Traveling at the speed of light, photons emitted by the Sun take just over eight minutes to reach Earth. The 93 million mile (150 million km) journey through the expanse of empty space is no obstacle to this light, but it does mean that when we look at the Sun, we see it as it is. was a short time ago, not as is instantly from our point of view. If the Sun were to disappear right now, we wouldn’t know – neither because of its light, nor because of its gravity – until eight minutes later. But what about from the point of view of the photon? We know that if you travel near the speed of light, Einstein’s special theory of relativity kicks in and time expands while lengths contract. Photons, however, do not travel near the speed of light but rather at its speed. So how much has a photon emitted by the Sun aged on reaching Earth?
If your intuition is to just say “eight minutes,” I would have a hard time arguing with you. After all, that’s how much the photon ages for us. If a 0.5 mile (0.8 km) walk to the store takes eight minutes and you walk to the store, you are eight minutes old. And if the shopkeeper watched you walk to the store, she would also know you in eight minutes. If all we did was adhere to the Newtonian definition of time – with the idea that time is an absolute quantity – this would be true for absolutely everything in the Universe: everyone, everywhere, would know the time that passes at the same speed in all circumstances. But if that was the case, the speed of light could not be a constant.
Imagine standing still on the ground, pointing a flashlight in one direction at an object a light second away. Now imagine that you are running towards that same object, shining the same flashlight. The faster you run, the faster you would expect this light to go: it has to move at the speed that the resting light is moving, more however fast you are running.
Why would this be a necessity?
I want you to imagine that you have a clock, only instead of having a clock where a gear spins and the hands move, you have a clock where a single photon of light bounces between two mirrors. If your clock is idle, you see the photon bounce back and the seconds go by normally. But if your clock is moving and you are looking at it, how will the seconds go by now?
Obviously, it takes longer for the bounces to occur if the speed of light is still constant. If time ran at the same speed for everyone, everywhere and under all conditions, then we would see the speed of light be arbitrarily fast as something moved faster. And what’s even worse is that if something moved very quickly and then turned on a flashlight in the opposite direction, we would see this light barely move: it would almost be at rest.
Since light does not do this – or in any way change its speed in a vacuum – we know this naive picture is wrong.
In 1905, Einstein presented his special theory of relativity, noting that the failed Michelson-Morley experiment and the phenomena of length contraction and time dilation would all be explained if the speed of light in vacuum were a constant universal, c. This means that the faster something is moving – the closer it gets to the speed of light – someone looking at it at rest will see their own times and distances as normal, but someone who is “riding” the object at rest. rapid movement will see that it has traveled a shorter time. distance and traveled less time than the observer remained at rest.
In fact, when you do that eight-minute walk to the store, thanks to Einstein’s relativity, the time on your watch – assuming it was very accurate and matched the merchant’s watch exactly before you left. – would now read just over seven femtoseconds behind that of the merchant. Look! The effects of relativity, although weak in most circumstances, are still in play.
The reason is that things don’t just move in space, and they don’t just move forward in time. This is because space and time are linked as part of a unified fabric: space-time.
This was first done by one of Einstein’s former teachers, Hermann Minkowski, in 1908, who said:
The views of space and time that I wish to present to you have emerged from the soil of experimental physics, and this is their strength. They are radical. Now space by itself and time by itself are doomed to fade into mere darkness, and only a kind of union of the two will maintain an independent reality.
The way it works is everyone and everything out there at all always move in space-time, and they always move in space-time with a very special relationship: you move a certain amount through the combination of the two, no matter how you move in relation to anything else.
If you move quickly in space from a certain point of view, you move less in time: this is why when you walk towards the store, your time travel is about 7 femtoseconds less. than that of the shopkeeper: you move in space faster than her. did, and so you’ve been through time a little less than she did. If you went faster, your clock would be even more advanced. In fact, if you were moving very close to the speed of light – if you were moving at 99.9999999% of the speed of light on that path to the store – no matter how far away from that store, the merchant would see that 22,000 times as much time spent for it as spent for you.
So now, with all of that in mind, let’s get to the photon itself. it does not move near the speed of light, but in fact To the speed of light. All our formulas to describe what it is for an observer gives us answers with infinities when it comes to asking what is going on To the speed of light. But infinities don’t always mean physics is bad; they often mean that physics is doing something unintuitive. When you move at the speed of light, it means the following:
- You absolutely can not have a mass; if you did, you would wear a infinite amount of energy at the speed of light. You must be massless.
- You will not experience any of your trips to space. All distances along your direction of movement will be reduced to a single point.
- And you won’t feel the passage of time; your whole trip will feel instantaneous.
For an observer here on Earth, the light will be emitted from the Sun about eight minutes (more like 8:20 am) before we receive it, and if we could “watch” the photon’s travel, it would appear to be moving around the corner. speed of light throughout its journey. But if there was a “clock” on board this photon, it would seem to us entirely stopped. While these a little over eight minutes would pass as usual for us, the photon would experience absolutely no passage of time.
It becomes especially worrying when we look at the galaxies far away in the Universe.
The light emitted by them takes Billions years to reach us from our point of view as observers in the Milky Way. During this time, the expansion of the Universe causes a stretching of space and a considerable drop in the energy of the emitted photons: a cosmological redshift. Yet despite this incredible journey, the photon itself experiences nothing of what we call time: it is simply emitted and then instantly is absorbed, experiencing the entirety of his travels through space in no time. Based on everything we know, a photon never gets old at all.