A photon emitted from a star a billion light-years away reaches the telescope without experiencing any time. Not a very short time. None.
That result is not a loose conjecture or a poetic way of speaking. This comes directly out of the mathematics of special relativity, and it points to something truly unique about the structure of the universe: time is not a fixed background against which events unfold. This is something that changes depending on how fast you go through space.
Two watches, one disagreement
The cleanest entry point into this problem is a thought experiment, even if it is a laboratory result.
Imagine two identical atomic clocks, synchronized and placed side by side. One remains constant. Another is brought back in a high-speed plane. When the travel clock turns back, it shows a little less elapsed time than it did when sitting back. This effect has been confirmed experimentally, most famously in a 1971 experiment by physicists Joseph Hafel and Richard Keating, who flew cesium clocks around the world and compared them to ground-based standards.
Physicists Joseph Hafele and Richard Keating flew cesium clocks around the world and compared them to ground-based standards. (Credit: Wikimedia / CC BY-SA 4.0)
The difference was small, measurable only because atomic clocks are extraordinarily precise. But it was real and matched the predictions of relativity. Time, it turns out, is not universal. How much it passes depends on the speed of the measuring clock.
The technical name for this phenomenon is time dilation. This results from a deceptively simple experimental fact about light.
Why light changes everything
In everyday motion, the velocities add up smoothly. A ball thrown from a moving car travels faster than a ball thrown from a stationary position, precisely by the momentum of the throw added to the momentum of the car. That’s Galilean relativity, and it works well for baseballs.
The light doesn’t help.
Measurements made in the late 19th century, and repeatedly confirmed since then, showed that the speed of light in a vacuum is the same for all observers, regardless of the speed of the source. A beam of light from a stationary flashlight travels at about 299,792 kilometers per second. Even a beam fired from a rocket moving at half speed travels at about 299,792 kilometers per second. The rocket’s velocity adds nothing.
Albert Einstein took this result seriously and published his special theory of relativity in 1905, which accepts the change of light speed as a basic postulate and derives the results. One of those results may not be universal at the time. If the speed of light is the same for everyone, measurements of time and distance must differ between observers at a relative speed just enough to keep that speed constant.
The mathematical framework that describes it was put on a firm geometric basis in 1908 by the mathematician Hermann Minkowski. In Minkowski’s formulation, space and time are unified into a four-dimensional structure called spacetime, and motion through spacetime is finite: everything moves at a speed equal to the speed of space combined with time and the total speed of light. Spend more of that rate on spatial speed, and less is available for temporal speed. Move fast through space, and time slows down.
Hermann Minkowski (front, left) was a brilliant mathematician who changed our ideas about space and time. (Credit: Wikimedia / CC BY-SA 4.0)
light clock
Physicists have a standard way of making this concrete. Imagine a clock made of two parallel mirrors with a pulse of light bouncing between them. Each round trip has one tick. It’s called a light watch, and while no one builds practical timekeepers this way, the geometry is transparent enough to make physics inescapable.
When the light clock is at rest, the light pulse travels straight up and down. When the same clock is observed moving horizontally, the light pulse must trace a diagonal path, because the mirrors are moving sideways as the light travels vertically. The speed of light is fixed, so it takes a long time to cover a long diagonal path. The ticks of a moving clock slow down as seen by a stationary observer.
This is not a malice or a trick of perspective. The geometry of motion actually changes the rate at which time accumulates.
A quantitative version of this effect involves a factor physicists call gamma, which is the square root of the speed of light divided by the square root of the velocity minus one. At daily speeds, gamma stays close to one, so time dilation barely shows up. Push to 90 percent light speed, and the gamma jumps to about 2.3, slowing the running clock to about 43 percent of normal. At the 99th percentile, gamma climbs to 7.1. At 99.9 percent, it rises to about 22. As the velocity approaches the speed of light, gamma increases without limit, and the rate of time for a moving object approaches zero.
