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Why “space” is hard to understand

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Artist concept of Gravity Probe B orbiting Earth to measure space-time/NASA

From Dan Falk at Nautilus:

In his popular book The Fabric of the Cosmos, physicist Brian Greene explains that although Einstein’s theory demolished Newton’s absolute space, it gave us something else in its place—a four-dimensional structure known as spacetime—and this, Greene argues, is absolute. You and I might disagree about the duration of a parade, or the distance that the marchers covered—but we’d agree on the total distance through spacetime between the start and end of the parade. This is hard to picture, since we can’t see in four dimensions, but it’s guaranteed by the equations in Einstein’s theory.

And yet, this is not Greene’s final word on the matter. Physicists now suspect the “Higgs field,” believed to endow particles with mass, permeates the universe. Although Einstein’s spacetime might serve as a reference frame against which accelerations can be measured, the Higgs field goes one better: By offering resistance to anything that passes through it, it may explain why objects have inertia in the first place.

Still another idea comes from Arizona State University physicist Paul Davies, who points out that “empty” space is actually a seething foam made up of short-lived subatomic particles popping in and out of existence. This quantum “vacuum frolic,” he says, could serve as a substitute for absolute space. More.

Quantum vacuum frolic?

Liquid space

In 1976 I began investigating what quantum mechanics might have to say. According to quantum field theory, the vacuum has some strange properties. Heisenberg’s uncertainty principle implies that even in empty space, subatomic particles such as electrons and photons are constantly popping into being from nowhere, then fading away again almost immediately. This means that the quantum vacuum is a seething frolic of evanescent “virtual particles”.

Although these particles lack the permanence of normal matter, they can still have a physical influence. For example, a pair of mirrors arranged facing one another extremely close together will feel a tiny force of attraction, even in a perfect vacuum, because of the way the set-up affects the behaviour of the virtual photons. This has been confirmed in many experiments.

So clearly the quantum vacuum resembles the ether, in the sense that there’s more there than just nothing. But what exactly is the new version of the ether like?

Far from abhorring a vacuum, nature may have worked very hard to create one. More.

That’s commendable on nature’s part. Oh, wait …

See also: Does space exist without objects? Nautilus: Instead of saying space brings order to the world, you can say that the world is ordered and space is a convenient notion for describing that order


Copernicus, you are not going to believe who is using your name. Or how.

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3 Replies to “Why “space” is hard to understand

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    Mapou says:

    Kant asked, “if space exists, where is it?” But why stop there? The same question can be asked of any number of things that physicists believe exist. If time exists, where is it? If infinity exists, where is it? If a number exists, where is it?

    Physics will come of age when physicists fully figure out the difference between abstract concepts such as spacetime and concrete objects like particles. If spacetime is abstract (i.e., if it does not exist), then everything is absolute. This is because all properties, including positions, are intrinsic to particles. The relative, too, is abstract.

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    mike1962 says:

    if space exists, where is it?

    If I create a “virtual reality” computer program that has an array of objects, where each object has a property of “location” relative to other objects in the virtual world, where is the “space”?

    If our universe is a virtual world, and there are good reasons to think it is, there is no positive space. There are only objects in relation as defined by their properties.

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