Physics has an ongoing obsession with time. Some of the most important discoveries about time have been made with so-called thought experiments. Gedankenexperimenten, as Albert Einstein called them, have advantages. They need not be practical. They use no apparatus. Anyone can play.

Many people have heard of the so-called twin paradox. It’s a Gedankenexperiment.You travel at almost the speed of light to a star forty light years from Earth, and head right back again. Your twenty-year-old twin stays home. He knows you will take forty years to get there and forty years to get back; total eighty. But, as Einstein showed, relative velocity shrinks all lengths, including the distance to the star. So you get to the star in a few months and return home a few more months later. When you arrive you find your twin has had eighty birthdays while you were away — as she expected. She is now a hundred. But you are still twenty. Einstein arrived at this conclusion with pure thought, but the phenomenon is real. Practical experiments have measured it. There is no paradox about it. Space is giving us an insight into time.

Let’s think our way through a more ambitious trip. Recently a British team of astronomers found some of the first stars in the universe forming eight-hundred-million years after the Big Bang. They found a clump of them in a galaxy thirteen-billion light-years away. The space that their light passed through to get to us was expanding and that expansion was accelerating. So now they are much further from us, more than forty-billion light-years away. But if we push our spaceship near enough to the speed of light we can shrink that distance so our velocity will cross it in a week. What will we find?

It takes us longer than a week. When we left Earth the space containing those first stars was moving away faster than the speed of light and we could no longer see them. We can catch their galaxy only because we move into space that is also moving away. When we arrive it is more than a hundred-billion light-years from Earth. Its stars are old. Those first stars were big and hot. They burned out fast and collapsed in a few million years, leaving cold remnants — black holes that formed in hypernovas. Their fierce gravitational and magnetic fields still whip thin interstellar gas into hot violence. Those remnants lasted more than a hundred-billion years, while the first stars’ light was wending its way to Earth and we were back-tracking to view their corpses.

How long will it take us to get home? Home? Neither twin nor home is waiting for us. After six-billion years the Sun morphed into a red giant and consumed the Earth. Then it became a white dwarf star long before our return. We arrive —still birthday-free—maybe five-hundred-billion years after we left. While we were traveling, space was still expanding. Was expansion still accelerating? When we clock the distances on our longer trip to find the answer, time is giving us an insight into space.

Both of our gedanken-journeys teach us a simple message: Time is motion, nothing else.

Other Reading:

Benjamin Botermann et al. (2014), “Test of Time Dilation Using Stored Li+ Ions as Clocks at Relativistic Speed”, Phys. Rev. Lett. Vol. 113, p. 120405; journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.120405

Image credit: Phys.org, http://phys.org/news/2014-09-ions-relativistic-dilation-precision.html