The Theory of General Relativity relates to gravity, while the Theory of Special Relativity is related to our concepts of space and time. Without these groundbreaking theories, technology such as nuclear energy and GPS navigation wouldn’t exist (or would have been developed much later).
What light is and how it behaves has intrigued scientists for thousands of years.
We know almost everything about how light “works” thanks to a few brilliant physicists such as Sir Isaac Newton, Christiaan Huygens, James Clerk Maxwell, Hans Christian Ørsted, Michael Faraday, and Albert Einstein.
Danish scientist Ole Rømer attempted to measure the speed of light in 1676. By observing Jupiter and its moons he calculated that light would take around 22 minutes to traverse the diameter of the Earth’s orbit.
Scientists believe Galileo Galilei was the first to try measuring the speed of light. He and an assistant stood on hilltops and uncovered lanterns to measure the time it took for light to reach the other person. Galileo concluded that light seemed to travel instantaneously.
What we know today
Evidence suggests that light is both a wave and a particle, which confused scientists for centuries, but in 2012 was directly observed for the first time. Light forms part of the electromagnetic wave spectrum, along with radio waves, X-rays, and microwaves.
Unlike sound waves, which need a medium to travel through, light waves are able to travel through a vacuum without dissipating. This explains why we can see starlight; those light waves can travel millions of years before reaching Earth.
Light is a form of electromagnetic energy that is composed of many tiny photons containing lots of energy. When light travels through a medium like water, some of it will be absorbed and lost as heat.
Einstein’s original theory
Before Einstein's time, how light behaves wasn’t well understood. It was thought that light had to move through a medium called an ether. In 1865, James Clerk Maxwell measured the speed of light, stating that it traveled at 186,000 miles per second.
The fact that light speed seemed to remain constant regardless of the Earth’s speed confused physicists and intrigued a young Albert Einstein.
Albert Einstein began thinking about the speed of light when he was 16 years old, and published his very first scientific paper at that age. He developed his theory of relativity while working as a patent clerk in Bern, Switzerland.
Einstein thought that if he ran fast enough, he would see light waves as if they were standing still. This contradicted findings of physicist James Clerk Maxwell, who stated that light isn’t able to stand still, no matter how fast a person travels.
Sir Isaac Newton’s theories suggested that if someone was traveling at the speed of light, then light itself would stand still. Maxwell’s calculations showed that this was impossible. No sane physicist would discard Newton’s work.
Luckily, Einstein wasn’t a physicist at the time of these calculations!
Once Einstein decided to discard Newtonian physics (a VERY controversial move), he concentrated on his perceived flaws in Maxwell’s equations.
Einstein dropped the idea of an ether and came up with a simple, but revolutionary idea: the speed of light in a vacuum is the same for all observers.
Light and time
In order to understand Einstein’s theories and understand why they are so revolutionary, we need to have an idea of what the previous theory was.
Sir Isaac Newton developed the accepted way of thinking about the physical world, time, and light in the mid–1600s, and it remained seriously unchallenged for over 250 years.
Most of what Newton developed is rooted in common sense, another reason why people get confused about Einstein's “nonsensical” theories.
Newton’s theory of gravity stated that objects fall because they are pulled by Earth’s gravity. Gravitational force is determined by the mass of the first object multiplied by the second, divided by the square of the distance between the objects.
Einstein incorporated Newton’s gravity theories into his theories.
Inertial frames relate to what is happening to the observer while making an observation. For example, a person walking in a moving rocket says they are moving 1 mph as the ground underneath them feels still. However, an observer watching from Earth says the person is traveling 601 mph as their inertial frame is the Earth itself.
In Newtonian physics, if you watched a rocket accelerating at 600 mph while standing still, the rocket’s speed would be 600 mph.
If you were in another rocket traveling at 500 mph, then the first rocket’s relative speed would be 500 + 600 = 1,100 mph. This assumption collapses when talking about light.
Einstein agreed with Maxwell’s equations that stated that no matter how fast an observer was traveling the speed of light will remain constant.
So even if a rocket was able to travel at 186,000 miles per second and the astronaut looked out the window, they wouldn’t see a “frozen” light beam; the light would still be traveling at 186,000 miles per second beyond the rocket.
Everything is moving
Due to these calculations regarding the speed of light, Einstein concluded that there are no fixed frames of reference. This led him to his theory that everything in the universe is moving relative to everything else.
Time is relative
Einstein worked out that time isn’t the same for everyone. Two people synchronize their watches. One person takes off in a rocket. The observer on the ground looks at the person’s watch using a telescope.
The ground observer notices that the rocket person’s watch seems to be running slower.
The person in the rocket is also looking at the Earth person’s watch through a telescope. According to them, their watch is working perfectly, and the Earth person’s watch is running slow! Amazingly, both people are correct. Time is relative to the person observing.
Measuring the length of a very fast moving rocket will yield a different result when measuring the same rocket at a standstill. Before takeoff, the rocket was measured.
During a very, very fast takeoff, the same rocket was measured using precise lasers, which found that the rocket was now shorter.
Length is relative
Like the watch example, both measurements of the rocket are correct. Length is also relative to the observer. At very high speeds, the faster an object is moving, the shorter it will appear to those watching it.
According to Einsteinian physics, the faster an object travels, the more mass it has.
Because of this increase in mass, nothing can travel faster than the speed of light; mass increases with velocity until the mass becomes infinite when it reaches light speed. Infinite mass needs infinite energy, an impossible feat.
Events that occur at the same time for one observer can occur at different times for another (and both would be correct). Einstein discovered that space and time are interwoven. He coined this term “space-time.”
Einstein discovered that truly massive objects such as suns and planets distort space-time. This is felt as gravity. Gradual changes in the orbit of the planet Mercury and NASA’s Gravity Probe B prove this part of Einstein’s theory.
Space-time is like a fabric which everything sits on. Imagine space-time is like a trampoline mat. A heavy bowling bowl would cause the mat to dimple around the bowling ball. This is what happens to space-time with a massive object like a sun.
Rolling a marble onto the dimpled trampoline mat causes it to circle around the bowling ball before eventually crashing into it. This is how planets end up orbiting massive objects like a sun, or a moon orbiting a larger planet.
Astronomers take advantage of these dimples to study objects that are far away. Light around massive objects such as a black hole is bent, causing it to act like a lens for any objects hidden behind it.