Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I'm reading about the (very near) equivalence of gravitational mass and inertial mass in my undergrad GR course, and the text (Lambourne) describes this equivalence as the inspiration for Einstein's falling elevator thought experiment. (i.e. one cannot tell the difference between free-fall and lack of gravity/upward acceleration and presence of gravity).

What was the significance of this thought? Surely physicists understood that the gravitational force caused things to accelerate before this, and therefore were aware of this kind of equivalence of motion and gravitation. Why was understanding gravity as an acceleration so big for Einstein (and physics as a whole)?

share|cite|improve this question
According to science historians Galileo's 1 pound weight vs 10 pound weight falling at the same rate was never tested but he did it as a thought experiment. His correct conclusions might indicate people did understand relative frames. But your question is probably not one that can be answered definitively. Newton and others understood how to model gravity (force, etc.) but we are still trying to understand gravity itself. Einstein certainly took our understanding much further as a property of space-time geometry. – user6972 Nov 24 '13 at 4:38
I'm not sure (hence a comment rather than an answer) but I think the main significance of the elevator gedankenexperiment is that light must be accelerated by gravity. More generally, Einstein turned the equivalence from a (known) fact about massive bodies into a general principle that applies to all physical laws. – Nathaniel Nov 24 '13 at 8:20
@Nathaniel I think the core of the issue was to show that one may not be able to tell gravity for any other form of acceleration in some circumstances. I can't remember right now whether Einstein considered geodesic deviation or not. – user34134 Nov 24 '13 at 15:47

Clearly, physicists thought gravity caused masses to accelerate. However, Einstein's thought experiment gave way to the reasoning that perhaps there is no difference between a constant gravitational field and a constant acceleration. The significant importance is that this allowed him to speculate that perhaps gravity affects massless objects, like photons.

Think about it. Assuming a constant gravity field is no different than a constant acceleration, then if a beam of light bends downward in an accelerating elevator, it must bend downward in an elevator in a gravity field. That's the breakthrough! We can easily understand a beam bending downward due to acceleration, but if gravity is causing a massless object to change direction just as if it had mass, then Newton's law of gravitation isn't complete. Under Newton, light with no mass experiences no force from gravity and thus doesn't bend.

With this new concept running around, it allowed physicists to speculate that gravity is more than just a force, but perhaps a result or a property even of the geometry of space. Perhaps massive objects cause space itself to curve, which allows even massless things like light to fall towards them.

Naturally one can see where that might lead... To a revolutionary way of considering the geometry of spacetime. That is why this one little notion was so massively important to our understanding of the universe

share|cite|improve this answer


  1. locally measured your time never differ, aka, 'c' relative your wristwatch for example, although some might want to argue that a acceleration is different there, which I don't agree too. But we can keep it to uniform motions and then also conclude that this fact is what makes 'repeatable experiments' work, as well as constants. Assume this wrong, and I would expect us to start living in 'interesting times'.

As for " Surely physicists understood that the gravitational force caused things to accelerate before this"

That is according to Einstein frame-dependent, gravity is transformed away for you when you're in a 'free fall'. So how people thought about it before Einstein is different from now. That a far away observer might define you as accelerating is a result of observer dependencies to me, different frames of reference.

Furthermore it's no longer a 'acceleration', that as you become 'weight less' in it. What he did was to define a equivalence between a planetary gravity (Earth), and a constant, uniform acceleration, aka, your rocket at one gravity.

You will always know a real acceleration, or gravity. That's when you find yourself gaining weight, without eating.

share|cite|improve this answer

English mathematician Sir Isaac Newton published Principia, which hypothesizes the inverse-square law of universal gravitation. He deduced that the forces which keep the planets in their orbs must be reciprocally as the squares of their distances from the centers about which they revolve. If he was reasoning this way about forces (F), he was also doing so for Masses (M) and Acceleration (A) since Newton was obviously aware that Force was related to Mass multiplied with Acceleration (F = MA).

The innovation Einstein brought to this was to consider the frame of reference (in this case the elevator) bringing it to extremes, such as accelerating the frame at the speed of light.

A couple of things came of this:

  1. The Cartesian frame of reference Newton presupposed turns out to be much more dynamic than Newton or anyone imagined (space bends). Newton's Cartesian space was static and rigid.
  2. Time's dimensional relationship to space was proven (time slows as acceleration increases)
  3. All things being equal, Newton's gravity was pretty good except as the boundary conditions were approached (extreme acceleration, etc)

So yes, prior to Einstein physicists did understand things accelerated due to gravity, at least since Newton who formulated an equation to describe this.

share|cite|improve this answer
This neither answers the question nor clearly explains the motivations and consequences of general relativity. – Jerry Schirmer Dec 24 '13 at 15:59

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.