What would happen if we travelled at the speed of light?

My youngest child is seven; he is a boy of many questions. Lately he has turned his attention to speed, specifically the speed of light, and what would happen if you travelled that fast.

The first question came at bed time (why is it always bed time??). He wanted to know what would happen if he travelled at the speed of light and would it change time. I answered as best I could (while trying to back out the door and turn off the light) and left it at that but the question has resurfaced and I know this little guy will not let it rest until he is sure he has full understanding of the answer. So, to satisfy my own son’s curiosity, and in case anyone else out there wanted to know… here is a quick low down on high speed.

Let’s start with the basics

Firstly, the speed of light is a staggering 299,792,458 metres per second (or approximately 299 792 kilometres per second). Albert Einstein may not have calculated this, but he was the one that recognised it as the fasted thing in our Universe, a cosmic speed limit.

This is the speed of light in a vacuum and is commonly denoted as c. Light travelling at different speeds depending on what it is travelling through, so for light to travel through anything other than a vacuum, it will travel a little slower. For example, light travels about 90,000 m/s slower in air (that’s about 0.03% slower).

In water light travels at 75% the speed it would in a vacuum.

It’s all relative

Einstein’s work on this cosmic speed limit led him to develop a little theory, calling it the Theory of Relativity.

Einstein’s Theory of Relativity…

E = mc2

E stands for energy, m is the mass of the object and c is the speed of light. But it still looks pretty confusing, right? Keeping it simple, this equation says two interesting things…

  1. it ties mass and energy together
  2. it says that nothing with mass can travel as fast as, or faster than the speed of light

You might like a refresher on what mass is… mass is basically a measure of how much matter (atoms) something is made up of, or how densely packed those atoms are. We usually talk about mass in terms of weight (kilograms) but when we do so, we are typically saying how much it weighs here on Earth.


Close, but not close enough

Light is made up things called photons and they have no mass. Everything else we can think of in our everyday lives does have mass.

Applying Einstein’s Theory of Relativity, the closer an object (with mass) gets to the speed of light, the more energy is required to keep it moving, until eventually the object would have an infinite mass and require and infinite amount of energy to move it… and that’s just not possible.

So nothing with mass, including us, or a big rocket, can move faster than the speed of light.

The fastest speed of a manned spacecraft to date was achieved by the Apollo 10 lunar module, on May 26, 1969 when it reached speeds of 39,897 km/h (about 11 km/s) before re-entering the Earth’s atmosphere.

Take your time

Where does time come into all this? Well, you might remember that the c in E=mc2 is a unit with distance and time in it, so time is part of the equation too.

What happens to time when we start to travel at close to the speed of light? The answer to that depends on where you are standing, in other words, it depends on where you are observing from.

Let’s take an example, and remember, this is all hypothetical… you are in a rocket travelling through space and you manage to travel at speeds approaching the speed of light. So for you, time slows down and you reach your destination in a relatively short space of time. You arrive, do whatever it is you went there to do and then head back to Earth (again at speeds close to the speed of light).

The main thing you would notice when you get back home is how old everyone is! People who were the same age as you when you left would be a lot older than you when you come back. Remember, as Einstein said, it’s all relative! It depends on where you are observing from; if you are on Earth then time continues as normal. But if you head off into space and travel at speeds that slow down time, then a little time for you will equal a lot of time back on Earth.

Scientists like to call this the twin paradox; if you took a set of identical twins and sent one travelling off in space at speeds close to the speed of light and left the other here on Earth, when the first twin returned from his cosmic travels he would be younger than his twin who remained on Earth.

In summary… we can’t actually travel at the speed of light, but if we could travel close to the speed of light then yes, time would slow down (for us anyway) but by the time we got back to Earth, everyone else would have aged more than us!

What did my son think of my explanation? I read this post to him last night and broke some of the theories down into seven year-old sized chunks of information and he was happy enough with the answer, he especially liked the twin paradox 🙂

Then he added some theories of his own… I’m not sure what Einstein would make of these but this guy certainly has some interesting ideas; Have a listen to a seven year-old’s theories on what else would happen if you travelled close to the speed of sound! 

Image sources: Rocket, time and light images were sourced on Pixabay.com


Science blogger and writer; Owner of Dr. How's Science Wows; Mother of three junior scientists who have taught me that to be a great scientist you need to look at life through the eyes of a child!

3 thoughts on “What would happen if we travelled at the speed of light?

Comments are closed.