What happens to a photon?
Light is made up of photons, which are massless. The laws of special relativity require massless objects to travel at exactly the speed of light, and anything with mass to travel slower. This is not a practical limit but a structural feature of spacetime.
The relevant quantity for understanding a particle’s experience of time is called proper time, the duration measured along the particle’s own world line through spacetime. For any object moving slower than light, the proper time is positive. The age of the item. The clock ticks on the board. Processes are revealed.
Time dilation (left) and length contraction (right) reveal a remarkable reality: when you go the speed of light, time slows down and distances shrink. (Credit: Wikimedia / CC BY-SA 4.0)
For a photon traveling at the speed of light, a proper time calculation yields zero.
This means that the photon has no elapsed time in its path. The emission event and the absorption event, however distant they are in space and time, are separated by zero proper time from the photon’s perspective, even when measured by observers at rest. A photon emitted by a star during the epoch when the first complex animals appeared on Earth, and absorbed by a detector today, spans billions of years in our calendar. In the photon’s spacetime trajectory, those events occur simultaneously.
This does not mean that photons have experiences or that they are somehow aware of their state. The proper time result is a calculation, not a statement about consciousness. What this means is that the mathematical structure of spacetime provides no temporal separation for events connected by the null path, which the photon traces.
Gravity and winding paths
Special relativity describes flat spacetime, when gravitational effects are negligible. The general theory of relativity, published by Einstein in 1915, extends the framework to curved spacetime, where massive objects distort the geometry through which everything moves.
One result is gravitational lensing: light traveling near a large object follows a curved path, which can be much longer than a straight-line path. Observers see it arriving later than light in a direct trajectory, an effect that has been observed and measured for sources ranging from nearby stars to distant quasars.
A longer path does not change the proper time calculation. In general relativity a zero path still has zero proper time, no matter how curved it is. Light takes a long time to arrive by our clock, but the proper time along its world line remains zero. Geometry bends; Basic results do not occur.
Applications that rely on having this right
Time dilation isn’t limited to thought experiments about distant stars. The Global Positioning System provides an everyday example of relativity at work.
GPS satellites orbit at an altitude of about 20,200 km and travel relative to the Earth’s surface at about 3.9 km per second. Their motion causes their onboard clocks to run about 7 microseconds per day slower than ground clocks, as predicted by special relativity. At their higher altitudes, where Earth’s gravitational field is weaker, their clocks run faster than surface clocks by about 45 microseconds per day, as predicted by general relativity. The net effect is that satellite clocks gain about 38 microseconds per day compared to ground-based clocks.
That sounds trivial. It’s not. GPS position calculations depend on the precise timing of signals traveling at the speed of light. An uncorrected 38-microsecond error per day would accumulate to position errors of more than 10 kilometers per day, rendering the system useless for navigation. Engineers consistently correct for both relativistic effects. Without those improvements, GPS would fail within hours.
Structure below
The deeper implication of all this is that there is no universal clock running in the background of the universe. Each object carries its own time, determined by its speed and gravitational environment. Two observers in relative motion will measure different time intervals between the same pair of events. Even two observers at different heights in the gravitational field will disagree. Neither is wrong. They are measuring different things, ie proper time on their own world lines.
A quantity all observers agree on is the spatial interval between events, a combination of spatial and temporal separations that remain invariant in reference frames. Events connected by a path where this interval is zero, such as light, have by definition zero proper time between them.
Light discovers the limits of causal structure in spacetime. No object with mass can reach or cross that boundary. Signals cannot travel faster than light, so the speed of light defines the limits of cause and effect. One event cannot affect another if it has to travel faster than light to get there.
At that boundary, where the edges are drawn due to spacetime, time does not slowly slow down and approach zero. It is zero. Travel, in any materially meaningful sense, is not manifest at all.
The original story “Light Can Travel Billions of Years But Experience No Time” appeared on The Brighter Side of News.
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