In space, no one can hear you scream. That’s according to physics textbooks and the tagline of the movie Alien. But it seems that in some circumstances, sound can jump between objects in a vacuum after all.
Sound waves are travelling vibrations of particles in media such as air, water or metal. So it stands to reason that they cannot travel through empty space, where there are no atoms or molecules to vibrate.
Now a theoretical analysis by Mika Prunnila and Johanna Meltaus, both of the VTT Technical Research Centre of Finland in Espoo, suggests that sound may be able to leap across a vacuum separating two objects made of piezoelectric crystals. These crystals generate an electric field when squeezed or stretched by sound waves or other forces, and deform in an electric field.
When a sound wave reaches the edge of one crystal, the electric field associated with it can stretch across the gap and deform the crystal on the other side, creating sound waves in that second crystal (Physical Review Letters, vol 105, p 125501). “It is as if the sound waves don’t even recognise the vacuum – they just go through,” says Prunnila.
It’s as if the sound waves don’t even recognise the vacuum – they just go through
The researchers say the gap need not be particularly small, and that the efficiency of transmission should vary with sound frequency and the angle at which the sound hits the first crystal’s edge. For some combinations the waves hardly lose any energy when they jump the gap.
The team hopes to show the effect experimentally soon. “Such work is interesting from a fundamental point of view,” says Gang Chen of the Massachusetts Institute of Technology.