# Why does the speed of light slow down on a cosmic scale?

The speed of light is an extreme fundamental measure of motion, the ultimate speed of our universe. It allows us to measure how far an object moves in a given amount of time. So can humans move at the speed of light? At first glance, one might think that velocity is relative because it can be measured at different values from different angles. However, special relativity changed our intuition in a fundamental way. One speed that never changes is the speed of light. To achieve the speed of light, we need a deeper understanding of the speed of light!

The speed of light is a constant in a vacuum that doesn't change no matter what angle you measure it from, and many of our explorations of modern physics actually started with light. As an electromagnetic wave, light has many properties. In 1842, Austrian physicist Christian Doppler discovered his famous Doppler effect in experiments that assumed a water tank with a wave source in the tank that produced circular water waves. At the same time, a man stands still in the water and watches the waves moving towards him. The frequency of the wave observed by the person should be exactly the same as the source of the wave.

But what happens when a person moves toward a wave source? The frequency starts to increase, which is called the Doppler effect. Light also has the Doppler effect. It changes the frequency of light due to the relative motion between the light source and the observer. As the light source moves away from the observer, the frequency received by the observer will decrease, resulting in a shift in the red end of the visible light spectrum, known as a redshift 39bet-đua chó-game giải trí -đá gà-đá gà trực tuyến-đánh bài. If the light source moves toward the observer, a blue shift is observed.

Another important property of waves is that their propagation speed depends only on the nature of the medium through which they propagate. Higher tones will have higher frequencies but smaller wavelengths, and vice versa, which will keep the propagation speed constant unless we switch mediators. If the source of the wave moves toward the observer, the person will perceive an increase in the frequency of the water waves. However, as long as the person remains still, the speed of the waves he senses will remain constant.

This is because the speed of water waves is independent of their source, but changes if people move together. With the source at the same velocity, the observed frequency is now equal to the source, although the wave is still compressed. This is because there is no longer relative motion between the person and the wave source, so the Doppler effect disappears. However, if the person is now moving relative to the water, the wave speed he feels should decrease, which follows the Galilean transformation. If the medium is shared by all passing objects, it must be considered as a global reference frame, that is, the Doppler effect behind the frequency change is due to the relative motion of different objects. And the Galilean transformation behind the wave speed change is actually the reference frame caused by the global absolute motion.

​Let us take it a step further. What happens if we put another wave source under the observer and let them all move together at the same speed? From a human point of view, two waves coming from two different directions should have the same frequency but different speeds. This is the famous Michael Simoli experiment, which replaced water waves with light. It postulates the existence of a medium called the aether, permeating the entire universe and acting as a carrier of light, much like a moving observer in water. The Earth moves in the aether, and as long as the Earth continues to move, light from different directions must have different speeds, which should be detected by measuring the speed of light in different directions at different times. But surprisingly, these experiments were negative.

But the speed of light does not show any difference in any direction, contradicting the Doppler effect of the wave. One possible explanation is that the aether is completely dragged by the Earth and therefore shares its motion. However, if the aether always moves with the Earth, then the global reference frame defined by the aether is equivalent to the local reference frame. So the Michelson-Morley experiment directly inspired the proposition of special relativity, and since then the global coordinate system of the whole universe has been replaced by the local coordinate system of a single object. In these local coordinates, the speed of light remains the same. However, it is not unconditionally equal to C.

In 1964, the American astrophysicist Irwin Shapiro devised a thought experiment: If we sent radio signals to distant plants and reflected them back via satellites orbiting them, we could know whether the speed of light had changed by calculating round-trip travel times and travel distances. Under normal circumstances, we shouldn't see any change. However, when the planet, the Earth and the sun are almost in the same line, we will see the light bending, traveling along a curved path. Light slows down as it passes by.

In 1976, scientists conducted an experiment that used the round-trip time of radio signals transmitted from Earth to the Viking spacecraft. In 2003, the Cassini spacecraft did the same experiment, and their results were consistent with the predictions of general relativity, which suggests that curved light does slow down as seen from Earth, which is what the so-called constant speed of light really means. If we place many observers along the path of light, then the speed of light measured in its local reference frame is always equal to C and does not change from the point of view of the Earth. The deflected light is slowed down, known as the Shapiro time delay, or colloquially as the gravitational time delay effect.

The gravitational time delay effect is caused by the expansion of space-time from the point of view of an outside observer, such as the Earth. However, each observer will locally measure light equal to C. Let's go back to our original thought experiment. If we use time as the Z-axis, the water waves will form a cone. The two edges of the cone represent the velocity of water waves. If we switch to the human point of view, then his world line will become a vertical line, and the cone is now tilting. For the person who represents the change in wave speed, if we replace water waves with light waves, this transformation will no longer apply.

Since the speed of light should remain constant in the local frame of reference, we need to make use of a new transformation: the Lawrence transform. This time, the two edges of the cone will remain the same, and the velocity of the coordinating cone begins to change. The speed of light is constant, but it is only measured locally as equal to c, in a small enough region that spacetime can be viewed as completely flat. This is why the Shapiro time delay can be observed on a cosmic scale.