The universe can be a very strange place. While groundbreaking ideas such as quantum theory, relativity and even the Earth going around the Sun might be commonly accepted now, science still continues to show that the universe contains things you might find it difficult to believe, and even more difficult to get your head around.
Theoretically, the lowest temperature that can be achieved is absolute zero, exactly −273.15°C, where the motion of all particles stops completely. However, you can never actually cool something to this temperature because, in quantum mechanics, every particle has a minimum energy, called “zero-point energy,” which you cannot get below. Remarkably, this minimum energy doesn’t just apply to particles, but to any vacuum, whose energy is called “vacuum energy.” To show that this energy exists involves a rather simple experiment– take two metal plates in a vacuum, put them close together, and they will be attracted to each other. This is caused by the energy between the plates only being able to resonate at certain frequencies, while outside the plates the vacuum energy can resonate at pretty much any frequency. Because the energy outside the plates is greater than the energy between the plates, the plates are pushed towards each other. As the plates get closer together, the force increases, and at around a 10 nm separation this effect (called the Casimir effect) creates one atmosphere of pressure between them. Because the plates reduce the vacuum energy between them to below the normal zero-point energy, the space is said to have negative energy, which has some unusual properties.
One of the properties of a negative-energy vacuum is that light actually travels faster in it than it does in a normal vacuum, something that may one day allow people to travel faster than the speed of light in a kind of negative-energy vacuum bubble. Negative energy could also be used to hold open a transversible wormhole, which although theoretically possible, would collapse as soon as it was created without a means to keep it open. Negative energy also causes black holes to evaporate. Vacuum energy is often modeled as virtual particles popping into existence and annihilating. This doesn’t violate any energy conservation laws as long as the particles are annihilated shortly afterwards. However, if two particles are produced at the event horizon of a black hole, one can be moving away from the black hole, while the other is falling into it. This means they won’t be able to annihilate, so the particles both end up with negative energy. When the negative energy particle falls into the black hole, it lowers the mass of the black hole instead of adding to it, and over time particles like these will cause the black hole to evaporate completely. Because this theory was first suggested by Stephen Hawking, the particles given off by this effect (the ones that don’t fall into the black hole) are called Hawking radiation. It was the first accepted theory to unite quantum theory with general relativity, making it Hawking’s greatest scientific achievement to date.
One prediction of Einstein’s theory of general relativity is that when a large object moves, it drags the space-time around it, causing nearby objects to be pulled along as well. It can occur when a large object is moving in a straight line or is rotating, and, although the effect is very small, it has been experimentally verified. The Gravity Probe B experiment, launched in 2004, was designed to measure the space-time distortion near Earth. Although sources of interference were larger than expected, the frame-dragging effect has been measured to an uncertainty of 15%, with further analysis hoping to reduce this further.
The expected effects were very close to predictions: due to the rotation of the Earth, the probe was pulled from its orbit by around 2 meters per year, an effect purely caused by the mass of the Earth distorting the space-time surrounding it. The probe itself would not feel this extra acceleration because it is not caused by an acceleration on the probe, but rather on the space-time the probe is traveling through–analogous to a rug being pulled under a table, rather than moving the table itself.
These spectacular pictures are shot by the Expedition 25 crew members aboard the International Space Station flying 220 miles above Earth on Oct. 28. The Expedition 25 astronauts used the ESA-built Cupola observatory module to make these amazing nighttime pictures of planet Earth.