# What makes light travel forever

## Why is it impossible to travel at the speed of light?

According to Albert Einstein's special theory of relativity, there is an absolute limit speed, the speed of light. More precisely, it means the speed of propagation of light in a vacuum. This is usually referred to as c and it is approx. 300,000 km / s. For comparison: a jumbo jet flies relatively slowly at a speed of no more than 0.25 km / s. In astronomy, knowledge of the speed of light is used to provide information about astronomical distances. Lengths are measured in light years. One light year corresponds to the distance that light travels in one year. With our jumbo jet it would take us 1.2 million years to cover this distance.

The question arises whether it is in principle possible for us to travel at the speed of light as well. We know that all massless particles and electromagnetic radiation travel at the speed of light. However, this is not possible for objects with mass, regardless of whether they are elementary particles or jumbo jets. According to the special theory of relativity, the mass of an object depends on its speed. It gets bigger the faster the object moves. However, our everyday experience teaches us that the heavier an object is, the more energy has to be expended to accelerate it.

Let's say we accelerated our plane to 75 percent the speed of light. Then its moving mass already corresponds to 1.5 times its mass at rest. If it moves at 99 percent of the speed of light, it is already 7 times faster. The closer we get to the speed of light, the more dramatic the increase and the associated energy requirements for further acceleration become. In the limiting case of the speed of light, the moving mass becomes formally infinite. Accordingly, an infinite amount of energy is required to achieve this speed. In this sense it is impossible to accelerate an object with mass to the exact speed of light.

Obviously, with objects like airplanes, we don't get anywhere near the speed of light. But how does it look on a very small scale? The current speed record for particles with mass is 99.988 percent of the speed of light. This was done at the Large Hadron Collider (LHC) at CERN in Switzerland, where protons, i.e. the nuclei of hydrogen atoms, are accelerated and made to collide. The moving mass of the protons is roughly 4250 times their rest mass.