The Sky’s the Limit on the Speed of Light

According to Einstein’s Special Theory of Relativity, no material object can move locally faster than light. Since 1905, modern physics has been ruled by the speed of light limit.

However, in September 2011 the OPERA (Oscillation Project with Emulsion-tRacking Apparatus) experimental team reported that neutrinos can travel faster than light. Given this claim, Wall Street traders could have made huge financial gains by responding to market changes with neutrino communication, faster than any competitors who are using traditional electromagnetic communication through regular internet channels.

The implications to our understanding of the cosmos would have been far more significant than the impact on the financial sector. The OPERA report resulted in numerous papers interpreting its consequences for fundamental physics. But within six months, the OPERA team concluded that a fiber optic cable was attached improperly and caused the apparently faster-than-light measurements in addition to a clock oscillator ticking too fast. Subsequently, five experimental follow-ups, including within the OPERA set-up, found neutrino speeds consistent with the speed of light limit.

The lesson from this experience is that any claim for new physics must be held to an extremely high level of experimental competence and must be reproducible. As I argued in a recent preprint, the dark objects observed by astronomers above Ukraine cannot be used as an argument for new physics if the distance measurements are highly uncertain.

Being the longest-serving chair of the Astronomy department at Harvard University, I am well aware that the best way to attract undergraduate students to astronomy is to take them on an observing trip. The view of the stars on a dark night is awe inspiring. Could humans not only stare at the stars but actually reach them within a human’s lifespan?

The distance to the nearest star, Proxima Centauri, is 4.2 light years. In order to reach it within the 122 years lifespan of the oldest person who ever lived, Jeanne Louise Calment, we need to launch a spacecraft that travels faster than 3.5% of the speed of light. Chemical rockets reach at best 0.01% of the speed of light because they carry their fuel and are subject to the tyranny of the rocket equation. But shortly after the laser was invented, Robert Forward suggested in 1962 that pushing a lightweight sail with a powerful laser beam can propel the sail close to the speed of light. Thanks to funding by the visionary entrepreneur and physicist, Yuri Milner, this idea materialized to the Starshot Initiative.

The Starshot concept envisioned a 100 giga-Watt laser shining on a human-size sail, to which a payload mass of a few grams is attached. Over a few minutes the sail can be propelled up to a fifth of the speed of light over five times the distance to the Moon. The system can launch many low-cost craft carrying smart electronic probes towards the nearest stars. The Scientific Advisory Board of Starshot — which I chair, concluded that there are no obvious “showstoppers” to this technology and that humanity could potentially build a working system later this century. By now, the research team identified promising materials and structures for the light sail, a path for developing lasers of sufficient total power, and methods for communication across the journey to the nearest stars. The technological challenges are great but so are the rewards.

The study of lightsails has a bright future ahead. In 2023 NASA is planning to launch an Advanced Composite Solar Sail System (ACS3) that will test new sail boom materials in a low-Earth orbit. The mission will deploy a lightweight sail about the size of a small apartment from a CubeSat, a spacecraft the size of a toaster oven.

But we must also keep in mind that standard physics offers a path to overcome the speed of light limit. According to Einstein’s General Theory of Relativity, space can expand so rapidly that it would separate two distant observers faster than light away from each other. The situation resembles a balloon which could be inflated so fast that stationary ants on its surface would be separated away from each other at a speed faster than their walking speed.

When the vacuum dominates the cosmic mass density, the expansion of space is accelerated, allowing for locally stationary observers to be separated faster than light. Any inhomogeneities in between are ironed out by the rapid expansion. As a result, the Universe becomes homogeneous and isotropic. An early phase of this accelerated expansion, known as cosmic inflation, was proposed four decades ago to explain why the universe has the same conditions on opposite sides that had no time to communicate with each other for a decelerating expansion, as expected without the gravitational repulsion of the vacuum.

The initial version of inflation developed in the early 1980s, explained the flat geometry, the near uniformity and isotropy of the Universe and the origin of structure in it as a result of quantum fluctuations. It made testable predictions and was falsifiable. But as the theory was explored further, it was realized that it can accommodate all possible outcomes. First, one can always reverse engineer an inflationary model to fit an observed outcome. For example, modelers came up with variants of inflation that can accommodate a non-flat universe. Second, it was realized that if one starts from some arbitrary initial conditions of large inhomogeneities, inflation may not be possible. Third, quantum fluctuations can lead to any outcome. In the words of inflation’s pioneer, Alan Guth: “In an eternally inflating universe, anything that can happen will happen an infinite number of times.” Given these circumstances, the theory is not easily falsifiable because without calibrating the likelihood of different outcomes there is no obvious way of ruling out the paradigm.

Gladly, there is a way out. In a new paper that I just published with Sunny Vagnozzi, we proposed an experimental test that could rule-out inflation. Without an inflationary epoch of vast expansion, the Universe would be filled with a radiation background of gravitational waves (gravitons) similar to the cosmic background of photons and neutrinos. In a strong magnetic field, some of these gravitons could be converted to observable electromagnetic signals. If this futuristic experiment, when done, detects the cosmic graviton background from the early universe, it would disprove all models of inflation.

Here’s hoping that the speed of light will continue to inspire future generations of physicists to pursue innovative ideas that are testable by reproducible experiments.

ABOUT THE AUTHOR

Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He chairs the advisory board for the Breakthrough Starshot project, and is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, is scheduled for publication in August 2023.

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Avi Loeb

Avi Loeb is the Frank B. Baird Jr Professor of Science and Institute director at Harvard University and is the bestselling author of “Extraterrestrial”.