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Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to curiouskidsus@theconversation.com.
What is space made of – what does gravity really bend? – Phil, age 12, Birmingham
What comes to mind when you think of space? Imagine a friend bragging about a big building, stadium or museum they recently visited. Do you imagine that the building is big and spacious? Is it just really big, or does it look empty?
The word does not necessarily mean empty expanse. It simply indicates that there is enough room to move around the objects inside it. Similarly, outer space is not completely empty. It’s vast and ever-expanding, but it’s got a lot of stuff.
As a child, I was thinking about questions like how far the clouds are from us, what is beyond the Earth and how big is space. This curiosity led me to pursue a master’s and Ph.D. degrees in astronomy. I have been teaching the concepts themselves for twenty years as a teaching professor of physics at the University of Texas, Arlington.
Where does space begin?
The Earth is surrounded by different layers of different gases. At sea level, the Earth’s atmosphere contains about 100 billion molecules per cubic centimeter. As we ascend, the atmosphere becomes thinner and thinner.
At an altitude of about 50 to 62 miles (80 to 100 km), there is not enough air for airplanes to fly. This boundary, which separates the Earth’s atmosphere from outer space, is called the Kármán Line, named after the Hungarian-American engineer and physicist Theodore von Kármán. Everything above the Kármán line is called space.
The density of space can vary, but, on average, it is only about 1 atom per cubic centimeter. Imagine that there are billions of air particles in a dice cube of the Earth’s atmosphere. But in space, there are only one or two particles in the same cube.
The interstellar medium and radiation
Space, or outer space, is a vast, nearly perfect void that contains very little matter. This vacuum contains very few particles compared to the Earth’s atmosphere. However, it is not completely empty.
Space is dotted with a diffuse material called the interstellar medium, which consists of hydrogen and helium atoms. These are the most common elements in space, and exist in both charged and neutral forms. The interstellar medium also contains cosmic dust – tiny particles of various elements, including carbon and silicon, scattered throughout space.
High-energy particles called cosmic rays – which are mainly protons and nuclei of atoms – travel through space at almost the speed of light. Cosmic rays come from various stars including our Sun, as well as supernovae, matter collapsing into black holes, colliding galaxies and more.
Space is filled with various types of radiation, including cosmic microwave background radiation. This is the residual heat from the origin of the universe. High-energy cosmic events such as supernovae and black holes also emit X-rays and gamma rays.
Magnetic fields generated by stars, planets and many other celestial bodies also travel through space. These fields affect the motion of charged particles by attracting or repelling them like magnets.
Dark matter and dark energy
Scientists predict that a significant portion of the mass of the universe is an as-yet-unseen type of matter that does not emit light or energy, known as dark matter. Researchers believe it exists because they see its gravitational pull on other visible matter.
Likewise, scientists predict that a mysterious form of energy called dark energy is driving the accelerated expansion of the universe. Unlike dark matter, dark energy is not associated with matter or gravitational forces, but is a property of space itself.
Imagine the globe as a balloon. Dark matter is like the material of the balloon, affecting its shape, and dark energy is like the air that is pumped into it. It does not change the content of the balloon, but it does affect how fast the balloon expands.
Does space shrink? Can gravity bend space?
Space can also be compact. Imagine that space is like a big extended trampoline. If you put a heavy ball, like a bowling ball, in the middle of the trampoline, it makes a big dip down. This dip is similar to how space goes around something large, like a planet or star. The bigger the ball, the stronger the gravity and the deeper the swim.
If you roll a smaller marble across the trampoline with the bowling ball in the middle, the marbles may start to float around the bowling ball’s center of gravity. The marble follows the curve of the dip, just as the planets follow the curve of space around the Sun.
Imagine you shine a flashlight across the trampoline. If the light passes near the dip that the bowling ball makes, it may bend slightly as it travels. This is similar to how light bends when it passes near a very large object in space, such as a galaxy.
Think of a black hole, which has enormous gravity, as you swim even deeper and deeper in the trampoline. If you rolled a marble too close to this dip too deep, it would fall in and disappear, just as objects might be drawn into a black hole in space and unable to escape.
Therefore, space can compact or bend around large objects with a lot of gravity, just like a trampoline bends when you place a heavy ball on it.
Space is more than emptiness. It contains a mixture of particles, radiation, magnetic fields and mysterious forms of matter and energy. Imagine space as a 3D playground, filled with things like the stars, planets, nebulae and galaxies that together make up our fascinating and complex universe.
Hello, strange children! Have a question you’d like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com. Please tell us your name, age and the city where you live.
And since curiosity has no age limit – adults, let us know what you think too. We won’t be able to answer every question, but we’ll do our best.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. It was written by: Nilakshi Veerabathina, University of Texas at Arlington
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Nilakshi Veerabathina does not work for, consult with, own shares in, or receive funding from any company or organization that would benefit from this article this, and has not disclosed any relevant connections beyond their academic appointment.
