What Are Tidal Forces
Tidal forces arise whenever a gravitational field varies in strength across an extended object. On Earth, we experience this as ocean tides caused by the Moon's gravity being slightly stronger on the near side of Earth than the far side. Near a black hole, this same effect is amplified to an extreme degree.
The tidal acceleration across a body of height Δr at distance r from a black hole of mass M is given by Δa = 2GMΔr/r³. The crucial feature is the r³ in the denominator — tidal forces grow extremely rapidly as you approach the black hole.
Spaghettification
The term 'spaghettification' was popularized by Stephen Hawking to describe what happens to an object falling into a stellar-mass black hole. The gravitational gradient stretches the object along the radial direction (toward the black hole) while simultaneously compressing it in the perpendicular directions. The result: the object is drawn out into a long, thin strand — like spaghetti.
For a 10 solar mass black hole, the tidal forces become lethal well outside the event horizon. An astronaut would be torn apart hundreds of kilometers before reaching the Schwarzschild radius. The process is not instantaneous but accelerates dramatically as the distance decreases.
The Supermassive Advantage
Here lies one of the most counterintuitive facts in black hole physics: supermassive black holes are gentler than stellar-mass ones at their event horizons. Because the Schwarzschild radius scales linearly with mass while tidal forces at a given radius scale linearly with mass too, the tidal force at the event horizon actually decreases with increasing mass (it scales as 1/M²).
For Sagittarius A* (4 million solar masses), the tidal acceleration at the event horizon on a 1.8-meter body is only about 0.009 g — completely imperceptible. An astronaut could cross the event horizon without any physical sensation of tidal stretching. For the ultramassive black hole TON 618 (66 billion solar masses), the tidal forces at the horizon are even more negligible.
The Point of No Return
Once past the event horizon, however, the tidal forces inevitably grow without bound as you approach the central singularity. Inside the black hole, the singularity is not a place in space but a moment in the future — inescapable, like tomorrow. The spaghettification that was avoidable outside a supermassive black hole becomes inevitable inside it. The exact nature of what happens at the singularity remains one of the great open questions in physics, likely requiring a theory of quantum gravity to resolve.