Daily Five universe and physics questionsanswers explained

Real basis

• How long does sunlight take to travel from the Sun's surface to the Earth? Answer: about 8 minutes 20 seconds. The average Earth–Sun distance is about 150 million km, and light takes about 499 seconds to cover it. The Sun you see is always the Sun of a little over 8 minutes ago.

• What is the temperature of the cosmic microwave background today? Answer: about 2.7 K. About 2.725 K. It was born when the universe was 380,000 years old and about 3000 K, and since then the expansion of the universe has stretched it and cooled it by a factor of about 1100.

• About how old is the universe? Answer: about 13.8 billion years. About 13.8 billion years. 4.6 billion years is the age of the solar system; 46 billion light-years is the radius of the observable universe, not an age.

• The universe is only 13.8 billion years old, yet the observable universe has a radius of about 46 billion light-years. Why? Answer: the universe kept expanding while the light was on its way. The light has traveled for 13.8 billion years, and in that time the place it set out from was pushed even farther away by the expansion. The speed of light never changed.

• Apart from the Sun, what is the brightest star in the night sky? Answer: Sirius. Sirius, about 8.6 light-years away. Polaris ranks only around 50th in brightness; it is famous because it hardly moves.

• About how long does a star of 20 solar masses live? Answer: about 10 million years. The more massive, the fiercer the burning: lifetime is roughly inversely proportional to mass to the power 2.5. The Sun lives about 10 billion years, and the smallest red dwarfs live trillions of years.

• What is the most abundant element in the universe? Answer: hydrogen. By atom count about 92% is hydrogen, and by mass about 74%. Most of it is left over from the first few minutes after the Big Bang.

• Where did the gold atoms in your jewelry most likely come from? Answer: violent events such as neutron star mergers. Elements heavier than iron need a lot of neutrons to build. The 2017 gravitational-wave event GW170817 let people see for the first time, directly, neutron star mergers forging heavy elements.

• What is the nearest star to the Sun? Answer: Proxima Centauri. Proxima Centauri, about 4.24 light-years away. It is the smallest red dwarf in the Alpha Centauri triple-star system, and a planet was found around it in 2016.

• What is the “event horizon” of a black hole? Answer: the boundary beyond which even light cannot escape. The horizon is not a physical surface. For a very large black hole, someone crossing the horizon may notice nothing at the time, but can never get out again.

• Which black hole was in the first image of a black hole, released in 2019? Answer: the black hole at the center of the galaxy M87. The Event Horizon Telescope linked radio telescopes around the world into one “Earth-sized” telescope. The image of the black hole at the center of the Milky Way was released in 2022.

• Why do stars “twinkle”? Answer: turbulence in the Earth's atmosphere. Air masses of different temperatures act like flowing lenses, making starlight stronger and weaker. A planet looks like a small disk, so it twinkles less.

• Why does the Moon always show the same face to the Earth? Answer: the Moon's rotation period equals its orbital period. This is called tidal locking. The Moon turns once in about 27.3 days, exactly the time it takes to orbit the Earth once.

• About how many stars are in the Milky Way? Answer: about 100 billion to 400 billion. The estimate lies between 100 billion and 400 billion. The range is so wide because faint red dwarfs are hard to count.

• What will the Sun eventually become? Answer: a white dwarf. The Sun is not massive enough, so it will first swell into a red giant, shed a planetary nebula, and finally leave behind a white dwarf the size of the Earth.

• Why was the universe “opaque” for its first 380,000 years? Answer: free electrons kept scattering photons. It was too hot then, and electrons had not yet been captured by nuclei. When it cooled to about 3000 K, atoms formed and light could travel freely. That light is today's cosmic microwave background.

• About what fraction of all matter in the universe is dark matter? Answer: about 85%. About 85%. The atoms we know make up only a small part of matter, and dark matter has so far been detected only through gravity.

• What did Hubble publish in 1929? Answer: the farther a galaxy is, the faster it moves away from us. In 1924 he first showed that Andromeda lies outside the Milky Way, and in 1929 he published the relation that recession speed is proportional to distance. Lemaître had already derived this relation in 1927.

• About how fast is Voyager 1 moving now? Answer: about 17 km per second. About 17 km per second. At that speed, reaching Proxima Centauri would take about 75,000 years.

• Which of these objects did not form in the solar system? Answer: ʻOumuamua. ʻOumuamua, discovered in 2017, is the first known interstellar object. It came from another star system and is now leaving.

• What does the “cosmological event horizon” mean? Answer: light emitted today by some galaxies will never reach us. The universe is expanding at an accelerating rate, and galaxies far enough away are receding so fast that the light they emit today can never catch up with us. The universe over there has not ended.

• What is the biggest difference between the first stars (Population III) and today's stars? Answer: they contain almost no elements heavier than helium. There were no heavy elements in the universe yet. They may have been extremely massive and extremely short-lived, and none has yet been confirmed by direct observation.

• About how much does a teaspoon of neutron star matter weigh? Answer: billions of tonnes. Neutron star density is about 4×10¹⁷ kg per cubic meter, so a teaspoon (about 5 mL) weighs about 2 billion tonnes.

• When gravitational waves were first detected in 2015, by how much did LIGO's 4 km arms change in length? Answer: about one-thousandth of a proton's diameter. About 10⁻¹⁸ m. To measure so small a change, LIGO has to isolate itself from earthquakes, traffic and even the vibration of ocean waves.

• Why is the sky blue? Answer: air molecules scatter short-wavelength light more easily. This is called Rayleigh scattering. At sunset the light passes through more atmosphere, the blue light is scattered away, and the red and orange light remains.

• What is most of an atom made of? Answer: empty space. The nucleus is only about one hundred-thousandth of the atom's diameter. If an atom were the size of a football stadium, the nucleus would be like a pea at the center of the field.

