Astronomy & Space · Unit 17

Relativity and Black Holes

Special and general relativity, black holes and gravitational waves

General relativity replaces gravity as a force with gravity as the shape of spacetime, and the prediction that follows is that space and time themselves are affected by mass.

The unit covers special relativity, general relativity, black holes, event horizons and singularities, and gravitational waves - predicted in 1916 and first detected in 2015.

This unit breaks down into 21 short steps and 120 questions, starting at difficulty 4 and building to 5. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps: 21
Questions: 120
Difficulty: 4-5

What this unit covers

General Relativity
Event Horizons and Singularities
Special Relativity
Black Holes
Gravitational Waves

Where this fits

Needs The Lives of Stars. Pairs with Physics's Modern Physics.

Where people slip

Black holes do not suck. Replace the Sun with a black hole of the same mass and the Earth's orbit would not change - it would simply get very dark.

How the unit is structured

Relativity and Black Holes runs as 21 short steps that unlock in order. 15 are practice rounds and 6 are challenge rounds that pull together everything before them. Questions start at difficulty 4 and climb to 5 as you progress.

Step 1 · easierStep 21 · harder

Challenge rounds

Example questions

30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 120 in the unit altogether.

Black Holes

Fill the blankLevel 4
1. The size of a non-rotating black hole's event horizon is set by its ____ radius.
- Schwarzschildcorrect
- Chandrasekhar
- Hubble
- Roche
The Schwarzschild radius grows in proportion to the black hole's mass.
Multiple choiceLevel 4
2. An object becomes a black hole when its escape velocity is greater than...
- the speed of lightcorrect
- the speed of sound
- Earth's orbital speed
- 11 kilometers per second
A black hole is a region where the escape velocity exceeds the speed of light, so nothing can get out.
Odd one outLevel 4
3. Which of these is NOT a category of black hole?
- Main-sequencecorrect
- Stellar-mass
- Intermediate-mass
- Supermassive
Main-sequence describes an ordinary star, not a black hole; the others are real black hole size classes.
Picture questionLevel 4
4. 🕳️ In astronomy, this stands for a region where gravity is so strong that not even light can escape. What is it?
- A black holecorrect
- A red giant
- A nebula
- A comet
A black hole is a region of spacetime with gravity so strong that nothing, not even light, can get out.
Choose all that applyLevel 5
5. Which of these are real ways astronomers detect black holes? (select all that apply)
- X-rays glowing from an accretion diskcorrect
- Stars orbiting an invisible massive objectcorrect
- Gravitational waves from mergerscorrect
- Direct light emitted by the singularity
We spot black holes by X-rays from hot infalling gas, by stars orbiting an unseen mass, and by their merger gravitational waves; the singularity itself emits no light.
Match the pairsLevel 5
6. Match each black hole with what it is known for.
Answer: Cygnus X-1 = First strong candidate; Sagittarius A* = Milky Way center; M87* = First imaged shadow
Cygnus X-1 was the first strong candidate, Sagittarius A* sits at the Milky Way's center, and M87* was the first to have its shadow imaged.

Event Horizons and Singularities

Build the sentenceLevel 4
7. Build a true sentence about a black hole's event horizon.
Answer: Not even light escapes past the event horizon
Not even light can escape once it crosses the event horizon.
Choose all that applyLevel 4
8. Which statements about a black hole's event horizon are correct? (select all that apply)
- It is the point of no returncorrect
- Light cannot escape from within itcorrect
- It is a solid surface you could stand on
- It grows larger as the black hole gains masscorrect
The horizon is the point of no return, traps light, and grows with mass, but it is not a solid surface you could stand on.
Fact or fibLevel 4
9. You could send a radio message back out from inside a black hole's event horizon.
Answer: False
Nothing, including radio waves, can escape from within the event horizon, so no signal could ever reach us.
Fill the blankLevel 4
10. Being stretched into a long, thin shape by a black hole's tides is nicknamed ____.
- spaghettificationcorrect
- sublimation
- ionization
- precession
Spaghettification happens because gravity pulls much harder on your near side than your far side close to a black hole.
Multiple choiceLevel 4
11. What is thought to lie at the very center of a black hole?
- a singularity, a point where density becomes infinite and known physics breaks down
- a giant solid ball of neutrons
- a hollow empty bubble of vacuum
- a doorway into another galaxy
At a black hole's center is a singularity, where our current physics can no longer describe what happens.
Guess the numberLevel 5
12. A black hole with the mass of the Sun would have an event horizon only about how many kilometers across?
Answer: 6 km
The Schwarzschild radius of a Sun-mass black hole is about 3 km, so the horizon is roughly 6 km across.

