We see light from distant galaxies which are billions of light-years away...so the light has been traveling for billions of years. Why doesn't it get tired?
Those are some pretty "deep" questions you pose there, Verginius Rufus.
Regards,
“does he explain why there is gravity to begin with?”
No, Newton had no explanation for it, he just found the equations to describe it (at least to some degree of accuracy).
Einstein did try to explain it, though I’m not certain that his explanation is anything more than a bit of sleight-of-hand. He basically said that gravitational attraction is kind of an illusion, that everything is still moving in a straight line when viewed in reference to “spacetime”, but the curvature of “spacetime” caused by the presence of mass makes it appear to us, observing in 3-dimensional space, like there is an acceleration between the two masses. As for why mass should deform “spacetime” (really just time, since the space component can’t be deformed), I don’t think Einstein really gave a good explanation for that.
“We see light from distant galaxies which are billions of light-years away...so the light has been traveling for billions of years. Why doesn’t it get tired?”
Well, that question is simple at least. Newton’s 1st Law of Motion: any object in motion will tend to stay in motion unless acted on by an outside force. Again, Einstein’s answer would be a bit different.