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1 Feasibility of measuring the Shapiro time delay over meter-scale distances Peter Shawhan (University of Maryland), Stefan Ballmer (LIGO - Caltech), Szabolcs Márka (Columbia University) American Physical Society Meeting April 14, 2008 LIGO-G080251-01-Z

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2 The Shapiro Time Delay Light or radio beam passing by limb of Sun is delayed by General relativity predicts =1 Radio ranging to planets and spacecraft; binary pulsar timing Tightest limit: – 1 = (2.1 ± 2.3) × 10 –5 Question: Does this also apply over short distances? 7×10 8 m 2×10 30 kg 1.5×10 11 m1.1×10 11 m Venus = 0.12 ms for Venus (one way) d = r 1 = r 2 =

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3 Laboratory Measurement of Time Delay ? Time delay for these parameters: ~10 –30 s Equivalent to a distance change of 3×10 –22 m How can we measure this ?? 0.5 m 10 4 kg 1 km

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4 Advanced LIGO as a Stopwatch Interferometer compares light travel times in the two arms ! Have to modulate the time delay Spin an asymmetric object Net change in time delay: 4 km d near d far

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5 Advanced LIGO Noise Curve Choose 50 Hz for signal frequency Integrate for a long time SNR grows as T

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6 A Rotating Mass Design: Simple Slab Spin frequency = 25 Hz Problems: –Only part of the mass comes close to the laser beam –Pushes the tensile strength limits of ordinary metals if the slab has a length of 3 meters, say

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7 A Better Rotating Mass Design Total mass = 30 tons total length = 10 meters, divided into several units In vacuum (separate from beam) Steel weight Carbon fiber composite shell 1.5 m Fig: Yoichi Aso

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8 Isochrones Contour labels: ×10 –22 m

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9 Time Delay Signal 5.5×10 –23 m 1.8×10 –31 s

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10 Signal Analysis Amplitude SNR for 1 year integration time: 6.5 4 2.3

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11 Some Implementation Issues Mechanical tolerances Determining distance to laser beam Vibration –Rotating mass is 2 km away from the interferometer mirrors –Vibrations will primarily be at the spin frequency –Possible bigger concern: vibrating beam tube & scattered light Electrical pickup Safety ! –Kinetic energy in rotation: ~750 MJ (comparable to largest experimental flywheels for energy storage)

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12 Summary Advanced LIGO could measure the Shapiro time delay produced by a rotating apparatus with mass ~30 tons Would test the curvature of space at short distances –However, certainly not a precision measurement The experiment is feasible, but challenging –Requires serious engineering Would be much easier at lower frequency –There is interest in future ground-based gravitational-wave detectors that would operate at lower frequencies (Paper in preparation)

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