Speed of Light: The upper limit.

Light. It's what enables us to see. It's what makes us see. Light reflects from objects around us and reaches our eyes. These objects are called secondary sources of light. When an object emits its own light due to explosions, or certain chemical reactions, it is called a primary source of light. The Sun is a primary source of light. Everything that reflects its light is a secondary source. Almost all the objects around us at this moment are secondary sources of light.

Light is said to have a dual nature, wave or particle nature. Light acts as a wave during some phenomena and as a particle during others. For example, in Young's double-slit experiment, light acts as a wave. A bit scientific, isn't it? Let's keep it simple.

In the early period, the light was assumed to be instantaneous. It was not considered to be a movement and it is justified. When you don't have a big enough scale to see that light is a movement, you can make a mistake like that. It wasn't a mistake at that time, but it's a mistake now. You cannot say that light is instantaneous. Light travels. It has a speed.

We never think of light as something that has a certain speed. That's because it is so fast. When you turn on the tube light, it feels like the light just exists now. We don't think that light traveled from the tube light to each and every corner of the room. It just doesn't click. That's why getting to know that there is a speed with which light travels is so fascinating, just as this blog is going to be. 2,99,792 km/s. Let that number sink in. For those who still don't get it. It's a LOT. That's 1,86,282 miles/second. Light can travel 2,99,792 kilometers or 186282 miles in one second. For instance, the diameter of Earth is 40,075 kilometers or 24,901 miles. So, light can go around the Earth multiple times in one second. That is beyond imagination. It is something we will never be able to experience. I will state the reason for that now.

Why can we not travel with speeds equal to or faster than the speed of light?

The genius Albert Einstein answered this question through his theory of relativity. A famous equation, Energy equals mass times the speed of light squared.

( Image credit: Energy Wave Theory EWT )

This equation suggests, rather shows that the Energy of a body is directly proportional to its mass. The more the mass of a body, the greater its energy. When we travel faster, we get heavier. Our mass increases. The faster you move through space, the more you put on weight lol. To travel at the speed of light, you will need to be pretty much massless. You need to have zero mass, basically. Particles of light, photons, are massless. So, I guess it's pretty clear that humans cannot travel at the speed of light, to be put very clearly and subtly.

Let's move on to why the speed of light changes everything, and how, about the universe.

Now, it's pretty clear that light has the greatest speed ever. Nothing that you have found or will find has a speed greater than light. Let's start with the Sun. The sun is very far away, obviously. But, how far is it really? Light takes 8 minutes to reach us from the sun. EIGHT MINUTES!!!!!!! That's nothing for us. But for light, that's too much. I mean, if I had a speed of 3,00,000 km/sec, I would be zooming past everything. It's like, having a speed of 100 km/hr and taking 20 hours to reach somewhere. Not very great when it comes to distance. The sun is 149.6 million kilometers or 92.96 million miles away from us. 149.6 million miles is 149600000 kilometers. That's a lot. All this drama but the sun is the closest star to us. It's like at walking distance. Not even walking distance. It's sitting right next to us on the couch on the scale of the universe. It's very easy to misunderstand how big the universe is.

The next closest star to us is Proxima Centauri. Stress on the word closest. Light's pretty fast is a well established fact by now. Very fast. Great. In one year, light travels a distance that is equal to 9.461 trillion kilometres. It is known as one light year. It's a universally used unit of distance. Don't mistake it to be a unit of time. It is just like kilometres except it's unfairly larger. Proxima Centauri, not to be surprised, is 4.3 light years away. That's 406823 with 8 zeros in front, in kilometres. Humans have never dealt with distances on this scale when it comes to travelling. That's why it is reasonable that we have a difficult time imagining how huge that distance is. At the speed of a 113 kmph or 70 mph, with an infinite amount of fuel, it would take us 40.6 million years to reach Proxima Centauri, the closest star to us, by car. How can that, in the whole wide world, be the closest?

Believe me, it gets crazier. Light is fast. I will say that a thousand more times. The milky way, our home galaxy, is Huge. It is 100,000 light years across which means that light, having a speed of 2,99,792 kilometres per second, takes 100,000 years to reach from one end of the milky way to the other end. In case of distance, it is 9.461 with 17 zeros. Your last brain cell stopped, didn't it? But when it comes to the universe, there is always more room for crazy. The Andromeda galaxy, our neighbouring galaxy is 220,000 light years across. That's 20,000 light years more than double the milky way. All of these are just our neighbours. As of right now, there is no way we can get out of our own galaxy within our lifetimes.

What if I tell you we haven't even scratched the surface in terms of how big the universe is?

I'll leave it up to you guys to contemplate with one fact. The universe is expanding. It's getting bigger and bigger and bigger every second. As I'm writing this blog, the universe is reaching infinities. It expands at a rate of 72 kilometres per second per megaparsec.(Parsec is also a unit of distance.) This means that, every 3.3 million light years further away you are from the earth, the matter where you are is moving away from earth 72 kilometres a second faster.

When we look up in the night sky, what do we actually see?

When you look at the night sky, you are actually looking at the past. Stars emit light. This light travels to us along the huge distances and reach the earth and eventually our eyes. But, the process, as discussed earlier, is not instantaneous. It takes time to reach us. If a star is 4 light years away from us, the light that it emits right now will be visible to us after 4 years. I'm writing this blog on 5th April 2023 at midnight. The light released by the star now will be seen from the earth on 5th April 2027 at midnight. So the light I am seeing now, from the star, was released 4 years ago i.e. 5th April, 2019 at midnight. 4 light years is a very small distance. Stars in the night sky are usually millions of light years away from us. So, we are seeing millions of years into the past. That's how we can see the big bang theoretically. The farther you see in the night sky, the farther behind you go in time. So, with a powerful enough telescope, we can actually see the birth of the universe. For that, the telescope should be able to see objects that are 13.8 billion years away from us, at the speed of light. Back to the seeing in the past topic. The shoulder of constellation Orion is a star named Betelgeuse. It is considered or atleast it is said to have exploded in a supernova. However, the light from the star hasn't yet reached us. It could reach us tomorrow or in the next 100,000 years. We're not sure because all we know is that it is a red giant star nearing the end of it's life.

All of that and we can only see a certain part of the universe. There are some part of the universe whose light will never reach us and we'll never knkw what's going on there. It is truly disappointing. Light has the upper hand when it comes to speed. Always.