Consider the strangeness of this everyday miracle: each time a GPS receiver locks onto a position or a spacecraft adjusts its trajectory across millions of miles of near-emptiness, it is relying on corrections that would be meaningless without a radically altered picture of the universe. Time does not tick uniformly everywhere. Space is not a neutral stage. Gravity bends both, subtly but persistently, enough to matter when precision is measured in microseconds and orbital trajectories in kilometers. That conceptual rupture belongs to Albert Einstein, whose theories of relativity continue to underwrite the most exacting technologies of the modern world while still resisting any sense of final completion. A century after their formulation, they remain both astonishingly successful and, in certain corners of physics, stubbornly incomplete.
The story of how those ideas moved from speculative insight to operational necessity is inseparable from decades of experimental verification carried out with increasing precision. One of the most elegant of these confirmations came in the form of a delay that should not, in a naive Newtonian universe, exist at all. When radar signals are bounced off planets or spacecraft and return slightly later than expected, the difference is not noise but structure: light itself is responding to the curvature of spacetime. This phenomenon, now known as the Shapiro time delay, revealed that gravity does not merely pull objects through space but reshapes the geometry through which signals travel. What looks like a trivial discrepancy in timing becomes, under scrutiny, a direct imprint of relativistic gravity.
At the center of this work stands Irwin Shapiro, who helped turn an abstract prediction into one of the most precise experimental tests of Einstein’s theory. His radar-based measurements of planetary distances in the 1960s and 1970s provided a striking confirmation that signals passing near massive objects take measurably longer to arrive than they would in flat spacetime. The effect is vanishingly small—measured in microseconds—but its implications are vast, reinforcing general relativity not as a conceptual framework alone, but as an empirically exact description of how the universe behaves at large scales. Shapiro’s career, spanning leadership at the Harvard–Smithsonian Center for Astrophysics and a long tenure at Harvard University, has been defined by this kind of quiet precision: experiments that do not overturn theory so much as tighten its grip on reality.
What emerges from his reflections is less a narrative of revolutionary rupture than of accumulating clarity. Einstein’s equations, once startling in their abstraction, have become embedded in the infrastructure of modern science—guiding satellite navigation, shaping cosmological models, and defining the framework for understanding black holes and gravitational waves. Yet their endurance is not the result of inertia or tradition. It is the consequence of repeated, exacting confrontation with the physical world. In that sense, Shapiro’s work does not merely honor Einstein’s legacy; it extends it, showing how a theory born in thought experiments continues to reveal itself through the slow accumulation of measurements, refinements, and surprises.
Charles Carlini: You're a professor of astronomy at Harvard University and the recipient of the 2013 Einstein Prize from the American Physical Society. How has Albert Einstein influenced your work in gravitational physics?
Irwin Shapiro: Einstein created the general theory of relativity, which I thought I would test. Clearly, his influence was fundamental to my studies.
CC: In 1966, you conducted your fourth test of Einstein’s theory of general relativity, which followed the three proposed by Einstein. Could you briefly summarize what the test involved?
IS: The test involved sending a radar signal from Earth to Mercury and detecting its echo when Mercury was on the opposite side of the Sun from Earth, near the superior conjunction. The theory of general relativity predicted that the round-trip travel time of the radar signal would be greater by up to about 0.2 milliseconds because the signal passed near the Sun’s limb, independent of the increase due to the coronal plasma. I realized from computation that the theory predicted such a (detectable) increase in the round-trip travel time due to the presence of the Sun’s mass near the line of sight traveled by the radar signal.
CC: Your test turned up a measurable delay (the eponymously named “Shapiro Delay”) and agreed with Einstein’s theory of general relativity. The results have held up over the years throughout repeated tests. Were you at all surprised by the results after the first test?
IS: No, I would have been greatly surprised—and pleased!—had the results differed from the predictions of general relativity.
