🌈 Pound and Rebka Experiment. Does gravity slow down time?

Einstein predicted three main effects of the general theory of relativity. Two were more or less clear: the deflection of light by the Sun was measured back in 1919, and astronomers had struggled with the strange motion of Mercury for half a century. But the third prediction - gravitational redshift (i.e., time dilation in a gravitational field) - could not be detected.

The effect is too small: for Earth it is only 10^-15, that is, one millionth of a billionth. It seemed impossible to measure in a laboratory.

But in 1959, Robert Pound and his graduate student Glen Rebka from Harvard decided to take the risk.

They took a tower 22.5 meters high, a source of gamma quanta (iron-57 nuclei), and the newly discovered Mössbauer effect, which allowed the emission of photons without recoil.

The idea was simple: send a photon from top to bottom and see if its frequency changes due to gravity.

If time flows slower at the bottom, the frequency should shift slightly. The problem: the expected shift is 2.46·10^-15, and the accuracy of the instruments is thousands of times worse.

So the physicists used a trick: they made the source vibrate at a known speed, creating an artificial Doppler shift, and observed how gravity added to it.

After accounting for thermal effects, the result matched the theory with an accuracy of 10%.

And a few years later, Pound and Snyder improved the accuracy to 1%.
Every time we turn on the navigator in our phone, it uses corrections for gravitational time dilation.

GPS satellites fly at an altitude of 20,000 km, where time flows faster by about 45 microseconds per day.

💥 If engineers had not accounted for this, the error in determining coordinates would grow at a rate of 10 km per day!

The Pound and Rebka experiment is the basis of modern navigation)

👌 Signal from noise, which was thousands of times stronger here