Can sound travel in space 1

Vienna Journal: How quiet is it in the universe?

Astronomy is a silent science. It gives the ears a vacation. The earth's axis does not squeak; the firmament turns in sublime calm. Even the satellites moving high above make no sound, although they are chasing around the earth at 40,000 km / h. In feature films, on the other hand, spacecraft shoot past the viewer with a dramatic roar. Even serious TV documentaries don't skimp on spacey sounds. In truth, there is only one sound that fits the cosmos: silence.

Sound waves propagate in elastic media. In our everyday life it is above all the air. Here these pressure and density fluctuations overcome around 340 meters per second. In the ideal vacuum, sound waves would have no chance. However, the cosmic vacuum is not perfect: the interstellar medium consists of at least 100, sometimes even billions, atoms per cubic meter. It can, albeit very weakly, transmit at least the longest sound waves. Their frequencies are, of course, billions of times lower than audible sound.

Assuming very high temperatures, sound waves travel thousands of times faster in the cosmos than on earth. This is proven by the aging Orion star Betelgeuse. In its hot environment, the speed of sound is around 40,000 km / h. The gas bubbles rising from its interior sometimes overcome the force of gravity of the inflated giant star. They hit the interstellar medium at supersonic speed. In the resulting shock fronts, the gas is slowed down, compressed and heated: infrared photos capture the glowing arcs.

Extremely high temperatures prevailed after the Big Bang. The universe initially expanded at an almost inflationary rate. Quantum fluctuations were projected into larger dimensions as with a slide projector. So there were slight fluctuations in density in the hot big bang plasma. Baryonic acoustic oscillations emanated from the denser zones: these sound waves traveled in all directions - at a speed that was only 40 percent less than that of light.

Big Bang music
After 380,000 years, the temperature had dropped significantly and the universe was transparent for the first time. The speed of sound dropped rapidly. This moment is captured by the cosmic background radiation, lyrically called "afterglow of the big bang". The former sound waves, sometimes referred to by romantics as "the music of the big bang", left their signature in it - in the form of slight temperature variations.

For Johannes Kepler, too, the space was supposedly filled with music. In Linz he calculated the fastest and slowest daily movement of every planet on its elliptical orbit - viewed from the sun. Then he put the two values ​​in relation to one another. In search of harmonic sounds, he also shared the string of a monochord with a movable bridge. He said he was measuring the same length ratios here as with the planets. After a few rounds, for example, the fifth (ratio 3: 2) matched Mars, the major third (5: 4) matched Saturn, and the minor third (6: 5) matched Jupiter. Kepler thought enthusiastically that he had taken a look at God's blueprint.

Listen to fluctuations in light
Sound needs a medium to propagate. Electromagnetic waves such as radio or light master cosmic expanses without such help. In principle, all signals can be converted electronically into audible sound - even the gravitational waves, which were first detected in 2015.
The transformation of starlight had been achieved long before that. Stars do not twinkle above the turbulent Earth's atmosphere. Space telescopes nevertheless capture rapid, periodic fluctuations in brightness in the sub-promille range. At least in the outermost zone of a star, the energy is not transported by radiation, but by convection. Similar to a saucepan, hot gas bubbles rise and cooler ones sink. This "seething" creates vibrations that reach deep into the star and build up. When set in vibration, stars expand a little rhythmically, only to shrink again immediately - which leads to a periodic decrease and increase in their temperature and thus their luminosity.

Seismologists use earthquake waves to research the structure of the earth's body. Astroseismologists use the aforementioned variations in brightness to look beneath the surface of strange suns. The method allows quite precise statements about sizes and masses: similar to organ pipes, giant stars swing more slowly than dwarf suns.

The light fluctuations can also be artificially compressed and transformed into tones. Transposed up about 19 octaves, it sometimes sounds like a heavenly organist is pressing several keys at once. Because stars vibrate in several frequencies at the same time. Our sun also swings like a bell, albeit in a five-minute rhythm and thus a hundred thousand times slower.

Radio nature
If our ears could perceive short-wave radio signals, they would hear the rustling of the sun and the center of the Milky Way. Also Jupiter: The giant planet is orbited by Io every 42 hours. Shooting through Jupiter's magnetic field, this volcanically active moon provokes aurora-like phenomena and also has a strong influence on the creation of Jupiter's radio waves. In the headphones of a suitable shortwave receiver, these signals sound partly like ocean surf, partly like popcorn.

After a supernova explosion, what is left of a massive star is usually an ultra-dense neutron star. Its mass exceeds that of our sun, but has been compressed to the size of a city. That is why such a star corpse often twirls around its axis dozens to hundreds of times per second. Extreme magnetic fields produce synchrotron radiation that is emitted in two opposing cones. If such a cone sweeps the earth, thumps or rattles in the loudspeaker of the radio telescope.

Our sun chases electrically charged particles into space. Sometimes this solar wind swells into a storm. The four cluster satellites of ESA recently recorded the magnetic waves that are generated in the earth's magnetic field. Converted into sound, one listens to eerie sounds; the pitch rises with the strength of the storm.
A mixture of longitudinal waves every morning that is reminiscent of the chirping of birds sounds more lovely. Isao Tomita and Pink Floyd opened their songs "Dawn Chorus" and "Cluster One" respectively. Longitudinal waves (VLF) vibrate 3000 to 30,000 times per second. Human hearing ranges from approximately 16 to 20,000 Hertz. A large part of the VLF frequencies are therefore suitable for the ears - provided that these electromagnetic waves are converted into sound beforehand with suitable receivers.

Whispering northern lights
Shooting stars penetrate the earth's atmosphere at 40,000 to 259,000 km / h. At heights between 120 and 80 kilometers, they ionize air molecules and turn them into electrically conductive plasma. This plasma tail reflects the radio signals from distant transmitters hidden behind the curvature of the earth. The Natural History Museum in Vienna then briefly receives signal echoes from the GRAVES radar station near Dijon in France.

In the open air, in rare cases, observers perceive a hissing, whirring, buzzing or crackling sound at the sight of a falling star - at the same time as it burns up. That would actually be out of the question because of the slow speed of sound. One possible explanation: synesthesia. The brain transforms one sensory stimulus into another - for example, a quick flash of light into a supposed noise.

In addition, luminous falling stars themselves produce electromagnetic VLF waves that propagate at the speed of light. They could cause objects close to the observer to vibrate mechanically. This creates audible sound of the same frequency. Telephone lines, glasses frames, trees or blades of grass have already been suspected.

Extremely bright falling stars flash dozens of times as they burn up in a matter of seconds. These flashes of light may heat suitable materials on the ground. This in turn causes small pressure oscillations in the surrounding air - i.e. sound waves. In addition to leaves or grass, supposedly dark clothing and frizzy hair are particularly suitable: In the best case, the volume of rustling leaves or whispers is achieved.
While many people would be happy to see the Northern Lights, a few have actually heard them! The occasional hissing, crackling, whistling or clapping may again be caused by electromagnetic VLF waves. Power lines or the discharge of static electricity in an inversion layer about 75 meters above the ground could be responsible for the conversion into sound. So far, the nocturnal silence has rarely been broken by hissing falling stars or whistling northern lights.

Links to sound samples:
www.klang.himmelszelt.at