Q&A: The Expansion of Space
The Expansion of Space
Question
Hello Rabbi,
It is commonly accepted in physics to say that space itself is expanding. That is, if today I am holding a ruler 30 cm long, tomorrow that same ruler will be larger because of the expansion of space. But seemingly I should not be able to notice this, because I have no scale against which to compare that growth. In order to notice that space is expanding, you would need to compare it to some absolute scale that is not itself expanding. In addition, I understood that the expansion of space is not subject to the limitation of the speed of light. That is, two objects in space can move away from each other faster than the speed of light if that distancing is due to the expansion of space. In articles that explain this idea, they insist on saying that space is not expanding into something, and that there is no additional external space outside the expanding space. They often bring the example of an inflating balloon with dots on it, or raisin bread rising, where the dots/raisins move away from one another during the inflation. But those examples are דווקא intelligible in a situation where there are two spaces, one static and one inflating. I cannot understand how there can be an inflating space without there being some other space relative to which that inflation is supposed to be measured. Do you perhaps have some explanation of this that could make a bit of order out of the confusion?
Best regards,
Answer
The measuring scales themselves do not expand. The meter rod would be longer, but its length would no longer be a meter. How exactly this would be measured is another question. For example, one can think about how long it would take light to cross that distance.
It is hard for me to explain this intuitively. Indeed, it is hard for us to accept expansion that is not into something already existing, but in mathematics they certainly distinguish between intrinsic and extrinsic curvature (relative to the surroundings) of surfaces and curves. That can serve as some kind of example. Perhaps if you think about someone living on the surface of the balloon: if the balloon inflated, he would have to travel a greater distance between two points that had been close, and so he would feel that the balloon had become larger, even though he would have no awareness at all of the space around him. That is not decisive, of course, because perhaps he would infer that there is a surrounding space even if he does not see it.
Discussion on Answer
The speed of light does not change.
But the person living on the balloon also grows, so it would still take him the same amount of time to get from one point to another.
(And isn’t the speed of light also related to space?)