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Physical laws are supposed to be invariant under choice of metric units. The meter and the second, for instance, are arbitrary choices.

If we had come upt with other units of measurements, the physical laws would be the same.

But as to how far can this be stated.

Suppose something very unordinary. We had choosen our unit of length to be the distance between us, and a far away galaxy cluster.

In old units, this galaxy system is about 5 billion lighthears away.

Don't worry about the fact that such a choice would be rather unpractical. We only hypothetize and theorize.

Suppose we would define that as our unit of measurement. Would all physical laws still be the same?

If so, the please explain why all kinds of matter seems to be contracting on the long time scale...

Is physical law invariant for all units of measurement?

We suppose for instance that things like atoms, don't change in size.

But how can we know that?