The search for extra-terrestrial life

Avi Loeb


Abraham “Avi” Loeb (born February 26, 1962) is an Israeli-American theoretical physicist who works on astrophysics and cosmology. Loeb is the Frank B. Baird Jr. Professor of Science at Harvard University. He had been the longest serving Chair of Harvard’s Department of Astronomy (2011–2020), Founding Director of Harvard’s Black Hole Initiative (since 2016) and Director of the Institute for Theory and Computation (since 2007) within the Harvard-Smithsonian Center for Astrophysics.

The search for extraterrestrial life is one of the most exciting frontiers in Astronomy. First tentative clues were claimed close to Earth: the weird interstellar object `Oumuamua and the cloud deck of Venus. Our civilization will mature once we find out who resides on our cosmic street by searching with our best telescopes for unusual electromagnetic flashes, industrial pollution of planetary atmospheres, artificial light or heat, artificial space debris or something completely unexpected.
We might be a form of life as primitive and common in the cosmos as ants are in a kitchen. If so, we can learn a lot from others out there.

The talk

The following are notes from the on-line lecture. Even though I could stop the video and go back over things there are likely to be mistakes because I haven’t heard things correctly or not understood them. I hope the Professor Loeb and my readers will forgive any mistakes and let me know what I got wrong.


The photographer, who took the photograph above left, asked the professor to write on his hand an important science question. He wrote “Are we alone?”


Herlinde Koelbl (born 31 October 1939) is a German photographic artist, author and documentary maker. (above left)

Her comprehensive work is characterised above all by long-term photographic projects, often complemented by in-depth interviews. She is particularly interested in creating portraits of milieus and people. She has received a number of awards for her photographic work, for example the Dr Erich Salomon Prize in 2001. Since 2009, she has regularly worked as an author and photographer for ZEIT magazine, in the column “What saved me”. (Above right)

The Berlin-Brandenburg Academy of Sciences and Humanities, abbreviated BBAW, is the official academic society for the natural sciences and humanities for the German states of Berlin and Brandenburg. Housed in three locations in and around Berlin, Germany, the BBAW is the largest non-university humanities research institute in the region.

The professor’s astronomical studies have given him a sense of modesty.

Matter takes new forms over time. We are all made up of matter that was once produced in massive stars that exploded. This matter is in the food that keeps us alive. Our bodies are just tiny shapes of matter that exist for a very short time on a tiny insignificant planet, in a very large Universe that has been around for nearly 14 billion years. We are insignificant, a tiny transient structure, and if we are lucky, our presence is recorded by other tiny transient structures.

Elements up to iron are formed in stars more massive than our Sun.


Artist’s illustration of the core of a massive star just prior to a type II supernova explosion. The core is a series of nested spherical shells, with each shell fusing a different element from hydrogen to helium, to carbon, through the periodic table to iron. Credit: Penn State Astronomy & Astrophysics

Not only do supernovae serve as the mechanism for the creation of these heavy elements, they also serve as the mechanism for their dispersal. Our Sun is a low mass star, so it will only ever create carbon and oxygen within its core. It will never achieve the conditions necessary to create iron. However, when we take a spectrum of the Sun, we see spectral lines from nitrogen, sodium, magnesium, iron, silicon, and even rare elements such as europium and vanadium. These elements observed in our Sun (and in many other stars) were created in ancient supernovae explosions. The elements got dispersed by the supernova explosion and became mixed in with the gas in molecular clouds. Thus, when the next generation of stars formed, the gas in the molecular cloud already contained some heavy elements. Since the Earth (and all of us!) are made of heavy elements, life as we know it would not be possible without the occurrence of supernovae prior to the formation of our Sun.

Elements heavier than iron are produced during other astronomical events.

Research so far has found more habitable Earths in the observable volume of the Universe than there are grains of sand on all of the beaches on the Earth.

About half of all the Sun-like stars host an Earth-size planet in their habitable zones (Bryson et al., October 2020)

The Occurrence of Rocky Habitable Zone Planets Around Solar-Like Stars from Kepler Data

We now know from the Kepler satellite that about half of the Sun-like stars (+/-30%) host an Earth sized planet at roughly the same separation as between the Earth and the Sun. In principle some of these planets could have surface liquid water and the chemicals of life (carbon, hydrogen and oxygen).

So, humans are not the centre of the Earth, Earth isn’t in the centre of the Solar System (as the ancient Greeks thought and this idea had lasted for over a thousand years), our Solar System is not the centre of our galaxy, the Milky Way, the Milky Way is not the centre of the Universe and we may not be the only planet to have life on it.

The Kepler space telescope is a retired space telescope launched by NASA to discover Earth-size planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched on March 7, 2009, into an Earth-trailing heliocentric orbit. The principal investigator was William J. Borucki. After nine years of operation, the telescope’s reaction control system fuel was depleted, and NASA announced its retirement on October 30, 2018.


Designed to survey a portion of Earth’s region of the Milky Way to discover Earth-size exoplanets in or near habitable zones and estimate how many of the billions of stars in the Milky Way have such planets, Kepler’s sole scientific instrument is a photometer that continually monitored the brightness of approximately 150,000 main sequence stars in a fixed field of view. These data are transmitted to Earth, then analysed to detect periodic dimming caused by exoplanets that cross in front of their host star. Only planets whose orbits are seen edge-on from Earth can be detected. During its over nine and a half years of service, Kepler observed 530,506 stars and detected 2,662 planets.

Ancient civilisations started to have theories of Earth’s place in the Universe as far back as the 16th century BCE (perhaps even before but as writing hadn’t been developed nothing was written down).

It was during the 4th century BCE that Aristotle proposed an Earth-centred universe in which the Earth is stationary and the cosmos (or universe) is finite in extent but infinite in time. (below left)


Aristotle (384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece.

Surprisingly Aristotle’s ideas weren’t totally accepted at the time, but his ideas prevailed until the 16th/17th century AD (above centre)

Aristarchus of Samos (c. 310 – c. 230 BC) was an ancient Greek astronomer and mathematician who presented the first known heliocentric model that placed the Sun at the centre of the known universe with the Earth revolving around it. He was influenced by Philolaus of Croton, but Aristarchus identified the “central fire” with the Sun, and he put the other planets in their correct order of distance around the Sun. Like Anaxagoras before him, he suspected that the stars were just other bodies like the Sun, albeit farther away from Earth. His astronomical ideas were often rejected in favour of the geocentric theories of Aristotle and Ptolemy. Nicolaus Copernicus attributed the heliocentric theory to Aristarchus

Claudius Ptolemy (; c. 100 – c. 170 AD) was a mathematician, astronomer, natural philosopher, geographer and astrologer who wrote several scientific treatises, three of which were of importance to later Byzantine, Islamic and Western European science.

It is important to point out that Several astronomers (mostly Islamic scholars) proposed a Sun-centred universe before the work done by Nicolaus Copernicus.

The idea of a Sun-centred was not accepted at the time as the Roman Catholic church had accepted the teachings of Aristotle (in fact they had made him an honorary Catholic). This was rather unfortunate for Galileo. (below left)


Nicolaus Copernicus (19 February 1473 – 24 May 1543) was a Renaissance-era mathematician, astronomer, and Catholic canon who formulated a model of the universe that placed the Sun rather than Earth at the centre of the universe. (Above centre)

Galileo di Vincenzo Bonaiuti de’ Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath, from Pisa. Galileo has been called the “father of observational astronomy”, the “father of modern physics”, the “father of the scientific method”, and the “father of modern science”.

In his 1615 Galileo defended heliocentrism, and claimed it was not contrary to Holy Scripture.

In February 1616, the Catholic Inquisition assembled a committee of theologians, known as qualifiers, who delivered their unanimous report condemning heliocentrism as “foolish and absurd in philosophy, and formally heretical since it explicitly contradicts in many places the sense of Holy Scripture.”

Galileo’s trial in 1633 involved making fine distinctions between “teaching” and “holding and defending as true”. For advancing heliocentric theory Galileo was forced to recant Copernicanism and was put under house arrest for the last few years of his life.

