**Nature of space, time and light**

- In the early 1900s, Einstein came up with a revolutionary idea that changed our understanding of space and time.
- His special theory of relativity states that space and time are not separate and absolute, but are intertwined and relative to each other.
- Motion affects space and time:
- According to classical physics, space is uniform and rigid, filling the universe like a background stage, and time passes at a monotonous, unchanging rate.
- However according to Special Theory of Relativity, measurements of space (as distance between 2 points) and time is affected by the motion of the observer.
- In short, space and time are not absolutes. They depend on how one is moving, and different values are measured for these quantities by someone moving differently.
- Idea of space-time: In addition to this, space and time are actually interdependent. In other words, they’re not two separate things, but time is intertwined with the three dimensions of space, and they are regarded as a single four-dimensional entity called spacetime. (more on this later)
- Constant speed of light: According to special theory of relativity the speed of light is constant and always same for all observers. This means that no matter how fast you are moving, you will always measure the same speed of light.
- E=MC^2: Another very important result is the mass-energy equivalence principle which is represented by the famous equation E=MC^2. This simply means energy of radiation is related to the mass of the object producing it.

**Predictions of Special Theory of Relativity**

- Time dilation: Einstein also showed that time is relative and can be affected by motion. This means that time can appear to move slower or faster depending on how fast you are moving relative to something else.
- Illustration: Imagine you are on a moving train and there is a light bulb, in the middle of the train, that shines in both directions.
- To you, it looks like the light hits the front and back of the train at the same time.
- But someone standing outside the train watching the light would see it hit the back of the train before the front, because the train is moving forward while the light is travelling.

- Length contraction: Objects can appear to be shorter or longer depending on how fast they are moving relative to the observer. This is called length contraction.

- Mass gain with increase in velocity: Faster an object moves more massive it becomes and requires more energy to accelerate.

**Real world applications of special theory of relativity**

__Navigation satellites__**:**Navigation satellites work on the principle of Speed=Distance/Time taken. Now since the satellites orbiting the Earth and the clocks on them are affected by their motion, time dilation would occur. Hence clocks on navigation satellites need to be adjusted accordingly for accuracy.: E=mc² is used to calculate the energy released in nuclear reactions, fusion or fission.__Nuclear Energy__PET scans involve detecting pairs of gamma rays emitted from a radioactive tracer that is injected into the body and measuring the time interval between the pair. The relativistic effects of time dilation and length contraction is very prominent in sub-atomic particles like positron and must be taken into account for accuracy.__Medical Imaging:__