In this blog post, we’ll take an accessible yet in-depth look at why we can’t travel faster than light, exploring the structure of the universe that imposes this limit and the core principles of the theory of relativity, based on the latest scientific findings.
There is a single, absolute rule in the universe: no matter how much energy is applied, no object with mass can travel faster than light. Scientists have used the most powerful particle accelerators to accelerate electrons to speeds extremely close to the speed of light, but they have not succeeded in exceeding the speed of light itself. Why is that? The answer physicists have come up with completely defies our intuition. The reason travel faster than light is impossible is not simply a lack of energy, but because the very structure of space-time in the universe is designed that way. For hundreds of years, humanity has struggled to measure the speed of light. Understanding why makes us realize even more vividly just how staggering the speed of light truly is.
Light travels approximately 299,792 km per second in a vacuum. This is a speed fast enough to circle the Earth’s equator about 7.5 times in just one second. Even the Parker Solar Probe—currently the fastest spacecraft ever built by humankind—reaches a top speed of about 690,000 km per hour, but this is only about 0.064% of the speed of light. The distance to the Moon is about 380,000 km, which light travels in about 1.3 seconds, and the average distance to the Sun—about 150 million km—is covered in about 8 minutes and 20 seconds. In contrast, the Parker Solar Probe takes about 90 days to travel the same distance. A bullet travels at about 3,600 km per hour; while the Parker Solar Probe is about 200 times faster than that, it is still thousands of times slower than light. Furthermore, based on the distance to Proxima Centauri—the star closest to Earth, at about 4.24 light-years—it would take the Parker Solar Probe approximately 6,300 years to reach it. Due to this overwhelming difference in speed, the experiments conducted by early scientists were bound to fail time and again.
A prime example is Galileo Galilei’s experiment. He attempted to measure the time it took for light to travel by using lanterns on two distant hills, but since light arrives in an extremely short amount of time—while human reaction time is at least about 0.15 seconds—he was unable to detect any difference. Ultimately, he could only confirm that light travels at a speed so immense that it is beyond human measurement. However, in 1676, the Danish astronomer Ole Rømer solved the problem in an entirely different way by utilizing the orbital phenomena of Jupiter’s moon Io. By analyzing how the timing of Io’s eclipses shifted—either becoming slower or faster—as the distance between Earth and Jupiter changed, he inferred the speed of light. His calculations showed a deviation of only about 25% from the actual value, which was an outstanding result for that time.
Since then, measurements of the speed of light have become increasingly precise, and in 1983, the General Conference on Weights and Measures redefined the meter based on the speed of light. Today, the speed of light is no longer merely a measured quantity but a fundamental constant that defines the International System of Units (SI). So why can’t we exceed this speed? The reason lies far deeper than a simple lack of energy. Albert Einstein introduced the concept of spacetime—which unites space and time—through his special theory of relativity, and Hermann Minkowski mathematically formalized it with great precision. The speed of light is not merely a rate of motion but a fundamental constant that defines the structure of spacetime itself. As seen in the equation E=mc², mass and energy are linked through the square of the speed of light, and this value is a key factor in determining the physical laws of the universe.
A key point in the concept of spacetime formalized by Minkowski is that all objects move through spacetime. However, the statement that “all objects always move through spacetime at the speed of light” is useful as an intuitive analogy but is not a strictly accurate physical description. More precisely, objects with mass move through spacetime according to their own proper time, and as their velocity in the spatial direction increases, the flow of time slows down relatively. In other words, when we stand still, there is almost no movement in space, but time flows normally. Conversely, the faster we move through space, the slower time flows—and this is precisely the phenomenon of time dilation described in the special theory of relativity. No object with mass can reach the speed of light, no matter how much energy is supplied to it; only massless photons always travel at the speed of light in a vacuum.
This theory is not a mere hypothesis but has been repeatedly verified through numerous experiments. In the 1971 experiment by Haffele and Keating, atomic clocks were loaded onto an airplane and flown around the Earth once; as a result, the time difference between the clocks on the plane and those on the ground—as predicted by the theory of relativity—was actually observed. Furthermore, the GPS systems we use every day would experience positional errors of several kilometers per day if they did not simultaneously apply time corrections based on both the special and general theories of relativity. In other words, the theory of relativity is not merely a theoretical concept but a reality that is utilized every moment within the core technologies of modern society.
Ultimately, the reason it is impossible to travel faster than light is that the very structure of spacetime in the universe is designed that way. As speed in space increases, the passage of time gradually slows down, and as an object with mass approaches the speed of light, the energy required increases to infinity. Therefore, an object with mass can never reach or exceed the speed of light. This is the conclusion of modern physics, confirmed through experiments and theory to date, and it remains one of the deepest and most important questions for understanding the nature of spacetime and the structure of the universe.