Between 1617 and 1621, Johannes Kepler developed a heliocentric model of the Solar System in which all the planets have elliptical orbits. (Above right)

Johannes Kepler (27 December 1571 – 15 November 1630) was a German astronomer, mathematician, and astrologer. He is a key figure in the 17th-century scientific revolution, best known for his laws of planetary motion, and his books Astronomia nova, Harmonices Mundi, and Epitome Astronomiae Copernicanae.

The Galileo affair did little overall to slow the spread of heliocentrism across Europe, as Kepler’s Epitome of Copernican Astronomy became increasingly influential in the coming decades. By 1686, the model was well enough established that the general public was reading about it.

In 1687, Isaac Newton published Philosophiæ Naturalis Principia Mathematica, which provided an explanation for Kepler’s laws in terms of universal gravitation and what came to be known as Newton’s laws of motion. This placed heliocentrism on a firm theoretical foundation.

In the mid-eighteenth century the Catholic Church’s opposition began to fade. An annotated copy of Newton’s Principia was published in 1742. In 1758 the Catholic Church dropped the general prohibition of books advocating heliocentrism from the Index of Forbidden Books.


Sir Isaac Newton PRS (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, theologian, and author (described in his own day as a “natural philosopher”) who is widely recognised as one of the most influential scientists of all time and as a key figure in the scientific revolution.

Our ancient ancestors could be compared to very little children before they go to nursery. These little children think the Universe centres around them. Then they go to nursery and find there are lots of other little children. All different in little ways but fundamentally the same.

It is difficult to accept that the soup of chemicals that produced life could only have occurred on Earth and that this life is the best possible. An analogy can be found in baking. The same set of ingredients can give you radically different cakes. Perhaps we are not the best “cake” in the Universe. After all you only have to look at the news to see some of the daft things we have done (Two world wars for a start).

In evolutionary biology, abiogenesis, or informally the origin of life (OoL), is the natural process by which life has arisen from non-living matter, such as simple organic compounds. While the details of this process are still unknown, the prevailing scientific hypothesis is that the transition from non-living to living entities was not a single event, but an evolutionary process of increasing complexity that involved molecular self-replication, self-assembly, autocatalysis, and the emergence of cell membranes. Although the occurrence of abiogenesis is uncontroversial among scientists, its possible mechanisms are poorly understood. There are several principles and hypotheses for how abiogenesis could have occurred.

Earth is our home, but only for a while. Even if we have good health and looked after the planet we will die at some point.



However, we help the “dying process” with climate change, nuclear accidents, nuclear wars, biological wars, chemical wars, pandemics, political or technological catastrophes. We have developed the means for our own destruction.

There are, of course risks, that we have no influence over such as earthquakes and volcanoes.

There are also risks from space such as asteroid impacts (dinosaurs became extinct due to one such impact), solar flares, exploding stars and of course our own Sun won’t last for ever.

The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary (K–T) extinction) was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the exception of some ectothermic species such as the sea turtles and crocodilians, no tetrapods weighing more than 25 kilograms survived. It marked the end of the Cretaceous period, and with it the end of the entire Mesozoic Era, opening the Cenozoic Era that continues today.

In the geologic record, the K–Pg event is marked by a thin layer of sediment called the K–Pg boundary, which can be found throughout the world in marine and terrestrial rocks. The boundary clay shows unusually high levels of the metal iridium, which is more common in asteroids than in the Earth’s crust.

As originally proposed in 1980 by a team of scientists, it is now generally thought that the K–Pg extinction was caused by the impact of a massive comet or asteroid 10 to 15 km wide, 66 million years ago, which devastated the global environment, mainly through a lingering impact winter which halted photosynthesis in plants and plankton.

Hopefully, as humans have more developed brains than dinosaurs, we should be able to identify incoming asteroids and do something about them.

At present, nearly half the hydrogen at the core of the Sun has been consumed, with the remainder of the atoms consisting primarily of helium. As the number of hydrogen atoms per unit mass decreases, so too does their energy output provided through nuclear fusion. This results in a decrease in pressure support, which causes the core to contract until the increased density and temperature bring the core pressure into equilibrium with the layers above. The higher temperature causes the remaining hydrogen to undergo fusion at a more rapid rate, thereby generating the energy needed to maintain the equilibrium.

In about 600 million years, the Sun’s brightness will have disrupted the Earth’s carbon cycle to the point where trees and forests (C3 photosynthetic plant life) will no longer be able to survive; and in around 800 million years, the Sun will have killed all complex life on the Earth’s surface and in the oceans. In 1.1 billion years’ time, the Sun’s increased radiation output will cause its circumstellar habitable zone to move outwards, making the Earth’s surface too hot for liquid water to exist there naturally. At this point, all life will be reduced to single-celled organisms. Evaporation of water, a potent greenhouse gas, from the oceans’ surface could accelerate temperature increase, potentially ending all life on Earth even sooner. During this time, it is possible that as Mars’s surface temperature gradually rises, carbon dioxide and water currently frozen under the surface regolith will release into the atmosphere, creating a greenhouse effect that will heat the planet until it achieves conditions parallel to Earth today, providing a potential future abode for life. By 3.5 billion years from now, Earth’s surface conditions will be similar to those of Venus today.

Around 5.4 billion years from now, the core of the Sun will become hot enough to trigger hydrogen fusion in its surrounding shell. This will cause the outer layers of the star to expand greatly, and the star will enter a phase of its life in which it is called a red giant. Within 7.5 billion years, the Sun will have expanded to a radius of 1.2 AU—256 times its current size.

As the Sun expands, it will swallow the planets Mercury and Venus. Earth’s fate is less clear; although the Sun will envelop Earth’s current orbit, the star’s loss of mass (and thus weaker gravity) will cause the planets’ orbits to move farther out. If it were only for this, Venus and Earth would probably escape incineration, but a 2008 study suggests that Earth will likely be swallowed up as a result of tidal interactions with the Sun’s weakly bound outer envelope.

As the Sun dies, its gravitational pull on the orbiting bodies such as planets, comets and asteroids will weaken due to its mass loss. All remaining planets’ orbits will expand; if Venus, Earth, and Mars still exist, their orbits will lie roughly at 1.4 AU (210,000,000 km), 1.9 AU (280,000,000 km), and 2.8 AU (420,000,000 km). They and the other remaining planets will become dark, frigid hulks, completely devoid of any form of life. They will continue to orbit their star, their speed slowed due to their increased distance from the Sun and the Sun’s reduced gravity. Two billion years later, when the Sun has cooled to the 6000–8000K range, the carbon and oxygen in the Sun’s core will freeze, with over 90% of its remaining mass assuming a crystalline structure. Eventually, after roughly 1 quadrillion years, the Sun will finally cease to shine altogether, becoming a black dwarf.

Professor Loeb’s journey to astronomer was slightly unusual. He was born and brought up on a farm in Israel where he had chores such as collecting eggs, but he liked to read books on philosophy. He liked philosophy because he thought it addressed fundamental questions but the journey through adolescence led him to physics and ultimately to astrophysics. As a professor at Harvard his research encompasses the Universe, but there are links to his original interest in philosophy in that there are fundamental questions using the scientific method which began during philosophic debates. After all physics was once described as “Natural philosophy”.

The scientific method is an empirical method of acquiring knowledge that has characterized the development of science since at least the 17th century. It involves careful observation, applying rigorous scepticism about what is observed, given that cognitive assumptions can distort how one interprets the observation. It involves formulating hypotheses, via induction, based on such observations; experimental and measurement-based testing of deductions drawn from the hypotheses; and refinement (or elimination) of the hypotheses based on the experimental findings. These are principles of the scientific method, as distinguished from a definitive series of steps applicable to all scientific enterprises.

In 2015 Professor Loeb had a visit from Yuri Milner.


Yuri Borisovich (Bentsionovich) Milner (born 11 November 1961) is an Israeli-Russian entrepreneur, venture capitalist and physicist.

In July 2015, Milner launched Breakthrough Initiatives, a scientific program to investigate the question of life in the Universe.

