WHAT IS SPECIAL RELATIVITY

• When: Developed in 1905
• By Whom: Albert Einstein
• Famous Formula: E=mc²

It is near the end of the year 2025, which marks 120 years since Albert Einstein published the Special Theory of Relativity. This theory was a crucial milestone in physics, demonstrating the equivalence of mass and energy, a relationship famously expressed by the equation: E=mc²

Almost every book I picked up covers the topic on Special Relativity, and it is perhaps one of the most important checkboxes to tick if you are new to physics.

What is Special Relativity?

• Special Relativity is a theory developed in 1905 by Albert Einstein.
• It explains how space and time are unified, creating a four-dimensional structure called spacetime.
• The theory shows that the laws of physics are the same for all inertial (non-accelerating) frames of reference, and that the speed of light in a vacuum is constant for all observers.
• This theory deals with flat spacetime, not curved spacetime (which is described by General Relativity), therefore gravity is not included in this theory.
  

Special relativity was discovered because classical physics (at that present time) couldn’t explain why the speed of light is always the same—and Einstein realized space and time had to adapt.

• Back in 1905, many physicists thought that light must travel through a an invisible substance called the luminiferous ether. If our planet moved through this ether, they believed that the speed of light should change depending on direction.

Through theoretical reasoning and experiments, Einstein explained that because the speed of light is constant, time itself must be relative. He introduced time dilation, length contraction, and mass–energy equivalence, which together led to one of the most famous equations in physics: E=mc².

Special Relativity is based on two core values:

1. The laws of physics are the same for all observers moving at constant velocity.
   (no experiment can tell you whether you’re “really” moving or at rest).
2. The speed of light in a vacuum is the same for all observers, no matter how fast the light source or observer is moving.

Is Special Relativity a Theory for Flat space?

Yes. Special Relativity deals with flat, non-curved spacetime.

Before the 20th century, including during Newton’s time, space and time were believed to be absolute and flat. When Einstein developed Special Relativity in 1905, he assumed spacetime was flat, and we know that because he did not include gravity in his special relativity theory (don’t worry, a few years later he came up with another theory which does include gravity).

Special Relativity therefore explains how space and time behave when gravity is almost not there and motion is uniform—no acceleration, no gravitational effects, just flat spacetime.

Why was General Relativity needed?

Space is not always flat; it can be curved by mass and energy, which we experience as gravity. Einstein later worked on a theory that could explain how mass and gravity bend spacetime.

One of the key predictions of this new theory was the bending of light by gravity, which was famously observed during a solar eclipse, when starlight passing near the Sun was deflected by the Sun’s gravitational field.

To explain gravity and curved spacetime, Einstein developed General Relativity, which applies to accelerated frames and includes gravitational effects.

Why still use Special Relativity if it deals with Flat Space?

Although General Relativity is more general, Special Relativity is still crucial, important, and widely used for several reasons:

• Spacetime is not strongly curved everywhere in the universe; curvature is significant mainly near very massive objects.
• Even near Earth, spacetime curvature is very small.
• In regions where gravity is weak, spacetime can be accurately approximated as flat.
• Special Relativity is much simpler mathematically and works perfectly in many practical situations.

Because of this, physicists often treat spacetime as flat when high precision gravitational effects are not required.

Can the formula E=mc² be used in everyday cases?

Yes, E=mc² applies universally, but its noticeable effects are rare in everyday life.

Examples include:

• The Sun and other stars– During nuclear fusion, a small amount of mass is converted into energy when hydrogen fuses into helium.
• Nuclear power plants – A tiny fraction of nuclear mass is converted into energy, producing heat that generates steam and electricity.

Thank you, I hope you enjoyed the article. Please note, I carefully research the topic before publishing, however some facts could have changed since publishing this article, so I appreciate your understanding. All articles published are thoroughly researched and inspired by published books. The list of resources are published in every article. Please be kind, and have a nice day.

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