(Absolute space has recently been re-conceptualized and re-named as space that has “background-independence.”)
The concept of absolute space is that space is no more than an empty stage within which events occur. Space and time form a fixed background against which material objects move. Is this the way that you think of it? Well, so do most other people. And it’s foundational to Newtonian physics. But, it’s not the way that physicists think of space today.
History of the concept of absolute space. The concept of absolute space was a subject of contention even in Newton’s day, in the late 1600’s. The renowned scientist, Gottfried Wilhelm Leibniz, a contemporary of Newton, for example, did not assume absolute space. Since the time of Newton and Leibniz, the debate about absolute space has continued.
Einstein’s Theory of Special Relativity, published in 1905, abandoned the concept of absolute space. In absolute space, any two observers would always measure the same stretch of space as having the same length. But, according to Special Relativity, measurements of length (as well as time) vary depending upon the speed with which one is traveling. For a person traveling a significant fraction of the speed of light (1/10 or more), distances shorten significantly compared to the measurement of the same stretch of space that would be made by someone traveling more slowly.
The other effect of speedy travel, the slowing of time, is described in the discussion of absolute time.
Einstein’s Theory of General Relativity includes the idea of measurements of length varying with speed but, at the same time, restores the idea of an unchanging stage. This stage is created by absolute spacetime in which matter and energy interact. A recent speculative theory, Loop Quantum Gravity, flip-flops again and abandons the concept of absolute spacetime.
Properties of absolute space. In Newtonian physics, space is thought of as a reference system against which the position and motion of an object can be measured. Absolute space is seen as having five properties:
- The same stretch of space will always be measured as the same length from all viewpoints in the universe.
- The size of the universe doesn’t change; the total size of all space is always the same over time.
- Space containing no material objects in it (no solids, liquids, nor gases) is completely empty.
- Space has no shape and cannot change in shape but is simply emptiness.
- Space does not interact with matter or energy.
In this view, space is as if seen as from the unchanging viewpoint of God looking down upon our universe. In fact, Newton believed that transparent absolute space was filled with “spiritual substance,” but that this substance was “no obstacle to the motion of matter, no more than if nothing were in its way.”* Later scientists abandoned the idea of space filled with a spiritual substance and, instead saw space as completely empty.
Newton’s view of relative velocities. Newton recognized that measurements of velocity of an object vary depending upon the observer’s position and velocity. For example, when a train passes Alice, who is standing still on a train station platform, she might measure its speed at 70 miles per hour. But, Bob, driving a car alongside the train in the same direction at 50 miles per hour, would measure the train’s speed at 20 miles per hour. This is Galilean Relativity, which was described by Galileo decades prior to Newton’s work.
However, Newton believed that the interaction of the train, Alice on the station platform, and Bob, driving his car, could all be viewed as if by God. From this privileged viewpoint, the viewpoint by which one perceives absolute space, one could identify all the true speeds involved—of the Earth, the train, Bob in the car, and Alice on the station platform.
In Einstein’s General Relativity, spacetime is absolute in a similar sense. It forms an absolute frame of reference against which true acceleration (not true velocity) can be measured.
Absolute space in contrast to Special Relativity, an example: Bob on Earth, sitting at his telescope, catches sight of Alice in her rocket ship streaking at 9/10 the speed of light right towards the sun. He realizes that, very unfortunately, Alice is only 10,000 kilometers from the sun. If he believed, like Newton, in absolute space, he would expect that rocketing Alice would also measure her distance from the sun as 10,000 kilometers. He would be wrong.
Special Relativity tells us that when traveling close to the speed of light, distances shorten. Alice in her rocket ship (if not yet baked to a crisp) measures her distance, even worse, as 4,000 kilometers from that great ball of fire.
Great balls of fire!!!
Absolute space paired with absolute time. The concept of absolute space is paired with another assumption of Newtonian physics, that of absolute time. Newtonian physics assumes that everyone, no matter where in the universe, would experience the same moment of time simultaneously. It is as if a giant clock ticked throughout the huge empty stage of the universe. Modern physics has abandoned the assumption of absolute time along with that of absolute space in favor of the absolute spacetime of General Relativity.
*Sir Isaac Newton, as quoted by Brian Greene, The Fabric of the Cosmos; Vintage Books, New York City, 2004; p. 29.