Milner wanted Professor Loeb to lead an initiative to visit the nearest star to our Solar System.

Proxima Centauri is a small, low-mass star located 4.2465 light-years (1.3020 pc) away from the Sun in the southern constellation of Centaurus.

A visit to this star would require transport capable of moving at a fifth of the speed of light if a person wanted to travel there without getting too old (the journey would take twenty years).

Professor Loeb pointed out to Milner that there wasn’t the technology at the moment to accomplish that feat.

The professor, along with his students and Postdocs (people who will have recently obtained a PhD), considered the proposition for six months and recommended a method/technology that could achieve the task.

The project was called Starshot, announced in New York City on April 12th 2016


Getty Images: Stephen Hawking, Mae Jemison, Freeman Dyson, Yuri Milner, Pete Worden, Avi Loeb

Breakthrough Starshot is a research and engineering project by the Breakthrough Initiatives to develop a proof-of-concept fleet of light sail interstellar probes named Starchip, to be capable of making the journey to the Alpha Centauri star system 4.37 light-years away. It was founded in 2016 by Yuri Milner, Stephen Hawking, and Mark Zuckerberg. (below left)

Stephen William Hawking CH CBE FRS FRSA (8 January 1942 – 14 March 2018) was an English theoretical physicist, cosmologist, and author who was director of research at the Centre for Theoretical Cosmology at the University of Cambridge at the time of his death. He was the Lucasian Professor of Mathematics at the University of Cambridge between 1979 and 2009. (below right)

Mark Elliot Zuckerberg (born May 14, 1984) is an American media magnate, internet entrepreneur, and philanthropist. He is known for co-founding Facebook, Inc. and serves as its chairman, chief executive officer, and controlling shareholder. He also is a co-founder of the solar sail spacecraft development project Breakthrough Starshot and serves as one of its board members.


The bigger picture


The distance from the Oort cloud to the interior of the Solar System, and two of the nearest stars, is measured in astronomical units. The scale is logarithmic; each indicated distance is ten times farther out than the previous distance. The red arrow indicates the location of the space probe Voyager 1, which will reach the Oort cloud in about 300 years.

The journey to the nearest triple-star system, Alpha Centauri, located about four light years away, will take tens of thousands of years using conventional chemical rockets (if rockets were to get there now, they would have had to start at the time when humans first left Africa). The edge of the Solar System is marked by the Oort cloud, stretching half the way to Alpha Centauri. Distances are labelled in units of Earth-Sun separation (1 Astronomical Unit (1AU)). In 2012 Voyager 1 crossed the heliopause, the surface where the Solar wind collides with interstellar gas.


Alpha Centauri AB is the bright star to the left, which forms a triple star system with Proxima Centauri, circled in red. The bright star system to the right is Beta Centauri.

Alpha Centauri is the closest star system and closest planetary system to Earth’s Solar System at 4.37 light-years (1.34 parsecs) from the Sun.

The Oort cloud sometimes called the Öpik–Oort cloud, first described in 1950 by Dutch astronomer Jan Oort, is a theoretical cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 2,000 to 200,000 au (0.03 to 3.2 light-years). It is divided into two regions: a disc-shaped inner Oort cloud (or Hills cloud) and a spherical outer Oort cloud. Both regions lie beyond the heliosphere and in interstellar space. The Kuiper belt and the scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth as far from the Sun as the Oort cloud.

The outer limit of the Oort cloud defines the cosmographic boundary of the Solar System and the extent of the Sun’s Hill sphere. The outer Oort cloud is only loosely bound to the Solar System, and thus is easily affected by the gravitational pull both of passing stars and of the Milky Way itself. These forces occasionally dislodge comets from their orbits within the cloud and send them toward the inner Solar System. Based on their orbits, most of the short-period comets may come from the scattered disc, but some may still have originated from the Oort cloud.

Jan Hendrik Oort ForMemRS (28 April 1900 – 5 November 1992) was a Dutch astronomer who made significant contributions to the understanding of the Milky Way and who was a pioneer in the field of radio astronomy.


Voyager 1 is a space probe that was launched by NASA on September 5, 1977. Part of the Voyager program to study the outer Solar System, Voyager 1 was launched 16 days after its twin, Voyager 2. Having operated for 43 years, 5 months and 9 days as of February 14, 2021 UTC, the spacecraft still communicates with the Deep Space Network to receive routine commands and to transmit data to Earth. Real-time distance and velocity data is provided by NASA and JPL. At a distance of 152.2 AU (22.8 billion km; 14.1 billion mi) from Earth as of January 12, 2020, it is the most distant human-made object from Earth.


An artist’s illustration of NASA’s Voyager 1 spacecraft, the farthest human-built object from Earth, which launched in 1977 and is headed for interstellar space. (Image credit: NASA)

If you imagine an Oort cloud around every star then they will be touching each other. So, if one star moves, so will its Oort cloud and any outer rocks will likely get ripped apart or ripped out of its solar system and go wandering about in outer space.

So interstellar space is likely to be populated by icy rocks from the Oort clouds of stars. The number of such rocks have been calculated and published in a paper more than a decade ago. It is unlikely that any of these will be seen with the Pan-STARRS Telescope.

The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1; obs. code: F51 and Pan-STARRS2 obs. code: F52) located at Haleakala Observatory, Hawaii, US, consists of astronomical cameras, telescopes and a computing facility that is surveying the sky for moving or variable objects on a continual basis, and also producing accurate astrometry and photometry of already-detected objects. In January 2019 the second Pan-STARRS data release was announced. At 1.6 petabytes, it is the largest volume of astronomical data ever released.


The image above is an artist’s impression of Proxima b, the nearest habitable planet outside of the Solar System. This planet, discovered in August 2016, has a mass roughly 1 to 2 times the mass of the Earth and revolves with a period of 11.2 days around Proxima Centauri, a dwarf star with 12% the mass of the Sun at a distance of 4.24 light years. Proxima b has a surface temperature comparable to that of Earth, but because of its proximity to its faint host star – is believed to be tidally locked with permanent day and night sides (Credit: ESO)

Because Proxima Centauri has a low surface temperature it emits mainly infra-red light so any animal with eyes would need to be able to see IR. If there were intelligent life then they probably wouldn’t want to come to Earth as their eyes wouldn’t be adapted to our visible light.


Artist’s conception of the surface of Proxima Centauri b. The Alpha Centauri binary system can be seen in the background, to the upper right of Proxima.

The habitability of Proxima Centauri b has not been established, but the planet is subject to stellar wind pressures of more than 2,000 times those experienced by Earth from the solar wind. This radiation and the stellar winds would likely blow any atmosphere away, leaving the subsurface as the only potentially habitable location on that planet.

The exoplanet is orbiting within the habitable zone of Proxima Centauri, the region where, with the correct planetary conditions and atmospheric properties, liquid water may exist on the surface of the planet.

Even though Proxima Centauri b is in the habitable zone, the planet’s habitability has been questioned because of several potentially hazardous physical conditions. The exoplanet is close enough to its host star that it might be tidally locked. In this case, it is expected that any habitable areas would be confined to the border region between the two extreme sides, generally referred to as the terminator line, since it is only here that temperatures might be suitable for liquid water to exist. If the planet’s orbital eccentricity is 0, this could result in synchronous rotation, with one hot side permanently facing towards the star, while the opposite side is in permanent darkness and freezing cold.

Another problem is that the flares released by Proxima Centauri could have eroded the atmosphere of the exoplanet. However, if Proxima b had a strong magnetic field, the flare activity of its parent star would not be a problem.

In order to find out if there is life on Proxima b then probes would need to be sent. That was the goal of the Starshot initiative.


Above left is an artist’s impression of Starshot, a project to push a lightsail by a powerful laser beam from Earth. Focusing a beam of 100 gigawatt on a sail with a few metres (roughly the size of a person) and the mass of 1 gramme, can launch it to a fifth of the speed of light, so that it would reach Proxima b within two decades. Feasibility studies are currently underway regarding the laser optics, the design of the sail, and the communication challenge because of the large distance involved. Above centre shows an example of the lightweight electronic device (StarChip) that could be attached along with a camera to the sail (all the equipment would have a mass equal to or less than one gramme and versions of these can already be found in a mobile phone). Above right is a photograph of the solar lightsail 2 deployed by the planetary society on July 23rd, 2019 with the Sun showing through the 32-square metre sail (Credit: Planetary Society). The Sun “pushed” the lightsail in this instance.

