A black hole is a region where gravity is so strong that not even light can climb out. A white hole is its mirror image in time: a region nothing can ever enter. Bend starlight with a black hole, then drop an astronaut in and compare their clock with a friend's who stays safely outside.
Mass
Event horizon radius
Horizon → singularity
Stretch at the horizon
01 · Lens lab
Mass curves spacetime, so starlight passing nearby follows a curved path. Drag the black hole across the stars and watch them smear into arcs, and into a full Einstein ring when one lines up exactly behind it.
Mass
–Spin a
0Change the mass and the picture stays the same. Every black hole has the same shape; only the scale bar changes. A black hole is fully described by its mass, spin and charge, and nothing else. Turn up the spin: the horizon shrinks, the disk creeps inward, and the shadow flattens on the side moving toward you.
02 · Fall in
An astronaut drops from rest. Their own watch and a distant friend's watch disagree, and near the horizon they disagree completely. Switch views and press play.
Distance from centre vs time
–
Start distance
–Mass
–03 · The journey
Two stories of the same fall, for a big, gentle supermassive black hole. For a small stellar one the trip ends much earlier (stop 2).
04 · Learn
When a star more than about 20 times the Sun's mass runs out of fuel, nothing can hold its core up against its own gravity. It collapses past every barrier, through the stage where a neutron star would form, and keeps going. Once the matter is squeezed inside its own Schwarzschild radius, rs = 2GM/c², even light moving outward is dragged back. That boundary is the event horizon.
Supermassive black holes, millions to billions of Suns, sit at the centres of galaxies. Ours is Sagittarius A*, 4.3 million solar masses, 26,000 light-years away. How they grew so big so early is still an open question.
A black hole is not a hole in the ground and not a cosmic vacuum cleaner. Far from the horizon its gravity is exactly that of any object with the same mass. It only becomes extreme close up, because the mass is packed into such a small region.
Stars whipping around Sgr A* on tight orbits (Nobel Prize 2020). Gravitational waves from merging black holes, detected by LIGO since 2015 (Nobel Prize 2017). And direct images of the shadow of M87* (2019) and Sgr A* (2022) by the Event Horizon Telescope, a radio telescope the size of Earth.
Real black holes spin, because the stars they formed from were spinning. A spinning (Kerr) black hole drags space round with it. Inside the ergosphere nothing can stand still, not even with infinitely powerful rockets; everything is swept along with the rotation. Spin also lets matter orbit closer before plunging in, so a fast-spinning black hole's disk can turn up to about 32% of the infalling mass into light, against 6% for a non-spinning one. Nuclear fusion manages just 0.7%. Try the spin slider in the lens lab.
Try this
The equations of general relativity work just as well with time reversed. Play a black hole backwards and you get a white hole: a region with a horizon that nothing can enter. Matter and light can only come out of it. A black hole is a place you can never leave; a white hole is a place you can never reach.
The full, eternal solution found by Schwarzschild in 1916 actually contains both: a black hole in the future and a white hole in the past, connected by an Einstein–Rosen bridge (a "wormhole") to a second, separate universe. See the spacetime map tab.
Nothing makes them. Black holes form when stars collapse. A white hole would have to exist from the start of the universe, with no known way to create one.
They are unstable. Matter that tries to approach piles up just outside the horizon. Its gravity soon becomes strong enough to make the whole thing collapse into an ordinary black hole. This is the Eardley instability, from 1974.
They are speculative ideas. Some physicists suggest that the Big Bang behaves a little like a white hole; that quantum gravity might make an evaporating black hole "bounce" into a white hole; and that the odd gamma-ray burst GRB 060614 was one exploding. speculative None of this is confirmed.
| Black hole | White hole | |
|---|---|---|
| Horizon | One-way: in only | One-way: out only |
| Singularity | In your future | In your past |
| Can you enter? | Yes, easily | Never |
| Can you leave? | Never | Everything leaves |
| Formed by | Collapsing stars | Nothing known |
| Stable? | Yes | No, collapses to a black hole |
| Observed? | yes | never |
You fall toward it but can never cross its horizon. Light and matter pour out of it, and from your point of view the horizon keeps receding ahead of you. You slow and pile up just outside it with everything else that ever fell toward it, bathed in a blinding flood of blue-shifted radiation from inside. Very soon the combined mass outside tips it over: the white hole collapses into a black hole, and you fall into that instead. So either you are blasted away, or the white hole turns into a black hole and swallows you.
A Penrose diagram squeezes all of infinite spacetime onto a page. Time runs up the page and light always travels at 45°, so every light cone is a V shape. Dashed lines are horizons. Click a region.
The bridge between I and III pinches off faster than light could cross it, so you cannot use it to travel. Real black holes form from collapsing stars, and the collapsing star replaces regions III and IV. Only I and II exist.
Replace the Sun with a 1 M☉ black hole and Earth's orbit would not change at all. Only the light and heat would go. Gravity is only extreme within a few horizon radii.
It is not made of anything. A falling astronaut notices nothing when crossing it. It is simply the point of no return.
Backwards. Tidal stretch at the horizon falls as 1/M². You would be shredded long before reaching a small one, but could cross the horizon of a supermassive one intact.
Only the idealised eternal solution has a bridge, and it cannot be crossed. Black holes made by collapsing stars have no second side.
Not for a non-rotating hole. Outside light reaching you is blue-shifted only modestly, and you hit the singularity first. The "frozen forever" effect is what your friend sees of you, not what you see.
Quantum effects make them glow very faintly (Hawking radiation) and slowly evaporate. For a stellar black hole this takes about 10⁶⁷ years, far longer than the age of the universe.
Each equation is worked through for the mass currently selected: –. G = 6.674×10⁻¹¹ m³/kg·s², c = 2.998×10⁸ m/s, M☉ = 1.989×10³⁰ kg.