Lightyear: You really misunderstood me!

What exactly is a light year?

Because of the "year" in it, the light-year as a distance unit can easily be considered as a time unit. Some netizens even think that a light-year is the time it takes for light to travel in one year. If you think about it carefully, saying that a light-year is the time it takes for light to travel in one year is like saying: My name is Zhang San, the Zhang in Zhang San and the San in Zhang San do not convey any useful information.

In fact, a light-year is a unit of length . It represents the distance that light travels in a vacuum in one year, which is about 9.46 trillion kilometers. Maybe you don't have much feeling about this number, but after reading the following description, you should have a deeper understanding of it.

If the speed of the car is 120 kilometers per hour, it will take us about 9 million years to travel a distance of 1 light year. If we take a plane, assuming the speed of the plane is 1,000 kilometers per hour, it will also take us more than 1 million years to travel this distance!

Why use light-year as a unit of length?

The speed of light in a vacuum is constant, so the distance that light travels in a vacuum in one year is also fixed. So the light year is not an approximate number, but a defined and exact value. This is very important as a unit of length.

The light-year has been around for a long time. In 1838, German astronomer Friedrich Wilhelm Bessel (the Bessel function commonly used in physics and engineering is named after him) first used the term "light-year" in astronomical measurement. Later, other astronomers found that this unit was very convenient for measuring the distance between celestial bodies, and it gradually became popular.

Similar units to light years include light minutes and light seconds, which represent the distance light travels in 1 minute or 1 second, respectively. So like light years, they are all units of length.

Light-years, light-minutes, and light-seconds are all units of distance.

(Image source: MathsIsFun)

As shown in the figure above, the distance from the moon to the earth is 1.28 light seconds, which is about 384,000 kilometers. The distance from the sun to the earth is 8.2 light minutes, which is about 148 million kilometers. The diameter of the Milky Way galaxy we are in is about 100,000 light years. What is the concept of 100,000 light years? The current record for the fastest man-made object is 200 kilometers per second, which was achieved by the Parker Solar Probe built by NASA. At this speed, the fastest man-made object will take about 150 million years to cross the Milky Way. This is much longer than the less than 10,000 years of human civilization.

The diameter of the Milky Way is about 100,000 light years. The Sun in the picture represents the location of the sun.

(Image source: Caltech)

Why is Light Year always "misunderstood"?

The distance that light travels in a vacuum in one second is about 300,000 kilometers, which is equivalent to the distance of four and a half circles around the earth's equator. Therefore, when we usually talk about distance, we rarely use the unit of light year. Some literary works may use the word light year to describe a very long distance for the purpose of exaggeration.

Because this unit of length is so large, it is usually only used in astronomy. It's like we often use Mach to measure the speed of fighter jets, but we don't use Mach to describe the speed of motor vehicles, because the speed of ordinary motor vehicles is only about 100 kilometers per hour, which is only about 0.1 Mach. For another example, the unit of mass - tons, we usually don't use "tons" to describe a person's weight. We would say that an adult male weighs about 75kg, but no one would say that an adult male weighs 0.075 tons. I believe no one would use anti-aircraft guns to kill mosquitoes!

Next time someone asks you how long a light-year is, you can firmly tell them: A light-year cannot be measured! It is the distance that light travels in a vacuum in one year, which is about 9.46 trillion kilometers!

Conclusion

In short, in order not to confuse this unit, the key is to understand the definition and purpose of this unit, so that you can use this unit correctly. I believe that in the future, when humans can really achieve interstellar travel, light years will become more popular as a distance unit in daily life, and it will not be so easy to confuse at that time.

Produced by: Science Popularization China

Produced by: Star Team

Producer: China Science Expo