# Is the speed of light relative

## Why are space and time relative?

Time dilation is one of the most confusing conclusions from the theory of relativity: time passes more slowly for fast moving clocks than for slow or motionless clocks. (At that time, Einstein could not have known that, strictly speaking, nothing is at rest because space is expanding, and it does not play a role for the SRT either.) Incidentally, such a time expansion can also be caused by gravity: clocks in the gravitational field tick more slowly than such isolated in space. But this is an effect of general, not special, relativity. The time dilation in rapid movements has caused heated discussions.

The phenomenon is often illustrated with the so-called twin paradox: According to this, an astronaut racing through space at high speed would have aged much less when he returned to Earth than his twin brother who stayed at home. Assume that a 27-year-old astronaut flies at 98 percent of the speed of light to the star Vega, which is around 25 light-years away, and back again. Then 10 years have passed on his return, so he is 37 years old - while his twin brother, who remained on earth, has already celebrated his 77th birthday and is now 40 years older than the astronaut. (The example is simplified because the time-consuming acceleration and braking phases were omitted.) At 98 percent speed of light, the time in the spaceship passed considerably more slowly than on earth.

### The TWIN PARADOXON is false

This age difference is irritating enough, but it is a measurable fact. It becomes paradoxical when one argues that the twin who remained on earth also moved away from their brother at 98 percent of the speed of light - after all, the special theory of relativity teaches that no reference system is preferred. Seen in this way, the spaceman from whom the earth moved would have to be 40 years older than his brother when the earth returned to him.

But that is wrong - as is the whole argument. Because the twins' movements must not be viewed as symmetrical. Only reference systems that are at rest or move at a constant speed are equal. But in the example, the spaceman first speeds up, then flies a curve at Wega to return, and finally brakes again near the earth. Such accelerated movements are not an object of the special, but of the general theory of relativity. However, if two astronauts were to fly past each other at high constant speed and compare their clocks several times, then they could both actually find that the other clock is ticking more slowly.

### JOURNEY INTO THE FUTURE

In principle, time dilation can even be used for a journey through time into the distant future. For example, if you were to fly to a star 500 light-years away at up to 99.9992 percent of the speed of light with the acceleration and braking pressure of 1 G - this corresponds to the force of gravity - you would only have aged almost 25 years while on the Earth 1000 years would have passed. A way back to their own youth would of course be blocked. So if you are planning a trip into the future, you should submit your tax return beforehand - otherwise the terrible impatience of the tax office awaits you when you return.

The length contraction, which is also a consequence of the constant speed of light, is complementary to time dilation. Because like time, distance is also relative. In the direction of movement, all scales are shortened by the same factor that time expands. For example, if an astronaut flies to Vega at 98 percent the speed of light, he is on the way for 5 years and has covered a distance of 5 times 0.98 light years, i.e. 4.9 light years, in his reference system, while from the perspective of the earth it is 25 light years .

The length contraction was described by George FitzGerald before Einstein in 1889 and Hendrik Antoon Lorentz in 1892. These physicists also tried to resolve the formal contradictions between classical mechanics and electromagnetism. However, they were still caught up in pre-relativistic thinking and wanted to explain the phenomenon through velocity-dependent forces in the ether between the atoms. In contrast, the relativity theory's length contraction does not mean that a meter stick will shorten, as if it were being compressed. Rather, the length contraction is a matter of the - thoroughly "objective" - ​​reference system. So if you want to lose weight, you can't just whiz through the world at almost the speed of light and trust that the length contraction will make your bulbous belly disappear. ■

January 18, 2011