Light can travel for billions of years but it has no time

A photon from a star billions of light-years away takes no time to reach a telescope. It’s not too little time. There is nothing.

That result is not a measured or poetic way of speaking. It comes directly from the mathematics of special relativity, and it points to another strange thing about the nature of the universe: time is not a fixed background in which events occur. It’s something that changes depending on how fast you’re moving in space.

A clean entry point into this problem is a thought experiment, even if it has become a laboratory result.

Imagine two identical atomic clocks, connected and placed side by side. One remains standing. One is taken on a high-speed plane and returned. When the moving clock comes back, it shows the time that has passed less than the time that is behind. This result has been confirmed by experiments, the most famous of which was the 1971 experiment by physicists Joseph Hafele and Richard Keating, who flew cesium clocks around the world and compared them with standards based on the ground.

The difference was small, measurable because atomic clocks are incredibly accurate. But it was true and consistent with the predictions of the relationship. Time is not a universal thing. How much it passes depends on the movement state of the measuring clock.

The technical name for this phenomenon is time dilation. It is the result of one simple experimental fact about light.

On a daily basis, the pace is clearly compounded. A ball thrown from a moving car travels faster than one thrown from a stationary position, with the direct velocity of the throw added to the speed of the car. That’s the Galilean relationship, and it works well for baseball.

Light does not interact.

Measurements made at the end of the 19th century, and confirmed repeatedly since then, have shown that the speed of light in empty space is the same for all observers regardless of the source’s motion. A beam of light from a stationary torch travels approximately 299,792 kilometers per second. Light from a rocket traveling at half that speed also travels at about 299,792 kilometers per second. The speed of the rocket adds nothing.

Albert Einstein took this result seriously and in 1905 he published the special theory of relativity, which accepts the invariance of the speed of light as the basis and results. One of those consequences is that time will not be everywhere. If the speed of light is the same for everyone, then the dimensions of time and space must vary between observers with relative motion, just enough to keep that speed constant.

The mathematical model that explains this was put into a solid geometric phase by the mathematician Hermann Minkowski in 1908. In Minkowski’s formulation, space and time are combined into a four-dimensional structure called spacetime, and travel through space-time is prohibited: everything moves through the combined dimensions of space and time at the full speed of light. Use most of that rate for spatial motion, and less is available for temporal motion. Move fast in space, and time slows down.

Physicists have a common way of making this concrete. Imagine a clock made of two parallel mirrors with light striking between them. Each return trip makes one mark. This is called a light clock, and while no one makes timekeepers that work this way, the geometry is transparent enough to make the physics inescapable.

When the timer is at rest, the sound of the light goes up and down. When the same clock is observed moving horizontally at high speed, the light beam must follow a diagonal path, because the mirrors slide sideways as the light travels upwards. The speed of light is constant, so covering a long diagonal path takes more time. Therefore, the ticks of a moving clock move as slowly as a person standing still.

This is not a malfunction or a visual trick. The geometry of motion has really changed the speed at which time accumulates.

Most of this type of effect involves something physicists call gamma, which is defined as the square root of one minus the velocity squared divided by the speed of light squared. At a daily rate, gamma stays close to one, so time dilation is not noticeable. Click on 90 percent of the speed of light, and the gamma jumps to about 2.3, slowing the clock down to 43 percent of normal. At 99 percent, gamma increases to 7.1. At 99.9 percent, it goes up to about 22. As the speed approaches the speed of light, the gamma increases without limit, and the time speed for the moving object approaches zero.

Light is made of photons, which have no mass. The laws of special relativity require anything that has no mass to travel at the speed of light, and anything that has mass to travel slowly. This is not a practical limit but a characteristic of the space-time structure.

The quantity involved in understanding the experience of particle time is called absolute time, time measured relative to the particle world in spacetime. For anything that moves slower than light, the correct time is good. Something gets old. The clocks on the wall tick. The process is ongoing.

For a photon traveling at the speed of light, the absolute time interval is zero.

This means that there is no elapsed time in the photon’s path. The emission event and the absorption event, regardless of how far apart they are in space and time as measured by observers at rest, are separated by zero exact time from the point of view of the photon. A photon emitted from a star around the time the first complex animals appeared on Earth, and absorbed by a detector today, spends hundreds of millions of years in our calendar. Depending on the photon’s position in space, those events occur simultaneously.

This does not mean that photons have experiences or that they somehow know their state. The result of the correct time is a calculation, not a statement about the mind. What this means is that the mathematical structure of spacetime does not provide a temporal separation to events that are related to the null path, which is what the photon follows.

Special relativity describes a flat atmosphere, which is ideal when gravitational effects are negligible. The general theory of relativity, published by Einstein in 1915, extends the framework to curved space-time, where massive objects distort the geometry in which all objects move.

One of the effects is a gravitational lens: light traveling near a massive object follows a curved path, which can be much longer than a straight path. Observers see the light arrive later than it would on a direct path, an effect that has been observed and measured for sources from nearby stars to distant quasars.

A long way does not change the exact amount of time. An idle path generally still has the right timing, no matter how curved it is. Light takes a long time to reach our clocks, but the exact time according to its global position is always zero. Geometry bends; the basic result is not.

Time dilation is not limited to thought experiments about distant stars. The Global Positioning System provides a daily model of the relationship at work.

GPS satellites orbit at an altitude of about 20,200 kilometers and travel about 3.9 kilometers per second relative to the Earth’s surface. Their motion causes their ship’s clocks to run 7 microseconds a day slower than the clocks on the ground, as predicted by special relativity. Their high altitude, where Earth’s gravity is weaker, causes their clocks to run faster than ground clocks by 45 microseconds per day, as predicted by general relativity. The effect is that satellite clocks gain about 38 microseconds per day compared to ground-based clocks.

That sounds trivial. It is not. GPS position calculations are based on the precise timing of signals traveling at the speed of light. An uncorrected 38-microsecond error per day can accumulate position errors of more than 10 kilometers per day, rendering the system useless for navigation. Engineers always adjust for both relativistic effects. Without those fixes, the GPS would fail within hours.

The bottom line of all this is that there is no universal clock running behind the universe. Everything has its own time, which is determined by its movement and its gravitational environment. Two observers in relative motion will measure different times between the same pair of events. Two observers at different points in the gravitational field will also oppose each other. Even if it’s a mistake. They measure different things, which is the right time according to their worldly ways.

The one quantity that all observers agree on is the spatial interval between events, a combination of spatial and temporal divisions that remain constant across reference frames. Synchronized events are such that this separation is zero, as for light, they have an exact time between them according to definition.

Light follows the boundary of the causal structure in spacetime. Nothing of mass can reach or exceed that limit. Signals cannot travel faster than light, which is why the speed of light defines the limit of cause and effect. An event cannot influence another event if getting to it would require traveling faster than light.

Along that line, where the causal sides of spacetime are drawn, time slows down and approaches zero. It is zero. Travel, in any meaningful physical sense, is not at all clear.