By Laura Crocioni

Imagine standing on a platform as a train rushes past at extreme speed. From your perspective, the train appears physically shorter, and the clocks would seem to tick more slowly. However, for the passengers on board, everything would feel completely normal. This simple scenario captures the essence of Einstein’s theory of relativity: space and time don’t behave the same for everyone. So, why do space and time behave differently depending on who’s observing?

Before Einstein proposed his Theory of Relativity, Newton’s theory of classical mechanics was the standard. This theory entailed that space and time were absolute, fixed and independent from each other (Rynkiewicz, 2004). In this framework, the universe behaved predictably: a second was always a second and a metre was always a metre. An observer’s movement made no difference to how space or time behaved. But this once celebrated theory quickly shattered, where experiments such as the Michelson-Morley test revealed inconsistencies which classical physics couldn’t explain (Michelson & Morley, 1887). Einstein’s work challenged this standard, revealing that time could stretch, space could compress, and both depended on how you moved (Einstein, 1905).

Einstein’s theory contradicted Newton’s belief. As Einstein understood, space and time are not separate entities at all, but part of a single, interconnected fabric known as space-time (Einstein, 1920). Further, changes in motion affected both space and time, which is why fast-moving objects experience time differently and appear shorter to outside observers. 

The time dilation phenomenon is rooted in Albert Einstein’s theory of special relativity. According to EBSCO, time dilation refers to time as not an absolute parameter; instead, it is influenced by factors such as speed and gravitational fields (EBSCO, 2024). Our perception of time is primarily based on the Earth’s rotational field, where 1 day is equivalent to 24 hours. However, this is not the standard across other planets in the universe. For example, 1 day on Jupiter lasts only 9.9 hours. This difference helps illustrate the concept of relativity, as the flow of time is not universal. Time dilation isn’t something which happens on distant planets; it happens right here on Earth. For example, when clocks are placed at higher altitudes, they tick faster due to a weaker gravitational field (Pound & Rebka, 1959). 

Surprisingly, time dilation isn’t simply just an observable idea. It is utilised in our everyday lives through GPS. The satellites which enable location services orbit Earth at high speeds and at much higher altitudes. Therefore, their clocks tick faster due to weaker gravity and simultaneously slightly slower due to their fast motion (Ashby, 2023; NASA, 2020).  If these tiny differences weren’t corrected, your location would be wrong by kilometres within a day. Therefore, every time you check Google Maps, you are watching relativity take place.

Just as time can stretch or slow down, space itself can change, too. This leads to a phenomenon known as length contraction, according to special relativity, objects moving at high speeds appear shorter along direction of motion (Taylor & Wheeler, 1992). It is not just an optical illusion; it is also a physical effect which completely depends on the observer’s frame of reference. To someone inside the fast-moving object, nothing changes, the object’s length appears to be completely normal. But to an outside observer watching it rush past, the object is measurably shorter, just like the train example from earlier.

Perhaps the strangest consequence of relativity is that different observers don’t agree on what happens at the same time. In classical physics, simultaneity was universal: everyone agreed on what counted as “now”. Einstein proved that this is not true. If two lightning bolts strike opposite ends of a moving train, an observer on the platform might see them occur simultaneously, while someone inside the train would see one strike before the other (Einstein, 1920). Neither perspective is incorrect, they are just moving differently. Therefore, this breakdown of universal simultaneity reinforces the idea that time is not absolute but depends entirely on your frame of reference.

From trains that appear to shrink to clocks that tick at different speeds, relativity reveals that space and time are far more flexible than we once believed. These effects are not tangible or distant; they shape our everyday world. We can see relatively everywhere, from the satellites that keep GPS working to the tiny differences in time which take place at different altitudes on Earth (Ashby, 2003). Einstein’s work replaced the comforting certainty of absolute space and time with a universe where reality depends on motion, perspective and gravity. Together, these ideas show that our intuitive sense of a fixed, universal “now” simply does not exist. Ultimately, the theory of relativity invites us to rethink the very structure of reality, reminding us that the universe behaves in ways far more powerful than our minds can allow.

References:

Ashby, N. (2003) ‘Relativity in the Global Positioning System’, Living Reviews in Relativity, 6(1), pp. 1–45.

EBSCO (2024) Time Dilation Overview. EBSCO Research Starters.

Einstein, A. (1905) ‘On the Electrodynamics of Moving Bodies’, Annalen der Physik, 17, pp. 891–921.

Einstein, A. (1920) Relativity: The Special and General Theory. London: Methuen.

Hafele, J.C. and Keating, R.E. (1971) ‘Around-the-World Atomic Clocks: Observed Relativistic Time Gains’, Science, 177(4044), pp. 166–168. (Optional; relevant if you want to include it.)

Michelson, A.A. and Morley, E.W. (1887) ‘On the Relative Motion of the Earth and the Luminiferous Ether’, American Journal of Science, 34(203), pp. 333–345.

NASA (2020) Relativity and GPS. NASA Factsheet.

Pound, R.V. and Rebka, G.A. (1959) ‘Apparent Weight of Photons’, Physical Review Letters, 4(7), pp. 337–341.

Rynkiewicz, R. (2004) Space, Time and Motion in Classical Physics. Warsaw: Polish Academy of Sciences.

Taylor, E.F. and Wheeler, J.A. (1992) Spacetime Physics. New York: W.H. Freeman.