Exploring the Journey of Light: Can It Reach the Universe's Edge?
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Chapter 1: The Nature of Light
When we illuminate the night sky with a flashlight for just a moment before extinguishing it, we might be left wondering about the fate of the emitted light. What happens to it? Let’s delve into this intriguing subject.
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Section 1.1: Understanding Photons
What we perceive as a beam of light is actually a result of photons scattering as they interact with minute particles in the atmosphere, such as dust and water droplets. When the flashlight is turned off, the photons that have not yet interacted with these particles continue their journey. In ideal conditions—clear skies and minimal impurities—some of these photons can escape the Earth’s atmosphere and venture into the vastness of space.
Photons exhibit both wave-like and particle-like characteristics. As quantized electromagnetic waves, they follow Maxwell’s equations, where a changing electric field generates a magnetic field, and vice versa. This alternating process does not deplete energy, allowing photons to travel through the vacuum of space at light speed without additional energy input.
Subsection 1.1.1: The Duality of Photons
In terms of their particle nature, photons are massless particles that can only move at light speed, as per special relativity. They possess an infinite half-life, suggesting that once they are released into space, they can theoretically continue traveling indefinitely, unless obstructed.
Section 1.2: The Universe's Vastness
The universe is unimaginably large, with a matter density estimated at about six protons per cubic meter. Consequently, the likelihood of photons colliding with obstacles is minimal. However, it is essential to note that the light from a flashlight does not travel in parallel lines; rather, as distance increases, the photons disperse. This raises the possibility that some of these photons could travel endlessly through space.
But can they reach the "edge" of the universe? Unfortunately, the answer is no. The question of whether the universe has an edge remains open. Some theories suggest it is infinite, while others propose it has boundaries. If the universe is boundless, the idea of an edge becomes irrelevant.
Chapter 2: The Expansion of the Universe
The first video titled "Can a Flashlight's Beam Reach the Edge of the Universe" explores whether the light emitted from a flashlight can ever reach the universe's edge, taking into account the nature of photons and their journey through space.
In addition, scientists have discovered that the universe is perpetually expanding, causing distant celestial bodies to move away from us. This movement, referred to as "recessional velocity," indicates that the farther away an object is, the faster it appears to move away.
According to the formula v = Hr, where "r" is the distance and "H" is the Hubble constant, the estimated value of this constant is around 42.12 miles per second per megaparsec (approximately 3.26 million light-years).
It's crucial to understand that recessional velocity reflects the rate at which a celestial body recedes due to the expansion of space, rather than its movement through space. As a result, a celestial body can recede faster than the speed of light when it exceeds a certain distance—approximately 14.4 billion light-years.
In this context, even though photons travel at light speed, they cannot keep pace with objects receding at faster speeds. Therefore, while some photons emitted by a flashlight may theoretically continue their journey through space indefinitely, they will never reach celestial bodies that are over 14.4 billion light-years distant.
The second video titled "How Far Can You Travel? Voyaging Billions of Light Years in a Human Lifetime" examines the vast distances involved in cosmic travel, providing insight into the limitations of our understanding of space.