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We say space-time is not rigid.

But if, e.g. space expands then even the 'meter' stick we use to measure distance expands - (I guess distance is nothing but number of times a meter stick is repeated to reach from pt A to pt B which then won't change) - then how will we ever know that space (i.e. distance between A and B) has 'expanded'

What is it that I'm missing in this - Why am I wrong?

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  • $\begingroup$ There is a big difference between the cosmological expansion of space since the Big bang and the appearance of distance to observers travelling at relativistic speeds. Which are you thinking of? $\endgroup$
    – Guy Inchbald
    Commented Mar 14, 2020 at 14:18
  • $\begingroup$ this answer of mine might help physics.stackexchange.com/questions/529799/… $\endgroup$
    – anna v
    Commented Mar 14, 2020 at 17:01

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But if, for e.g. space expands then even the 'meter' stick we use to measure distance expands - (I guess distance is nothing but number of times a meter stick is repeated to reach from pt A to pt B which then won't change)

This is not correct. The number of meter sticks from A to B does increase. If A and B are each stationary with respect to the cosmic microwave background and are exchanging light signals they will each find that the other’s signals are redshifted and progressively delayed. Since the meter is based on the speed of light, this indicates an increasing number of meters between them

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  • $\begingroup$ To a relativistic observer it is correct: objects near lightspeed do appear to compress in the direction of travel. $\endgroup$
    – Guy Inchbald
    Commented Mar 14, 2020 at 14:13

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