Q: Why do heavy objects bend space and what is it they are bending?

Physicist: Scientists can generally answer “medium complicated” questions.  The really hard ones are too damn hard, and the really simple ones are often “just the way things are”.  Or they may be really, really difficult questions in disguise.  You’ve found one of the fundamental questions that no one seems to have a good answer for.  We also don’t know why the mass, M, we use when talking about inertia (as in F = MA) is the same as the mass we use when talking about gravity (as in F = \frac{GMm}{R^2}).  The best theory I’ve ever heard is that matter is actually made of bunched up “knots” of space-time, and even that isn’t a terribly good a theory.

The second half of your question we do have an answer for, it’s just a little mind bending and hard to picture.  What heavy objects are bending is space itself. The way you detect space (follow me here) is with rulers or stretched out strings, or stuff like that.  Anything that measures distance.  Near heavy objects the distance between points is greater than you would expect, and you can use this fact to detect and measure the stretching of space.

The usual trick when trying to picture higher dimensional space is to knock off a couple of dimensions, and picture that instead.  3 dimensions is too complicated, and 1 is stupid, so here’s 2!

A circle and it's diameter in flat space

A circle and its diameter in flat space. Here D=6.5'', C=20.4'', and 20.4/6.5=3.13 ≈π.

A circle and its diameter in curved space

A circle and its diameter in curved space. Here D=7.2'', C=20.4'', and 20.4/7.2=2.83<π

Take the circumference, C, and the diameter, D.  You’ll notice that on paper (flat space) \frac{C}{D} = \pi.  Which makes sense, since that’s the definition of \pi.  However, on the balloon (curved space) \frac{C}{D} < \pi, since the diameter is longer.  This effect is huge (infinite) for things like black holes, but for an object like the Earth the effect is tiny (ignore the hell out of it).  The stretching of space caused by the Earth’s gravity (well… that is Earth’s gravity) increases the diameter of the Earth by about 18mm.

That is, the diameter you get by measuring the equator and dividing by \pi (D = \frac{C}{\pi}) is the “flat space diameter”, the diameter you get by physically dropping a rope through the planet is the true, “curved space diameter”, and the difference between these two is 18mm.

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54 Responses to Q: Why do heavy objects bend space and what is it they are bending?

  1. The Physicist The Physicist says:

    You’re exactly right, I should have written “spacetime”.

  2. Ray says:

    Q. please help..maybe I’m just thick but In which direction is space” bent “by a moving body of given mass and gravitational force ? In all the programs I,ve watched the diagrams all indicate that the moving mass tents to bend it’s surrounding near space beneath itself, which indicates that the body’s mass distorts the space beneath it.However if space itself is void of mass how can it be distorted in any direction let alone in only one direction.If space is not a true void, i.e.space is made up of some form of mass then surely a moving body travelling through space would distort space all around it.Yes I know I’m thick!!! Can anyone clarify? Thanks.

  3. Don says:

    The statement “space is distorted below” is not totally accurate. I believe it is the result of how the distortion in three dimensions is conveyed to the reader/viewer. And it is typically displayed with a weight in the middle of a rubber sheet. Again, this is done to help convey the principle of what happens.

    I don’t know how much correlation there is between the spacial distortion and the rubber sheet demonstrations. Therefore, take this with a grain of salt … If the rubber sheet had grid lines on it, as some images do, then dropping in the weight causes the grid lines to be distorted towards weight. I take this to mean that space is distorted towards (or pulled towards) the mass causing the distortion. So the actual spacial distortion is “towards” the mass, not “below” it.

  4. Orien igney says:

    Many brilliant minds have mulled this question over for centuries without success. Only Einstein came up with the theory of gravitational lensing which was later proven as fact, that something?, actually bends the track of photons around celestial objects but does not necessarily bend space itself. Personally I believe the innate property of magnetism in all matter will be proven as the catalyst holding the world together, one way or the other.

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