The cost of the lightsail is reasonable. Most of the cost of the project is down to the laser infrastructure.

Solar sails (also called light sails or photon sails) are a method of spacecraft propulsion using radiation pressure exerted by sunlight on large mirrors. A number of spaceflight missions to test solar propulsion and navigation have been proposed since the 1980s.

Lots of low powered lasers would combine their beams to produce a more powerful coherent beam on the sail, roughly the size of a few metres, to push it away from the Earth.


Above left: the lower powered lasers. Above right: the launch of the Lightsail


The Lightsail would be launched above the atmosphere so that wouldn’t be the friction to cause it to slow down.

The mothership could launch more than one sail.

A few minutes after the sail has been launched (this is how long the laser beam acts on the sail) the twenty-year journey towards Proxima b begins.


Artist’s impression of the Lightsail approaching proxima b when it is hoped the camera will take some pictures. These might show signatures of life.

Because the images will be transported at the speed of light it will take four years for the images to reach Earth (24 years in total). The signal will be weak but hopefully technology (better telescopes than now) will be able to pick them up.

The Planetary Society is an American internationally active, non-governmental, nonprofit. It is involved in research, public outreach, and political advocacy for engineering projects related to astronomy, planetary science, and space exploration. It was founded in 1980 by Carl Sagan, Bruce Murray, and Louis Friedman, and has about 60,000 members from more than 100 countries around the world.

The Society is dedicated to the exploration of the Solar System, the search for near-Earth objects, and the search for extra-terrestrial life. The society’s mission is stated as: “To empower the world’s citizens to advance space science and exploration”. The Planetary Society is also a strong advocate for space funding and missions of exploration within NASA. They lobby Congress and engage their membership in the United States to write and call their representatives in support of NASA funding.


Planetary Society founders – 1980 photo. Clockwise from bottom left: Bruce Murray, Louis Friedman, Harry Ashmore (advisor), Carl Sagan

Starshot is a new opportunity in improving laser technology and the miniaturisation of electronics.

Are humans the most intelligent life in the Universe?

If not, we have to hope that these other civilisations are nice so that we can learn from them. Perhaps allowing us to do away with years of research.

What evidence is there?

Perhaps we should look at our own Solar System first. Perhaps objects have already made the trip. Perhaps their journeys have taken millions of years. Perhaps we can learn from the civilisations that produced them.

We should examine them as this would be quicker than sending objects out of the Solar System.

ʻOumuamua is the first known interstellar object detected passing through the Solar System. Formally designated 1I/2017 U1, it was discovered using the Pan-STARRS telescope at Haleakalā Observatory, Hawaii, on 19 October 2017, 40 days after it passed its closest point to the Sun on 9 September. When it was first observed, it was about 33 million km (0.22 AU) from Earth (about 85 times as far away as the Moon), and already heading away from the Sun.


Combined telescope image of the first interstellar object ʻOumuamua, circle in blue as an unresolved point source at the centre. It is surrounded by the trails of faint stars, each smeared into a series of dots as the telescope tracked the moving ʻOumuamua. Credit: ESO/k.Meech et al.


Sky path of ʻOumuamua, labelled by date, as seen from Earth. The relative size of each circle gives a sense of the changing distance of ʻOumuamua along its apparent trajectory. Also shown are the direction of motion of the Sun in the Local Standard of Rest (purple, labelled “Solar apex”), Venus (green), Mars (red), Uranus (turquoise) and the opposite direction to the motion of the Sun (purple, labelled “Solar antapex”). ʻOumuamua trajectory moved from the local Standard of Rest to south of the ecliptic plane (marked by the thin yellow line) of the Solar System between September 2nd and October 22nd, 2017.

The interesting feature is that at the top right of the above image ʻOumuamua was at “rest” at that point in its trajectory. The relative velocity of the Sun and ʻOumuamua had the same velocity of the Sun in the local standard of rest.

This is surprising because only one in five hundred stars is so much at rest relative to the local Standard of Rest as ʻOumuamua was, and since we assume that rocks are torn apart by the Oort clouds around stars, they should inherit the initial speed of the star because they are very loosely bound and any relative speed, they have compared to the star is negligible. They basically inherit the speed of the star when they are being torn apart an external perturbance.

So, finding ʻOumuamua at rest in that frame is really surprising. It’s a low probability event. This is the first object to be found from interstellar space. Astrophysicists know that because it moves too fast and it cannot be bound to the Sun.


Hyperbolic trajectory of ʻOumuamua through the inner Solar System, with the Sun at the focus, showing its position every 7 days. The planet positions are fixed at the perihelion (closest approach to the Sun) on September 9, 2017. Shown from a three-quarter perspective, roughly aligned to the plane of ʻOumuamua’s path.

It was still approaching us on July 2017. Unfortunately, this wasn’t known at the time and astronomers were a bit miffed as they could have sent a camera into space to photograph it. ʻOumuamua wasn’t actually spotted until October 2017.


The solar apex, or the apex of the Sun’s way, refers to the direction that the Sun travels with respect to the local standard of rest. This is not to be confused with the Sun’s apparent motion through all constellations of the zodiac, which is an illusion caused by the orbit of the Earth.

The local standard of rest is the local reference that you get to when you average the motions of all the stars in the sky within the neighbourhood of the Sun. It’s like the Sun’s parking lot and ʻOumuamua parked there for a bit.

In astronomy, the local standard of rest or LSR follows the mean motion of material in the Milky Way in the neighbourhood of the Sun (stars in radius 100 pc from the Sun). The path of this material is not precisely circular. The Sun follows the solar circle (eccentricity e < 0.1) at a speed of about 255 km/s in a clockwise direction when viewed from the galactic north pole at a radius of ≈ 8.34 kpc about the centre of the galaxy near Sgr A*, and has only a slight motion, towards the solar apex, relative to the LSR

The solar antapex, the direction opposite of the solar apex, is located near the star Zeta Canis Majoris.


The trajectory of ʻOumuamua through the Solar System is unlike all asteroids or comets observed before. Its orbit is not bound by the Sun’s gravity. ʻOumuamua originated from interstellar space and will return there with a velocity boost as a result of its passage near the Sun. The Sun deflected it because of its gravitational force. Its hyperbolic orbit was inclined relative to ecliptic plane of the Solar System and did not pass close to any of the planets on the way in. The Sun caused it to leave the Solar System by a different direction to the one it entered. Credit: K. Meech et al.

An analogy of the Sun and ʻOumuamua would be to have them on the ocean. ʻOumuamua is just a buoy sitting there and the Sun is a ship that comes along and knocks it, through its “gravitational force” into a different direction.

Another unusual property of ʻOumuamua


ESO/K. Meech et al. – at,

Variation in brightness of ʻOumuamua as observed by various telescopes during three days in October 2017. Different coloured dots represent measurements through different filters in the visible and near infra-red bands of the colour spectrum. The large range of brightness/amount of reflected sunlight changed periodically by a factor of ten (2.5 magnitudes) because of its very elongated shape and the fact that it rotated every 7.3 hours. This implied that it had an extreme shape which meant it was at least ten times longer than it was wide when projected on the sky. The dashed white line shows the curve expected if ʻOumuamua were an ellipsoid with a 1:10 aspect ratio (the deviations from this line are probably due to irregularities in the object’s shape or surface albedo). However, the best fit to the light curve from its tumbling motion implies a flattened, pancake-shaped configuration rather than an oblong, cigar-shaped object as commonly depicted in the media. Credit: ESO/K. Meech et al.

1:10 aspect ratio means that it was ten times longer than its width.


