reverse definition: 1. to change the direction, order, position, result, etc. of something to its opposite: 2. to drive a vehicle backwards: 3. to make a phone call that.
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Great Images of Star Explosions ]. Gamma-ray bursts are some of the highest-energy explosions ever detected, shining brighter than a million trillion times the output of Earth's sun, according to NASA.
They produce more energy output than anything else that emits light, " said study lead author Jon Hakkila, an astrophysicist and associate dean of the Graduate School at the College of Charleston in South Carolina. When two neutron stars collide, they send out short gamma-ray bursts as they form a black hole. Supernovas, or star explosions, produce longer gamma-ray bursts as the dying stars collapse into black holes. For both types of gamma-ray bursts, "most of their energy comes in the form of pulses," or blips, Hakkila said.
When Hakkila took out the main, brightest pulse from the data to better see the rest of the light signal, he found that "the blip actually had some little side blips," he said. Every pulse, the researchers found, had three distinct peaks where light increased and then decreased in intensity a few times over each pulse. As the scientists looked at the data, they happened to find that the structure of these peaks looked like reflections in a mirror — the parts of earlier pulses that came out first were coming out last on subsequent pulses.
While looking at six of the brightest gamma-ray bursts detected by NASA's Compton Gamma Ray Observatory as part of the Burst and Transient Source Experiment in the s, the team found that the bursts contained time-reversed light signatures. In other words, "they all have this signature of brightness that fluctuates and then it turns around and goes backwards in time," Hakkila said. This is true for both short-lived and long-lived gamma-ray bursts, Hakkila said.
To better picture this, imagine if you turned on three light switches: To really see this, the researchers took the entire signal, stretched it and folded it right down the middle like a piece of paper. That "folding" process aligned the rise of the signal with the fall of the signal. The two ends lined up really well. Though the blast is probably not "reversing time" with some radiation mechanism as it would in a science-fiction movie, "I'm not going to exclude any kind of weird thing," he said.
However, a likelier explanation may come from looking at how a blast wave moves through matter, Hakkila said. When a star explodes, a big blast wave can move outward through material and light it up as it goes. First, it lights up clump A, then clump B, then clump C. In ancient Greek monarchical coinage, the situation continued whereby a larger image of a deity , is called the obverse, but a smaller image of a monarch appears on the other side which is called the reverse. In a Western monarchy , it has been customary, following the tradition of the Hellenistic monarchs and then the Roman emperors , for the currency to bear the head of the monarch on one side, which is almost always regarded as the obverse.
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This change happened in the coinage of Alexander the Great , which continued to be minted long after his death. After his conquest of ancient Egypt , he allowed himself to be depicted on the obverse of coins as a god-king , at least partly because he thought this would help secure the allegiance of the Egyptians, who had regarded their previous monarchs, the pharaohs , as divine. The various Hellenistic rulers who were his successors followed his tradition and kept their images on the obverse of coins.
A movement back to the earlier tradition of a deity being placed on the obverse occurred in Byzantine coinage , where a head of Christ became the obverse and a head or portrait half or full-length of the emperor became considered the reverse. The introduction of this style in the gold coins of Justinian II from the year provoked the Islamic Caliph , Abd al-Malik , who previously had copied Byzantine designs, replacing Christian symbols with Islamic equivalents, finally to develop a distinctive Islamic style , with just lettering on both sides of their coins.
This script alone style then was used on nearly all Islamic coinage until the modern period. The type of Justinian II was revived after the end of Iconoclasm , and with variations remained the norm until the end of the Empire.
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Without images, therefore, it is not always easy to tell which side will be regarded as the obverse without some knowledge. After Islamic coins avoided all images of persons and usually contained script alone. The side expressing the Six Kalimas the Islamic profession of faith is usually defined as the obverse. A convention exists typically to display the obverse to the left or above and the reverse to the right or below in photographs and museum displays, but this is not invariably observed.
Obverse and reverse
The form of currency follows its function, which is to serve as a readily accepted medium of exchange of value. Normally, this function rests on a state as guarantor of the value: Traditionally, most states have been monarchies where the person of the monarch and the state were equivalent for most purposes. For this reason, the obverse side of a modern piece of currency is the one that evokes that reaction by invoking the strength of the state, and that side almost always depicts a symbol of the state, whether it be the monarch or otherwise.
If not provided for on the obverse, the reverse side usually contains information relating to a coin's role as medium of exchange such as the value of the coin.
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Additional space typically reflects the issuing country's culture or government, or evokes some aspect of the state's territory. Regarding the euro , some confusion regarding the obverse and reverse of the euro coins exists. Officially, as agreed by the informal Economic and Finance Ministers Council of Verona in April , and despite the fact that a number of countries have a different design for each coin, the distinctive national side for the circulation coins is the obverse and the common European side which includes the coin value is the reverse.
A number of the designs used for obverse national sides of euro coins were taken from the reverse of the nations' former pre-euro coins. Several countries such as Spain and Belgium continue to use portraits of the reigning monarch; while the Republic of Ireland continues to use the State Arms, as on its earlier issues. Following ancient tradition, the obverse of coins of the United Kingdom and predecessor kingdoms going back to the middle ages almost always feature the head of the monarch.
By tradition, each British monarch faces in the opposite direction of his or her predecessor; this is said to date from , with Charles II turning his back on Oliver Cromwell. Hence, George VI faced left and the present Queen faces right. The only break in this tradition almost occurred in when Edward VIII , believing his left side to be superior to his right, insisted on his image facing left, as his father's image had.
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No official legislation prevented his wishes being granted, so left-facing obverses were prepared for minting. Very few examples were struck before he abdicated later that year, but none bearing this portrait ever were issued officially. When George VI acceded to the throne, his image was placed to face left, implying that, had any coins been minted with Edward's portrait the obverses would have depicted Edward facing right and maintained the tradition. Current UK coinage features the following abbreviated Latin inscription: The United States specifies what appears on the obverse and reverse of its currency.