In the beginning of the year 1905, Albert Einstein was an unknown 25-year-old clerk in the Swiss patent office. By the end of the year, he had published four papers of extraordinary importance. His third paper introduced his ‘Special Theory of Relativity’ which revolutionised the classical concept of space and time.
Einstein illustrated his concepts through ‘thought experiments’:
Consider a train moving in a straight line with a uniform speed comparable to the speed of light. Two lightning bolts strike the train, one at each end, A’ and B’. Each bolt also leaves a mark on the ground at A and B.
Stanley is stationary on the ground at O, midway between A and B. Mavis is moving with the train at O’ in the middle of the passenger car, midway between A’ and B’.
Both Stanley and Mavis see flashing from the points where the lightning strikes.
Suppose the light from the two bolts reach Stanley at O simultaneously. Mavis at O’ is moving to the right with the train, so she runs into the light from B’ before the light from A’ catches up to her. She therefore concludes that the lightning bolt at B’ struck before the one at A’.
Stanley at O measures the two events to be simultaneous, but Mavis at O’ does not!
This shows that whether or not two events at different locations happen simultaneously depends on the state of motion of the observer. It also follows that the time interval between two events may be different for different frames of reference. This is known as the ‘relative nature of simultaneity’.
Other similar thought experiments can be used to illustrate two important concepts in relativity.
If two events occur at the same point in space, the time interval between them as measured by a clock on the train and a clock on the ground will vary. The train clock’s reading will be smaller than the one on the ground. In brief, moving clocks appear to run slow. This effect is called ‘time dilation’.
Even the distance between two points may depend on the observer. It can be shown that the measured length of a body is greater in the train than on the ground. This effect is called ‘length contraction’.
Thought experiments like the train-and-lightning scenario showcase Einstein’s theory of time and distance relative to the frame of reference, providing deep insights into the nature of reality.
Rona La Bella is a mathematics lecturer at the University of Malta Junior College.
Sound Bites
• In 2021, Dan Wilkins and his team of astrophysicists from Stanford University reported a groundbreaking achievement: they observed X-ray emissions from behind a black hole for the first time ever. This observation, made possible by the Event Horizon Telescope, confirmed a prediction of Einstein’s theory of general relativity. The black hole’s event horizon, a boundary beyond which nothing, not even light, can escape, was effectively probed through gravitational lensing.
• Anupama B and P K Suresh (2024) present a novel theory of quantum gravity aiming to reconcile quantum physics with Einstein’s general relativity, potentially resolving the long-standing cosmological puzzle of the universe’s expansion rate inconsistency (the Hubble Parameter).
For more science news, listen to Radio Mocha on www.fb.com/RadioMochaMalta/.
DID YOU KNOW?
• The twin paradox is a thought experiment in special relativity where one twin travels through space at relativistic speeds and returns younger than their stationary sibling due to the effects of time dilation.
• E = mc2, a fundamental equation of special relativity, expresses the equivalence of mass m and energy E, revealing that mass can be converted into energy and vice versa, governed by the speed of light c. This equation has profound implications for nuclear physics and energy generation.
• Relativity does not invalidate the laws of Newtonian mechanics but rather generalises them. All principles of Newton’s laws are retained as limiting cases at low speeds relative to the speed of light.
• General relativity extends special relativity by incorporating gravitational effects, describing how mass and energy curve spacetime, influencing the structure of the universe. Black holes and gravitational waves are both consequences of a prediction of Einstein's general relativity.
For more trivia, see: www.um.edu.mt/think.