Two atoms were out for a run at the speed of light; one says to the other
Well that’s just it; they wouldn’t be able to say anything. Atoms communicate by exchanging messenger particles, photons; it is this exchange which is the basis of chemistry, biology, even consciousness. Photons are, among other things, particles of light; as such they travel at the speed of light. You might think of the speed of light, c, in absolute terms, as an inviolable law of nature; or you can just accept it as the speed at which photons happen to travel. Either way, if the two atoms are travelling at the speed of light, it is all a photon can do to keep up; there will be no exchange of photons, there will be no chemical, biological or conscious processes. It would be as though time stood still for any such pair of atoms, the same goes for collections of 6 or 7 billion, billion, billion atoms such as you or me.
Of course, Einstein’s theory of Special relativity demonstrates that atoms can’t travel at the speed of light, but as they accelerate towards it, the paths that messenger photons have to take become longer. Suppose the two atoms were one above the other. If they are at rest a photon can take the shortest route between the two, straight up or straight down. For a similar pair of atoms moving left to right, were a photon to travel vertically, it would miss, because it’s target has moved on. If it is to reach the target atom, a photon has to aim ahead and set off at an angle; it’s speed remains c, but now it has a horizontal as well as a vertical component. The greater the speed of the target, the further ahead the photon must aim and the greater the angle. This increases the horizontal component of the photon’s speed, at a cost to it’s vertical component. When such a system is compared to a stationary one, there are fewer exchanges; less chemistry, less biology, less consciousness.
What is not clear is whether there is any less time. Certainly a body in motion will age less compared to one that is stationary, but does this mean there is a discrete substance that the moving atoms have had less of, or is it simply that less has happened to them? Of all people, it was Plato who first argued that time is no more than the passage of events and although he is one of the ugliest blights on western civilization, on this occasion he may have a point.
Einstein though said that time is not discrete, that it is bound to space creating space-time, the warping of which, it is said, gives rise to gravity. General relativity was given support by the bending of starlight during a total eclipse. The position of stars was a matter of record when viewed at night time from one side of the sun, the eclipse allowed their position to be charted from the other side and the two were found to be different. The photons that made up the starlight experienced a slight deflection towards the sun; in other words, their speed adopted a component that was aimed at the sun. In general, anything that travels through a gravitational field experiences a deflection towards the source. Our two atoms for instance; though as atoms they might be stationary, their components, the electrons and quarks that make them up are not.
Particles travelling through gravitational fields behave as though they were being refracted by a denser medium. Gravity is supposed to be universal; all objects are attracted to all others. Since the universe is expanding it follows that the gravitational field of every object is also expanding and has been doing so for the last 14 billion years, with no apparent effect on the strength of gravity. This gives the impression of a particle as an inexhaustible source of gravitational field that rarefies as it spreads. But to call it gravitational is misleading, because clearly it is a repulsive force which in condensed states causes localised attractive anomalies; better to call it stuff. It is the repulsive nature of stuff that provides the thrust of photons and the energy of atoms and they, like all particles, are just knots and eddies.