# Galileo's dictum and how light cannot violate it

Okay. So I've been told that the speed of light is constant and cannot violate Galileo's dictum, but even if it weren't constant (in a vacuum), how would it violate it anyway?

Say you are on a train and you are stationary. Another train is traveling towards you with velocity v and shoots a bullet with a velocity j. The apparent speed from your point of view would be v+j.

If you were instead traveling towards the train, at a velocity v, and they shot the bullet at velocity j, the perceived velocity of the bullet would still be v+j.

Thus, it must be impossible to tell if you are moving towards something or if something is moving towards you, even if the bullet's speed from an observer's point of view is different (in one example, it is v+j, in the other it is just j)

So if you replace the bullet with light, it would not have to be measured as being the same speed by an observer for the people on the trains to not be able to tell whether they were moving.

Can someone explain why it has to be constant not to violate Galileo's Dictum?

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