tests is known as the recoiling-slit experiment, a thought experiment, an experiment imagined in the mind, with a movable slit placed before a pair of slits. Bohr held that a particle's wave nature and its path cannot both be seen in a single experiment, and the test was meant to get around that rule. [3]
In a simplified summary, Bohr's answer was that the slit's position and motion cannot both be known well enough for Einstein's trick to work. Physicists still write papers about this exchange today, and one recent analysis argues that Bohr's reply left out an important feature, the link between the particle and the moving slit. [3]
4. How Did Bohr Answer Einstein's Photon Box?
At the Solvay conference of 1930, Einstein came back with a new thought experiment, the "photon box." Imagine a box full of light, with a clock-controlled shutter that lets a single photon, a particle of light, escape at a chosen moment. Weigh the box before and after, and the missing weight tells you the photon's energy. Einstein's aim was to know the energy and the time of escape both very precisely, which the energy-time uncertainty relation does not allow. [4]
Dieks and Lam, two physicists in the Netherlands, write that according to Bohr's report the debate revolved around whether that relation is valid. [4] Summaries of the debate say that Bohr replied using Einstein's own general theory of relativity: weighing the box means it hangs in a gravitational field, and gravity changes how fast clocks run, so the clock cannot be read as precisely as Einstein assumed. [4]
In 2007, Dennis Dieks and Sander Lam worked through the physics in detail and wrote that their calculation justifies Bohr's qualitative arguments in a precise way. They also showed that the photon box looks ahead to a later, famous thought experiment by Einstein and two colleagues. [4] So the debate was not only a duel. Each side's challenge pushed the other to say more exactly what the theory meant.
5. Why Did Bohr Have to Flee Denmark in 1943?
Germany had occupied Denmark since April 1940. [5] By autumn 1943, Bohr, who was among the most famous scientists in the world, was no longer safe there. He escaped to Sweden and was flown on to Britain in a Mosquito, a fast British bomber that had no seats for passengers, so he rode in the bomb bay. [5]
Several popular accounts say that his flying helmet was too small for his large head, so he did not hear the pilot's order to switch on his oxygen. He lost consciousness, and the pilot dropped to a lower altitude, where Bohr came around. [5] Sources disagree on the exact date and landing place, so this article does not give them. After reaching Britain, Bohr went on to visit the Los Alamos laboratory in the United States, the secret center of the atomic bomb project. [5]
6. How Can You Explore Bohr's Atom Yourself?
No lab is needed. Look up "hydrogen emission spectrum" in a school science site, and you will find a picture of its bright colored lines. Try to match each line to one electron jump between two of Bohr's orbits. Notice that a smooth rainbow would mean any jump was allowed, which is exactly what Bohr's atom rules out.
If you want to read the original thinking, start with the Nobel Foundation's biography of Bohr, then the arXiv paper on the photon box. The paper is written for physicists, but its introduction retells the story in plain words.