Simulation of ʻOumuamua spinning and tumbling through space, and the resultant light curve. In reality, observations of ʻOumuamua detect the object as a single pixel — its shape here has been inferred from the light curve

The albedo of a surface is the fraction of the incident sunlight that the surface reflects. Radiation that is not reflected is absorbed by the surface. … The surface albedo is a key ingredient in the remote sensing of surface and atmospheric properties from space.

What were the properties of ʻOumuamua?


There are a number of properties that make it different from any object that has appeared in the Solar System before:

It wasn’t anticipated that an interstellar object would be discovered by Pan-STARRS. Calculations done a decade earlier indicated that Pan-STARRS would see nothing from interstellar space. So, the mere detection was a surprise.

It originated from the “Local Standard of Rest” of nearby stars; 1 in 500 chance (Mamajek 2017).

Its abundance was larger than expected by 2-8 orders of magnitude.

Its brightness changed by a factor of ten as it spins and it had an extreme shape,

The most favoured fit of the light curve indicated it was pancake-shaped not cigar shaped.

No heat/infrared radiation observed by the Spitzer Space Telescope. Its size smaller than ~200m with a reflectivity higher than ~20%. A shiny small object.

It was known that it was heated because astronomers knew its trajectory and its temperature was only dependent on its distance from the Sun. So, detecting no heat implied that it was small.

20% reflectivity was at the high end compared with objects that are usually seen in the sky.


An artist rendering of the Spitzer Space Telescope.

The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope launched in 2003 and retired on 30 January 2020.

ʻOumuamua orbit deviated from a Keplerian orbit (Micheli et al. 2018)

An extra push 0f 0.1% of gravitational force by the Sun would require an evaporation of ~10% of the object by a rocket effect (to conserve momentum). But no outgassing was detected: (i) no cometary tails of dust following deep observations from the Spitzer Space Telescope (Trilling et al. 2018); (ii) no emission by CO and CO2 (no carbon-based molecules); (iii) no change in spin or sudden kicks as observed for comets (Rafikov 2018).

The most peculiar fact about ʻOumuamua was that it exhibited a deviation from an orbit shaped just by the Sun’s gravity. There was an extra push and that force away from the Sun varied inversely from (distance)2 and it was smooth. But there was no way to explain this force through the rocket effect, mentioned above. The usual explanation in the case of comets. ʻOumuamua didn’t have a tail behind it like a comet would have and it didn’t lose mass.

What gave ʻOumuamua this extra push? The simplest explanation was reflection of sunlight. However, for this to happen the object needed to be thin.

Light carries momentum that can push on an object. When a light beam reflects or scatters off an object, the object will recoil.

At school students are taught that momentum equals mass x velocity (p = mv) but the mass of light is zero, so how can it have momentum? Einstein’s general theory of relativity states that something can have momentum even if it has no mass – it just has to have some amount of energy.

So, an alternative theory to outgassing was that ʻOumuamua had to have a push from sunlight and for this to happen it would require a thickness of less than a millimetre and be no longer than 20m (for a perfect reflector). Could it possibly a light sail of artificial origin? Nature doesn’t make light sails.

The mainstream community divides into three parts. There are people who write blogs and popular books on science, but they don’t practise science. They haven’t written a scientific paper. So, Professor Loeb doesn’t read what they have written about ʻOumuamua.

Then there is the bulk of the scientific community that is simply not working on rocks, asteroids, comets, the Solar System etc. They just say “don’t bother us with the details as it is probably a rock, we don’t want to get involved with something controversial”. So “business as usual”.

Then there are people who really paid attention to the anomalies and tried to explain them. So, what did they come up with? They looked at the evidence and tried to explain it.

One suggestion was that ʻOumuamua was a dust bunny spinning over eight hours. Made of dust particles that were loosely bound so it is was a very porous object with a mean density a hundred times lower than air (Moro-Martin 2019; Sekanina 2019; Flekkoy et al. 2019) – but it is unclear how to maintain the integrity of a 200 metre “steam cloud” over an interstellar journey lasting millions of years with an eight-hour spin period. So, Professor Loeb isn’t convinced by this suggestion. The object wouldn’t have enough strength to survive passage near the Sun, being heated to hundreds of degrees and also travel through interstellar space.….157…86M/abstract’Oumuamua_as_a_Fractal_Dust_Aggregate

Dust bunnies have been used as an analogy for the accretion of cosmic matter in planetoids. You find versions of these at home but a cosmic one can be the size of a football field.

Another suggestion that was made that ʻOumuamua is a fragment from a tidal disruption of a bigger object (Zhang & Lin 2020; Rafikov 2018). The problem is these objects are usually elongated/cigar shaped. Not pancake shaped. Also, most objects won’t pass close to a star. So why would we see the most common object, which happens to be the first one we discover, that is coming from a disruption by a star, if most objects never pass close to a star.

However, with a 90% confidence rating the shape is flat rather than cigar-shaped (Maschenko 2019). This would be very rare.

Another proposal was that ʻOumuamua was a molecular hydrogen iceberg (Seligman & Lauglin 2020). It would produce a tail but hydrogen is transparent. So, you can get cometary activity without seeing it. So that would explain the extra push.

However, such objects would evaporate during interstellar travel (Hoang & Loeb 2020). A hydrogen iceberg the size of ʻOumuamua would evaporate very quickly along the journey by absorbing starlight. So, it would never make it to our Solar System. Coupled with the fact that astronomers have never seen a hydrogen iceberg.

Another analogy is that ʻOumuamua could be a “plastic bottle” (an artificial object) amongst “natural seashells” (asteroids and comets). The chance of finding the “plastic bottle” simply reflects the number of plastic bottles per unit area around the seashore. Similarly, in space and nothing to do with the Drake equation, it depends on how much “trash” there is in interstellar space by other civilisations. Space junk, because most of it is billions of years old and will have stopped working. The two voyager probes (and others) were sent fully functional into space, but they will stop working. After a billion years they too will become space junk.

The Drake equation is a probabilistic argument used to estimate the number of active, communicative extra-terrestrial civilizations in the Milky Way galaxy.

Astronomers could search for items that were just thrown out.

It turns out there was another object, discovered a few months ago. It was bound to the Sun, in an orbit similar to the Earth, but it also exhibited an extra push away from the Sun as a reflection of sunlight and it didn’t have a cometary tail. It was given the name 2020 SO.


The orbit of 2020 SO around Earth and Sun from Nov. 2020 to Mar. 2021

2020 SO is a near-Earth object identified to be the Surveyor 2 Centaur rocket booster launched on 20 September 1966 to go to the Moon (Lunar Lander surveyor 2). The object was discovered (as an unusual “asteroid”) by the Pan-STARRS1 survey at the Haleakala Observatory on 17 September 2020. The object was initially suspected to be an artificial object due to its low velocity relative to Earth and later on the noticeable effects of solar radiation pressure on its orbit. Spectroscopic observations by NASA’s Infrared Telescope Facility in December 2020 found that the object’s spectrum is similar to that of stainless steel, confirming the object’s artificial nature.

Surveyor 2 was to be the second lunar lander in the uncrewed American Surveyor program to explore the Moon. It was launched September 20, 1966 from Cape Kennedy, Florida aboard an Atlas-Centaur rocket. A mid-course correction failure resulted in the spacecraft losing control. Contact was lost with the spacecraft at 9:35 UTC, September 22.

The rocket booster was a hollow structure, with very thin walls and that is why it exhibited a push away from the Sun as a result of reflecting sunlight. An object’s orbit exhibits an extra push by solar radiation pressure without a cometary tail.

So, this illustrates that we can tell the difference between a rock and an artificial object. In this case we know it is artificial because we produced it.

In the case of ʻOumuamua we don’t know who produced it.

Now the second interstellar object that was reported was found by a Russian amateur astronomer, Gennadiy Borisov.


Gennadiy Vladimirovich Borisov (is a Crimean telescope maker and amateur astronomer who discovered the first known interstellar comet, 2I/Borisov.

