Einstein’s Final Riddle

Why does the Universe expand the way it does? Why does it accelerate? Einstein’s equations offer an explanation of gravity that works on the scale we know, but do they work on the grandest scales of space and time? Humanity now has a way to find out. The General Theory of Relativity predicts the behaviour of gravity, and includes a term known as the cosmological constant.  Einstein added this term to make the universe static and unchanging, as he believed it was.  But when the expansion of the universe was discovered by Edwin Hubble, Einstein regarded it as ‘the greatest blunder...

Do You Weigh Less When the Moon is Above You?

Everything in the universe that has mass, has gravity.  It’s easy to understand that the Earth has gravity and pulls on us all day every day.  It’s just as easy to understand that the Moon and the planets and the Sun have gravity too, and they are all pulling on us. It’s much harder to understand that every person on Earth has gravity too.  You exert a force on the Earth, and on every other human.  You also exert a force on every rock, tree, and creature that roams the Earth, and they all exert a force on you.  The...