Time travel

Toss a ball straight up in the air while walking, and it will fall back into your hand just as it would when standing. The ball has only up and down motion for you. For a stationary onlooker, however, your ball tracks more quickly through a longer parabolic arc, with up, down and forward motion. The frame of reference thus determines the dynamics of motion. Galileo perfectly understood this basic principle of relativity1; Newtonian physics entirely accommodates it2.

If you travel fast and far enough, on a long-haul flight for example, then you might detect relativity also in the ticking of clock time3. Bring along an atomic clock on an eastward flight around the world, outpacing Earth’s own eastward rotation, and you will find at journey’s end that time itself has passed detectably more slowly for you than for those you left behind, and yet more slowly than for those flown westwards4. Although you won’t feel it, you will have aged fractions of a microsecond less than them.

Board a rocket to space, and you might detect another influence on time, explained by gravity5. The atomic clock next to you will tick incrementally faster the further you lift up and away from Earth’s gravitational pull6. Satellite communications and GPS only work by accounting for dependencies of time itself on the frame of reference7.

Suppose that you wish for your space odyssey to carry you ever faster from Earth8. As your relative speed increases, so also does your relativistic mass3, ever raising the energy requirement to go faster. You approach only asymptotically towards the speed of light, meaning that you never quite attain its 299,792 kilometres per second in the vacuum of space9. Special relativity reserves this ‘universal’ speed – constant across all frames of reference10 – for the photon, the massless quantum of all electromagnetic radiation11,12. You can always radio home to Mother Earth, then. But beware the greater passage of time back there, for you will be reaching years into your future13.

Galileo Galilei’s understanding of basic relativity, 1632, built on the definition in Aristotle’s Physics (c. 350 BCE) of an entity’s place in terms of the surroundings that contain it. Aristotle’s relational concept of place led him to reject the possibility of empty space in the sense of an absolute void between things. Galileo subsequently allowed for the void as a local condition, and Newton went on to treat it as a foundational property.

Isaac Newton’s classical mechanics, 1687, specifically Newton’s First Law which would define your and the onlooker’s perspectives as alternative inertial frames of reference.

At the age of 16, Albert Einstein wondered what it would be like to run after a wave of light at the speed of light. He was tempted to think the wave would appear frozen in front of him. But that notion was incompatible with experiments published a few years earlier. In 1887, Albert Michelson and Edward Morley had demonstrated that light always has the same speed for whoever is measuring it, regardless of their motion10. Einstein faced an apparent contradiction, which 10 years later would bring him to a radical solution: the universal speed of light meant that time itself must vary between observers in relative motion. Time was not absolute.

Let’s suppose that Einstein is falling behind just a little in his pursuit of the light wave. He shines his torch in front of him. Does the light from his torch catch up with the light wave ahead? No, it can’t travel faster than the universal speed of light. The light wave he’s chasing and the light wave sent out from his torch both travel at the same speed. This can be true for Einstein’s own measurements only if something else changes: his pocket-watch would run more slowly and distance would shorten along his direction of motion, compared to the measurements of observers moving relative to him. In such cases of steady relative motion, each observer finds that the other’s clock runs more slowly. (This happens without contradiction because the observer’s frame of reference dictates what counts as the same time when comparing distant clocks). As much as time stretches, distance must shrink in compensation to preserve light’s universal speed, measured as distance over time. Space and time thus form a single dynamical structure: spacetime. (And since simultaneity is relative to the frame of reference, physics rejects the concept of a universal present moment between past and future).

Einstein published his resolution as the special theory of relativity in 1905, introducing the ideas of time-dilation and length-contraction for moving objects, relative to any observer.

In 1972, Joseph Hafele and Richard Keating flew four atomic clocks around the world on commercial airliners, once eastward and once westward. On their eastward round-trip in the direction of Earth’s rotation, the clocks lost a few hundredths of a microsecond compared to a ground-based reference clock. On the westward trip against Earth’s rotation, they gained on the reference by a few tenths of a microsecond.

