"It would be hard to imagine a better guide to this difficult subject." -- Scientific American In Three Roads to Quantum Gravity, Lee Smolin provides an accessible overview of the attempts to build a final "theory of everything." He explains in simple terms what scientists are talking about when they say the world is made from exotic entities such as loops, strings, and black holes and tells the fascinating stories behind these discoveries: the...

Energy flows all around you. It warms the air and powers our machines. In this book, readers in grades 4-9 will discover how power from the sun journeys to Earth's surface and back out to space. The sun's energy can be reflected, absorbed, used by plants and animals, given off, or trapped to warm the atmosphere. Learn how heat in the atmosphere is rising and what people can do about it. Earth's energy flow affects every living creature on the planet--including...

A crack of lightning. The blue glow of a phone screen. A blinking firefly. The blazing sun. A camera's rapid flash. No matter the source, light travels in waves, and it does so at a speed faster than anything else in the universe. But what is light, and how do we study it? Read about light waves and light's properties and how people--and animals--manipulate light for their own purposes.

"Squeak! Grrr! CRACK! Shhh! Our world is filled with sounds big and small, pleasant and annoying. But how is all that sound created and what can it do? In this book, readers in grades 4-9 will learn how sound travels and the many roles it plays in our world. Sound helps us communicate, but it can do so much more. You might be amazed at how powerful sound waves can be when they let us see into the human body, set off a massive avalanche, or discover...