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How alien astronomers could find earth: we could make it easier for our counterparts on distant planets to find evidence of our existence. But it would be a massive undertaking for us.

On Valentine's Day in 1990, before closing its robotic eyes forever, Voyager 1 turned them back to its planet of origin. Though we have seen pictures of Earth from space before, most notably from the Apollo missions, Voyager 1's portrait provided the first glimpse of how a distant observer might see Earth--as a pale blue dot nearly lost in the inky darkness of the cosmic ocean.

Voyager 1 knew where to look. Could another civilization, perhaps also attempting to find the first signs of life elsewhere in the universe, find Earth? If so, would they just detect the planet, or might they infer humanity's existence?

We can divide these questions into two parts. The first is how much Earth emits naturally, which would allow another civilization to detect the planet itself. The second part is how much radiation our civilization produces or could produce, which would allow them to infer our existence.

Natural Emissions

The Sun and Earth emit the bulk of their natural radiation at visible and infrared wavelengths. The Sun's light output peaks in the green-yellow part of the spectrum. Earth reflects a portion of this visible radiation, as anybody who has seen earthshine on the Moon can attest. Even today, astronomers observe earthshine in an effort to understand how we might someday study the reflected light of extrasolar planets.

The amount of light that Earth reflects varies dramatically, depending upon a variety of factors. These include whether one is viewing primarily the Northern or Southern Hemisphere (the Northern Hemisphere has more land; the Southern Hemisphere has more water), the amount of cloud cover, and the amount of snow on the ground.

Nonetheless, a remote observer attempting to find Earth has an incredibly difficult job. Consider an astronomer only 10 light-years away, essentially on our cosmic doorstep. At this distance, the Sun and Earth are separated by no more than 0.1 arcsecond (about 30 millionths of a degree). Even though Earth is a fairly good reflector, reflecting about one-third of the Sun's incident light, the Sun is extremely bright. At visible wavelengths, the Sun is about 10 billion times (about 25 magnitudes) brighter than Earth.

The situation improves somewhat if we switch to infrared wavelengths. With a temperature of about 15[degrees]C (290 K), Earth's light output peaks in the infrared, while the Sun is not as bright as it is in the visible. The result is that Earth is "only" 10 million times (about 18 magnitudes) fainter than the Sun.

Astronomers are currently studying advanced space telescope concepts that would block or cancel out a star's light, which would allow us to detect any orbiting Earth-like planets. If civilizations only slightly more advanced than ours exist around nearby stars, they may have already identified at least some of the Sun's planets, including Earth.

Even if a nearby civilization has detected Earth, that doesn't necessarily mean they know we exist. For example, Venus, with its thick clouds, is an extremely efficient reflector of sunlight, so another civilization might find it. But Venus's surface is inhospitable to life as we know it. Today's most ambitious telescope concepts aim not only to detect Earth-like planets around nearby stars, but to determine their atmospheric gases. Because of life's effects on Earth, the atmosphere has far more oxygen and far less carbon dioxide than either Venus or Mars. Indeed, oxygen is such a chemically reactive element that if all life on Earth were suddenly to disappear, most of the atmospheric oxygen would be removed by various processes within about 300 years. Thus, a nearby Earth-like planet with atmospheric oxygen would be a telltale indicator of a life-bearing world.

Measuring the chemical composition of an exoplanet's atmosphere is not as far-fetched as it might seem. Even with today's telescopes, astronomers are beginning to probe the atmospheres of some giant planets that transit in front of their host star. The advanced telescope designs being studied today could do the same for nearby Earth-size planets in habitable zones, even if they don't transit their stars.

Optical Transmissions

If another civilization has measured Earth's atmospheric composition, it still couldn't determine if the life was in the form of plants, dinosaurs, the Roman Empire, or today's jet-setting crowd. How could another civilization detect us?

Anyone who has ever seen a picture of Earth at night might wonder if they could detect our light pollution. Our stray light is obnoxious to amateur astronomers and wildlife, but it pales in comparison to the amount of light that Earth reflects from the Sun. Simply put, our host star is so bright that in order to be detectable, our civilization has to compete with the Sun. A civilization with an extremely large and advanced optical telescope might be able to detect our city lights, but if our streetlights aren't up to snuff, there's a way we could compete with the Sun and do so in a way to make it completely obvious that an intelligent civilization is signaling.

