IB Physics Special Relativity (HL): FAQs

You need three things solid: the two postulates (light speed is constant for all observers, physics laws hold in every inertial frame), the consequences — time dilation, length contraction, mass–energy equivalence — and use of the Lorentz factor. According to the IB Physics guide (first assessed 2025), this sits in Topic A.5, Galilean and special relativity, which is HL only.

Special relativity is HL only — SL Physics students never study it and won't meet it on either exam paper. It sits in Topic A.5 of the current guide, examinable across Paper 1 and Paper 2 for HL candidates, so if you're doing SL you can skip this topic completely.

Yes — it's one of the more conceptually demanding HL topics, mostly because it contradicts everyday intuition about time and space. The maths itself is manageable (mostly algebra and substitution into given equations), but students often struggle to identify which frame is 'moving' in a problem, which is where marks get lost.

In the current DP Physics guide, only special relativity is examinable — the equivalence principle, gravitational time dilation and black holes that appeared under the old Relativity option have been dropped from the 2025 syllabus. Topic A.5 covers postulates, time dilation, length contraction and relativistic energy-momentum only, all within inertial frames.

No — special relativity in IB Physics HL is entirely algebra-based. You won't differentiate or integrate anything; you'll substitute values into the Lorentz factor and related equations from the data booklet. What trips students up isn't the maths, it's correctly identifying proper time, proper length and which observer is measuring what.

Every relativity equation you need — the Lorentz factor, time dilation, length contraction, relativistic momentum and energy, and the velocity addition formula — is printed in the IB Physics data booklet under Topic A.5. You don't memorise them; you need to recognise which one fits the scenario described in the question.

Special relativity questions appear in Paper 1 (multiple choice) and Paper 2 (short and extended response) for HL candidates only — it isn't tested in Paper 3, which focuses on experimental and data-based skills across the whole HL course. Expect at least one multi-mark Paper 2 question most sessions, often paired with a numerical calculation.

Time dilation uses Δt = γΔt₀, where γ = 1/√(1 − v²/c²) and Δt₀ is the proper time measured in the object's own rest frame. Work out γ first from the given speed, then multiply by the proper time — always check v is expressed as a fraction of c before substituting.

Start by mastering which quantity is 'proper' in each scenario — proper time is measured by a clock at rest relative to the event, proper length by an observer at rest relative to the object. Practise past-paper Paper 2 relativity questions until identifying frames becomes automatic, then check your γ calculations for careless rounding, the single biggest way marks disappear.

The most common mistake is mixing up which frame is 'moving,' which flips whether you should be dilating or contracting a quantity. Second most common: rounding v/c too early, giving a γ that's out by enough to cost the final mark. Third: forgetting length contraction only applies along the direction of motion.

Special relativity is one topic within a much bigger HL course, so it shouldn't be the deciding factor either way. HL Physics adds roughly 90 extra guided teaching hours of content — including rigid body mechanics and relativity — so the real question is whether your child manages abstract, maths-heavy reasoning comfortably across the whole syllabus.

Yes, HL Physics is a genuine step up — it covers everything SL does plus additional topics like special relativity and rigid body mechanics, with correspondingly higher demand in Paper 1 and Paper 2. A student who finds SL maths and physics reasoning straightforward usually adapts fine; one already stretched at SL will likely find HL a significant jump.