2I/Borisov, originally designated C/2019 Q4 (Borisov), is the first observed rogue comet and the second observed interstellar interloper after ʻOumuamua. It was discovered by Gennadiy Borisov on 30 August 2019 UTC (29 August local time.) Observed close to the Sun

2I/Borisov has a heliocentric orbital eccentricity of 3.36 and is not bound to the Sun. The comet passed through the ecliptic of the Solar System at the end of October 2019, and made its closest approach to the Sun at just over 2 AU on 8 December 2019. The comet passed closest to Earth on 28 December 2019.[15] In November 2019, astronomers from Yale University said that the comet’s tail was 14 times the size of Earth, and stated, “It’s humbling to realize how small Earth is next to this visitor from another solar system.” In the middle of March, 2020, the comet was observed to fragment; and later, in April, even more evidence of fragmentation was reported.


It was a regular comet with a cometary tail. Professor Loeb was happy that this interstellar object was a comet, but ʻOumuamua didn’t look like a comet.


Above left: Trajectory of Borisov (yellow) crossing the ecliptic plane; ‘Oumuamua (red) shown for comparison. Above centre: Borisov approaches the ecliptic plane between the orbits of Jupiter (pink) and Mars (orange). Above right: Borisov’s trajectory and position (white) as of 13 October 2019 (top view).

So, the fundamental question is, is ʻOumuamua a natural or artificial object. There is a very simple way to find out. Get a camera close enough to take a photograph. They say a picture is worth a thousand words and Professor Loeb wouldn’t have had to write his book.


Artist’s impression of two possible shapes for ʻOumuamua. The object’s length is estimated to be between tens to hundreds of metres, up to the size of a football field. It is either an oblong, cigar-shaped rock – as depicted in the upper left image (Credit: ESO/M.Kornmesser), or a flattened, pancake-shaped object, as shown in the upper right image (Credit: Mark Garlick). The pancake shape provides the best fit to ʻOumuamua’s light curve at 90% confidence and represents the highest (and most likely) excitation energy for a tumbling object. Even a razor-thin object, like a flat sheet of paper, would appear to possess some width wen projected at a random orientation on the sky, so the intrinsic aspect ratio of ʻOumuamua can be much smaller than the value of 1:10 inferred from its light curve.

So, the lesson is that all other interstellar objects that look as weird as ʻOumuamua, that will be discovered in the future, need to be followed up by a camera that takes up close-up photos of them. So that we can tell if they are “plastic bottles” or natural rocks. It is a new way to search for extra-terrestrial civilisations and it is different from the traditional way of looking for radio signals. Radio signals are just like speaking on the phone. You can only have a conversation with someone who is alive. But another way of communication is through the mail. You can receive, in principle, a letter from someone who is dead. That is an advantage because we can find evidence for civilisations that are dead, not around culturally (this goes for our own history as well as extra-terrestrial history), within the Milky Way. If those civilisations deposit space trash then we can find it. If we can’t have a conversation over the “phone” with them we can look for their relics (on Earth this would be an archaeological dig), particularly the ones that end up on our “doorstep” from outside the solar system. Space archaeology.

To keep things in perspective


Timeline of the history of the Universe. Our Solar System formed relatively late, only 4.6 billion years ago. Modern technology appeared only appeared over the last century, namely 0.0000001 billion years ago. Many civilisations could have appeared and disappeared before we developed our modern telescope technologies to detect them. Even on Earth we have had civilisations appear, only to disappear.

Stars would have been born and died before our Sun came into existence. If these stars were similar to ours and they had an Earth-like planet with technologically able living things. Then these living things will have probably deposited things into space. If we find these things, we can learn from them.

The habitable zone distance changes as a function of stellar temperature (vertical axis) and the amount of energy from the star that hits the planet (horizontal axis). This plot shows the limits for both the “conservative habitable zone,” which are based on one-dimensional climate model calculations (Kopparapu et al., 2013), and for the “optimistic habitable zone,” which are based on observations that Mars once had liquid water at the surface and Venus used to have more water, possibly contained in oceans. (Credit: Chester “Sonny” Harman, using planet images published by the Planetary Habitability Lab at Aricebo, NASA, and JPL)


The habitable zone constitutes the region around a star where liquid water may exist on the surface of a rocky planet, like the Earth, and allow the chemistry of life-as-we-know-it. Water oceans would boil off if a planet is too close to a star and would freeze to solid iced if it is too far. The diagram shows the habitable zone boundaries around stars with different surface temperatures (vertical axis), ranging from the most abundant dwarf stars, like Proxima Centauri, to rare gas giant stars, like Eta Carinae. The horizontal axis shows the flux of light shining on the planet’s surface relative to sunlight on Earth. Various known planets are labelled in the diagram. The nearest habitable planet, Proxima b, appears near the bottom right.

Different sized stars and stars with different temperatures will have different sozed habitable zones. These zones are just the right distance from the star to have similar temperatures to Earth and therefore liquid water.

If such habitable planets have technological civilisations we could search for their signatures (aliens be afraid, be very afraid. Humans are coming).

For example, instead of just searching for oxygen as the marker of primitive life we could search for industrial pollution. Complex molecules like CFCs from industry and refrigerating systems. We could potentially see these molecules when a planet transits a star. We could also search for beams of light that are used for propulsion of light sails, because there would be some leakage of the light around the sail, and when that beam sweeps across our sky it will appear as a flash of light. So we could search for flashes of light that look unusual. We could search for structures around stars or planets. A swarm of satellites.

Space Exploration Technologies Corp. (SpaceX) is an American aerospace manufacturer and space transportation services company headquartered in Hawthorne, California.

SpaceX is planning of launching 4,425 satellites capable of beaming the Internet to the entire globe (astronomers are not happy).

We could look for evidence of photovoltaic cells and artificial light on he dark sides of planets.


Examples of artificial structures from alien intelligence around stars and planets. They could include Dyson spheres – a hypothetical megastructure constructed around a star to harvest its light; an infrastructure for the transfer of cargos with laser-pushed light sails in an Earth-Mars type planetary system; a swarm of communication satellites; industrial pollution of planetary atmospheres; coating the permanent dayside of a tidally-locked planet, like Proxima b, with photovoltaic cells to generate light and electricity for the permanent nightside; giant radio transmitters for communication.

A Dyson sphere is a hypothetical megastructure that completely encompasses a star and captures a large percentage of its power output. The concept is a thought experiment that attempts to explain how a spacefaring civilization would meet its energy requirements once those requirements exceed what can be generated from the home planet’s resources alone. Only a tiny fraction of a star’s energy emissions reaches the surface of any orbiting planet. Building structures encircling a star would enable a civilization to harvest far more energy.


Above left: A conventional crystalline silicon solar cell (as of 2005). Electrical contacts made from busbars (the larger silver-coloured strips) and fingers (the smaller ones) are printed on the silicon wafer. Above right: Symbol of a Photovoltaic cell.

A solar cell, or photovoltaic cell, is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon. It is a form of photoelectric cell, defined as a device whose electrical characteristics, such as current, voltage, or resistance, vary when exposed to light. Individual solar cell devices are often the electrical building blocks of photovoltaic modules, known colloquially as solar panels. The common single junction silicon solar cell can produce a maximum open-circuit voltage of approximately 0.5 to 0.6 volts.

Decisions have to be made to look for these signs. If we don’t look, we will never find anything.

Physicists have been searching for dark matter, the greatest constituent of the Universe. We don’t know what it is but a lot of money has been set aside for its discovery.

If the same amount of money had been allocated to search for technological signatures of other civilisations then we might have found them.

In principle, one can launch a light sail close to the speed of light without a laser beam. By using a natural source of light, like an exploding star (a supernova). If you park light sails about 100 times the Earth-Sun separation around a massive star that is about to explode then you can wait and hover at that distance just like surfers wait for a giant wave, except in this situation you are waiting for a wave of light, a flash of light. Once the star explodes the flash of light will carry the sails close to the speed of light, moving through the Milky Way galaxy very quickly, like dandelion seeds carried by the wind.


Artificial “dandelion seeds”: Light sails hovering at 100AU can reach a fraction of the speed of light after the explosion.