If we observe a relatively nearby Sun-like star through a modest amateur telescope, we receive about 1 billion photons every 10 seconds. Observed for a much shorter interval, say, 1 billionth of a second, we probably would not receive a single photon from the star. If we could produce, on average, more photons in a short interval than the Sun does, it would signal a distant civilization that we're here.

We have built lasers, such as the National Ignition Facility's Petawatt Laser (located at the Lawrence Livermore National Laboratory in California) and the University of Texas at Austin's Texas Petawatt Laser, that for brief instants produce extremely energetic pulses of light. If we used one of the 10-meter Keck telescopes in Hawaii not to receive starlight but to transmit the light from a petawatt laser, over interstellar distances the flash would greatly outshine the Sun for a brief interval of time.

None of these petawatt lasers is currently hooked up to a large telescope. But knowing that we could construct such a system, professional and amateur astronomers have conducted modest searches for analogous signals from other civilizations. Known as optical SETI, these systems typically use relatively small telescopes equipped with ultra-fast detectors (S&T: November 2010, page 22).

High-Energy Transmissions

The Sun dims rapidly at wavelengths increasingly shorter than visible light. It produces enough ultraviolet (UV) light to cause a nasty sunburn, and its magnetic fields interact to produce X-rays and gamma rays, but the bulk of the Sun's energy output is at visible wavelengths. Is Earth brighter than the Sun at high energies, or could we make it so?

Thunderstorms can generate small amounts of X-rays and gamma rays, but not in sufficient quantities to detect over interstellar distances. Fortunately, our civilization doesn't generate much X- or gamma-ray emission either, because these energetic photons damage our cells. Moreover, Earth's atmosphere is opaque to X- and gamma rays, meaning that any attempt to send or receive signals must be done in space. Interestingly, gamma rays share an important attribute with radio waves: These are the only two kinds of electromagnetic radiation to which our galaxy is essentially transparent. Other wavelengths of light suffer varying amounts of absorption from the giant clouds of gas and dust in the Milky Way's disk.

A straightforward means of generating an X- and gamma-ray signal is to detonate a nuclear device in space. Most of a nuclear explosion's energy is initially emitted as X- and gamma rays; it is only upon interaction with Earth's atmosphere and surface that a nuclear explosion assumes the standard mushroom-cloud shape.

Our civilization no longer produces such signals because the 1967 Outer Space Treaty prohibits the detonation of nuclear devices in space. Nonetheless, suppose we decided to try to broadcast our existence via X- and gamma rays, perhaps by detonating nuclear devices on the far side of the Moon. NASA's Fermi Gamma-ray Space Telescope is currently detecting gamma-ray bursts (GRBs), brief flashes of gamma rays that originate in the destruction of distant stars. U.S. Department of Defense Vela satellites discovered GRBs in the 1960s while monitoring Earth and the immediate space environment for clandestine Soviet nuclear tests. Could we use nuclear explosions to signal a nearby civilization that has a Fermi-like telescope?

An obvious tactic would be to produce an explosion similar to but clearly different from GRBs. We could explode more than one device from the same location (a nuclear Morse code), which would clearly not be expected from natural GRBs. But the X- and gamma-ray signals from individual nuclear explosions are so weak at the distance of the nearest stars that the number of nuclear devices we would need to explode vastly exceeds the largest stockpiles maintained during the Cold War.

Non-Photonic Communication

What about other means of interstellar signaling and communication? All light is transmitted via photons, the carriers of the electromagnetic force--one of the four fundamental forces. Alternative communication methods must rely on nonphotonic (non-electromagnetic) means.

We can dismiss one of the other four forces quickly. The gravitational force is transmitted via gravitational waves, and since gravity is the weakest force, these waves are extremely difficult to detect. Astronomers can indirectly detect gravitational waves from binary systems consisting of two neutron stars (S&T: August 2010, page 28). Ongoing upgrades to the Laser Interferometer Gravitational-Wave Observatory (LIGO) may advance to the point that we will directly detect gravitational waves from the collisions of neutron stars and black holes, thereby opening up an entirely new way of viewing the universe. Although the laws of physics do not prevent a sufficiently advanced civilization from using gravitational waves for communication, it will be decades before we could hope to listen in on such communications, to say nothing of taking part.

The other possibilities involve the strong and the weak nuclear forces. The strong force binds an atomic nucleus together. One could imagine sending atoms as a means of communication. In a sense, we have already done this, as NASA's Voyager 1 and 2 and Pioneer 10 and 11 spacecraft head out of the solar system. But traveling at only about 0.005% of the speed of light, it will take millennia for these spacecraft to cover the distance to even the nearest star.

