The Present and The Future of Interstellar Communication

Recently I stumbled on the screening of "Passengers" movie on TV. I've seen it before, and the first question which arose in my mind on first viewing was: "Why the hell he decided to wake up the writer, not the hibernation capsules engineer?!" At the second viewing in my head was arouse a more scientific question.

At the beginning of the movie, the main character sends a video message to the Earth, hoping to get help. Joyful a robotic voice informs that the message was sent and will reach the Earth in 19 years, and the answer will come back in 36 years…

Sooo, you have a fully autonomous spacecraft, equipped with something similar to AI, which flies with a speed of ½ the speed of light and has everything needed to colonize a new planet. I.e., you can conduct a full analysis of far planet (in another system) and understand that it is suitable for life. But, people still have not been able to establish a high-speed space communications. Is it really so difficult task? Let's try to find out.

Radio waves faster than you thought

Today, all spacecrafts on the Moon, on Mars, near the Titan or on the edge of the solar system use radio waves to transmit signals. The radio wave- sounds like something from the last century. The fact is that in space vacuum radio waves move almost with the speed of light (300 000 km/s) - and this is the maximum known speed today.

Even such high speed of information transfer, does not solve the problem with signal delay. For example, the delay of signal transmission from Mars to the Earth ranges from 3 to 21 minutes. This is due to the fact that the distance between the planets varies from 56 to 226 million km.

Voyager 1- the most remote spacecraft, transmits data with a delay of 18 hours! Over 36 years of flight, it overcame a 19 billion kilometers.

The bandwidth of the past era

The second major drawback of radio is low bandwidth. Do you remember the days of the Internet with the modem connection that was making strange sounds? Here in space now the same garbage. The maximum throughput of a single channel does not exceed 256 Kbps. And that's assuming no interference and loss of signal transmission (great rarity).

Lack of space in infinity

During transmission radio wave should not come across obstacles to reach receivers on the Earth in a readable form. But, the celestial bodies are constantly in motion. Every 780 days, the Sun occupies the position between Mars and the Earth, and kills any connections.

The greater the distance from the receiver to the transmitter, the more obstacles. It is impossible to talk about continuous communication with spacecraft at the edge of our system, not to mention about interstellar roaming.

Interesting fact: every modern spacecraft uses its own software and communication protocols, and has its own information channel. Maybe it's the echo of the space race, but we have no uniform format for space communications. Instead of creating a single network, where any spacecraft could serve as a repeater or extender, we have just a bunch of separate devices, with which we often lose connect.

To save Chris Pratt, we must to solve three problems:

• The delay of signal transmission from several minutes to several years.
• The low bandwidth signal transmission (256 kbps).
• Constant loss of signal or its complete absence due to the influence of celestial bodies.

Some of these tasks we can solve today, using modern technologies.

"Bread Crumbs"

To eliminate the impact of cosmic bodies, it is sufficient to use bread crumbs. For example, the same spaceship during the flight leaves a trail of small communication satellites. Which form a continuous chain and transmit a signal sequentially to each other and so to the Earth. This approach is also beneficial in that it relieves of the need to equip the spaceship with a powerful telecommunications system. Which will consume large amounts of energy to generate a powerful signal directly from a spaceship to the Earth.

Inside the Solar system this problem is solved even easier. We need to place a few spacecrafts in orbits that are not associated with the orbits of the planets and not fall into the trajectory of other celestial bodies. But, keep in mind the fact that such spacecrafts must have large reserves of fuel and energy, to maintain their own orbits.

The problem with the corruption of data packets, when interruption of the communication channel, has actually been resolved. DTN was successfully tested in 2008 and is used today for the ISS. In simple words: its protocols support the storage of information packets, until the emergence of a stable communication channel, instead of sending packages to nowhere.

Laser Time

OK, we have a stable communication channel, but its bandwidth is only 256 kbps. Imagine that you are trying to fill a glass with water from the tap with the speed 1 drop per second - not very productive. Unlike radio waves, a laser beam can carry much more information, with less energy consumption.

And we've already taken the first steps in this direction. In 2013, as part of the LADEE project, scientists successfully used the laser communication between the Earth and the Moon. The received speed of data transmission amazes: 622 Mbps on the download (from the Moon to the Earth) and 20 Mbps on the upload (From the Earth to the Moon). Such bandwidth not available even in some regions on the Earth.

Faster Than Light

We have a permanent channel of communication with the spaceship, which can effectively broadcast even video signal. But what's the point if we receive information with a delay of a few hours or decades? Any information obtained from the constellation alpha Centauri will not be actual. We are able to transmit a signal at the speed of light, but it is not enough. Can something move faster than light?

According to the theories of Einstein: in the Universe there is no object or particle that can move faster than 300,000 km/sec. But, in quantum mechanics, some events may occur at a speed above the speed of light.

One of these events- is the phenomenon of quantum entanglement. Without going into the full theory: We have two interconnected particles. The change in the state of one particle in the same time is displayed in second particle, regardless of the distance at which they are located. Even if these two particles are at opposite ends of the Universe, the destruction of one leads to instant destruction of another.

It sounds wonderful, like magic. Unfortunately, even if this theory is true, anyway it is impossible to transmit any information using quantum entanglement. This restriction follows from another law of physics - the destruction of the wave function.

Each particle is described by a wave function that indicates its possible location, speed and other properties. But, at the time of particle measurement, all values of its properties are destroyed, and the obtained data cease to exist.

The data you received is no longer relevant. Imagine that this is a wave that formed in the heart of the ocean, and you could see it only on the shore, faced with the shore, the wave disappears.

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In fact, if we can find a way to transmit information faster than the speed of light, communication with the spacecraft will be a small achievement, against all the other opportunities. Exceeding the speed of light means the control of space and time, we will become the Masters of the Universe. Until then, Chris Pratt is forced to wait for a response from the Earth for years.

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sources: Communications in Space: A Deep Subject, Deep Space Communications, Space roaming, Radio wave, , Delay-tolerant networking, WiKi, Geektimes, Physics of The Universe: Questions with no Answers, Quantum entanglement, images from Google search.

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