Planck units

A tightrope walker steps onto the wire. Every move is a calculation: shift your weight just right, or you’ll fall. In the world of quantum particles described by the Dirac equation, there’s a different kind of abyss — the wave function’s furious oscillations.

In the classical picture of the world, gravity is the smooth bending of space and time. But at the microscopic level, where quantum mechanics rules, these bends should turn into a grainy ripple. The particles of such a ripple are called gravitons. The trouble is that straightforward calculations with gravitons always run into infinities — the theory loses its meaning.

Two massive spheres slowly drift apart in empty space. From the standpoint of ordinary mechanics, nothing special is happening. But Einstein's equations say otherwise: accelerating masses stir spacetime itself, and from them spread gravitational waves — ripples that stretch and squeeze everything in their path.

Two astronomers look at the same universe but see it differently. One measures the expansion rate using ancient light that has been traveling toward us for nearly 14 billion years. The other uses recent stellar explosions in nearby galaxies. The first gets 67 kilometers per second per megaparsec, the second gets 73.

Two great theories describe reality: one — how massive bodies cause curvature of space, the other — the Standard Model — governs elementary particles. Reconciling them remains elusive, like trying to describe an ocean storm in the language of a diamond lattice.

Cosmic rays are charged particles that fly to us from the depths of the Galaxy. If you plot how many particles arrive at each energy, the line runs smooth, then suddenly drops sharply.