Alternatively, we could shoot particle beams toward stars at nearly the speed of light, such as those being created at the Large Hadron Collider (LHC) in Europe. We have instruments looking for naturally occurring high-energy particles from space (cosmic rays). A charged particle beam pulsed in some non-random fashion would be a clear signal of our existence.

A significant problem in using beams of protons or electrons for communication is that these charged particles are deflected by magnetic fields. The amount by which a proton is bent from a straight-line path by a magnetic field depends upon both the strength of the magnetic field and its orientation. Unfortunately, we know too little about the interstellar magnetic fields near the Sun to accurately "aim" a proton or electron beam toward any nearby star. If we built a space-based LHC (where Earth's atmosphere wouldn't interfere), we wouldn't know which (if any) stars would be in the beam's path. And even if we decided that we don't need to know where recipients might be, a space-based LHC lies many years in the future.

We could also transmit a beam of neutral particles. Such a beam could be aimed at a star, and it would travel undeflected by magnetic fields. One candidate would be the neutron. Curiously, the neutron is an unstable particle in isolation. The half-life of a bare neutron is only 11 minutes. Eleven minutes after launch, half the neutrons in the beam would have decayed to other particles, 22 minutes later, only 1/4 of the original number of neutrons would remain, 33 minutes later, only Vs, and so forth. Recalling Einstein's special theory of relativity, however, 11 minutes to an observer riding with the neutron beam need not be 11 minutes to those of us back on Earth. We would want the beam to travel relativistically anyway.

Could we produce a neutron beam so that 11 minutes in the neutron beam's frame of reference would be 4 years in our frame of reference, thereby allowing about half of the neutrons to make it to the nearest star? Unfortunately, no. Scientists would have to accelerate neutrons to energies comparable to particles accelerated near the surfaces of neutron stars, which is at least 1,000 times beyond the LHC's capabilities.

The final force is the weak interaction, which governs radioactive decay. We have already detected weakly interacting particles from the cosmos in the form of neutrinos from the Sun and Supernova 1987A. Large neutrino detectors have been built and others are under construction for conducting neutrino astrophysics, and our particle accelerators can produce neutrino beams. Thus, another civilization at a comparable level of development might also have neutrino detectors.

Neutrino communication has peculiar advantages and disadvantages. Because the weak force is indeed weak, neutrinos barely interact with other forms of matter. A neutrino beam can travel through Earth with almost no neutrinos being absorbed or deflected. But the fact that these ethereal particles can pass through an entire planet with incredible ease also makes reception extremely difficult. Our largest neutrino detectors are immense, approaching 1 cubic kilometer in volume.

A European team recently reported that some neutrinos might travel faster than light. Even in the unlikely event that this result is confirmed, it's not obvious it helps us signal other civilizations because the difference between the apparent neutrino speed and the speed of light from the experiment was only about 0.0025%.

Unfortunately, producing a sufficiently luminous neutrino beam lies well beyond our current capability. Fewer than two-dozen neutrinos were detected from Supernova 1987A in the Large Magellanic Cloud. Suppose that we wanted to communicate with a civilization much closer, say, 160 light-years instead of the 160,000 light-years to the LMC. Moving SN 1987A this much closer would result in it being 1 million times more neutrino luminous, but a supernova also involves the conversion of about 10% of a star's mass to energy. Producing enough neutrinos so that a civilization about 160 light-years distant would still detect about two-dozen neutrinos would require liberating the energy stored in a body about the size of Mars. Clearly, effective neutrino transmission will remain beyond our capabilities for a bit longer.

Limited Capabilities

Could "They" see us now? Certainly we are developing the technology to see other planets around nearby stars. If technological civilizations exist around nearby stars, they may have already observed (and named) Earth. Depending upon their level of technology, they may have even determined the approximate composition of Earth's atmosphere and deduced that some kind of life is present.

As we have seen, detecting our civilization is difficult, or, equivalently, our capability to signal other civilizations remains fairly limited. In the few cases for which we have the technology, we have not chosen to deploy it in a way to signal or communicate with others. For now, any nearby civilizations might also be trying to figure out if they are alone in the universe.

Should we try to signal other civilizations

Stephen Hawking recently remarked, "If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the native Americans." True, but if extraterrestrials have the incredibly advanced technology to traverse interstellar distances, they surely have space telescopes that would have identified Earth as a life-bearing world long before the dawn of human civilization. If extraterrestrials suddenly appear on our doorsteps, it will almost certainly be because they wanted to explore a life-bearing world up close, not because they heard from us.

