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The same thing will happen with a wormhole, only with radiation instead of sound. As soon as the wormhole expands, natural radiation will enter it, and end up in a loop. The feedback will become so strong it destroys the wormhole. So although tiny wormholes do exist, and it may be possible to inflate one some day, it won't last long enough to be of use as a time machine.
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Any kind of time travel to the past through wormholes or any other method is probably impossible, otherwise paradoxes would occur. So sadly, it looks like time travel to the past is never going to happen. A disappointment for dinosaur hunters and a relief for historians.
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Unit 8 Why We're Fat
So why is obesity happening? The obvious, clichéd-but-true answer is that we eat too much high-calorie food and don't burn it off with enough exercise. If only we had more willpower, the problem would go away. But it isn't that easy.
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When warned about the dangers of overeating, we get briefly spooked and try to d o better. Then we're offered a plateful of pancakes smothered in maple syrup, our appe tite overpowers our reason, and before we know it, we're at it again. Just why is appeti te such a powerful driver of behavior, and, more important, how can we tame it?
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itWhin the past few years, science has linked our ravenous appetites to genes and hormones. Among the hormones that fuel these urges are ghrelin and leptin, known as the "hunger hormones." Ghrelin is produced mostly by cells in the stomach lining. Its job is to make you feel hungry by affecting the hypothalamus, which governs metabo lism. Ghrelin levels rise in dieters who lose weight and then try to keep it off. It's alm ost as if their bodies are trying to regain the lost fat. This is one reason why it's hard to lose weight and maintain the loss.
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Leptin turns your appetite off and is made by fat cells. Low leptin levels increase your appetite and signal your body to store more fat. High leptin levels relay the oppo site signal. Many obese people have developed a resistance to the appetite-suppressin g effects of leptin and never feel satisfied, no matter how much they eat. Basically, yo
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ur body uses these hormones to help you stay at your weight and keep you from losin g fat ¡ª which is another reason why dieting can be so difficult.
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Lack of sleep promotes obesity by messing with these hunger hormones. If you sk imp on sleep, ghrelin levels rise, making you hungry, and leptin levels dip, which sign als a need for calories. During my internship, I was chronically sleep-deprived becaus e I had to be at the hospital and stay up all night every third night. I gained a lot of wei ght that year; now I know why. Years later, after I gave birth to my son Alex, I put on a lot of pounds, too ¡ª twenty pounds left over from the pregnancy, and twenty pound s from being up all night with him. He had colic (fussiness) and never slept more than twenty minutes at a time the first six months of his life. My weight started coming off more easily when I got more sleep.
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Besides ghrelin and leptin, many other hormones play a role in appetite. Scientist s have been looking for ways to control all these hunger hormones, but so far nothing usable has panned out.
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Hunger isn't the only reason people eat and eat and eat. Stress, depression, boredo m, loneliness, and even joy all come into play. And some of us may just be inclined to gain weight while others are not, due to genetics ¡¡ ´Ë´¦Òþ²Ø£º4865×Ö£¬È«²¿ÎĵµÄÚÈÝÇëÏÂÔØºó²é¿´¡£Ï²»¶¾ÍÏÂÔØ°É ¡¡
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