When you look up at the sky during the day, you will typically see an expanse of blue. The colour results from sunlight being scattered after reaching the Earth’s atmosphere, with blue light being scattered more than other colours as it travels in short waves. Meanwhile, the sky looks black or empty when the sun goes down at night.
To the human eye, these differences are stark, with light and darkness appearing to be opposites. This notion leads many to believe that both are independent entities that can be defined separately—which means that if there is a speed of light, there should also be a speed of darkness.
The debate between light and dark, in the world of physics, is similar to the debate between Fahrenheit and Celsius. Hence, to understand why, we must first define what light and dark are.
What is light and dark?
Light is made up of particles called photons, which are small packets of electromagnetic energy. Scientists have long established that it is an independent entity with its own physical characteristics; for example, it possesses a speed of 300,000 kilometres per second in a vacuum. When in contact with other mediums like glass or water, the speed of light shifts. Additionally, light originates from sources like the sun and fire and can be produced through electrical systems.
On the other hand, darkness is not its own entity: it can only be defined in relation to light. The night sky is dark only because the sun, or the primary light source, goes down; you cannot create or add darkness—you can only remove the light. Therefore, darkness is more a lack of a thing than a thing itself.
Thus, light and darkness are bound to each other and cannot be defined separately.
So, which travels faster?
Determining whether light or darkness travels faster requires us to compare the speed of each. However, darkness is not a unique entity with individual characteristics. It does not travel relative to other bodies and thus, on the most literal level, does not have a speed.
However, if you were to calculate the speed of darkness as the speed of light receding, then darkness can be considered to move at the speed of light. To understand this, it would help to imagine what would happen if the sun and all other light sources disappeared. The world would not instantly go dark—it would take a while before the last bit of light reaches the Earth. In other words, darkness appears right after light vanishes, so darkness (or the absence of light) spreads at the speed of light.
It is challenging to compare the speed of light with that of darkness because they are not equivalent in terms of definition and characteristics. While light is an independent entity, darkness can only be defined as the absence of light and thus does not have a speed of its own.
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