An astronaut falls toward a supermassive black hole while a distant observer watches. Which statement correctly describes both perspectives simultaneously?
AThe astronaut feels a sharp jolt as they cross the event horizon; the distant observer sees them fall in and disappear instantly
BThe astronaut notices nothing unusual at the horizon crossing and passes through; the distant observer sees the astronaut appear to slow, redden, and fade — never quite reaching the horizon
CBoth observers see the astronaut cross the horizon at the same moment, after which communication becomes impossible
DThe astronaut is destroyed at the event horizon by the intense physical boundary; the distant observer confirms this by seeing a flash of radiation
For a supermassive black hole (where tidal forces at the horizon are gentle), the infalling astronaut experiences nothing special at the horizon — it is a causal boundary, not a physical surface. But the astronaut's outgoing light signals are increasingly gravitationally redshifted and time-dilated as seen by the distant observer, making the astronaut appear to slow and fade without ever visibly crossing. The two perspectives are both physically correct and consistent with general relativity.
Question 2 Multiple Choice
A stellar-mass black hole has mass M and Schwarzschild radius Rs ≈ 3 km. A new black hole forms with mass 10M. What is its Schwarzschild radius?
AStill about 3 km — greater mass is more compressed, keeping the event horizon constant
BAbout 9 km — radius scales as M^(2/3) like a normal dense object
CAbout 30 km — the Schwarzschild radius is directly proportional to mass
DAbout 300 km — radius scales as M² because curvature grows faster than mass
Rs = 2GM/c² is linear in mass. If M increases by a factor of 10, Rs increases by a factor of 10. This is a distinctive property of black holes: unlike normal objects where density is roughly constant, black hole event horizons grow linearly with mass. A 10 solar-mass black hole has Rs ≈ 30 km; the Milky Way's 4-million solar-mass black hole has Rs ≈ 12 million km.
Question 3 True / False
An observer falling into a sufficiently massive black hole would not experience anything physically dramatic at the exact moment of crossing the event horizon.
TTrue
FFalse
Answer: True
The event horizon is a causal boundary defined by spacetime geometry, not a physical surface. For a supermassive black hole, tidal forces at the horizon are actually gentle (they scale inversely with the square of the Schwarzschild radius, which is large). The infalling observer would detect no special local physics at the crossing — the dramatic consequences (inability to return or communicate) only become apparent afterward, as their future light cone no longer intersects the exterior universe.
Question 4 True / False
The event horizon of a black hole acts as a physical solid boundary that infalling matter collides with and can seldom pass through.
TTrue
FFalse
Answer: False
The event horizon is a causal boundary in spacetime geometry — a surface defined by which regions of spacetime can send signals to distant observers — not a physical surface with material properties. Matter and observers freely cross it without experiencing any local barrier. The horizon's significance is entirely about the causal structure of future trajectories: after crossing, no worldline can return to the exterior. There is nothing to 'hit.'
Question 5 Short Answer
Why can a distant observer never actually witness an astronaut crossing a black hole's event horizon, even in principle with arbitrarily powerful telescopes?
Think about your answer, then reveal below.
Model answer: As the astronaut approaches the event horizon, light signals they emit take exponentially longer to climb out of the increasingly deep gravitational well. Gravitational time dilation stretches the intervals between successive photons emitted by the astronaut, and gravitational redshift shifts them to longer and longer wavelengths. From the distant observer's perspective, the astronaut asymptotically approaches the horizon and their signals become infinitely redshifted and infinitely time-dilated — the distant observer would need to wait an infinite amount of proper time to receive even a single photon emitted at the moment of crossing. The interior of the event horizon is causally disconnected from the exterior.
This is the observational consequence of the event horizon being a one-way causal boundary. General relativity predicts both that the infalling observer crosses smoothly (in finite proper time) and that the distant observer never sees this crossing — both are simultaneously true without contradiction.