The 5 step guide to understanding space-time

Updated: Dec 10, 2020

The concept of space-time a great intuition that came from a rare genius of the human mind. As I took my baby steps into the realms of Physics, space-time is where I struggled the most; I think it is natural for most people. We all have a hard time wrapping our minds around the beautiful geometry of space-time.

Evaporation is a physical phenomenon a fifth-grader understands; it is because the child sees most of what happens in front of its eyes. We humans don't see space every day and feel how it behaves, nor we are born equipped to do so. Don't feel bad if you are having a hard time.

This post is an attempt to simplify the concept of space-time and how it behaves, without getting too technical about it. Instead of jumping headfirst into Lorentz contraction, time dilation and other stuff, it is better to understand space and time separately and then combine their intrinsic nature.

Time and space are modes by which we think and not conditions in which we live.
-Albert Einstein

Step 1: Letting go of Newtonian space and time

The Newtonian view or the classical physics view of the universe is outdated. Newtonian mechanics helps us to understand the world to a great precision; it is good enough for day-to-day life. But when we get closer to the grand scheme of things and understanding reality itself, it gets complicated. The classical world view merely blurs our understanding of nature.

According to classical physics, space and time are absolute features of reality; they are universal and do not change at any circumstance. To take a step forward, you must take two steps backwards and change your notion of a flat, eventless space where our planet is sitting and the flow of time that is, same for the entire universe.

Step 2: Understanding the aspects of Einstein's space:

As we read in the previous post about the weirdness of gravity, in the absence of any matter or energy, space could be flat like the smooth surface of a table-when a massive body like the Sun emerges, it causes space to warp. It acts like a rubber sheet that curves when a bowling ball falls on it.

A massive object (like the Sun) in space warps its surroundings, this affects other stuff (like planets) in its vicinity and forces them to follow a curved path. Einstein explained how gravity is nothing but a geometric consequence of curved space. The more massive the object, the more distortion it causes; more distortion means more gravity.

Einstein's space is dynamic. It is not just a platform for events to take place, but a live arena that reacts to matter and energy.

Even in empty space, time and space still exist.
- Sean M. Carroll

Not only mass and energy cause space to distort, but acceleration too has an effect. Gravitational waves occur because of disturbances in the curvature of space, generated by aggressively accelerated massive bodies (like neutron stars or black holes) that propagate as waves outward from their source at the speed of light.

All these effects convert the empty Newtonian space into the bending, curving and fluctuating fabric of space we are all embedded into. We have now pretty much got the idea of how the fabric of space responds to matter, energy or acceleration. Now let's talk about time.


Above: Newton's fixed space vs. Einstein's flexible spacetime, from the film "Testing Einstein's Universe" by Norbert Bartel. [More]

Step 3: Letting go of the universal time

According to Newton, absolute time and space are solely aspects of reality. The pure mathematical time, also called duration, flows equally throughout the universe with no external influence. It exists independently and advances at a steady run, no matter what. This means absolute time does not depend upon physical events, but it is a backdrop or stage within which events occur.

While Newtonian arrow of time flows in a single direction at a steady rate, Einstein's time differs. It is very dynamic, depending on gravity and velocity.

Step 4: Grasping 'time' according to Einstein

According to General Relativity, time does not flow at a constant rate throughout the universe, but it dilates. Time dilation is the slowing down of time that occurs under the effects of gravity or motion.

Time dilation because of motion:

To understand time dilation due to motion, you need to understand dimensions. Our universe has three dimensions, this means our realm of reality allows objects to have a length, a breadth and a height. Einstein, with a stroke of genius, added time as the fourth dimension, apparently connecting time with the other three dimensions; it led to the concept of space-time ( a. k. a Minkowski space)

To understand the effects of velocity on time, we need to understand shared motion among different dimensions.

Have you ever noticed the fact that you are always in motion through the fourth dimension, that is 'time'? Even if you are sitting idly and binge-watching your favourite show on Netflix, you are still moving through time; falling into the hands of the future one second at a time.

Now imagine you are moving in a speeding car and binging Netflix; this time you are not only moving through time but you are also moving through space. Your constant motion through time gets shared among all four dimensions; this causes your time to slow down.

Therefore, a moving clock ticks slower than a stationary clock.

Can you imagine something mind-blowing than this? But there is something! We all know nothing travels faster than the speed of light. A photon (that is light) has the maximum velocity in three-dimensional space; such a particle has no momentum left in itself to move through another dimension. The photon doesn't move through time because it moves through space in the maximum velocity possible. This is why light does not age.

A photon produced at the big bag is, to this very moment zero years old.





Gravitational time dilation:

General relativity predicts that time flows slower when you are near a massive object like Earth. Closer you are to a strong gravitational field, slower your time flows. This is how General relativity gets rid of absolute time.

For example, let's consider a pair of siblings who are twins. Think that one of them goes to live on the top of a mountain while the other stays somewhere around the valley. The first twin, living at the mountain top would age sooner than the second. Thus, if they met again, one would be older than the other.


the twin paradox is a thought experiment in special relativity involving identical twins, one of whom makes a journey into space in a high-speed rocket and returns home to find that the twin who remained on Earth has aged more
The twin paradox

Here, the difference in ages would be insignificant, but it would be much greater if one twin went for a long trip in a spaceship travelling close to the speed of light. When she returns, she would be much younger than the one who stayed on Earth. This is the twins' paradox. This is how gravity affects time. The gravitational field is so strong near the singularity of a black hole, time practically stops inside it.

Time dilation is not just theory, we use this concept in everyday life. The difference in the speed of clocks at different heights above Earth is now of great importance.

We use very accurate navigation (GPS) systems based on signals from satellites nowadays. If you ignore general relativity, the position that a GPS satellite calculates would be wrong by several miles!

Step 5: Embracing space-time:

Space and time are now far away from being static and absolute and have become dynamic quantities: when a body moves, or a force acts, it influences the curvature of space-time—and the geometry of space-time influences how objects move and forces act.

Space-time not only affects everything but also gets affected by everything that happens in the universe. The new understanding of space-time has revolutionized our view of the universe. This is the story of how the notion of a dynamic space forever replaced the initial idea of an essentially unchanging universe, that could have existed forever. This fascinating idea has now given us an expanding universe with a finite past and a finite future.

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References:

  1. Greene, B. The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory., 1999. Print.

  2. Hawking, Stephen, 1942-2018, A Brief History of Time. New York: Bantam Books, 2017.

Image credits:

cover image: ESO/L. Calçada.

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