1.2 Einstein's thought experiments and postulates

Einstein's thought experiments laid the groundwork for special relativity. The light clock experiment showed how time slows down for moving objects, while the train experiment revealed that simultaneity depends on the observer's perspective.

Einstein's postulates revolutionized physics. The principle of relativity states that physical laws are the same in all inertial frames, while the constancy of light speed challenges our understanding of space and time.

Thought Experiments

Light Clock Experiment

Top images from around the web for Light Clock Experiment

• 

  Temps et espace – Le Gout du sens View original

  Is this image relevant?

• 

  Simultaneity And Time Dilation | Physics View original

  Is this image relevant?

• 

  Simultaneity And Time Dilation | Physics View original

  Is this image relevant?

• 

  Temps et espace – Le Gout du sens View original

  Is this image relevant?

• 

  Simultaneity And Time Dilation | Physics View original

  Is this image relevant?

1 of 3

Top images from around the web for Light Clock Experiment

• 

  Temps et espace – Le Gout du sens View original

  Is this image relevant?

• 

  Simultaneity And Time Dilation | Physics View original

  Is this image relevant?

• 

  Simultaneity And Time Dilation | Physics View original

  Is this image relevant?

• 

  Temps et espace – Le Gout du sens View original

  Is this image relevant?

• 

  Simultaneity And Time Dilation | Physics View original

  Is this image relevant?

1 of 3

• Imagine a light clock consisting of two mirrors facing each other with a light beam bouncing between them
• In the frame of reference where the clock is at rest, the light beam travels a vertical path and the clock ticks at a regular interval
• In a moving frame of reference, the light beam appears to follow a diagonal path as the clock moves horizontally
• The diagonal path is longer than the vertical path, causing the moving clock to tick slower than the stationary clock
• This thought experiment demonstrates time dilation, where time passes more slowly for an object in motion relative to a stationary observer

Moving Train and Simultaneity

• Consider a train moving at a high velocity relative to an observer on a platform
• Two lightning bolts strike the front and back of the train simultaneously from the perspective of the observer on the platform
• However, from the perspective of a passenger sitting in the middle of the train, the lightning bolt at the front of the train is perceived to occur before the one at the back
• This is because the train is moving towards the point where the front lightning bolt struck and away from the point where the back lightning bolt struck
• The thought experiment illustrates the concept of simultaneity, showing that events that appear simultaneous to one observer may not be simultaneous to another observer in a different frame of reference

Postulates of Special Relativity

Principle of Relativity and Invariance of Physical Laws

• The principle of relativity states that the laws of physics are the same in all inertial reference frames
• An inertial reference frame is a frame of reference in which an object remains at rest or moves with constant velocity unless acted upon by an external force
• In other words, there is no preferred or absolute frame of reference, and all inertial frames are equivalent
• Consequently, the laws of physics, such as Newton's laws of motion and Maxwell's equations of electromagnetism, are invariant (remain unchanged) in all inertial frames
• This postulate implies that no experiment can distinguish between two inertial frames of reference, and there is no absolute state of motion or rest

Constancy of the Speed of Light

• The speed of light in a vacuum, denoted as $c$, is constant and independent of the motion of the light source or the observer
• Regardless of the relative motion between the source and the observer, the speed of light always remains at approximately 299,792,458 meters per second
• This postulate challenges the notion of absolute time and space, as the speed of light is invariant and does not follow the classical rules of velocity addition
• The constancy of the speed of light led Einstein to develop the theory of special relativity, which revolutionized our understanding of space, time, and the relationship between them
• The invariance of the speed of light has been confirmed through numerous experiments, such as the Michelson-Morley experiment and measurements of cosmic rays