The light in the void could travel eternally, what happens is that the universe is full of things and the light interacts with those things. But, let us start at the beginning. It is important that you know that light is an electromagnetic wave, that means that it is composed of an electric field and a magnetic field that oscillate with a certain frequency. This oscillation propagates, travels, and that causes it to have a spatial periodicity, which we call wavelength and which, together with frequency, is what characterizes electromagnetic waves. To understand it, there is a very graphic example: think about when the spectators make a wave in a football stadium. Each person gets up, raises his arms and sits down, does not move from his place, but when you see him from a distance what you observe is a movement that spreads, a wave that goes from one place to another. Something similar happens with electromagnetic waves. In a moment you can see the distance between one crest and another, that distance is what we call wavelength. If the ridges are closer, the wavelength will be shorter and if they are farther away, it will be longer.
What we call light is radiation, or wave, electromagnetic whose wavelength is in a range visible to the human eye. But there are others that we do not see. For example, there are ones with a very small wavelength, the size of atoms, which are called X-rays, or there are also radio waves and telephony or microwaves that we use at home to heat milk. All of them are electromagnetic waves. All electromagnetic waves, and only them and gravitational waves, can travel in a vacuum at the speed of light, 300,000 kilometers per second, because they have no mass.
In a vacuum, any electromagnetic wave, including light, could travel in a straight line eternally and at that speed. What happens is that there is no vacuum, in our universe there are many things, such as galaxies, stars, planets and also dust, gases, ions, and those things are what prevent this eternal and linear journey of light. Einstein's theory of relativity explains what happens. Although electromagnetic waves do not have mass, stars and planets attract them and that causes their trajectory to curve. Even if the light passes too close to a black hole it will catch it. In fact they can not even get the light out and that's where their name comes from, black hole.
The way in which electromagnetic waves interact with all those things in the universe depends on their energy. For example, visible light bounces (reflects) on a white wall, that bounced light is what we see and what tells us that that wall is there and that it is white. If the wall is black, it totally absorbs the light and it no longer travels, it transforms into heat in the wall. However for a radio wave, which has a longer wavelength than visible light, (between 1 km and 10 cm for radio and TV stations), the wall is transparent whether white or black, passes through it and that's it. However, those same radio waves bounce off the Earth's ionosphere. Or, for example, the microwaves of our appliances are absorbed by the water of food and metals, and this is the reason why we should not put metals in the microwave because the metal will be damaged and, worse, if it has edges powerful sparks will form. There are other types of electromagnetic waves such as gamma rays that come from radioactive or astronomical processes and have an enormous energy capable of crossing our bodies and causing serious damage to cells.
When light travels in a material medium such as atmosphere, water or a glass its speed changes, it is reduced more or less depending on the medium in question. It also happens that there is a change of direction when passing from one medium to another, it is what is called refraction. So when we see a spoon placed in the water it seems to be bent just at the point of interface between the air and water.
As a summary we can say that light could travel eternally in a vacuum, what happens is that it encounters many obstacles in its path that influence both its speed and its direction. But even so, the fact is that light makes very, very long trips. The light of the farthest star that has been captured, by the telescope Hubble, and that comes from a star called Icarus, has been traveling 9,000 million years. That light that has made a long journey to the telescope informs us of how Icarus was 9,000 million years ago and therefore it is almost certain that this star has long since disappeared or has been transformed into a black hole.
Alicia de Andrés She holds a PhD in Physical Sciences and a Research Professor at the Materials Science Institute of the CSIC.
Coordination and writing: Victoria Toro.
Question asked via email Adrián Garay Vergara
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