How to pick up messages after they’re gone

Scientists say there’s a way to retrieve light-encoded messages even after the light has disappeared


Today, astronomers read the history of the universe by looking at the light emitted by hot, distance objects. Sometimes, however, those light sources give out — and their light disappears. Good news: A new technique might be able to still read the information that had been carried in that long-gone light.

Diedre Hunter (Lowell Obs.) et al., HST, NASA

SAN ANTONIO, Texas — Light may travel at the speed of light. That does not, however, mean that the data it carries has to. Scientists have just proposed a way to read light-based messages long after the light itself has flown by.

The new technique relies on measuring electromagnetic “echoes” in space. One day, astronomers might use the new process to glean details about distant stars and galaxies without directly measuring their light. Doing that, however, is a ways off.

Even so, this new approach to “reading” long-gone messages is piquing the interest of scientists. Physicists study energy and matter. And they had believed that to share information using light, there had to be a source that sent the light and a receiver to absorb it (and its message). The new technique would now prove an exception to that supposed rule.

Scientists described the new technique March 2 at a meeting, here, of the American Physical Society. Additional details will appear soon in Physical Review Letters.

How it would work

Much communication, today, already depends on messages encoded in electromagnetic radiation — or light. It’s what allows Internet users to chat through fiber-optic cables. It also underpins radio broadcasts. A radio antenna, for instance, broadcasts photons. These are particles of electromagnetic energy. Photons travel at the speed of light. The radio in your home or car absorbs that energy and translates it into sound. If those photons don’t strike your radio, it can’t play the breaking news bulletin or music you tuned in to hear.

Indeed, there should be no way to pick up the information carried by photons once those photons have passed by. But Robert Jonsson, Eduardo Martín-Martínez and Achim Kempf figured out how to do it anyway. These three theoretical physicists work at the University of Waterloo in Canada. And they knew photons always leave some mark on their surroundings. Even in the emptiness of the vacuum of space. And that’s because even a vacuum is never truly empty. It is full of fleeting electromagnetic energy (radiation).

The three physicists have now demonstrated — mathematically, anyway — that when a sender generates photons to broadcast a message, those photons produce what might be thought of as an afterglow. And it can be “viewed” by measuring fluctuations — variations — in the radiation present in a vacuum.

What this means: Someone could still “tune in” to a light broadcast even if the photon carrying it had whizzed by long ago.

Listening would take very sensitive “ears”

This technique is truly new. In fact, the sender never directly transmits energy to the receiver, explains Jorma Louko. He’s a theoretical physicist at the University of Nottingham in England. For the Waterloo technique to work, he says, the receiver has to use energy. That energy measures disturbances to the background radiation by the long-gone photon broadcast.

“The receiver has to actively do something to see something,” he explains.

Detecting the changes in the background energy would require both the sender and receiver to use special antennas. These would consist of atoms that appear to have multiple amounts of energy at the same time — a state known as quantum superposition, Martín-Martínez says. Such technology is not yet available for consumers. But it is available in some physics labs.

Martín-Martínez is talking to other physicists who might be interested in trying to demonstrate his team’s new technique. Those scientists would use chilled superconducting circuits for their system’s antennas.

Eventually, Martín-Martínez hopes this research will lead to grander uses. His team outlines one in a paper posted online January 7 at In it, the group argues that light emitted during the dawn of the universe should have left an afterglow. And it may be “visible” to a new breed of telescopes. If successful, these telescopes might detect objects billions of light-years away — even if the photons those objects emitted passed by Earth long ago.

Power Words

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antenna  (plural: antennae) Devices for picking up (receiving) electromagnetic energy.

astronomy    The area of science that deals with celestial objects, space and the physical universe as a whole. People who work in this field are called astronomers.

atom   The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

electromagnetic radiation  Energy that travels as a wave, including forms of light. Electromagnetic radiation is typically classified by its wavelength. The spectrum of electromagnetic radiation ranges from radio waves to gamma rays. It also includes microwaves and visible light.

galaxy  A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.

light-year  The distance light travels in one year, about 9.48 trillion kilometers (almost 6  trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.

photon  A particle representing the smallest possible amount of light or other electromagnetic radiation.

physicist  A scientist who studies the nature and properties of matter and energy.

protocol   An accepted or agreed-upon procedure for doing something.

quantum theory  A way to describe the operation of matter and energy at the level of atoms. It is based on an interpretation that at this scale, energy and matter can be thought to behave as both particles and waves. The idea is that on this very tiny scale, matter and energy are made up of what scientists refer to as quanta — miniscule amounts of electromagnetic energy.

quantum physics  A branch of physics that uses quantum theory to explain or predict how a physical system will operate on the scale of atoms or sub-atomic particles.

radiation (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

radio  To send and receive radio waves; or the device that receives these transmissions.

radio waves  Waves in a part of the electromagnetic spectrum; they are a type that people now use for long-distance communication. Longer than the waves of visible light, radio waves are used to transmit radio and television signals; it is also used in radar.

star  The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

superconductor  Materials that have no resistance to the flow of electricity, typically only when they are cooled below a certain temperature. Superconductors also repel all magnetic fields, which allows them to float in the air when they are placed inside a strong magnetic field.

telescope    Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.

theoretical    An adjective for an analysis or assessment of something that based on pre-existing knowledge of how things behave. It is not based on experimental trials. Theoretical research tends to use math — usually performed by computers — to predict how or what will occur for some specified series of conditions. Experimental testing or observations of natural systems will then be needed to confirm what had been predicted.

vacuum  Space with little or no matter in it. Laboratories or manufacturing plants may use vacuum equipment to pump out air. 


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