A spectacular demonstration of the Equivalence Principle

The Equivalence Principle enunciated by Albert Einstein says that any two objects in a uniform gravity field will fall at the same rate if dropped from a height. Of course, if we drop a feather and a bowling ball, that is not what we see and we explain that by saying it is because the air resistance slows down the feather much more than it does the bowling ball. But if you could drop the two items in a vacuum, they should fall at the same rate.

The catch is that getting a vacuum in a large enough volume is not easy. Astronaut David Scott, when on the moon in 1971 as part of the Apollo 15 mission, ran a rough experiment using a hammer and a feather and showed that they both hit the ground at approximately the same time.

But here is a more spectacular demonstration using a bowling ball and feathers done on Earth with more precision and using high-speed cameras to capture the action. To do that, they used a facility near Cleveland consisting of a massive chamber that is capable of being evacuated to something approximating outer space, no mean feat when you consider the enormous pressure that the surrounding air exerts on a high-vacuum chamber. The resulting video is quite beautiful. What is interesting is how, even though we have so much confidence in the Equivalence Principle, demonstrating it still causes such surprise and pleasure.


  1. Jean says

    That’s is really fascinating to see but I’m really disappointed. They don’t even show a complete fall at full speed. Who’s ever going to be able to see a feather reach such a velocity? Why not spend the few seconds on the video to show that too?

  2. Trickster Goddess says

    I remember watching Neil Armstrong doing this experiment on the moon with a hammer and a feather.

    I agree with Jean. I would like to see the feathers go splat in real time.

  3. lpetrich says

    Very nice demonstration. There’s a simpler version of it that you people can try. Hold a pen and a book at the same height, then release those two items. They will fall at close to the same rate. You could try using a video camera on your experiment, like one in a smartphone, and then play it back much slower or frame by frame.

  4. Lassi Hippeläinen says

    That is the Galilean Equivalence Principle, not Einsteinian.

    Einstein’s principle states that a bowling ball in a static elevator on Earth hits the floor in the same time as a bowling ball in a zero gravity elevator somewhere in space that is being accelerated at 1 g.

    The Galilei version is easy to understand from the Einstein version: if a ball and a feather are in a zero gravity vacuum box, pulling the box at any acceleration means the contents hit the opposite wall at the same time.

  5. Rob Grigjanis says

    Lassi @4:

    Einstein’s principle states that a bowling ball in a static elevator on Earth hits the floor in the same time as a bowling ball in a zero gravity elevator somewhere in space that is being accelerated at 1 g.

    No, that’s just a restatement of the weak (Galilean) principle, because you’re talking about the gravitational trajectories of objects. The Einstein version adds to the WEP the statement that the outcome of any non-gravitational experiment* in a freely falling lab, is independent of the velocity and spacetime location of the lab. In other words, it is the WEP plus local Lorentz invariance, plus local position invariance.

    *e.g. measuring the electric force between two charges.

  6. says

    After a fruitless discussion with a moon landing denialist, I once asked a Hollywood special effects artist how hard it would be to fake this demonstration (recall, they did it on the moon) and he rattled off a couple, effortlessly. My favorite was that the floor, background, and camera would all be on a hydraulic lift, with the objects suspended on fine black wire, and at the moment of “release” the lift would rise.

  7. Who Cares says

    Not a stranger to high vacuum vessels. The difference is the absolute enormous size of this one.
    You can fit the building where I’ve seen/worked with vacuum vessels into this one. That is more what leaves me going whoa then oh they fall at the same time.

  8. Mano Singham says


    That is a nice demo. But one rule of demos is that the bigger, bright, and louder it is, the greater the impact!

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