Next time you watch an airplane jetting off to some exotic destination, remind yourself that the most extraordinary phenomenon you are witnessing is not the miracle of flight, but the reality of time travel. Those passengers will, in fact, arrive at their destination physically a tiny bit younger than if they had stayed at home.
This captures Einstein’s special theory of relativity in an everyday event. As it turns out, light, not time, is the universal constant. To us the conclusion is obvious once you accept two premises: first, the speed of light is the same for every observer (no matter how fast they are moving); and second, the laws of physics must look the same in every inertial frame of reference (a viewpoint that is not accelerating). At this juncture the only remaining “variable” that can adjust is time. However, in Einstein’s day this conclusion required overthrowing centuries of settled opinion that time alone is the fixed constant.
Following the publication of the special theory of relativity in 1905 it would take a decade and a genius mind to make this revolutionary insight orthodox. So how did it come about in the first place?
It literally began as a daydream. As a 16-year-old, Einstein found himself running alongside a beam of light, at the speed of light. Now according to classical physics, if you matched the speed of the light wave, the electric and magnetic fields should appear frozen in place — quite an extraordinary sight. Yet by this time Maxwell’s prediction had already established that the speed of light is an invariable constant. This vexing conundrum stayed with the young Einstein for another ten years, until 1905 — the year he later called his annus mirabilis, his “year of miracles” (Interestingly paralleling Newton roughly two-and-a-half centuries earlier).
That year he published four groundbreaking papers, the third of which was on special relativity. The latter began with a reflection on Faraday’s law of induction: when a magnet and a conducting coil move relative to one another, an electric current is produced. The size of the current depends only on the relative motion, not on which object is actually moving. Yet classical physics offered two entirely different explanations for this identical observed current; another conundrum with which Einstein was to wrestle.
The observation made its way into the opening paragraph on special relativity where he asks: “What if we take the relativity of motion seriously and demand that the laws of physics look the same no matter which object we consider to be moving?” A simple question whose profundity is easy to miss. This was followed through with another thought experiment: A train is moving through a station at high speed. An observer standing on the platform sees two lightning bolts strike the front and rear of the train at exactly the same moment. Inside the train, a passenger sitting in the middle compartment sees the lightning from the front strike first and the lightning from the rear strike slightly later. The conclusion, time dilates with motion. (Wow!)
From these insights Einstein distilled two simple postulates that subsequently became the foundation of special relativity. First, the laws of physics are the same for every observer moving at constant speed. Second, the speed of light in vacuum is the same for all observers, regardless of their motion. Everything else — the relativity of time, the disappearance of absolute simultaneity, and the strange behavior of moving clocks and rods — follows directly from these two statements.
It is hard to imagine the reception of this paper. The back-story of a man in his mid-twenties challenging centuries of settled science must have sent the academic world buzzing. Unsurprisingly, the paper was not instantly and universally accepted, but neither was it ignored or rejected. Thanks largely to Max Planck’s early advocacy and the theory’s internal consistency, it moved from “interesting new idea” to “generally accepted” among physicists in roughly five to six years — remarkably fast for a theory that overthrew the absolute nature of time and space.
What began in the imagination of a teenager is, today, the foundation of vital technology.
The global positioning system (GPS) would quickly drift into uselessness if it did not continually correct for the very effects Einstein predicted — the slowing of time on orbiting atomic clocks caused by their speed, and the slight speeding up of time caused by their weaker gravity. Miniature atomic clocks, the heart of that system and of many other modern applications, are the practical legacy of the insight that time itself is not absolute.
Let’s for a moment imagine what would have happened had special relativity never been discovered. Without Einstein’s insight, the engineers who built GPS would have launched their satellites only to discover that the world’s most precise clocks were mysteriously racing ahead by 38 microseconds every day. This may seem incredibly minor, but the practical impact would have been disastrous. After only a few hours the position error would already be hundreds of meters. After one day it would exceed 11 kilometers making the entire system wholly unusable.
Imagine the scene. Engineers scurrying around hunting for hardware faults, temperature effects, radiation damage, magnetic fields or software bugs. Eventually they would have realized the error was consistent and linked to orbital speed and altitude. They would then have measured it carefully and swiftly inserted an ad hoc correction. Yet, without a theoretical framework they would never have been certain the correction was complete or how it might change under different conditions. A headache for sure.
Thankfully, this is not the case. Special relativity was firmly established decades before GPS. The world’s first practical atomic clock was built in 1955; in 1967 the second itself was redefined as 9,192,631,770 periods of the radiation corresponding to a transition in the caesium-133 atom; and Coordinated Universal Time (UTC) — replacing Greenwich Mean Time — began on 1 January 1972. One of the more intriguing consequences of UTC has been the 27 leap seconds added since then to keep atomic time roughly in step with the Earth’s actual rotation.
What these events make clear is that without understanding the whole picture, engineers will always be at a disadvantage. “Understanding” here means a grasp of the first principles that define how a system fundamentally works. Our use of the plural “engineers” is deliberate. Even Einstein worked within a community of ideas, and today, both truths — the need for first principles and the power of collective insight — hold more strongly than ever.
That is especially true when the system in question is a miniature atomic clock. These devices sit at the intersection of quantum physics, precision timing, and extreme packaging constraints. Success depends not only on the physics package itself, but on the ability to assemble it with the accuracy, hermeticity, thermal control and long-term reliability that the application demands.
This is the territory in which Palomar’s Advanced Solutions Division works every day — partnering with teams developing chip-scale atomic clocks, advanced timing modules and related quantum devices, and helping move them from laboratory prototype to robust, production-ready hardware. Our engineers stand ready to help turn ambitious ideas into practical, reliable reality. You may contact our Principal Engineer here.
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Dr. Anthony O'Sullivan
Palomar Technologies
Senior Director of Strategic Marketing