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When I was in the National Air and space museum in washington, I read that that there was a message sent to the space, so someone can hear and answer it, Carl Sagan made the recordings. How can we send that radio signal, if it is a wave and waves need to make something vibrate, in here is the atmosphere what vibrates, but in space there is nothing to vibrate.

This is a good question, and one that baffled scientists for quite some time. In fact, the discovery that light apparently does not need a medium to travel through, unlike other types of waves, was one of the driving forces for the development of the theory of relativity. (It's a long story.) The short answer to your question is that light is both like a wave and like a particle. Particles don't need anything through which to travel; in fact, you might expect particles to travel better when there's nothing for them to interact with. Light is like that, too. Light travels faster in a vacuum than through any material.

Richard Barrans Jr., Ph.D.

Electromagnetic waves (such as radio, TV and light waves) do not require a medium. The electric and magnetic fields oscillate even in vacuum. This is how we communicate with space explorers on the moon and spacecraft that travel to Venus and Jupiter.

Dr. Bradburn

I think that message you are talking about was saved as a recording on a gold disc that was placed on the Voyager exploration craft. The Voyager was launched back in the 70's and went past Jupiter , Saturn, and Neptune in the 80's It left our solar system a couple of years ago. As for radio waves, they are a form of electromagnetic radiation and do not require an atmosphere to transmit a wave. Sound waves are produced by vibrations of the molecules of air around your ear drum. That is why sound cannot travel in a vacuum. However, electromagnetic waves like radio, x-rays, gamma rays, and light do not care whether they go through a vacuum of not. That is one of the properties of electromagnetic waves. We on earth are pounded daily with electromagnetic waves, with sun light being the most obvious. If you go to your public library, you can find a book on physics that will explain this in more detail and provide you with an electromagnetic spectrum that shows all of the known frequencies of waves.

Dr. C Murphy

There are a number of messages to ET floating out there. On the sides of Pioneers 10 and 11, launched in 1971 and 1972, and the first spacecraft to achieve solar escape velocity -- and hence leave the Solar System -- there are plaques with a pictographic message to aliens. They were designed largely by Carl Sagan and associates, and you can read about the plaques here:

There are also sound recordings (greetings from Presidents, whale noises, etc.) on Voyagers 1 and 2, launched in 1977, see:

But what you are probably thinking about is a radio message sent out from the Arecibo National Radio Observatory in November 1974:

The Planetary Society, at, maintains quite complete online records of all the Searches for Extraterrestial Intelligences (SETI).

Your question about radio is apt, and pondered at length by 19th century scientists. They proposed that the Universe was filled with a "something" they called luminiferous aether, which was what vibrated when light or radio waves passed.

It turns out that aether is unnecessary. The thing that is vibrating when a radio wave passes is the electric field. The electric field fills the Universe at all times, and it can be described as "the force on a test electric charge located at that point in space and time". Almost always and everywhere the electric field has an amplitude near zero, meaning a test charge would feel no force, but the field exists even when its amplitude is zero.

Air has little effect on the vibrations of the electric field that correspond to radio waves. The reason is that the waves are of such slow frequency that nothing in the ocean of atoms of air wiggles at anywhere near the same frequency. This is good: it means that air poses little barrier to radio transmissions, as we know from the fact that we can communicate by radio with spacecraft.

Dr. C Grayce

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