How to Make Light Late express
Mass curves spacetime like a heavy stone pressing into an asphalt road. Light passing by skirts around this dent, its path lengthens, and it arrives late. Astronomers have observed this Shapiro effect many times near the Sun and planets.
A new plan proposes to recreate this delay on a lab bench. Two beams are simultaneously launched into a closed fiber-optic loop: one clockwise, the other counterclockwise. A massive object placed nearby curves space within the loop, and one of the beams arrives with a delay of billionths of a second. The technology is well-honed: similar devices already detect vibrations of space.
Instead of satellites and telescopes, Einstein's theory of gravity could be tested in an ordinary room. The precision is enough to spot deviations hinting at new physics. Remarkably, the light delay near bodies was originally proposed by Einstein himself in 1916—long before the first spaceflights.
Original abstract
as written by the authors arXiv:2601.00468v2
The Shapiro time delay is one of the four classical tests of Einstein general theory of relativity and is commonly interpreted as a constrain on the parametrized post Newtonian (PPN) parameter gamma, which is exactly unity in general relativity. To date all measurement of the Shapiro time delay have been confined to astrophysical and solar system observations, yielding constrains on the PPN parameter gamma at the 10-5 level. In this work, we propose a fiber based Sagnac interferometer for precision terrestrial measurements of the Shapiro time delay, enabling a laboratory scale determination of gamma with an intrinsic sensitivity approaching 10-9. This approach provides an independent test of general relativity in a previously unexplored regime.
The strongest Shapiro effect from our star: a signal to Venus and back is delayed by about 200 microseconds due to the Sun's gravity.