The Crab Nebula is the remnant of a supernova explosion observed on Earth in 1054 from a distance of about 6,000 light years. The remnant is a neutron star, the Crab Pulsar, near its centre that spins 30 times every second and pulsates like a lighthouse. (Credit: ESO)

The Crab Nebula (catalogue designations M1, NGC 1952, Taurus A) is a supernova remnant and pulsar wind nebula in the constellation of Taurus.

A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich.

Neutron stars have a radius on the order of 10 kilometres and a mass of about 1.4 solar masses. They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past white dwarf star density to that of atomic nuclei.

The Crab Pulsar (PSR B0531+21) is a relatively young neutron star. The star is the central star in the Crab Nebula, a remnant of the supernova SN 1054, which was widely observed on Earth in the year 1054. Discovered in 1968, the pulsar was the first to be connected with a supernova remnant.

The Milky Way is the galaxy that contains our Solar System, with the name describing the galaxy’s appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye.


The Milky Way arching at a high inclination across the night sky, (this composited panorama was taken at Paranal Observatory in northern Chile), the bright object is Jupiter in the constellation Sagittarius, and the Magellanic Clouds can be seen on the left; galactic north is downward


Diagram of the Sun’s location in the Milky Way, the angles represent longitudes in the galactic coordinate system.

Fermi’s Paradox: Enrico Fermi once asked “Where is everybody?” If there are extra-terrestrial civilisations why don’t we see them.

Well, we might not be interesting enough for them to visit us.

They might have reached a technological level that caused them to destroy themselves.

Perhaps there is social distancing on a cosmic scale. We might infect or negatively influence their quality of life (which wouldn’t surprise me in the least).

However, they can’t completely hide away. According to the second law of thermodynamics they have to deposit “trash”.

Physicists could find out about these civilisations by examining their trash.


Enrico Fermi (29 September 1901 – 28 November 1954) was an Italian (later naturalised American) physicist and the creator of the world’s first nuclear reactor, the Chicago Pile-1. He has been called the “architect of the nuclear age” and the “architect of the atomic bomb”. He was one of very few physicists to excel in both theoretical physics and experimental physics. Fermi was awarded the 1938 Nobel Prize in Physics for his work on induced radioactivity by neutron bombardment and for the discovery of transuranium elements. With his colleagues, Fermi filed several patents related to the use of nuclear power, all of which were taken over by the US government. He made significant contributions to the development of statistical mechanics, quantum theory, and nuclear and particle physics.

The second law of thermodynamics means hot things always cool unless you do something to stop them. It expresses a fundamental and simple truth about the universe: that disorder, characterised as a quantity known as entropy, always increases.

The second law of thermodynamics is perhaps the most profound of the three laws of thermodynamics. Its importance is best expressed by sketching out a situation which violates it. Imagine placing 20 coins, heads up, on a tray, filming it as you give it a shake and then playing the film backwards. The coins start out as a jumbled mess, but all jump and eventually come to rest with the same side up – an unreal, slightly creepy sequence. Similarly imagine an egg yolk and white reassembling themselves after you’ve cracked it open, or even a world where it’s just as easy to pair up your socks in the right pairs as it is to jumble them up.

Entropy increase is so universal that many physicists propose it is why we see time flowing. It is certainly why our hearts must constantly pump blood, supplying our cells with energy as a temporary stay against the inevitable onset of decay and disorder.

The most likely reason why we can’t find any living aliens is because their star was born before ours and they will have died out. One reason for this is that their technology caused their self-destruction. We could use space archaeology to look for artifacts of dead technological civilisations on other planets, such as mega-structures, polluted atmospheres and relics flying through the Solar System. This could tell us why they are not around anymore.

Professor Loeb’s hope is that by finding relics of civilisations who perished as a result of climate change or ward would inspire us to get our act together and avoid a similar fate.

The search for life elsewhere goes on.


Mars 2020 is a Mars rover mission by NASA’s Mars Exploration Program that includes the Perseverance rover and the Ingenuity helicopter drone. It was launched on 30 July 2020 at 11:50 UTC, and will touch down in Jezero crater on Mars on 18 February 2021.

The mission will be looking for traces of life that existed in the past, or may still exist now.

Questions and answers

1) If the planet we found with life had technology that was 11 million years ahead of us why wouldn’t they have found us already instead?

Well, I don’t think we are that interesting. Where not intelligent enough to attract attention.

2) Would it be possible to send a probe to take a picture of ‘Oumuamua or is it impossible to catch up now?

It wasn’t possible from the beginning because it was moving faster than all the rockets that we can launch, but right now it is already a million times fainter than it was when close to the Sun. We don’t know exactly where it is. If we wanted to chase it even if we had a spacecraft that can reach that speed, we would need to equip it a telescope.

We shouldn’t really obsess about this object but there are bound to be more of them out there. We should just search for more of the same. We could deploy cameras around the Earth so that the next interstellar object that comes along could be photographed.

3) What is the most likely source of ‘Oumuamua and have we pointed a telescope at it to look for signatures of life?

We don’t know where it came from because it was in the Local Standard of Rest and it didn’t move like any other object and moreover if it had come from another planetary system there are so many different directions that it could have travelled along. It could have been travelling for billions of years which would make it to difficult to figure out where it came from unless of course we take a photograph or we land on it and find a statement about where it came from. If it is artificial it is quite possible it will have a label.


The items we humans launch into space have labels on them (I don’t know if Blackpool rock has gone into space). Voyager 1 went into space with a very special “label”.

4) If ‘Oumuamua is a designed probe has it given you new ideas for a potential design for an interstellar vehicle that we could send from Earth?

Well, if it is a light sail then it encourages us to produce light sails but the problem is that we didn’t have enough information about this object to eb able to tell what kind of technology it included.

The more evidence we have the better so in the future we should get as much data as possible about the next interstellar object that looks like ‘Oumuamua. Looks weird like a “plastic bottle on a beach”. Out of curiosity we should find out as much as possible about it. Rather than just saying “everything is just rocks”

5) Given that the speed of light has a maximum speed and that radio waves can be chaotic in the grand scheme of things have you thought about exotic means of communication that aliens might be using such as particle entanglement or gravitational waves?

Quantum entanglement is a physical phenomenon that occurs when a pair or group of particles is generated, interact, or share spatial proximity in a way such that the quantum state of each particle of the pair or group cannot be described independently of the state of the others, including when the particles are separated by a large distance. The topic of quantum entanglement is at the heart of the disparity between classical and quantum physics: entanglement is a primary feature of quantum mechanics lacking in classical mechanics.

Things are entangled when information about one improves our knowledge of the other.

Gravitational waves are disturbances in the curvature of spacetime, generated by accelerated masses, that propagate as waves outward from their source at the speed of light.

As a result of ‘Oumuamua I changed my view about how best to search for other civilisations. The community as a whole, for 70 years, was looking for radio signals, laser signals. The problem is that when you have a phone conversation the other person needs to be alive. For instance, you couldn’t have a

conversation with the Minoan culture because they don’t exist anymore. So, we use archaeology to find the relics and I think the discovery of ‘Oumuamua, and potentially other relics in the future opens up a completely new approach which is far better because you can “communicate” with civilisations which are dead by now. You can find evidence of the kind of technologies they use, how they lived, finding “messages in a bottle” and I think that would be a very affective way as long as deploy cameras and find ways to land on these objects or wait for the ones that collide with the Earth and appear as interstellar meteors. So, we can track the speed of meteors when they enter the Earth’s atmosphere and we can tell if one of them comes from outside the Solar System and we can find out more if it leaves a relic on the ground. If it is bigger than the size of a person it won’t burn up completely when travelling through the atmosphere. Then we can put our “hands” on it. So far nobody has attempted to do that. We see meteorites but nobody really goes in search of them. They sort of get discovered by accident (unless they do a lot of damage) so we don’t know if any of them are interstellar meteors.

The Minoan civilization was a Bronze Age Aegean civilization on the island of Crete and other Aegean Islands, flourishing from c. 3000 BC to c. 1450 BC and, after a late period of decline, finally ending around 1100 BC. It represents the first advanced civilization in Europe, leaving behind massive building complexes, tools, artwork, writing systems, and a massive network of trade.

