“Everyone should be very grateful radioactivity exists at all. It can kill you, yes, but without it you wouldn't have been born in the first place. On Earth, deep under your feet, our planet happens to contain many atoms that do decay, all the time. Less so now than in the past, but still, Earth's mantle is radioactive. When atoms decay there, the particles they emit bump into their neighbours and generate heat, the very heat that contributes to keeping our planet warm. Without radioactivity, there would be no seismic or volcanic activity. The surface of the Earth would have been dead cold billions of yeras ago. Life as we know it would probably not exist at all.” LifeEarthPastBornColdBirthGratefulWarmSurfaceHeatDeepAtomsDeadDecayKillNowPlanetEveryoneRadioactivityExistsEmitSeismicVolcanismMantleVolcanic Book:The Universe in Your Hand: A Journey Through Space, Time, and Beyond Source: The Universe in Your Hand: A Journey Through Space, Time, and Beyond
“Even today, more than eighty years after Oort's bold guess, we still don't have a clue what this dark matter is made of. We know it exists. We know where it is. We have maps of its presence within and around galaxies throughout the universe. We even have stringent constraints on what it is not, but we have no clue what it is. And yes, its presence is overwhelming: for every one kilogram of ordinary matter made out of neutrons and protons and electrons, there are five kilograms of dark matter, made out of who-knows-what.” MatterUniverseMapsOverwhelmingClueGalaxyPresenceKnowConstraintsWhatElectronsExistDark MatterGalaxiesMapWhereNeutronsGuessExistsProtonsOort Book:The Universe in Your Hand: A Journey Through Space, Time, and Beyond Source: The Universe in Your Hand: A Journey Through Space, Time, and Beyond