MCQ Detailed View

The correct answer is Velocity of light. According to Albert Einstein’s theory of relativity, one of the fundamental postulates is that the speed of light in a vacuum (c = 3 × 10⁸ m/s) is constant and independent of the motion of the source or the observer.

This principle is revolutionary because in classical mechanics, velocities simply add up. For example, if a car moves at 60 km/h and throws a ball at 20 km/h, the ball’s speed relative to the ground is 80 km/h. But light does not follow this rule. Whether the source is moving towards you, away from you, or you yourself are moving, the measured speed of light remains the same — always constant in a vacuum.

This constancy has important implications:

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  Time Dilation: Time does not remain constant. A moving clock ticks slower compared to a stationary one, as shown in experiments with fast-moving particles and satellites.

  
  


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  Length Contraction: Length also does not remain constant. An object moving close to the speed of light appears shorter in the direction of motion.

  
  


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  Space and Time are relative: Both space and time depend on the motion of the observer. Together, they form a four-dimensional continuum called spacetime.

  
  

Thus, the only quantity that always remains unchanged in relativity is the speed of light in a vacuum. This concept is the foundation of modern physics, GPS satellite systems, nuclear physics, and even cosmology.

🔑 Key Facts:

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  Speed of light (c = 3 × 10⁸ m/s) is a universal constant.

  
  


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  Independent of observer or source motion.

  
  


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  Time and length are relative and can change depending on velocity.

  
  


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  Foundation of special relativity and modern physics.

  
  


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  Verified experimentally through Michelson–Morley experiment and modern particle physics.

  
  

👉 Final Answer: According to relativity, the quantity that always remains constant is the Velocity of light.