• Why is water H₂O and not HO? Answer: an oxygen atom needs two more electrons to be stable. Oxygen has 6 electrons in its outer shell, and sharing 2 more fills it, so it shares one pair of electrons with each of two hydrogen atoms.

• By mass, what is the most abundant element on Earth? Answer: iron. Counting the core, iron makes up about a third of the Earth's mass. Looking at the crust alone, the most abundant is oxygen.

• What are diamond and graphite both made of? Answer: carbon. They are the same carbon atoms arranged differently: in diamond each carbon atom is bonded to four neighbors, and in graphite the atoms form layers of hexagons.

• What was the first element to be made artificially? Answer: technetium. It was found in 1937 in discarded molybdenum foil from a cyclotron. Its name comes from the Greek for “artificial”.

• Why is iron the end point of stellar fusion? Answer: fusing iron no longer releases energy. The nuclei around iron-56 are the most tightly bound. Fusing anything heavier absorbs energy instead, so once the core of a massive star fills up with iron, it collapses.

• About how fast does light travel in a vacuum? Answer: 300,000 km per second. The exact value is 299,792,458 meters per second. Since 1983 the meter has been defined using the speed of light.

• What does E = mc² say? Answer: mass and energy can be converted into each other. Converting 1 gram of matter completely into energy gives about 9×10¹³ joules, equivalent to about 20,000 tonnes of TNT.

• Where was the Higgs boson discovered in 2012? Answer: the Large Hadron Collider. The Large Hadron Collider at CERN. Its discovery confirmed the mechanism by which particles get mass, and the 2013 Nobel Prize in Physics went to the physicists who proposed that mechanism.

• What is absolute zero in degrees Celsius? Answer: −273.15 °C. −459.67 is in degrees Fahrenheit. Absolute zero can be approached without limit but never reached; this is the third law of thermodynamics.

• About how many solar neutrinos pass through an area the size of your fingernail each second? Answer: about 60 billion. About 60 billion per square centimeter per second. Almost all of them pass through you and through the Earth, leaving no trace.

• Can quantum entanglement be used to send information faster than light? Answer: no. The measurement results at the two ends of an entangled pair are correlated, but each end on its own looks random. You must compare the two sets of results using an ordinary signal before the correlation can be seen. This is called the no-communication theorem.

• What was “Schrödinger's cat” originally meant to show? Answer: pushing quantum superposition up to the macroscopic world looks absurd. Schrödinger proposed it in 1935, meaning it as a criticism of the then-popular interpretation of quantum mechanics, but it became the best-known image of quantum mechanics.

• What does the second law of thermodynamics say? Answer: the entropy of an isolated system does not decrease. Heat flows spontaneously from hot to cold, and not the other way around. It gives time a direction.

• Why does water boil below 100 °C on a high mountain? Answer: the air pressure is low. The lower the pressure, the more easily water molecules escape. On the summit of Mount Everest, water boils at about 70 °C.

• Why does a mirror swap left and right but not up and down? Answer: what a mirror really flips is front and back. The mirror flips the direction perpendicular to its surface. We imagine the person in the mirror as someone who has turned around to face us, and that is why left and right seem reversed.

• The protons in a nucleus all carry positive charge, so why don't they push each other apart? Answer: the strong interaction holds them together. At the scale of the nucleus the strong interaction is far stronger than electromagnetic repulsion, but its range is extremely short, and outside the nucleus it almost vanishes.

• What is a “half-life”? Answer: the time it takes for half of the nuclei to decay. The half-life of carbon-14 is about 5,730 years, so it can be used to date organic material up to a few tens of thousands of years old.

Related research

These studies relate to this topic but are not direct proof of this page.

• The universe is expanding: Hubble found that the farther a galaxy is, the faster it recedes. The universe isn't static; it is expanding.

  Source: Hubble, PNAS 1929, "A relation between distance and radial velocity among extra-galactic nebulae"

• The primeval atom: Lemaître independently proposed that the universe is expanding, and later imagined it began as a “primeval atom,” the forerunner of Big Bang theory.

  Source: Lemaître, Annales de la Société Scientifique de Bruxelles 1927, "Un Univers homogène de masse constante et de rayon croissant"

• Finding the Higgs particle: The Large Hadron Collider found a new particle at about 125 GeV, confirming a prediction made 48 years earlier.

  Source: ATLAS Collaboration, Physics Letters B 2012, "Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC"

• Cosmic microwave background: Two Bell Labs researchers couldn't get rid of the noise in their antenna, and finally realized it was the afterglow of the Big Bang. They won the 1978 Nobel Prize.

  Source: Penzias & Wilson, The Astrophysical Journal 1965, "A measurement of excess antenna temperature at 4080 Mc/s"

• Shadow of a black hole: Radio telescopes around the world formed an Earth-sized “virtual telescope” and photographed the shadow of the black hole at the center of M87.

  Source: Event Horizon Telescope Collaboration, The Astrophysical Journal Letters 2019, "First M87 Event Horizon Telescope Results. I. The shadow of the supermassive black hole"

• Gravitational waves: Two black holes merged 1.3 billion years ago, and ripples in spacetime were finally heard by humans, confirming Einstein's prediction from a century earlier.

  Source: Abbott et al. (LIGO & Virgo), Physical Review Letters 2016, "Observation of gravitational waves from a binary black hole merger"

• Recipe of the universe: The Planck satellite measured the tiny fluctuations in the cosmic microwave background: ordinary matter makes up about 5% of the Universe's total energy, dark matter about 27%, and the rest is dark energy.

  Source: Planck Collaboration, Astronomy & Astrophysics 2020, "Planck 2018 results. VI. Cosmological parameters"