Gravity Changes Colors

  • By Greg Kestin
  • Posted 04.14.16
  • NOVA

Einstein predicted that gravity changes the color of light all around us. Did this ingenious experiment prove him right? Find out in this episode of What The Physics?!

Running Time: 02:42


Gravity Changes Colors

April 14, 2016

Greg Kestin: You know what happens when you drop something…it starts accelerating down.

But what happens if you drop light? Or, shine light downward? It’s already moving at the speed of light, so it can’t move faster.

Einstein predicted that it actually changes color, but nobody knew if he was right. Here, where I work, an ingenious experiment was done to test this.

I’m Greg, and What The Physics?!

The experimenters, Robert Pound and Glen Rebka, started down here.

They punched holes through all the floors from the lowest point in the building, this sub-basement, up to the attic. They placed some foil made of iron-57 atoms down here in the basement, and another foil of iron-57 atoms up here in the attic.

This iron is really special because it has a favorite color. Like how I only do my research in “razamataz,” and NEVER in “jazzberry jam,” iron-57 wants to emit and absorb specifically 86 picometer wavelength light.

Here in the attic, the iron emits that favorite color down 5 stories through the holes, but the iron in the basement refused to absorb some small amount of that light. Was Einstein right? Was that because the light changed color as it fell, and was no longer the iron’s favorite color?

The trick to figuring out if he was right was to use something called the Doppler effect. When an ambulance moves away from you it sounds lower because the wavelength of the sound is stretched. That works for light too, but instead of getting lower, the light changes color. If something’s moving away from you, the light coming off of it is red-shifted, and if something’s moving toward you, the light coming off of it is blue-shifted.

Einstein predicted a different kind of redshift. A redshift that occurs as light moves up in a gravitational field, and a blueshift that occurs as light moves down in a gravitational field.

So if the iron in the basement was getting light that was blue-shifted, then we should use just the right amount of Doppler effect to redshift it before it fell, to counteract that blue-shift.

So what they did was to take the iron and attach it to a speaker that vibrated, continually moving the iron that was emitting the light toward the basement and then away from the basement. And indeed, when the speaker moves the iron up at just the right speed, it red-shifts the light perfectly, so as the light falls back down, it gravitationally blue-shifts back to the favorite color, and the iron in the basement does absorb it.

This was the first confirmation that light actually changes color as it falls.

Maybe find some seamless way to incorporate the snow…but it has to be seamless.



Host, Writer, Animator, Editor
Greg Kestin
Special thanks
Tyler Howe
Ari Daniel
Lissy Herman
Stuart McNeill
Lauren Aguirre
Kristine Allington
Anna Rotschild
Allison Eck
Lauren Miller
Karishma Desai
Paul Horowitz
Seeta Joseph
Sara Tewksbury
Funding provided by
Foundational Questions Institute


provided by APM




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