【Introduction】
"There is a scientific basis for a day in the sky and a year in the earth."
Einstein's general theory of relativity states that the greater the gravitational pull, the slower time becomes.
Over the past few decades, physicists have been exploring this theory and giving proofs, but the spatial scale of experiments is very large.
Today's Nature cover article proves that even if the height difference is only 1 millimeter, the time difference effect exists, which is the smallest scale of proof of general relativity in the world today.
General relativity proves that the time at both ends of a millimeter is different
According to general relativity, the Earth or any massive object is distorting space-time in a way that slows down time.
If you have a twin brother who was born in the plains and he went to Mount Everest, then your brother will be older than you, because time passes more slowly in the plains, and this difference is measured by putting an atomic clock at the top of the mountain.
And the paper published this time by nature, from the University of Colorado and the Joint Laboratory of the National Institute of Standards and Technology, Ye Jun's research group, measured the difference in time flow between the top and bottom of an atomic cloud that is a millimeter high.
Another important implication of this work is that it lies at the intersection of general relativity and quantum mechanics, and the two theories are notoriously incompatible, and that the new clock is basically a quantum system — an atomic clock — and wraps it up with gravity.
In the experiment, Ye Jun's team used an optical lattice clock — a cloud of 100,000 strontium atoms that can be triggered by a laser. If the frequency of the laser is just right, then the electrons moving around each atom will be excited into a higher-energy orbit.
Because only a very small range of laser frequencies cause the electrons to move, measuring this frequency can provide extremely precise time. It's like a quantum old-fashioned clock, with the ticking sound coming from the oscillation of the laser, not the swing of the pendulum.
The atomic clocks at Ye Jun's laboratory feature a blue laser beam that activates a cloud of strontium atoms inside the round window.
The researchers split their clocks into two parts — they looked at the clouds on the camera and then drew two imaginary boxes around the upper and lower parts. They then compared the ticking frequencies of the upper and lower halves and found that the atoms at the top of the cloud experienced 0.000000000000001% less than the atoms at the bottom.
Because the excitation frequency is the same, it should be the same if the time is not offset, but now there is a difference, indicating that the bottom is indeed slow.
Their method of measuring displacement is special, comparing two parts of the same cloud – which allows them to cancel out many of the measured noises common to both parts.
It's like measuring a sailboat on a rough sea. Even when it is unpredictably rocking up and down, the distance between the keel and the mast will always remain the same. Although clocks made of atomic clouds drift due to many factors – the electric field, the magnetic field, the laser itself, the ambient heat – but the frequency difference at the top and bottom of the cloud is the same.
Measuring this discrepancy reveals the effect of gravity on time, said Andrew Thompson, an atomic clock expert at the National Institute of Standards and Technology. Andrew Ludlow said, "It's not trivial. ”
Quantum mechanics and relativity have a point of convergence
Relativity describes a space-time in which an object has definite properties and can move predictably from one location to another. In quantum theory, by contrast, an object can be in a "superposition state" of multiple properties at the same time, or it can suddenly jump to a specific location.
The two descriptions match their respective fields of application well, but together they seem contradictory. So, how do you explain it when quantum mechanics and relativity describe a phenomenon at the same time?
Take, for example, the superposition of a massive object in two possible positions at the same time:
General relativity says that any object with mass should bend the structure of space-time. But what if the object is in a superposition state? Is the space-time around it also superimposed?
To study these questions, physicists are always looking for systems where gravity and quantum mechanics coexist, and clocks naturally cross the line between quantum mechanics and relativity. They tell time that this is an intrinsic relativistic problem, but at the same time essentially quantum: electrons transition from one energy level to another through the superposition of two energy levels.
If Ye Jun's team can increase the sensitivity of their circadian clock by a factor of about 10, they can begin looking for gravitational effects in atomic behavior. Ye Jun said that an accurate atomic clock will open up the possibility of exploring quantum mechanics in curved space-time, such as the physical state of particles distributed in different positions in curved space-time.
Ye Jun's Profile: Graduated from the Department of Applied Physics of Shanghai Jiao Tong University with a bachelor's degree, and graduated from the University of Colorado with a ph.D. under the tutelage of Nobel Laureate in Physics John Hall.
Perhaps it is this tiny frequency difference that breaks the quantum coherence and makes macroscopic time classical (it all seems to have a definite state).
Reference Links:
https://www.quantamagazine.org/an-atomic-clock-promises-link-between-quantum-world-and-gravity-20211025/