Ask Ethan: Is the light always moving at the same speed?

The image of the center of the galaxy in several wavelength ranges shows such radiation sources as stars, gas, black holes, etc. But the light emanating from all these sources, from gamma radiation to the visible and radio bands, always moves through empty space at the same speed: the speed of light in a vacuum

No matter how fast you move, one thing you can never catch: light. The speed of light is not only the maximum speed with which anything in the Universe can move, it is also considered to be a universal constant. Whether we turn on a flashlight, look at the Moon or the Sun, or measure the parameters of a galaxy located billions of light years away, the speed of light is the only thing that remains the same. But is it always like this? This is what our reader wants to know:

Does the light move at the same speed all the time? If something slows him down, will he remain slow after this influence disappears? Will he accelerate back to the speed of light?

To begin with, what is a light at a fundamental level: quanta.


Electric and magnetic fields oscillating in one phase and propagating at the speed of light determine electromagnetic radiation. The smallest unit of electromagnetic radiation, a quantum, is known as a photon.

Light can be unlike particles, if you observe a light source such as a light bulb, a flashlight, a laser pointer, or the sun — but this is because we cannot see its individual particles. If we use electronic photodetectors instead of our eyes, we will find that all the light of the Universe consists of the same particles, or quanta — photons. It has several properties that are the same for all photons:

• weight equal to 0;
• speed, always equal to, the speed of light;
• spin, a measure of the internal angular momentum, always equal to 1;

and one very important property that is different for different photons: energy. Of all the photons visible to the human eye, violet light has the most energy, and red has the least. Even less energy in photons from the infrared, microwave and radio bands, and more - from ultraviolet radiation, x-rays and gamma radiation.


Size, wavelength and temperature / energy scales corresponding to different parts of the electromagnetic spectrum

Through the vacuum of space, regardless of energy, they always move at the speed of light. And no matter how fast you try to move in the direction of the light, or towards it: the photon velocity you observe will always be the same. Instead of speed, their energy will change. Move towards the light and it will appear bluer and its energy will be greater. Move away from him and he will appear red and his energy will be less. But no matter how you move, how the light moves, how you change energy - the speed of light will not change. The photon of the highest or lowest of all the observed energies will always move at the same speed.


All massless particles move at the speed of light, including photons, gluons and gravitational waves, transporting electromagnetic, strong nuclear and gravitational interactions, respectively.

But if you want to move from vacuum to some material, the light can be slowed down. Any material that is transparent to light will allow photons to move inside it - be it water, acrylic resin, crystals, glass, and even air. But since in these materials there are charged particles - electrons - they will interact with photons, and thus slow them down. The light, although it has no charge, behaves like a wave. A photon, moving in space, causes oscillations of the electric and magnetic fields, because of which it can interact with charged particles. These interactions slow it down, forcing it to move at a speed slower than the speed of light as it moves in the environment.


The behavior of white light passing through a prism demonstrates how light of different energies moves at different speeds in the medium — but not in a vacuum.

Different photons will have different energy, which means that their electric and magnetic fields will oscillate with different frequencies. In vacuum, the speed of different types of light is the same, and in the environment may differ. Enlighten white light consisting of all colors, a drop of water or a prism, and high-energy photons slow down more than smaller photons, causing the light to separate into colors.


Primary (bright) and secondary (dim) rainbows appear due to the interaction of sunlight and water droplets, and additional - due to reflections in the water. The colors are separated due to the different speeds of photons of different energies moving in the medium - in this case, in water

This is how light, passing through water droplets, creates a rainbow - photons of different energies interact with charged particles of the medium, and are slowed down in different ways.


Multiple reflections of light in a drop of water lead to the separation of light from different angles, when the red light in the water medium moves faster and the violet light moves more slowly.

It is important to remember that with this, no properties of light change. He does not lose energy, does not change his intrinsic properties, does not turn into anything. Only the surrounding space is changing. When this light leaves the environment and returns to the vacuum, it again moves at the speed of light in a vacuum: 299,792,458 meters per second. In fact, the definitions of distances and time - meter and second - are calculated through the speed of light. Atoms can absorb or emit light, depending on the electron transitions inside the atoms.


The atomic transition from the 6s orbital, Δf ₁ , determines the meter, second and speed of light

Cesium, the 55th element of the periodic table, has 55 electrons in a single, stable, neutral atom. The first 54 electrons usually exist in the state with the lowest energy, but the 55th has two possible levels of energy that it can occupy, located extremely close to each other. If he moves from what is slightly higher to that which is slightly lower, then the transition energy passes to the photon with a completely defined energy. If you take 9,192,631,770 cycles of this photon, you get 1 second. If we take the distance that it will cover in 30.663319 cycles (9 192 631 770/299 792 458), then we get 1 meter.

A surprisingly profound thing follows from this: as long as the atoms throughout the Universe are exactly the same, our definition of time, distance and speed of light will not change, regardless of where in the Universe we use them.


No matter how far we look into the Universe, the physics that control the atoms and determine the length, time and speed of light will remain unchanged.

So, what did we learn in the end?

1. Light, regardless of whether its energy is high or low, always moves at the speed of light, as long as it moves in the vacuum of empty space.
2. No change in your movement or the movement of light changes this speed.
3. By sending light to a medium other than vacuum, you can change its speed while it is moving in that medium.
4. Light of different energies will vary in speed in different ways, depending on the properties of the medium.
5. Leaving the environment and returning to the vacuum, the light begins to move again at the speed of light.
6. According to our knowledge and best measurements, the speed of light is maintained at 299,792,458 m / s in all places and at all times of the Universe.

In many respects, light is the simplest particle in the universe. And although it always moves at the speed of light, it does not always move in absolutely empty space. As long as transparent material remains in the Universe, you will not be able to avoid slowing down the light in it. But as soon as the light returns to empty space, it again moves at the speed of light, and each photon moves as if it never moved at a different speed!

Ethan Siegel - astrophysicist, popularizer of science, blog Starts With A Bang! He wrote the books Beyond The Galaxy , and Treknologiya: Star Trek Science [ Treknology ].

FAQ: if the universe is expanding, why aren't we expanding ; why the age of the universe does not coincide with the radius of the observed part of it .