Merging quantum with relativity has been a 100 year slog as one describes the subatomic while the other describes the universe but there may be a chance to finally unify the two using a clock equipped with unimaginable precision.
Relativity describes a space-time in which objects have well-defined properties and move predictably from one location to another. In quantum theory, by contrast, an object can be in a “superposition” of many properties at once, or it can suddenly jump into a particular location. These two descriptions match their respective realms of reality well, but they’re incongruous when taken together.
So what happens when both quantum mechanics and relativity are necessary to describe a phenomenon?
Take the case where a massive object is put into a superposition of two possible locations at the same time. General relativity says that any object with mass should bend the fabric of space-time. But what if that object is in a superposition? Is the geometry of space-time also in a superposition?
In order to study such questions, physicists are always looking for systems where both gravity and quantum mechanics are important. “Clocks are for sure one of the most promising systems to test these types of features,” said Flaminia Giacomini, a theoretical physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. Clocks naturally straddle the line between quantum mechanics and relativity. They tell time, which is an inherently relativistic concept. They’re also fundamentally quantum: The way the electrons move from one energy level to another is by passing through a superposition of being in both levels.
It's all about "noise".
Decoherence is responsible for the transition from the weird world of quantum mechanics to the ordinary world of everyday experience. Each time the environment interacts with a quantum system, it can be seen as a tiny measurement made on the system — a way for the environment to learn something about the quantum system and destroy its “quantumness.” Physicists have gotten very good at shielding their quantum experiments from anything in the environment that would disturb them. But they can’t shield them from gravity.
The end game???
The atomic clock in Jun Ye’s laboratory features a blue laser beam that excites a cloud of strontium atoms inside the round window. G.E. Marti/JILA
As the atoms in Ye’s clock move up and down in the cloud, experiencing a variation in the flow of time, gravity will alter the way they interact with each other and cause an observable change in their dynamics. It still won’t be quantum gravity per se, where gravity is quantized into fundamental particles called gravitons. But it would be a valuable instance of quantum mechanics and gravity interweaving to cause a new phenomenon.
Tick, Tick, Tick ...
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