In this blog post, we take an accessible yet in-depth look at why we cannot travel faster than light, exploring the structure of the universe that imposes this limit and the core principles of the theory of relativity.
This is a proofread and edited version that reorganizes the paragraphs to flow more naturally and seamlessly.
There is a single, absolute rule in the universe: no matter how much energy we pour into it, nothing can ever travel faster than the speed of light. Even when scientists used the most powerful particle accelerators to push electrons to 99.999% of the speed of light, they could never cross that final 0.001%. Why is that, exactly? The answer physicists have come up with completely defies our intuition. The reason travel faster than light is impossible isn’t due to a lack of energy, but because the very structure of the universe is designed that way. Humanity has tried to measure the speed of light for hundreds of years but has failed time and again; once you understand why, you’ll truly grasp just how absurdly fast this speed is.
Light travels approximately 299,792 km per second. That’s a speed fast enough to circle the Earth’s equator seven and a half times in a single second. Even the Parker Solar Probe—the fastest object humanity has ever created—has a top speed of about 690,000 km/h, which is a mere 0.064% of the speed of light. The distance to the Moon is about 380,000 km; light covers that distance in just 1.3 seconds, and it takes only 8 minutes and 20 seconds to reach the Sun, which is about 150 million km away. In contrast, the Parker Solar Probe takes about 90 days to travel the same distance. A bullet travels at about 3,600 km/h, but the Parker Solar Probe is about 200 times faster than that, and even the probe is thousands of times slower than light. Based on the distance to Proxima Centauri, the star closest to Earth, which is about 4.24 light-years away, it would take the Parker Solar Probe approximately 6,300 years to reach it. Due to this overwhelming difference in speed, early scientists’ experiments were bound to fail time and again.
A prime example is Galileo’s experiment. He climbed two hills and attempted to measure the time it took for light to travel using a lantern, but since light reaches its destination in less than a microsecond, while human reaction time is at least 0.15 seconds, he could not detect any difference. Ultimately, he could only conclude that light either traveled instantaneously or moved at a speed beyond imagination. However, in 1676, the Danish astronomer Ole Rømer solved the problem in a completely different way using Jupiter’s moon Io. He inferred the speed of light by observing how the timing of eclipses varied depending on the distance between Earth and Jupiter. This result was remarkably accurate, differing from the actual value by only about 25%.
Since then, measurements of the speed of light have become increasingly precise, and in 1983, the General Conference on Weights and Measures defined the meter based on the speed of light. Now, the speed of light is no longer a quantity to be measured but a fundamental standard of the universe. 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—where space and time are unified—through his special theory of relativity, and Hermann Minkowski mathematically formalized it. The speed of light is not merely a velocity; it is a 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 determining the physical laws of the universe.
A crucial fact revealed by Minkowski is that all objects are constantly moving through spacetime at a constant speed, and that speed is precisely the speed of light. When we are at rest, we do not move in the spatial direction, but we are moving in the temporal direction at the speed of light. Therefore, when we begin to move rapidly in space, our movement in the temporal direction decreases accordingly, and this is precisely why time slows down as speed increases. Ultimately, to achieve a spatial velocity equal to the speed of light, movement in the time dimension must be exactly zero, which means time stops. Only massless photons can reach this state; objects with mass can never reach that limit.
This theory is not merely a hypothesis but has been verified by actual experiments. In the 1971 experiment by Haffele and Keating, atomic clocks were placed on an airplane and flown around the Earth, confirming that time inside the plane flowed more slowly than on the ground. Furthermore, the GPS systems we use daily would accumulate errors of several kilometers per day if they did not account for the theory of relativity. In other words, the theory of relativity is not merely a theory but a reality already at work in our daily lives.
Ultimately, the reason travel faster than light is impossible is that we are already moving through spacetime at the speed of light. As spatial movement increases, temporal movement decreases, and the moment we reach the speed of light, time stops. This is an absolute limit set by the universe, and at the same time, it remains one of the deepest questions that modern physics has yet to fully explain.
