When a flashlight shines photons into the night sky, will it reach the end of the universe?

On a clear night, the sky is full of stars. If we hold a flashlight or laser and shine it into the sky, we can see a beam of light flying into the distance. If you turn off the flashlight, the beam of light becomes invisible immediately. So, where does the light from the flashlight go? Did these lights disappear, or did they fly into the universe and continue to spread?

First of all, we can see light pillars because dust particles in the air scatter photons. After the flashlight is turned off, no photons continue to enter our eyes, so the light column disappears immediately. In addition to part of the light emitted by the flashlight that we see, a large part of the photons will always travel at the speed of light.

However, even if the weather is clear, there are still a lot of invisible air and dust particles on the earth. So, will the flashlight light be completely absorbed by the atmosphere? Can these photons pass through the atmosphere? Regarding this issue, let’s take a look at sunlight.

Sunlight in space can penetrate the earth's atmosphere and reach the surface, bringing us light and heat. As it passes through the atmosphere, some of the sunlight is absorbed by the atmosphere. In space, where the Earth is, the intensity of solar radiation is 1368 watts/square meter. After reaching the surface vertically, the intensity of solar radiation weakens to 1000 watts/square meter, which means that about 27% of the sunlight is absorbed by the atmosphere.

Therefore, the light from the flashlight does not completely dissipate in the atmosphere, and some of the photons can enter into space. So, how far can these photons travel in the universe? Can they go all the way to the end of the universe?

Light has both particle and wave properties. They are electromagnetic waves. A changing electric field can excite a magnetic field, and a changing magnetic field can excite an electric field. This repeated alternation forms electromagnetic waves, which is what we call light.

Electromagnetic fields can exist in vacuum and do not require a medium, so light can propagate in vacuum. Not only that, the interactively transforming electromagnetic field does not consume energy, and the electromagnetic field is established at the speed of light. Therefore, photons always propagate forward in space at the speed of light, without consuming energy during this period. The moment photons are generated, their speed can reach the speed of light, and their movement does not require a power source.

Even if photons are absorbed by an object, they do not disappear in a sense. Because after an object absorbs photons, the electrons in the object will jump to a higher energy level. This state is unstable, and the electrons will jump back to a stable energy level and radiate new photons at the same time.

Unless photons encounter a black hole, they will disappear completely into the universe. According to Einstein's general theory of relativity, a black hole is an extremely distorted space-time with its mass concentrated at a central singularity. If a photon falls into a black hole, even if it can travel at the speed of light, it will not be able to escape the distorted space-time of the black hole. This is why the black hole itself is completely invisible.

But in the vast universe, the space is extremely empty, and most photons will not encounter objects or black holes, which means that they can always propagate in the universe and will not disappear out of thin air. . The earliest photons were produced 380,000 years after the Big Bang. They have not disappeared even after 13.8 billion years, and are still spreading throughout the universe.

In other words, the photons we shined into the sky with flashlights are still moving forward in the universe. It takes one year for these photons to leave the solar system and enter interstellar space. If photons travel in a direction perpendicular to the galactic plane, it will take them more than a thousand years to leave the Milky Way and enter the vast intergalactic space.

However, the photons from the flashlight will never reach the end of the universe. This is not because they do not have enough energy, but because the universe is expanding faster than the speed of light, and the expansion rate is still accelerating, and photons can never reach the distant universe. Because of this, photons from the distant universe can never come to Earth, where they will never be visible to us.