Space-time fabric

Before Einstein, the viewpoint of scientists was that Time was Absolute. It does not change for anyone. You can imagine it as the whole universe having a single pace of time. So if an hour has passed for me, an hour would pass for anyone travelling with some velocity, an hour for someone on top of the Mt. Everest, or an hour for 2 passing trains.

Einstein’s view was that Space and Time are interlinked, and one has no bearing without the other. They do not exist independent of each other. Also, time is relative. It is not the same for everyone; time passes at different pace for different conditions. One must keep their own watch/ clock to know precisely what time it was, relative to others. This bugged Einstein as it contradicted Newton, but Einstein was right. In Classical physics, we ignore the effects of relativity. In relativistic context, time and space cannot be separated, and rate at which time passes depends on the object’s velocity and gravitational field.

If the velocity and gravitational field are different for me, my time would pass at a different rate than yours! Time, slows down with an increase in gravity or velocity. So, you would live younger if you are living on the space station, or in an increased gravitational environment, like Jupiter. Time would pass quicker at the top of Mt. Everest, so you would age more there. At the speed of light, time stops.

To describe a mathematical model of space-time continuum, a coordinate system (Euclidean coordinates) was defined, which consists of 3 spatial dimensions and 1 temporal (time) dimension. Since it is impossible for us to visualise a 4D space, it is generally plotted with Space on the x-axis, and time on the y-axis, known as the Minkowski space-time. I discussed briefly in my post ‘What’s wrong with Gravity’, that matter bends space time fabric, which caused other bodies to feel the pull of gravity.

Gravitation

 Image courtesy: ‘Clear Science’

The perception of passage of space and time can be done through visioning the Light Cone.

Imagine a paper or a screen (2D), with a point in the center, representing an event. Now, when light from this event spreads out, it forms circles, just like ripples in a pond when a stone is thrown in the water, as shown in this animation below.

Ripples simulation

Image courtesy Motion Addicts

If you then plot the vertical axis as time axis, then the intersection of these ripples with time passage will cause the formation of the shape of a cone, just like the one below.

Light Cone 1

So basically, this cone is the locus of light through space and time. For forward time, this is the future light cone. If you reverse the direction of time, the cone would be inverted, and you are in the past. Light cone converges as observed in the present time.

Light Cone

Everything must lie within the light cone as the speed of light is the ultimate speed, the maximum speed at which an object can travel through space (Special theory of relativity)

Of course, we only assumed 1 spatial dimension, and in reality there are 3 spatial dimensions, hence the ripples would look like expanding spheres, just like an explosion, or while filling up a balloon with air. To represent this on a 4-D space is impossible, hence we use the light cone as the simplified version.

There was a young lady named Bright,
Whose speed was far faster than light;
She started one day in a relative way,
And returned on the previous night.

-A. H. Reginald Buller in Punch (Dec. 19, 1923): 591.

Where science doesn’t reach, philosophers do. Thanks for reading! Leave a comment below and let me know what you would like to discuss!

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