Astro News
- Recent Updates of Astro News
- Active Mercury(07/09)
- Hubble Status Report: Directly Observes a Planet Orbiting Another Star(01/09)
- A Non-trivial Answer to a Trivial Astronomical Question-The Origin Of Absolute Magnitude(07/08)
- Assault by a Black Hole(04/08)
- New Lakes Discovered on Titan(01/08)
- ˇ§Deviant Behaviourˇ¨ in the Solar System(10/07)
- Cosmic Ripples - Cosmic Microwave Background - CMB(07/07)
- Interplanetary Superhighway(04/07)
- Is Pluto a Planet?(01/07)
- Breathing Moonrocks(10/06)
- My Thoughts on the Theory of Relativity, Quantum Mechanics, Superstring Theory and Dark Matter(07/06)
- Space-time Vortex(04/06)
- Radio Astronomy(01/06)
- Neutrino Astronomy(10/05)
- The Active Earth(07/05)
- What is Dark Energy?(04/05)
- The Mysterious Black Holes(01/05)
- Intermediate-Mass Black Holes And Quasisoft X-Ray Sources(10/04)
- Time Travel: From a Scientific Approach(07/04)
- What is Astrobiology?(04/04)
- Black Hole: From Fantasy To Reality (II)(01/04)
- Black Hole: From Fantasy To Reality (I)(10/03)
- From The Oldest Light In The Universe To The Fate Of The Universe(7/03)
- The Cosmic HERO(4/03)
- Quaoar - the Tenth Member of the Solar System?(1/03)
- The First Chinese Telescope in Space(10/02)
- Diamonds and Other Stardust(7/02)
- Supermassive Black Hole in Andromeda Galaxy(4/02)
- Detection of Solar Neutrinos(1/02)
- Simultaneous Multiple Wavwlength Observation(10/01)
- Celestial Distance(7/01)
- Solar-Terrestrial Relations(7/00)
- Fundamental Particles in Astronomy(4/00)
- The Solar Maximum in 2000(1/00)
- Hubble Constant(10/99)
- New Findings on Cosmology(7/99)
- Strange Stars(4/99)
- How Strong Stellar Magnetic Field Can Be?(1/99)

Important notices

What is time travel? The story lines of many movies and TV programmes are often on time travel. Usually that is what happens: the leading character walked into a time machine. After some sophisticated or simple operations, the machine was activated. Our hero and everything near him passed slowly in terms of time. For example, he did not grow old apparently. But the time outside the machine experienced a sea change. He now could freely go back into the past or go into the future. In the strict discipline of science, can we construct a time machine like that?

Early in 1895, English novelist H.G. Wells has already published a science fiction
which talked about time travel.

Scientists discovered that time travelling in the forward or backward directions are totally different in nature. Time travel to the future is easy while going back into the past is extremely difficult or may even be impossible. Let's first discuss about the time travel to the future. According to Albert Einstein's theory of relativity, a clock moving at high speed will run slower than a stationary clock. For example, our watch may tick twice that translates into 2 seconds. However, during the same period of time, a clock which is moving at high speed may only tick once and hence, for this moving clock, only one second is passed. The higher the speed of the moving clock, the slower it ticks. The theory of relativity points out that nothing travels faster than light. In addition, the closer the speed of the clock to light, the slower it ticks. This phenomenon is not restricted to a clock. Time runs slow for any high-speed moving object. 

If we can build a spaceship which can travel at high speed in relative to the Earth, the astronauts inside are time travelling in reference to the Earth, though strangely they will find nothing unusual. For example, they still report that, as usual, it takes an hour to watch a TV drama. It is because everything inside the spaceship will slow down at the same rate. However, people on Earth will see that every action inside the spaceship is carried out in slow motion. A TV drama will drag on much longer than an hour. In simple words, this spaceship is the time machine we need to go into the future. For when it returns to Earth, the astronauts onboard will think that it is only a short journey, notwithstanding the Earth has already passed a long time. Up to now we are still not able to construct a spaceship that can travel with speed high enough that the relativity effect can be easily appreciated. But Scientists are already able to accelerate small particles to speed near that of light and find that short-lived particles can survive much longer than their natural life as it is travelling in a time machine.

From the aforesaid, we have full theoretical and experimental support to build a time machine to the future. Yet we still find no scientifically feasible way to go back into the past. In popular science publications, occasionally scientists might propose time travelling methods to the past. But with detailed study, all such methods are actually impractical in one way or other. For example, some may suggest to use the relative of black hole, wormhole, as time travelling vehicle. The problem is: wormhole is a one-way spacetime tunnel with no exit. Some think of cosmic string. Then they soon realize that this will require the total energy from the universe to construct the time machine out of cosmic string and hence is also impossible. That leads to the suggestion by many scientists of the not-yet-proven "Chronology Protection Conjecture", which says that the laws of physics do not allow time machines to the past. 

The "Chronology Protection Conjecture" was proposed by the famous physicist
Stephen Hawkings who believed that the universe will protect itself
from time paradox by not allowing time machines to be made.

Though scientists right now cannot rule out the possibility of ever building a time machine to the past, we are sure that even such machines are possible we can only go back to the moment when the first time machine was fabricated. In other words, our distant descendants cannot go back to our time for we cannot at present build such a machine yet. This is easy to understand. If that is not the case, our world should already be laden with tourists from the future.

If there is a machine that enables us to go back to the past, then two kinds of logical paradoxes will be created. The first is the famous paradox of patricide. If one has a time machine, he should be able to use it to go back in time and kill his father before he was conceived, thereby preventing himself from being born. The problem is then who went back in time to kill his father in the first place? The second is the paradox of creations. Say if a future man comes back to the beginning of the 20th century and tells Einstein about the theory of relativity. Then who is the one first to put forward the theory of relativity? Knowledge is passed from generation to generation. To trace the origin, we all learn the theory of relativity from Einstein. But if Einstein learned it from the future man, everything is going into a circle and we will be astonished to find that no one ever discovered it. Could knowledge be created out of nothing?

Scientists cannot solve these logical paradoxes by laws of physics as at present understood. Neither are they clear whether time machines are doomed to associate with such paradoxes. For example, can we build a time machine that can be void of all logical fallacies? Or time machines to the past are simply forbidden by the physical laws? These are the questions that scientists have to study.

Artist Concept of wormhole