Đề thi HSG lớp 12 môn Anh Tỉnh Lào Cai năm 2026

Time travel took a small step away from science fiction and toward science recently when physicists discovered that sub-atomic particles known as neutrinos - progeny of the sun's radioactive debris - can exceed the speed of light. The unassuming particle - it is electrically neutral, small but with a "non-zero mass" and able to penetrate the human form undetected – is on its way to becoming a rock star of the scientific world.

Researchers from the European Organization for Nuclear Research (CERN) in Geneva sent the neutrinos hurtling through an underground corridor toward their colleagues at the Oscillation Project with Emulsion-tRacking Apparatus (OPERA) team 730 kilometers away in Gran Sasso, Italy. The neutrinos arrived promptly - so promptly, in fact, that they triggered what scientists are calling the unthinkable - that everything they have learnt, known or taught stemming from the last one hundred years of the physics discipline may need to be reconsidered.

The issue at stake is a tiny segment of time - precisely sixty nanoseconds (which is sixty billionths of a second). This is how much faster than the speed of light the neutrinos managed to go in their underground travels and at a consistent rate (15,000 neutrinos were sent over three years). Even allowing for a margin of error of ten billionths of a second, this stands as proof that it is possible to race against light and win. The duration of the experiment also accounted for and ruled out any possible lunar effects or tidal bulges in the earth's crust.

Nevertheless, there's plenty of reason to remain skeptical. According to Harvard University science historian Peter Galison, Einstein's relativity theory has been "pushed harder than any theory in the history of the physical sciences". Yet each prior challenge has come to no avail, and relativity has so far refused to buckle.

So is time travel just around the corner? The prospect has certainly been wrenched much closer to the realm of possibility now that a major physical hurdle - the speed of light - has been cleared. If particles can travel faster than light, in theory travelling back in time is possible. How anyone harnesses that to some kind of helpful end is far beyond the scope of any modern technologies, however, and will be left to future generations to explore.

Certainly, any prospective time travelers may have to overcome more physical and logical hurdles than merely overtaking the speed of light. One such problem, posited by René Barjavel in his 1943 text Le Voyageur Imprudent is the so-called grandfather paradox. Barjavel theorized that, if it were possible to go back in time, a time traveler could potentially kill his own grandfather. If this were to happen, however, the time traveler himself would not be born, which is already known to be true. In other words, there is a paradox in circumventing an already known future; time travel is able to facilitate past actions that mean time travel itself cannot occur.

Other possible routes have been offered, though. For Igor Novikov, astrophysicist behind the 1980s' theorem known as the self-consistency principle, time travel is possible within certain boundaries. Novikov argued that any event causing a paradox would have zero probability. It would be possible, however, to "affect" rather than "change" historical outcomes if travelers avoided all inconsistencies. Averting the sinking of the Titanic, for example, would revoke any future imperative to stop it from sinking - it would be impossible. Saving selected passengers from the water and replacing them with realistic corpses would not be impossible, however, as the historical record would not be altered in any way.

A further possibility is that of parallel universes. Popularized by Bryce Seligman DeWitt in the 1960s (from the seminal formulation of Hugh Everett), the many-worlds interpretation holds that an alternative pathway for every conceivable occurrence actually exists. If we were to send someone back in time, we might therefore expect never to see him again - any alterations would divert that person down a new historical trajectory.

A final hypothesis, one of unidentified provenance, reroutes itself quite efficiently around the grandfather paradox. Non-existence theory suggests exactly that - a person would quite simply never exist if they altered their ancestry in ways that obstructed their own birth. They would still exist in person upon returning to the present, but any chain reactions associated with their actions would not be registered. Their "historical identity" would be gone.

So, will humans one day step across the same boundary that the neutrinos have? World-renowned astrophysicist Stephen Hawking believes that once spaceships can exceed the speed of light, humans could feasibly travel millions of years into the future in order to repopulate earth in the event of a forthcoming apocalypse. This is because, as the spaceships accelerate into the future, time would slow down around them (Hawking concedes that bygone eras are off limits - this would violate the fundamental rule that cause comes before effect).

Hawking is therefore reserved yet optimistic. "Time travel was once considered scientific heresy, and I used to avoid talking about it for fear of being labelled a crank. These days I'm not so cautious."

Over millions of years, the world's climate has undergone a series of dramatic changes that have most likely resulted from oscillations, or pendulum-like swings, in the rotation of the Earth. One such dramatic climate change was what is commonly called the Ice Age and was in reality a series of phases of colder and warmer weather that commenced approximately two million years ago. What we today refer to as the Ice Age was actually a sequence of weaker and stronger glacial periods during which sheets of ice expanded and moved varying distances south followed by interglacial periods when the glaciers melted to varying degrees and retreated to the north. These successive periods of colder and warmer weather have had striking effects on the world's landscape and on the life forms that inhabit the land. Each glacial period in the series of colder and warmer phases had a different impact, depending on how far south the ice extended, how long it lasted before melting, and how much it changed the landscape and sea levels.

The effect on the world's landscape that resulted from the changing climate was dramatic. Vast sheets of glacial ice in the northern latitudes of the world cut deep U-shaped valleys that still exist today and caused huge boulders and enormous amounts of clay and dust to be deposited across North America, Europe, and Asia and to reshape the landscape of these areas. As the glacial sheets of ice crept southward on the northern continents, they trapped large amounts of water. This served to reduce the amount of moisture in the atmosphere that could fall as rain or snow, reducing the amount of rainfall and leading to further aridity and the expansion of deserts in the tropical and subtropical regions. In addition, with so much water held in the glacial ice, the sea level fell, resulting in the exposure of land that today is covered with water. During glacial periods there was a land bridge from France to England, the islands of Japan and Java were connected to the Asian mainland, and Asia was linked to North America over what is today the Bering Strait. Then, during interglacial periods, when the glacial ice retreated to the north, more water became available as glaciers melted. Vegetation expanded with the increase in rainfall, and coastlines changed with the increase in ocean water; during interglacial periods, many areas that today are landmasses were covered with water.

[I] Prehistoric people seem to have adapted well to these environmental changes. [II] They could be found living in the grasslands of Asia, in the forests of Southeast Asia, and in the temperate areas of southern Europe, and they moved into and out of the more northerly regions as the glacial regions retreated and advanced. [III] Their diet changed to adapt to the changes in plant and animal life during succeeding glacial and interglacial periods, and the use of stone tools, the building of shelters, and the use of clothing came about during this period. [IV] It was these sorts of adaptations that enabled humans to survive and progress while many other species failed to survive and thrive during this period in the way that prehistoric people did.