CC: Deciding to conduct your test was far from beating a dead horse: Researchers largely ignored Einstein’s theory of relativity throughout the early 20th century, and many claimed that they had virtually no observations to test it. When your tests showed agreement with the theory of general relativity, what was the reaction? Did anyone dispute your findings?
IS: No "mainstream" physicist disputed our results, but many non-scientists (mostly those loosely termed the "fringes") disputed our findings.
CC: Since 1967, you’ve been teaching science and physics at institutions such as Harvard and the Massachusetts Institute of Technology. You’ve also expressed involvement in pre-science education. Are there any common issues that you’ve noticed students encounter when studying Einstein’s work?
IS: Most of the students I’ve taught general relativity related to the fourth test on a gravitational field of the Sun do not seem to have much trouble comprehending either the relevant math or physics. There are, of course, many levels of understanding, but I believe that most of those whom I have taught—graduate students at the time—gain a good level of understanding. I have not, however, kept statistics and, therefore, cannot make reliable quantitative statements.
CC: In a 2011 NOVA article, Peter Tyson notes that the theory of relativity has “one last hurdle” to clear: the existence of gravitational waves, but no one has directly observed them yet, even though Einstein predicted them. Setting aside the arguable confirmation of the existence of gravitational waves via the binary pulsar observations, how close are we to the direct observation of these waves via a Laser Interferometer Gravitational-Wave Observatory (LIGO) type of observation? Will these direct observations be able to be used successfully as a new type of “TELESCOPE” to study cosmology or astrophysical objects?
IS: It is reasonable to think that, in the next five years or so, LIGO and its fellow detectors will improve enough to reliably detect evidence of gravitational waves, opening a new window on the universe.
Certainly, most cosmologists believe they will gain new insights into cosmological issues and relevant astrophysical objects.
CC: In addition to your work in astrophysics, you’ve also served as a member of the Radio Science teams for several NASA missions. Over the past century, what sort of practical implications has the general theory of relativity had for astronomers? And how has Einstein’s theory affected the development of spacecraft?
IS: The general theory of relativity is the basic model of cosmology, although it is facing a serious challenge due to so-called dark matter and dark energy, which are still rather mysterious. For example, through gravitational lensing, predicted by general relativity, this theory has also yielded many insights into astrophysical phenomena. The development of spacecraft is another matter; for example, in performing their basic function, GPS spacecraft take into account general relativistic effects on clock behavior, which are needed for applications with nanosecond-level timing accuracy.
CC: Einstein’s image in the public consciousness is a relatively straightforward one: his name is synonymous with genius, but the average person knows him for the E=mc² equation and little else. How does the scientific community today typically remember Einstein?
IS: I believe the physics community remembers Einstein’s enormous contributions to the early days of quantum theory; he was a true pioneer, even though he did not fully accept quantum mechanics. Furthermore, people recognize his creation of special and general relativity as an amazing achievement of the human mind.
CC: Do any of his theories still provoke controversy?
IS: I think it is fair to say that among professional scientists, there is no longer any controversy, even on the quantum mechanical issue on which Einstein was on the 'losing' side.
CC: Your 1966 test was not the last one; scientists have been testing the theory of general relativity in various ways. Why is it important to keep testing the theory?
Physics is fundamentally an experimental science, and scientists must test the predictions of its models, such as general relativity, to increasingly higher levels of accuracy. At some level of accuracy, even general relativity will certainly be found wanting.
CC: What other tests are being prepared or were recently completed? And what is the status of the effect known as gravitational lensing, which is one of the predictions of Einstein’s general theory of relativity?
IS: A Lens-Thirring test, based on the orbits of the LAGEOS and LARES spacecraft orbiting the Earth, is currently in process. I am unaware of any other tests currently being carried out. Gravitational lensing is a well-studied prediction of general relativity and is applied in many astronomical and astrophysical situations, including in the large-scale structure of the universe.
CC: What do you feel is Einstein’s lasting legacy?
IS: His imprints are all over our thinking, our quantum theory, and our theory of the universe—theories of the very small and of the very large.
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