General Relativity

Build the sentenceLevel 4
13. Build a true sentence about how gravity works in general relativity.
Answer: Mass and energy curve the fabric of spacetime
In general relativity, mass and energy curve the fabric of spacetime, and that curvature is gravity.
Fact or fibLevel 4
14. GPS satellites must correct for gravitational time dilation to give accurate positions.
Answer: True
Clocks on GPS satellites run slightly faster in weaker gravity, so the system corrects for general relativity to stay accurate.
Fill the blankLevel 4
15. Clocks run ____ in stronger gravitational fields, an effect called gravitational time dilation.
- slowercorrect
- faster
- backward
- at exactly the same rate
The deeper a clock sits in a gravity well, the slower it ticks compared with clocks farther away.
Match the pairsLevel 4
16. Match each general relativity idea with what it means.
Answer: Curved spacetime = Gravity; Equivalence principle = Free fall; 1919 eclipse = Starlight bending
General relativity links gravity to curved spacetime, ties gravity to acceleration, and was confirmed by starlight bending.
Multiple choiceLevel 4
17. In general relativity, what we feel as gravity is best described as...
- the curving of spacetime by mass and energycorrect
- a magnetic pull between heavy objects
- a beam of particles fired between planets
- the spinning of the Earth on its axis
General relativity describes gravity as the curvature of spacetime produced by mass and energy.
Choose all that applyLevel 5
18. Which of these are confirmed predictions of general relativity? (select all that apply)
- Bending of starlight by the Suncorrect
- Gravitational time dilationcorrect
- Gravitational wavescorrect
- The constant speed of light in all frames
Starlight bending, gravitational time dilation, and gravitational waves are all general relativity; the constant speed of light in all frames is special relativity.

Gravitational Waves

Fact or fibLevel 4
19. Gravitational waves were first predicted by Einstein's general theory of relativity.
Answer: True
Einstein predicted gravitational waves in 1916 as a consequence of general relativity, about a century before they were detected.
Fill the blankLevel 4
20. Gravitational waves gently stretch and squeeze ____ itself as they pass by.
- spacecorrect
- sound
- iron
- air
A passing gravitational wave alternately stretches and squeezes space, though the effect is tiny.
Guess the numberLevel 4
21. Einstein predicted gravitational waves in 1916, but they were not directly detected until 2015. About how many years passed in between?
Answer: 99 years
It took roughly 99 years, from Einstein's 1916 prediction to LIGO's 2015 detection.
Match the pairsLevel 4
22. Match each gravitational-wave term with the right description.
Answer: LIGO = Merging black holes; Neutron star merger = Kilonova; Gravitational wave = Ripple in spacetime
LIGO detects merging black holes, neutron star mergers can create a kilonova, and a gravitational wave is a ripple in spacetime.
Multiple choiceLevel 4
23. Gravitational waves are ripples that travel through...
- spacetime itselfcorrect
- the Earth's magnetic field
- the solar wind
- the air around us
Gravitational waves are ripples in spacetime itself, set off by violent events like merging black holes.
Choose all that applyLevel 5
24. Which of these are true of gravitational waves? (select all that apply)
- They travel at the speed of lightcorrect
- They stretch and squeeze spacecorrect
- They were first directly detected in 2015correct
- They are ripples in Earth's atmosphere
They travel at light speed, stretch and squeeze space, and were first directly detected in 2015; they are not ripples in the air.

Special Relativity

Build the sentenceLevel 4
25. Build a true sentence about the key rule of special relativity.
Answer: The speed of light is constant for all observers
The constant speed of light for all observers is the starting point of special relativity.
Choose all that applyLevel 4
26. Which of these are predictions of special relativity? (select all that apply)
- Time dilation for moving clockscorrect
- Length contraction along the direction of motioncorrect
- Mass and energy are equivalent (E=mc^2)correct
- Mass curves the spacetime around it
Time dilation, length contraction, and mass-energy equivalence are special relativity; curved spacetime from mass is general relativity.
Fact or fibLevel 4
27. A fast-moving object is measured to be shorter along its direction of motion.
Answer: True
This shrinking of length along the direction of travel is called length contraction.
Fill the blankLevel 4
28. According to special relativity, as an object approaches the speed of light, the ____ needed to accelerate it further grows toward infinity.
- energycorrect
- temperature
- charge
- spin
The energy required keeps rising without limit, which is why a massive object can never quite reach light speed.
Guess the numberLevel 5
29. The speed of light is a fixed value in special relativity. About how many thousand kilometers per second is it?
Answer: 300 thousand km/s
Light travels at about 300 thousand kilometers per second, or roughly 300,000 km/s.
Match the pairsLevel 5
30. Match each special relativity effect with what it means.
Answer: Time dilation = Moving clocks slow; Length contraction = Moving lengths shrink; E=mc^2 = Mass-energy equivalence
Time dilation slows moving clocks, length contraction shrinks moving lengths, and E=mc^2 links mass and energy.

Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.

How you practise

This unit mixes 14 different question formats, so you are recalling and applying rather than recognising the same layout every time.

Build the sentence
Choose all that apply
Fact or fib
Fill the blank
Guess the number
Match the pairs
Multiple choice
Odd one out
Picture question
Put in order
Sort into groups
Spell it
True or false
Type the answer

Practise Relativity and Black Holes

120 questions across 21 steps. Start with step one and crawl at your own pace.

Play this unit

Read about Relativity and Black Holes

Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.

What Is a Black Hole?How gravity gets strong enough to trap light, what the event horizon is, and how astronomers found and photographed them.August 28, 2026 · 6 min read

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