Sound waves are mechanical waves. They can travel only through matter. They cannot travel through empty space. This was demonstrated in the 1600s by a scientist named Robert Boyle. Boyle placed a ticking clock in a sealed glass jar. The clock could be heard ticking through the air and glass of the jar. Then Boyle pumped the air out of the jar. The clock was still running, but the ticking could not be heard. Thats because the sound couldnt travel without air particles to transfer the sound energy. Is there any other place where sound cannot be heard? Here is a hint: is there any other place that does not have air molecules? Thats right, in space sound cannot be transferred. So can astronauts hear each other? Yes, they can, because they are in enclosed space stations or pressurized suits.In both cases, they are surrounded by air molecules. If they left their space ship and did not use their electronics, they would not be able to hear each other. Next time you watch a science fiction movie, think about this factoid. When spaceships are destroyed by hostile alien beings, there would be no sound heard. Sound waves can travel through many kinds of matter. 

How do astronauts hear each other?


General relativity includes a dynamical spacetime, so it is difficult to see how to identify the conserved energy and momentum. Noether's theorem allows these quantities to be determined from a Lagrangian with translation invariance, but general covariance makes translation invariance into something of a gauge symmetry. The energy and momentum derived within general relativity by Noether's presecriptions do not make a real tensor for this reason. Einstein argued that this is true for fundamental reasons, because the gravitational field could be made to vanish by a choice of coordinates. He maintained that the non-covariant energy momentum pseudotensor was in fact the best description of the energy momentum distribution in a gravitational field. This approach has been echoed by Lev Landau and Evgeny Lifshitz, and others, and has become standard. The use of non-covariant objects like pseudotensors was heavily criticized in 1917 by Erwin Schrodinger and others. 

Which of the scientists believed non-covariant energy momentum pseudotensor was the best description of energy momentum in a gravitational field?


Dubai's Crown Prince Sheikh Mohamed Bin Rashid Al Maktoum inaugurated a free zone for e-commerce today, called Dubai Internet City. The preliminary stages of the project, the only one of its kind according to its designers, are estimated at $200 million. Sheikh Mohamed, who is also the Defense Minister of the United Arab Emirates, announced at the inauguration ceremony that "we want to make Dubai a new trading center." The minister, who has his own website, also said: "I want Dubai to be the best place in the world for state-of-the-art technology companies." He said companies engaged in e-commerce would be able to set up offices, employ staff and own equipment in the open zone, including fully-owned foreign companies. The e-commerce free zone is situated in north Dubai, near the industrial free zone in Jebel Ali, the top regional and tenth international leading area in container transit. The inauguration of Dubai Internet City coincides with the opening of an annual IT show in Dubai, the Gulf Information Technology Exhibition (Gitex), the biggest in the Middle East. 


Why was an e-commerce free zone inaugurated in Dubai?