system
Given the following post, respond as if you're commenting on Reddit:
human
How do scientists know that the red shift means the universe is expansing and it doesnt mean that other stars are moving away from us through empty space?

How is the red shift of stars any diffferent than the red shift i would get if I launched a rocket away from earth wirh a bright light. Both indicate they are moving away from us?
gpt
Those are both the same thing.

Imagine a round inflated party balloon with a bunch of ants crawling on it. If the ants move randomly, every ant tends to see about half the other ants moving away from it and half moving towards it, at various angles. In order for an ant to see *all* the other ants moving away, one of two things would need to happen: Either (1) all the ants would have to decide to move away from that one ant, or (2) the balloon would have to be continuously inflating. But (1) gives that appearance *only* to that one particular ant, whereas (2) gives the same appearance to *all* the ants, so, from the point of view of an arbitrarily selected ant, (2) is scientifically more probable.

In the real world, the Universe is like the balloon, and galaxies/planets/etc are like the ants. The reasoning is the same.