6) If the recent discoveries on Venus turn out to be caused by living things, How, likely do you think that it represents a single genesis that is the same as life on Earth versus an independent beginning of life.

On Earth, the toxic gas is produced by microbial life. Could the same be true on Venus? Now, the debate begins.

Phosphine (IUPAC name: phosphane) is a colourless, flammable, very toxic gas compound with the chemical formula PH3, classed as a pnictogen hydride.

Well in principle we can find out by sending a probe that will scoop some material up the cloud deck (the upper surface of a cloud) of Venus and check what kind of life is present. I should say that since the original announcement, made on the 14th September 2020, some other scientists have claimed that the significance of the results is lower and the original discovery team have reduced the significance of their findings. Also, some other people claim it is a different molecule, SO2, which is producing the observed signature. So, there is still a back-and-forth discussion of what it all means. But sometime in the future there may be better data and we will infer that it is phosphine, but not necessarily coming from microbes. The only way to find out is to send a mission to scoop some of the cloud and find whether there are microbes in it, inside the droplets. Once the scooping is done, we can see if there is any DNA and whether it is similar to that found on Earth. And it is possible to transfer life between Venus and Earth through rocks. Rocks that graze the atmosphere of the Earth can scoop biological material from the lower atmosphere on Earth and then deliver it all the way to Venus. There is an exchange of material between Earth and Venus and Mars that could have transferred life. We could all be Martians. Life could have started on Mars first. The Mars rover Perseverance could find out.


7) Why do you think that so many missions to find evidence of extra-terrestrial life have been so conservative in their aims. Why search for the conditions that support life rather than extra-terrestrial life itself?

I think that many scientists are trying to protect their image by never getting too entangled in a controversial discovery or discussion. So, they avoid risk and

as a result, they do not innovate, and in my opinion (Professor Loeb) they are boring because they are not willing to put any “skin in the game” (take risks). The whole fun of doing science is to push forward progress, push forward the frontier of knowledge and to take some risk. You make some predictions and then you test everything and you gain evidence. Evidence is really the guiding light and science is based on evidence not about showing you are smart. If you want to show you are smart you would avoid making predictions for things that can be falsified. You would work on things like string theory that cannot be proved experimentally, because then you can show that you are mathematically smart and skilful. You can get honours and awards and people would admire you but you would not predict what nature is like, and that is not physics. Physics is about taking risks. Einstein was wrong three times in the last decade of his career despite being considered the most experienced physicist. He said there were no black holes in the Universe, he wasn’t sure about gravitational waves and quantum mechanics doesn’t have “spooky action at a distance”. The reason for this was that the information wasn’t clear at the time


Albert Einstein (14 March 1879 – 18 April 1955) was a German-born theoretical physicist who developed the theory of relativity, one of the two pillars of modern physics (alongside quantum mechanics).

All his mistakes were corrected by experiments later on, but it shows that if you are working at the frontier sometimes things are not clear and you will make mistakes. This is completely legitimate. It’s part of the learning experience. Scientists should be more like children because science is about wondering, figuring out. It’s not about boosting the image or getting awards. Who cares about that (a lot of people I would think, comment by Helen)? Let’s figure out what environment we live in. Who lives in our neighbourhood? Trying to know enriches your life and it can be a lot of fun. It gives a lot of pleasure in life

8) Where do you think the first definitive alien life will be found?

It could well be with the Mars rover, Perseverance. Something could be found on the surface of Mars. We know that Mars had an atmosphere early on and likely had liquid water on the surface and I wouldn’t be surprised that Perseverance finds life in the coming months. Either there will have been life in the past and evidence of it will be found or perhaps something that is still living. To be honest I am more interested in the “smartest kid on the block”. I don’t care if that “kid” isn’t around anymore. I just want to know that there was a “smartest kid on the block”. I’m almost sure that we are not the smartest, but perhaps we can learn something from whoever existed around us.


Nasa Mars Rover published August 7, 2012 by Oliver Schopf


Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury. In English, Mars carries the name of the Roman god of war and is often referred to as the “Red Planet”. The latter refers to the effect of the iron oxide prevalent on Mars’s surface, which gives it a reddish appearance distinctive among the astronomical bodies visible to the naked eye. Mars is a terrestrial planet with a thin atmosphere, with surface features reminiscent of the impact craters of the Moon and the valleys, deserts and polar ice caps of Earth.

9) Would a civilisation, say within 100 light years from us, with the same technology as us, be able to detect us?

Definitely. My advice, in general, is if you enter a room full of strangers you better keep quiet and listen, because you never know what the risks are. But we have been speaking loudly. We have been transmitting radio signals for a hundred years and there is a bubble around us, such that anyone with radio telescopes similar to ours can detect signals. So, we might hear back. It will take decades to hear back, but we should be checking to see if any signals are coming towards us, from a nearby star that says something about them coming to get us!

10) Can we assume that an alien civilisation would be benign or can we not?

All we can do is imagine experiences based on human history and if you look at European history. Europeans making the first trips to the Americas. It’s quite frightening.

But we don’t know about the aliens. We need to find out. Let’s listen and not speak for a while.

11) What is the primary purpose for looking for extra-terrestrial life and extra-terrestrial intelligence?

As far as I’m concerned, the biggest benefit from it would be to bring modesty to the human race. Humans tend to be attracted to power. They are arrogant and have feelings of superiority relative to each other. Once you see something that is more advanced than you it will bring a sense of modesty just like when children first go to nursery. This would certainly benefit a lot of politicians.

It will change our aspirations for the future and could affect religious beliefs. There would be huge impacts on society.

This is the biggest scientific question that we face in terms of our long-term future and I very much hope that in my life-time that young people find the answer.

Helen’s views

I hope that Professor Loeb doesn’t mind me making a few comments here.

My main view is that we should be investigating our past civilisations rather than going to look for extra-terrestrial ones. There is a saying that if you don’t learn from the past you are doomed to repeat it.

He may think that experiments trying to find dark matter might be a waste of time but we can gain things from the pursuit. It took a long time to find the Higgs Boson and we had to build powerful colliders to do it, but a result of this work is the world wide web and better medical treatments (and many other uses besides).

I have a physics degree but I am also a practising Catholic (although I’m a bit naughty and don’t go to church as often as I should) and my faith gives me a sense of modesty. However, I am sure that even non-believing physicists, even if the wider population consider them geniuses, (by the way I am not a genius, although if I were given three wishes one of them would be to make me a genius) have crises in their own abilities. It is natural for the majority of humans to present a front to those around them, but it is also natural to focus on the failures.

We rely on theoretical physicists to come up with new ideas. They are often responsible for whole new experiments that weren’t thought of before. As mentioned above the Higgs boson took a long time to be found and sadly some of the theoreticians never lived to see it. String theory may turn out to be pointless but at the moment we have very few ideas to link gravity with the standard model. The link with the small quantum universe and the large relativity universe still needs to be found.

Physics beyond the Standard Model (BSM) refers to the theoretical developments needed to explain the deficiencies of the Standard Model, such as the inability to explain the fundamental parameters of the standard model, the strong CP problem, neutrino oscillations, matter–antimatter asymmetry, and the nature of dark matter and dark energy. Another problem lies within the mathematical framework of the Standard Model itself: the Standard Model is inconsistent with that of general relativity, and one or both theories break down under certain conditions, such as spacetime singularities like the Big Bang and black hole event horizons.

Theories that lie beyond the Standard Model include various extensions of the standard model through supersymmetry, such as the Minimal Supersymmetric Standard Model (MSSM) and Next-to-Minimal Supersymmetric Standard Model (NMSSM), and entirely novel explanations, such as string theory, M-theory, and extra dimensions. As these theories tend to reproduce the entirety of current phenomena, the question of which theory is the right one, or at least the “best step” towards a Theory of Everything, can only be settled via experiments, and is one of the most active areas of research in both theoretical and experimental physics.

Now I suppose that Professor Loeb could argue that an alien civilisation may have already figured this out, but I am sure that humans are quite capable of solving it themselves at some point in the future and they will be a lot happier doing it. It will be a sense of achievement.

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