**Earth Science: Physics and Technology – Physics – Theory of Relativity**

*“Relativistic” physics began with a failed experiment: in the late 19th century, people believed that waves were disturbances in a medium called the ether. This was dis- credited by the groundbreaking work of Albert Einstein.*

**Earth Science: Physics and Technology – Physics – Everything is Relative**

*Even today, most people assume that time and space are absolute, unchanging points of reference. Yet since the publication of Albert Einstein’s special theory of relativity in 1905, we have known that this is not true.*

In his renowned theory of relativity, the German-born Nobel Prize-winning theoretical physicist Albert Einstein (1879-1955) discarded the now discredited theory of ether and came to a logical conclusion: the speed of light remains constant, regardless of a light source’s motion.

Instead, space and time are altered in accordance with the object’s movement. For instance, as a rocket accelerates, time passes more slowly for the rocket, and its length shortens. Conversely, from the rocket’s point of view, clocks on Earth advance more quickly, and the planet takes on more of an egg shape (the Earth’s radius shrinks in the direction of the rocket’s movement).

But which version is really true? According to the principle of relativity, both are true, since space and time are always relative to an object’s motion.

**The speed of light**

Not only is the speed of light always and everywhere the same, it is also the absolute maximum speed limit, as un-reachable as the low temperature limit of absolute zero. What happens if we take an electron that is already moving very rapidly and try to accelerate it further? It becomes more massive, and thus, because of its Newtonian inertia, it opposes further acceleration with ever stronger resistance.

In the most extreme case, its mass can become infinitely large. The precise speed of light can only be achieved by particles with no

mass, such as photons. However, these particles can never stand still; out- side of a lab the speed of light is unvarying.

**Mass and energy**

In this context, a clear description of the properties of mass is important. On the one hand, it is a measure of inertia. On the other hand, it continuously increases as the motion energy of a body rises. This means, however, that mass is actually a form of energy.

In 1905 Einstein was able to calculate the total energy E of a mass m, using the speed of light c, resulting in the ground-breaking equation E = mc2. This formula, the mass-energy equivalence equation-surely the most famous in physics—summarizes one of the most important

insights in the history of relativistic physics in just a few symbols.

**ALBERT EINSTEIN: THE FATHER OF MODERN PHYSICS**

Albert Einstein (1879-1955), one of the greatest physicists of all time, transformed our understanding of time and space. His world renowned theory of relativity and the mass-energy equivalence equation are his most well-known works: however, he published works on fields as diverse as probability and quantum theory.

In 1921 he was awarded the Nobel Prize in physics for his discovery of the photoelectric effect.

**THE TWIN PARADOX**

If one twin sister travels through space at a high speed while the other remains on Earth, time passes more slowly for the traveling twin, and upon returning, she is younger than her sister.

This seeming paradox has been confirmed both theoretically and experimentally (with elementary particles). It only appears paradoxical, however: it would contradict natural laws only if time were absolute, flowing at the same rate in all systems. This is precisely what does not occur.