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This is the most common cited example of paradox to a scientific theory. The noted writer, H. G. Wells wrote “The Time Machine, An Invention” in 1895. This fired up imaginations of supporters of quantum mechanics, who stated that such machine is possible. According to quantum mechanics theory, there are many parallel universes which co-exist, and one can travel through different universes to go backward in time. Thus according to the theory, one can visit half-a-century ago in time and meet his grandfather.
The opponents of such theory point out that what if the scientist travels backward in time and kills his grandfather, who is still a young boy? This is called grand father clause or grand father paradox. Once the grand father is murdered at the young age, he will not have any children, and thus the scientist would not have born. |
(A promotional poster of 2002 movie “Time Machine”, based on novel by H. G. Wells)
This seemingly contradiction is answered by supporters of quantum theory, who argue that since quantum mechanics is governed by probabilities, an unmeasured entity (in this case, your historical grandfather) has numerous probable states. When that entity is measured (his living status is verified), the number of its probable states singularises, resulting in a single outcome (in this case, ultimately, you). Therefore, since the outcome of your grandfather is known, you killing your grandfather would be incompatible with that outcome. Thus, the outcome of one's trip backwards in time must be complementary with the state from which one left.
For further reading:
No turning back
http://news.yahoo.com/s/livescience/20070309/sc_livescience/youcanttravelbackintimescientistssay
Slowing down the clock while travelling at a speed of light
 | According to Einstein’s general theory of relativity, a clock in a spacecraft travelling at a speed comparable to the speed of light slows down. This was verified by US govt. when two atomic clocks (with a precision of a billionth of a second) were tested. One of the clocks was resting on the earth and other was placed in a supersonic aircraft. In this famous experiment (done during middle of 20th century), the clock in the aircraft ran slower than one on the earth, thus proving Einstein’s theory.
Einstein added that not just mechanical clocks, but biological clocks also tend to run slower when travelling at a high-speed. This means that if a scientist travels in a spacecraft at a speed almost equal to the speed of light, and movers around universe, he can remain young for ever. When he returns to the planet earth after several centuries, all his cousins and relatives will be dead, but he will be alive. |
Albert Einstein (1879-1955) on TIME’s cover page
Some scientists challenge the reference frame selected for the theory, as everything is relative in the universe. They argue that when spacecraft leaves, why not it is considered stationary and earth as moving? In that case, scientist should be dead first. No has found answer to this argument yet.
Nature and speed of light
 | Sir Isaac Newton was a genius and great mathematician, who invented calculus. However, he had trouble explaining his physics theories to fellow scientists. He made the erroneous statement the speed of sound (344 m/s or 770 mph or 1238 km/h) is the maximum possible to achieve by any man-made craft. Today we know about supersonic aircraft and 35,000 Km/hr spacecrafts.
He also proposed the theory that just like sound waves, light waves also requires a medium to travel. To support this argument, he stated that the universe is filled with medium called ether. When asked whether it will not slow down the speed of rotation of planets, he assumed ether to be of negligible density, thus offering zero resistance.
Now we all know that light waves are electro-magnetic waves, and do not need any medium to travel. They are consisting of particles, called photons, which have weights. Einstein proved that light bends under the influence of gravity, hence carry weights. He also stated that weight of an object increases as it travels near the speed of the light, which is about 300,000 Km/sec. |
Sir Isaac Newton (1643-1727)
As a corollary to the theory of general relativity, Einstein proposed a fourth dimension called time, in addition to the 3 space dimensions, which is now known as space-time theory. According to this theory, it is impossible for nay object to travel faster than the speed of the light. This theory is true for now.
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