A solar gravitational lens will be humanity's most powerful telescope (2022)

Solar Gravitational Lens (SGL): Concept & Limits

• SGL uses the Sun’s gravity as a huge lens; focal region starts ~550–850 AU, i.e., well beyond Voyager 1.
• You must be far enough that the Einstein ring exceeds the apparent size of the Sun, otherwise you just see brightening, not a usable ring.
• The “lens” doesn’t form a normal focused image; it distorts light into a ring/band, and the effective field of view is extremely narrow (km-scale patch on the target).
• Each SGL spacecraft is effectively tied to one target; retargeting to a different exoplanet is described as essentially infeasible.

Planetary / Atmospheric Lensing & Alternatives

• Using planets as lenses is weaker: requires even larger distances and offers less collecting area.
• Earth’s atmosphere might be used as a refractive lens with focal points inside the Earth–Moon system.
• Other proposed approaches: large synthetic apertures / interferometry (e.g., “New Worlds Imager”) that avoid going to 500+ AU but need massive fleets and extreme formation precision.

Interferometry & Timing Challenges

• Idea: multiple satellites sampling the lensed light closer in and combining data later.
• For optical/IR, required timing (0.1 ns) and positional precision (100 nm) over AU baselines is seen as extremely hard.
• Advanced atomic clocks can in principle hold needed precision with periodic resynchronization, but synchronization and data volume are major engineering hurdles.

Propulsion & Mission Design

• Concepts include solar sails doing close solar flybys (“sundiver”), ion drives powered by nuclear reactors, and Oberth maneuvers near the Sun.
• Estimates range from ~10–25 years to reach ~550–700 AU with aggressive sail or nuclear-electric concepts.
• Some suggest “drive‑by” imaging with fleets of probes rather than stopping; others emphasize the difficulty of tracking a moving, rotating planet from 500+ AU.

Data Return & Autonomy

• Six‑month integrations imply huge raw data volumes; DSN bandwidth is a limiting factor.
• Strong preference in astronomy for returning raw data, but several comments argue on‑board processing and compression will be necessary.
• Optical (laser) links and large ground telescopes are discussed as more realistic than radio at 500+ AU; missions may need high autonomy and minimal real‑time commanding.

Physics Debates: How Light Bends

• Extended argument over whether gravitational lensing is best understood as:
  • Light following straight paths in curved spacetime with locally constant c (standard GR view), vs.
  • Light “slowing” in gravitational fields / varying effective c, with photons exchanging momentum with massive bodies.
• Participants disagree on whether “bending spacetime” is fundamental or just a computational trick, and on how strictly “speed of light is constant” applies in GR.
• No clear consensus is reached; labeled here as unresolved within the thread.

Neutrino Lensing

• Speculative idea: use the Sun’s core as a lens for neutrinos, with a focal region between Uranus and Pluto.
• Major obstacles: the Sun is a dominant neutrino source (background), neutrino detection requires enormous, shielded mass, and transporting such detectors to the focal distance is seen as impractical with current technology.

Economic & Practical Feasibility

• Enthusiasm for SGL’s scientific payoff: potentially ~25 km surface resolution on exoplanets, signs of habitability, time‑resolved maps.
• Skepticism centers on: extreme distances, propulsion, comms limits, need for many one‑target probes, and unclear funding/ROI for commercial or VC backing.
• Some argue mass‑production of probes and cheaper launch (e.g., heavy‑lift reusable rockets) could eventually make such missions realistic.