Students may find it difficult to grasp Einstein’s theories of relativity, comprising special relativity and general relativity. Special relativity applies to all physical phenomena in the absence of gravity. Einstein proposed this theory to explain cases where Newtonian physics could fail to deal successfully with phenomena. This theory was revolutionary as it proposed vast changes in human conceptions of time, space, and gravity.
The Special Theory of Relativity
There are 2 central tenets to the Special Theory of Relativity: (1) that the speed of light is constant for all observers, and (2) that observers moving at constant speed should be subject to the same physical laws. Thus, Einstein posits that time must change according to the speed of a moving object relative to the perspective of an observer. Basically, Einstein says that both space and time are relative instead of absolute, which was said to hold true in a special case – which is when a gravitational field is absent.
General Relativity
As for Einstein’s theory of general relativity, matter causes space to curve. Einstein suggests that gravitation is not a force (as understood in Newtonian physics) but instead a curved field (which is an area of space under the influence of a force) in the space-time continuum which is actually created by the presence of mass.
According to Einstein, this theory can be tested by measuring the deflection of starlight travelling near the Sun. Einstein correctly said that light deflection would be twice than what was expected by Newton’s laws. The theory of general relativity also explains why the light from stars within a strong gravitational field would be closer to the red end of a spectrum than those in a weaker gravitational field.
We can also observe Einstein’s theory of relativity in our daily lives. For example, magnetism is a relativistic effect, and this can be seen in generators. If a loop of wire is moved through a magnetic field, an electric current is generated. The charged particles in the wire are affected by the changing of the magnetic field, which forces some of the particles to move and creates the current.
However, when the wire is at rest and the magnet is moving, the charged particles (electrons and protons) in the wire are not moving anymore, which means that the magnetic field should not be affecting them. This is not the case, as the magnetic field affects these particles and the current still flows. This experiment demonstrates that there is no privileged frame of reference.
Moreover, the principle of relativity can show Faraday’s law, which states that a changing magnetic field creates an electric current. This is the basic principle behind transformers and electric generators, and thus, anyone using electricity will experience the effects of relativity.
Another area where relativity can be observed is in electromagnets. When a direct current of electric charge flows through a wire, electrons drift through the material. Normally, the wire would seem electrically neutral, with no net positive or negative charge. This is because the wire has roughly the same number of positively charged protons and negatively charged electrons. However, if another wire with a direct current is placed next to it, the wires attract or repel each other, depending on the direction in which the current is moving.
If we assume that the currents are moving in the same direction, the electrons in the second wire are not moving when compared to the electrons in the first wire if we also assume that the currents are roughly the same strength. However, the protons in both wires are moving in comparison to the electrons in both wires. Due to the relativistic length contraction, they seem to be closer together, and thus, there will be more positive charge than negative charge per length of wire. As like charges repel, the 2 wires also repel.
Currents in opposite directions result in attraction, as compared to the first wire, the electrons in the other wire are packed more closely together, which creates a net negative charge. Meanwhile, the protons in the first wire are creating a net positive charge, while opposite charges attract.
