Can anyone tell me what space-time is? Is it composed of strings (if so, what medium do strings vibrate in?) If not, what is it made of?

Also, if the universe (space-time) is expanding, does that mean that the distance between everything in the universe is increasing, even at the atomic level? Is that measurable on a local level e.g. I assume my commute to work gets longer every day.

Can anyone tell me what space-time is? Is it composed of strings (if so, what medium do strings vibrate in?) If not, what is it made of?

Space-time is a model, nothing more, nothing less. It captures our notions of three spatial dimensions and time to form a concept of distance. It is not a material thing, its not made up of anything. What is material is the distribution of mass-energy in a given area, volume, or--generalized to n-dimensions--content.

Also, if the universe (space-time) is expanding, does that mean that the distance between everything in the universe is increasing, even at the atomic level? Is that measurable on a local level e.g. I assume my commute to work gets longer every day.

You're thinking of space-time as a material thing that is the fundamental constituent of matter. A more intuitive way to think of expansion is to think of individual particles in an expanding cloud of dust. Only in the case of curved space-times, our definition of distance changes as the cloud of dust moves from one positional state to another.

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Space-time is a model, nothing more, nothing less. It captures our notions of three spatial dimensions and time to form a concept of distance. It is not a material thing, its not made up of anything. What is material is the distribution of mass-energy in a given area, volume, or--generalized to n-dimensions--content.


I usually disagree with people that say that spacetime "it's not a material thing". Take as example the photons of the CMB. Their wavelenght is stretched because spacetime is expanding, so this is an indication that the photons are interacting somehow with space-time, and this interaction is dwindling the energy of the photons. What's your explanation for that fact?

I usually disagree with people that say that spacetime "it's not a material thing". Take as example the photons of the CMB. Their wavelenght is stretched because spacetime is expanding, so this is an indication that the photons are interacting somehow with space-time, and this interaction is dwindling the energy of the photons. What's your explanation for that fact?

The same way you'd explain it if space were not expanding; that is to say if the metric describing distance in the large scale structure of the universe was Euclidean. The problem with the "spacetime as material" analogy is its misleading depiction of curvature as a property that is only analyzed extrinsically, that space-time really is like a curved rubber sheet or a ball or whatever embedded in an ambient Euclidean R^4. On the other hand, we capture a lot more about both general relativity and the underlying math and its consequence in cosmology if we think just take the FLRW metric at face value and say "distance in space-time depends on the distribution of mass-energy in space and, more importantly, over time." This is a fancy way of saying that distance, like the position and time of an event, is relative.

Do you think of some aether-like ambiance when you consider that position in space and time in special relativity are...well...relative. Why should this be any more confusing in general relativity?


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