Einstein was right! Atomic clocks and space-time symmetry

Einstein was right, that is what two research groups concluded based on their collaborative work with atomic clocks. Well, to be precise, he is at least right to the degree of 10^-21 (which is 0.000000000000000000001, so super accurate). What was he right about? He was right about that the speed of light is exactly the same regardless of the direction it would travel.

Who cares that the speed of light is the same in any direction? Well, Einstein’s theory of general relativity actually hinges a lot on the speed of light. If it would be different dependent on which direction you choose for it to move, it would make this theory a lot harder to work with, or possibly impossible to work with. Our predictions and understanding of the universe become obsolete, and you’ll be surprised how much in everyday life depends on Einstein’s principles.

Okay, so how did they work this out? Two research groups, one from Germany (Physikalisch-Technische Bundesanstalt) and one from the US (University of Delaware), used their atomic clocks to run an experiment for 1,000 hours! These clocks are based on ytterbium ions, or Yb+ for the chemists.

Two research groups used their atomic clocks to run an experiment for 1,000 hours. One clock was in Germany, the other one in the US. (Photo by Pixabay on Pexels.com)

Ytterbium is one of those special elements we call Rare Earth Elements; you can find it on the periodic table in the Lanthanide group. Normal Yb+ ions have electrons circling around the protons and neutrons in its core in a regular, symmetrical way – plainly said, they move around the nucleus in circles.

To make things more interesting, you can get the ytterbium ions excited. This is not because of something fun, but because of letting the electrons absorb extra energy in the form of radiation like light. When YB+ becomes excited, or their electrons rather, these electrons travel in longer ways in one direction compared to another. They start moving around with longer wave lengths, or more elliptical motion

When YB+ becomes excited, their electrons travel in longer ways in one direction compared to another.

With magnetic fields you can measure this direction. So the two research groups got their clocks in action at two different parts of the world. They got their ions excited and oriented them the same way. They then let them slowly rotate at the same speed as the Earth rotates around its own axis. So after exactly one day, the excited Yb+ ions would be in the same position again.

Great, so what did that tell them? That no matter where you are in a space (different parts of the world in this case), the Yb+ ions kept moving at the same pace. They could measure this down to the very tiniest of detail, that if the clocks were ever to run out of sync, this could only happen by 1 second after 10 billion years!

You’ll be surprised how much in everyday life depends on Einstein’s principles. – Click to Tweet this

No one’s ever lived that long, and even the universe itself is slightly older than that (by 3.5 billion years). It is pretty safe to say that Einstein was right and we don’t have to chuck out our mobile phones along with his theory of general relativity. Oh, did I forget to mention that it was important for our phones?


This research was published in the journal Nature:
Christian Sanner, Nils Huntemann, Richard Lange, Christian Tamm, Ekkehard Peik, Marianna S. Safronova, Sergey G. Porsev. Optical clock comparison for Lorentz symmetry testing. Nature, 2019 DOI: 10.1038/s41586-019-0972-2

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