How do photons perceive time?

Everyone has a dream; I would like to live until dawn, but I know that I have less than three hours left. It will be night, but it does not matter. Dying is easy. For this, no light is needed. So be it: I will die by the light of the stars.
- Victor Hugo

Moving at the speed of light, the photons emitted by the sun reach the earth in about 8 minutes. The path of 150 million kilometers of empty space is not an obstacle to light, but for us this means that looking at the Sun, we see it as it was shortly before, and not as it is at the moment. If the Sun disappeared instantly right now, we would not know about it - either because of the light or because of gravity - until eight minutes passed. But what does this look like in terms of a photon? It is known that if you move at a speed approaching the speed of light, Einstein’s service station turns on, time slows down, and length decreases. However, photons move at a speed not close, but equal to the speed of light. And how much then does the photon emitted by the Sun age by the time it reaches the Earth?

It will be difficult to argue with you if you intuitively answer “for eight minutes”. In fact, for us, this photon has aged so much. If a walk to the store with a length of 0.8 km takes eight minutes and you went to the store, you are eight minutes old. If the seller noticed that you go to the store, for him you will also be aged eight minutes. And if we simply adhered to Newton's definition of time - as an absolute value - this would be true for everything else. Everybody everywhere would feel that time is running at the same speed in all circumstances. But in this case the speed of light could not remain constant.



Imagine that you are standing on the ground and shining a lantern in one direction, in which an object is located at a distance of one light-second. Now imagine that you are running towards this object, shining with the same lantern. The faster you run, the faster, in your opinion, should the light move - at the speed of light combined with the speed of your run.
')
Why is this necessary?

Imagine that you have a watch with you, but not one in which gears rotate and arrows move, but ones in which a single photon of light is reflected up and down between two mirrors. If your watch is at rest, the photon moves up and down, and seconds go as usual. But if the clock moves, how will the situation change?



Obviously, the time between reflections will increase if the speed of light remains constant. If time flowed at the same speed for everyone, everywhere and in any conditions, then the speed of light could be any, the higher, the faster someone moved. What is even worse, if someone, moving very quickly, turned the flashlight back, we would see that the light almost does not move: it almost rests.

But the light does not behave this way, and does not change its speed in a vacuum in any situations, so we know that such a representation is wrong.



In 1905, Einstein proposed his special theory of relativity, noting that the failure of the Michelson-Morley experiment, as well as the change in the length and speed of time flow, can be explained if the speed of light in a vacuum is a universal constant, c. This means that a fast moving observer will see that he has traveled a shorter distance and has traveled less than it seems to a resting observer.


The Soyuz spacecraft, docked to the ISS Pierce module, will return astronauts who have aged a little less to earth than if they remained on the earth, thanks to a relativistic extension of time.

In fact, after your trip to the store, thanks to Einstein's theory of relativity, your clock — if it were perfectly accurate and initially matched the seller’s clock — would be almost two nanoseconds behind the seller’s clock. The effects of relativity, although they are usually small, always work.

The reason is that objects do not just move in space, and not only move forward in time. This is because space and time are combined into a single fabric of space-time.



For the first time, one of Einstein's teachers, Hermann Minkowski, thought of it in 1908, after which he wrote:

The views on space and time that I want to present to you have grown on the basis of experimental physics, and this is their strength. They are radical. Consequently, space itself and time itself is destined to disappear into the shadows, and only the union of the two will remain in the role of an independent reality.

It works as follows: everything and everything that exists always moves through space-time, and always in a certain way: you move a certain distance in space-time, no matter how fast you move relative to everything else.


The stretching of the time on the left and the shortening of the distances on the right show how time seems to go slower, and distances shorten when approaching the speed of light.

If from some point of view you move in space quickly, then you move slower in time: therefore, the trip to the store took you 2 nanoseconds less than the seller: you moved through the space faster than him, which means that after time you moved slower If on the way to the store you move at a speed very close to the speed of light - about 99.9999999% of the s - then it would be 22,000 times longer for the seller than for you.



Mindful of all this, let us return to the photon. It moves not close to the speed of light, but exactly at the speed of light. And all our formulas when trying to describe the behavior of an observer moving at the speed of light, give endless answers. But infinity does not necessarily mean the mistakes of physics - sometimes it means that physics contradicts intuition. When you move at the speed of light, it means that:

• You basically can’t have mass. If you had it, you would carry an infinite amount of energy. You must be massless.
• You will not accept your journey through space. All distances in the direction of your movement will be reduced to a point.
• You will not accept the passage of time. Your journey will seem instant.



For an observer on Earth, the light is emitted by the Sun eight (8:20) minutes before he sees it, and if we could watch the photon travel, it would move at the speed of light all the way. But if we were on board this photon watch, for us they would seem to have stopped. These eight minutes would pass for us as usual, but the photon would not perceive the passage of time.

This is especially surprising when you turn to distant galaxies of the universe.



The light emitted by them takes billions of years to reach us, from the point of view of an observer located in the Milky Way. During this time, the expansion of the Universe causes space to stretch, and the energy of the emitted photons drops noticeably: a cosmological redshift occurs. But, despite this amazing journey, the photon does not at all perceive what we call time: it is simply emitted, and then instantly absorbed, and the entire journey takes place for it literally instantly. Apparently, what we know, the photon does not age at all.