Space is not a Rubber Sheet.

You have probably been told this is what gravity does... but the rubber sheet is an overly simplified representation. Gravity acts more like an “omnidirectional vacuum cleaner” than a “rubber sheet”. The real players are mass and spacetime. Mass is what creates gravity, and gravity warps space and time, spacetime. Scroll down to see what's really happening.

In 1915, Einstein published the general theory of relativity.

In 1915, Einstein published the general theory of relativity, which evolved from his previous work, special relativity, developed a decade earlier. Special relativity unified space and time into a single fabric known as spacetime.

The 1915 theory demonstrated how mass interacts with spacetime. It predicted that spacetime curves around mass, and this curvature is what we perceive as gravity. Along with this primary prediction, several other intriguing predictions emerged, predictions that have made technology like GPS possible.

Gravitational lensing
Shifting orbits
Gravitational redshift
Time dilation
Gravitational waves
Black holes
Frame dragging

We have successfully proven each of these predictions. But relativity is far from done… it is incompatible with quantum mechanics, and the unification of the theories has eluded not just Einstein, but every other scientist since.

Space is not the final frontier. It is the eternal frontier, expanding faster than the speed of light.

Space in its simplest explanation is the combination of distance and time ruled by energy. Energy is also where mass comes from: E = mc². You could think of it as liquid, in fact the math to describe the properties of liquids oddly applies to space, but unlike a liquid it compresses and stretches.

Gravity means that any object with mass attracts other objects with mass.

Newton says the apple, and imperceptibly the Earth, simply traveled to meet one another. In relativity, an apple falls because its internal clock is going faster than the planet’s clock. Why? Because it’s in less dense space. Since density is determined by mass and distance from that mass, the apple falls simultaneously to Earth, and through time. Spacetime.

But it is mostly falling through time.

What? You say. Yes. The pace of time at the surface of the Earth is a number. Two meters up, that number is bigger, by about one part in 10¹⁶ per meter: a sixteenth decimal place difference and basically unnoticeable. But multiply it by a constant, in this case the value of one second expressed as distance, c, and we can see how it falls through time rather than space. Mathematically a fall has two axes: the time you fall for, and the speed you reach. Each one carries a second. Use c on one axis and c on the other: c².

c = 3×10⁸ (one second, expressed as distance; happens to be the speed of light) c × c = 9×10¹⁶ (one c per axis) gain = 1.1×10⁻¹⁶ (clock speed gained per meter, measured; the time difference between clocks) g = c × c × gain ≈ 9.8 m/s²: speed gained each second; gravity calculated as a rate of acceleration

We have the angle and the area so we can use a triangle and good old algebra to determine the fall duration: 0.64 seconds. That time as distance is 191,000 kilometers. The apple fell two meters through space and half way to the Moon through time.

Mass compresses and stretches spacetime all at once.

Making it “denser” at the center and less dense on the edges, creating an omnidirectional curvature around a body. The greater the mass, the greater the curvature. The size of an object is not always related to how much gravity it has... for instance Saturn would float in a big enough ocean, making it a “light” planet. Notice how strong gravity bends light, creating a gravitational lens that lets an object hidden behind another one be seen.

Time Travel

But wait. What about a ship traveling to Alpha Centauri? Earth ages 4 years the ship 2... and time travel. Right. Time flows relative to the speed you are traveling so the faster you go, the less time you experience. And time travel... who knows... maybe time flows backwards relative to something, but nothing we can check for now.

Call a point in space where all gravity from all sources cancels out, the baseline.

Near a mass, space’s “density” increases and it gets denser the closer you get to the center. Going outward it thins back toward the baseline, and it reaches the baseline only at infinite distance. Which means that every object in space is influenced by one another. Though the effects could never be measured.

Mass & Gravity

Time is the unintuitive one: compression makes time slow, but the numbers might lead you to believe that time increases. Why? Because space is denser. Like denser water, the harder it is for something to pass through. So the number, in case you were wondering, is the compression rate for both time and space. (Nothing is to scale here, by the way. It is just an illustration.)

About

I work in IT. Astronomy and physics have been the passion alongside it for as long as I can remember.

I built this because the rubber sheet does not explain gravity properly. It shows a ball denting a surface, which is a picture of something falling into a hole. It is not a picture of what mass does to space.

Relativity is also hard to grasp if you do not understand that you need a point of reference. That was my own sticking point while making this. Things kept getting described as relative without the thing they were relative to ever being named. Once you fix the reference, it stops being slippery.

That is why the stars are here. They give you frames of reference for navigation, and they do exactly the same job for time dilation.

A word on scale. Anything that can be shown truthfully is: the Moon is drawn at its real size beside Earth, the axial tilt is the real 23.44 degrees, the stars sit at their catalog positions, and the clocks run at real gravitational rates with no exaggeration anywhere. Some things cannot be shown truthfully and still be seen at all. The orbit is drawn far tighter than it is, Earth’s wobble about the barycenter is magnified, and above all the grip mass has on space is increased by an enormous factor; at its true strength the lattice would look perfectly straight. Wherever a figure is exaggerated the panel says so and shows you the real one beside it. The shape is honest. The size of it is not.