RELATED ARTICLE: Could aliens listen to our radio or watch our TV?

HOW BRIGHT is our civilization at radio wavelengths? Consider the Arecibo planetary radar system. It has led to a number of discoveries, such as water ice on the poles of the Moon and Mercury and improvements in our understanding of the properties of potentially hazardous asteroids. In addition to using the world's largest radio telescope (305 meters in diameter), the Arecibo planetary radar system will soon be using a transmitter with a power of 1,000 kilowatts (1 MW), operating at a frequency of 2380 MHz. For comparison, a typical TV or FM radio transmitter, operating around 100 MHz, might broadcast with a power of 50 kW, some 20 times less powerful.

The Arecibo planetary radar transmitter would have a radio brightness of about 50 Janskys to a civilization some 25 light-years distant. For comparison, typical celestial radio objects have brightnesses of about 1 Jy, while a cell phone at a distance of about 1 km would have a radio brightness of millions of Janskys. A radio brightness of 50 Jy is easy to detect with instruments such as the Expanded Very Large Array (EVLA) in New Mexico, and even on galactic scales, two Arecibo-like systems could engage in a conversation, albeit one with a very long time lag.

There are complications to any civilization detecting transmissions from the Arecibo planetary radar system. The first is that the system doesn't operate all of the time. The Arecibo telescope is used for other astronomical and atmospheric research. Clearly, no matter how sensitive another civilization's telescopes are, if we are not transmitting, they cannot detect us.

The second complication is that the Arecibo telescope is attached to the rotating Earth when transmitting. Because of the telescope's large diameter, its radar transmissions are strongly beamed, not unlike a lighthouse beacon. Only if the other civilization is looking at exactly the correct moment as the Arecibo radar beam sweeps across their planet will they be able to detect it. Even worse, if another civilization is monitoring our planet continuously, but the orientation between their radio telescope and the Arecibo telescope is incorrect, they will never detect Arecibo. (No matter how often an astronomer watches the sky in Minnesota, she will never see Alpha Centauri.)

A final complication is that the planetary radar system hasn't been operating in its current form over Arecibo's entire life span; previous systems were lower power, which means they could not have been detected as far away. The extraterrestrial civilization must also be watching the sky at the correct frequency.

One possibility to improve the odds of detection would be to produce a signal that is "on" continuously. Perhaps the next most powerful radio signal on the planet, and one that is transmitted continuously, is the Air Force Space Surveillance System (AFSSS). This radar system tracks satellites and has been operating for almost 40 years. This system is always operational, but, like Arecibo, the AFSSS is also attached to a rotating planet, so a distant civilization would still have to be observing at the correct time and have the appropriate orientation between the AFSSS and their telescope.

However, AFSSS's power is slightly less than that of Arecibo and its transmitting beam is not as focused. The less-focused beam is directly related to its mission to establish a "space fence" that orbiting satellites must cross as they orbit Earth. The combination of less power and wider beam means that it would be perhaps 10,000 times weaker, but likely still detectable at about 25 light-years by a radio telescope comparable to the EVLA or the Giant Metrewave Radio Telescope (GMRT) in India.

A common worry about Earth's transmissions is that other civilizations will intercept TV transmissions such as Cilligan's Island and conclude that no intelligent life exists on Earth. But receiving TV transmissions over interstellar distances is even more difficult. TV and FM radio transmissions are not only typically much lower in power, they tend to be far less strongly beamed (so that many people here on Earth can receive them). The combination of low power and wide beams means that any nearby civilization must have telescopes at least 100 times more sensitive than our current radio telescopes to have any hope of detecting our TV signals.

Taking all of these considerations into account, it's easy to appreciate why SETI programs are still in their beginning stages. Only with much larger telescopes, operating with "piggyback" SETI programs all the time (such as the SETI@home program at Arecibo), are we likely to have a reasonable chance of detecting our cosmic neighbors.

Joseph Lazio is a radio astronomer at the Jet Propulsion Laboratory. He observes frequently with many of the world's premier radio telescopes, including the Very Large Array and the Green Bank Telescope, and leads searches to find natural radio emissions from known extrasolar planets.


To listen to a wide-ranging audio interview with author Joseph Lazio about interstellar communication, visit DetectingEarth.
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Title Annotation:Interstellar Communication
Author:Lazio, Joseph
Publication:Sky & Telescope
Geographic Code:1USA
Date:Jan 1, 2012
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