These findings accorded with predictions of special and general relativity for asymmetrical movements that brought the travelling clocks back to the same starting point. For speeds of motion as they would be perceived by an inertial observer looking down on the North Pole from a great distance, the fastest-moving (eastbound) clocks ticked some 60 nanoseconds more slowly than the reference clock, which ticked some 270 nanoseconds more slowly than the slowest-moving (westbound) clocks. Thus did time dilate in accordance with special relativity. Compared to the reference, time gained by the westbound clocks greatly exceeded time lost by the eastbound clocks because the high altitude of their flights weakened the effect of time dilation due to Earth’s gravitational pull. This prediction of general relativity was expressed as an additive effect for the westbound clocks and a cancelling effect for the eastbound clocks.

The tiny overall time differences between all the atomic clocks illustrate the almost negligible effects of special and general relativity at these scales, allowing us to perceive time as effectively absolute in everyday experience.

Albert Einstein’s 1915 theory of general relativity, provided a framework for identifying the geometry of spacetime with the gravitational field, leading to the contested conclusion that spacetime is the gravitational field. Since that field pervades even regions empty of matter, the theory is consistent with Aristotle’s rejection of an absolute void. By extension, the relational nature of reality means there can be no edge of the Universe outside of which to view its entirety. The Universe closes on itself such that it surrounds us wherever we find ourselves within it.

Optical lattice clocks, invented in 2015, have now achieved an accuracy of 1 second in 260 billion years; they tick detectably faster with each millimetre of altitude.

GPS was first developed in 1973 by the US Department of Defense to enhance the positioning capability of its military forces.

Only slightly more wishful than the Starshot lightsail, 2018.

The Danish astronomer Ole Rømer achieved the first quantification of the speed of light, in 1676, by timing the eclipses of Jupiter’s moon Io, as it orbited behind Jupiter every 42.5 hours. He noticed that the eclipses appeared some minutes later when Jupiter was furthest from Earth, and earlier when it was closest to Earth. This discrepancy led him to estimate the time taken by light to cross Earth’s orbit, at 22 minutes. He thereby provided the first observational proof that light is not instantaneous, as Galileo had suspected, but has measurable speed.

Rømer’s measurements applied to the speed of light as it passes through the vacuum of space. Light is slowed by passage through substances, including air which takes 90 km/s off its speed, water which reduces its speed by one-quarter, and glass which reduces its speed by one-third. This slowing is caused by interference with electromagnetic waves of jiggled electrons, and full speed is restored instantly upon return to a vacuum. The pace changes caused by substances have the effect of refracting the light, for example making a straight stick appear bent at its point of entry into water.

Michelson-Morley experiment, 1887. They used a half-silvered mirror to split a single beam of light into two beams travelling at right-angles to each other. Each beam reflected back and forth off mirrors to finally recombine, producing an interference pattern of light and dark fringes that was sensitive to tiny differences in travel times. They hypothesised that the speed of light would combine with the motion of Earth as it orbits the Sun at about 30 km/s, causing the beam travelling along Earth’s path to take slightly longer than the one travelling across it (just as a swimmer takes longer to swim up river and back than the same distance across and back). They mounted the whole apparatus on a platform floated in a bath of mercury, allowing them to change its orientation relative to Earth’s motion. Rotating it should have shifted the interference fringes, but they saw far less than the predicted shift. Light appeared to travel at the same speed regardless of its direction relative to Earth’s motion, a result that puzzled physicists until Einstein’s 1905 theory of special relativity.

James Clerk Maxwell’s 1865 theory of the electromagnetic field explained how electromagnetic radiation such as light propagates through space as oscillating electric and magnetic fields. Einstein’s subsequent theory of general relativity revealed how the energy and momentum of these self-sustaining ripples in the electromagnetic field contribute to the geometry of spacetime, which is the gravitational field. Gravity in turn determines the path of electromagnetic radiation, for example bending light close to massive objects such as the Sun (as confirmed by Arthur Eddington in 1919).

Albert Einstein’s 1905 theory of light quanta, later named as photons. Having no mass, a photon is not an object but a mode of interaction of the electromagnetic field with whatever senses it.

For every hour you experience on a spacecraft travelling at one-quarter the speed of light, about 1 hour and 2 minutes will pass on Earth. Your hour will correspond to about 2 hours on Earth when your speed achieves 87% of light speed, and about 7 Earth hours at 99% of light speed.