Which way does light travel in a homogeneous medium?


See what Einstein said in his 1920 Leyden Address: "This space-time variability of the reciprocal relations of the standards of space and time, or, perhaps, the recognition of the fact that 'empty space' in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gμν), has, I think, finally disposed of the view that space is physically empty".

Einstein was saying space was a medium, an "aether", and that a gravitational field was a place where space was neither homogeneous nor isotropic. Light curves downwards in a gravitational field, where space is inhomogeneous. If space is homogeneous, light doesn't curve at all. It goes straight.

You might think this aether is archaic, but it isn't. Julian Schwinger wrote a paper in 1949 called quantum electrodynamics II : vacuum polarization and self-energy. He was saying space is a polarizable medium. You can also find modern authors saying much the same thing. See for example the 2008 paper Inhomogeneous vacuum: an alternative interpretation of curved spacetime. Also see the Wikipedia aether theories article and note the 2005 quote by Robert B Laughlin: "it is ironic that Einstein's most creative work, the general theory of relativity, should boil down to conceptualizing space as a medium when his original premise [in special relativity] was that no such medium existed". Laughlin also said space is more like a piece of window glass than ideal Newtonian emptiness. He finished up saying this: "the modern concept of the vacuum of space, confirmed every day by experiment, is a relativistic ether. But we do not call it this because it is taboo".

How likely is it that aliens reproduce differently?

It is probably likely to be so, considering that aliens would have evolved separately from Earth organisms. However, it is not impossible that some alien animals might also reproduce similarly to us, due to the fact that a reproduction cycle with more than two sexes might become too unwieldy, since you'd require all

Is human a threat for AI?

An interesting question that turns the common one on its head.Perhaps 'human' is a step on the evolutionary ladder for AI.Just as the JCB needed the weak physical body of humans in order to appreciate the advantages of the power given by hydraulics,

What are the most alien-looking places on Earth?

The Deep Ocean.We know more about planets that are 40 million miles away that we do about our own ocean.When it comes to planets, like Mars, we know quite a lot of stuff about it. We know how far away it is, its general shape and size are totally