Uchicago
If you could uninvent one thing, what would it be — and what would unravel as a result? – Inspired by Eitan Fischer, Class of 2027
Raymond Damadian was a physician working in a lab that had been experimenting with nuclear magnetic resonance equipment. This lab’s particular focus was the hydrogen nuclei relaxation times associated with various materials. During their experiments, Raymond gained insight: what if there was a distinct relaxation time associated with cancerous and non-cancerous tissue? To test this, he included samples of cancerous and non-cancerous tissues. He found that the relaxation time between the two tissues was significantly different. The promising results persuaded him to imagine a machine that could scan the whole body for cancer detection. This invention would come to be known as the MRI machine.
Nuclear Magnetic Resonance relies on atomic precession. Precession is the spin around an axis due to torque. Atomic precession occurs in specific nuclei that have a property called spin, which results from the orientation of their angular momentum. This property causes atoms to behave like magnets. Within a strong external magnetic field, the nuclei will precess. They will precess at a frequency coined the Larmor frequency. In a magnetic field, certain atoms, such as hydrogen, have two possible spin states. They may be parallel to the field, in a lower-energy state, or antiparallel, in a higher-energy state. A net magnetization can be measured by its energy; a small net magnetization results from an orientation parallel to the magnetic field direction. When a radio-frequency pulse at the Larmor frequency is applied to the atoms, their spins gain energy. They will become perpendicular to the magnetic field. Following the pulse, they relax to their aligned orientations, and when this happens, a radio signal is detected by the Nuclear Magnetic Resonance Machine.
Physicist Isidor Isaac Rabi knew that atomic nuclei have a magnetic moment due to their spins. This moment, however, could not be measured yet. Beginning in the late 1930s, he began experimenting with a molecular beam apparatus that sends atoms into a strong magnetic field. Depending on the atom in question, he noticed a tiny magnet-like alignment. This was a monumental discovery in itself, but the Larmor frequency was the essential insight to developing a reliable analytical process for measuring this magnetic moment. He found that if a weaker oscillating magnetic field were induced on the atoms at the Larmor frequency, they would flip, switching energy states in the process. His insight lay in the deflection of the atomic beam, an indicator of an absorbed energy state within the atoms, or a resonance. He had discovered that nuclei absorb energy at a specific resonance frequency, known as the Larmor frequency.
The Larmor frequency is the rate at which a magnetic moment precesses around an external magnetic field. It is the key to finding the harmony between the induced field and the particle's own behavior. It is derived from the gyromagnetic ratio, which relates a particle's dipole moment to its angular momentum in an external magnetic field. At its core the Larmor frequency is dependent on the introduction of Faraday's theory of electromagnetic induction and the observations first taken by Physicist Peter Debye in the study of dipole moments in 1912.
Since the invention of the MRI, an estimated 8 billion scans have been performed globally. Each year, 100-150 million MRI scans are performed.
Today, the MRI machine relies on many more scientific and computational discoveries than the precession of the atom, but its primary role is to measure this precession. Were I to uninvent the discovery of the Larmor frequency, we would lose the method by which to measure this precession; this would require me to uninvent the gyromagnetic ratio, or Faraday's theory of electromagnetic induction, so that they may not be related to equate a model for precession in the first place and without a model for precession a frequency of resonance may not be found for nuclear magnetic resonance to take place.
It’s evident that if a single detail were to be removed in our understanding of physics and engineering, we would lose a large amount of the inventions we have today. One example is the toaster oven; take one of its 600+ components globally and the methods of manufacturing and logistical hurdles to then create this cheap toaster and, well, you don't have a viable toaster anymore. Specific details matter more than others. The most important detail is the discovery of phenomena that later become parts of blueprints that grow to encompass such large machines that, in turn, make the details of the phenomena they harness a tiny part of itself. These details all stack from the most basic discoveries to become parts of entire theories, theories to experiments, and experiments to creations with purpose and application.
In his constant effort to foster invention, Da Vinci observed an important axiom about the nature of creation when he said, “From the part comes the whole and from the whole comes the part.” Our models of nature break its chaotic rhythms to communicate an exchangeable truth of its behavior. If we were to break this chain of dependencies, in either the theoretical or the physical, we would find ourselves without much of what we call technology today.