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An endless journey through space?
Light is an electromagnetic wave... and it has no mass. It is precisely this lack of mass that allows it to travel at a speed of 186,000 miles per second (300,000 km/s) without ever slowing down, as long as it remains in a vacuum. It thus escapes all forms of friction and gravity that usually slow down material objects.
In the vastness of space, the vacuum is nearly perfect. There is almost nothing to disturb the light. As a result: it crosses the Universe over the greatest distances, intact. But beware: when it encounters matter, such as Earth's atmosphere or an interstellar gas cloud, it can be absorbed or scattered.
Thanks to this ability, astronomers can observe distant galaxies and, in a way, travel back in time by capturing light emitted billions of years ago. They are therefore seeing the past of the Universe.
Relativity: when time stretches... and stops
And what if we add a touch of relativity? According to Einstein, time is not fixed but depends on speed and gravity. This is time dilation.
An object moving at high speed or near an intense gravitational field will experience time passing more slowly than an object at rest or far from that gravity. For example, the clocks on GPS satellites must be adjusted to account for this discrepancy.
For a photon (the particle of light), it's even more counterintuitive: by traveling at the speed of light, time completely stops from its perspective.
In other words, its journey from one point in the Universe to another seems instantaneous to it. Its "life" is limited to its birth (emission) and immediately its death (absorption), even if its birthplace and place of death are billions of light-years apart. For us, it will have taken billions of years to cover that distance. For the photon, it's instantaneous.
These notions, as bewildering as they may be, are crucial for understanding our Universe... and our modern technologies, like GPS.