Is Time Travel possible?
Traveling forward in time is easy enough. If you move close to the speed of light or sit in a strong gravitational field, you experience time more slowly than other people do--another way of saying that you travel into their future.Traveling into the past is rather trickier. Relativity theory allows it in certain spacetime configurations: a rotating universe, a rotating cylinder and, most famously, a wormhole--a tunnel through space and timeOur best understanding of time comes from Einstein's theories of relativity. Before this, time was widely regarded as absolute and universal, the same for everyone no matter what their physical circumstances were. In his special theory of relativity, Einstein proposed that the measured interval between two events depends on how the observer is moving. Crucially, two observers who move differently will experience different durations between the same two events. The effect is often described using the "twin paradox." Suppose that Paul and Sebastian are twins. Paul boards a rocket ship and travels at high speed to a nearby star, turns around and flies back to Earth, while Sebastian stays at home. For Paul the duration of the journey might be, say, one year, but when she returns and steps out of the spaceship, she finds that 10 years have elapsed on Earth. His brother is now nine years older than she is. Paul and Sebastian are no longer the same age, despite the fact that they were born on the same day. This example illustrates a limited type of time travel. In effect, Paul has leaped nine years into Earth's future.In science fiction, wormholes are sometimes called stargates; they offer a shortcut between two widely separated points in space. Jump through a hypothetical wormhole, and you might come out moments later on the other side of the galaxy. Wormholes naturally fit into the general theory of relativity, whereby gravity warps not only time but also space. The theory allows the analogue of alternative road and tunnel routes connecting two points in space. Mathematicians refer to such a space as multiply connected. Just as a tunnel passing under a hill can be shorter than the surface street, a wormhole may be shorter than the usual route through ordinary space. Theorizing the concept of time travel is great exercise for the brain. It is not necessarily the question of "can we do it?" but "should we do it?
Strange Physics Theories
Left Brain/Right Brain/Overloaded Brain
Wave Particle Duality
Matter and light have properties of both waves and particles simultaneously. The results of quantum mechanics make it clear that waves exhibit particle-like properties and particles exhibit wave-like properties, depending on the specific experiment. Quantum physics is therefore able to make descriptions of matter and energy based on wave equations that relate to the probability of a particle existing in a certain spot at a certain time.
Einstein's Theory of Relativity
Einstein's theory of relativity is based upon the principle that the laws of physics are the same for all observers, regardless of where they are located or how fast they are moving or accelerating. This seemingly common sense principle predicts localized effects in the form of special relativity and defines gravitation as a geometric phenomenon in the form of general relativity.
Quantum Probability & the Measurement Problem
Quantum physics is defined mathematically by the Schroedinger equation, which depicts the probability of a particle being found at a certain point. This probability is fundamental to the system, not merely a result of ignorance. Once a measurement is made, however, you have a definite result.The measurement problem is that the theory doesn't completely explain how the act of measurement actually causes this change. Attempts to solve the problem have lead to some intriguing theories.
Heisenberg Uncertainty Principle
The physicist Werner Heisenberg developed the Heisenberg Uncertainty Principle, which says that when measuring the physical state of a quantum system there's a fundamental limit to the amount of precision that can be achieved.For example, the more precisely you measure the momentum of a particle the less precise your measurement of its position. Again, in Heisenberg's interpretation this wasn't just a measurement error or technological limitation, but an actual physical limit.
Quantum Entanglement & NonlocalityIn
In quantum theory, certain physical systems can become "entangled," meaning that their states are directly related to the state of another object somewhere else. When one object is measured, and the Schroedinger wavefunction collapses into a single state, the other object collapses into its corresponding state ... no matter how far away the objects are (i.e. nonlocality).Einstein, who called these influences "spooky action at a distance," illuminated this concept with his EPR Paradox.
Unified Field Theory
Unified field theory is a type of theory that goes about trying to reconcile quantum physics with Einstein's theory of general relativity. The following are examples of specific theories that fall under the heading of unified field theory:Quantum GravityString Theory / Superstring Theory / M-TheoryGrand Unified TheoryLoop Quantum GravityTheory of EverythingSupersymmetry
The Big Bang
When Albert Einstein developed the Theory of General Relativity, it predicted a possible expansion of the universe. Georges Lemaitre thought that this indicated the universe began in a single point. The name "Big Bang" was given by Fred Hoyle while mocking the theory during a radio broadcast.In 1929, Edwin Hubble discovered a redshift in distant galaxies, indicating that they were receding from Earth. Cosmic background microwave radiation, discovered in 1965, supported Lemaitre's theory.However, today some scientists believe the universe is expanding and will eventually contract, indefinitely.
Dark Matter & Dark Energy
Across astronomical distances, the only significant fundamental force of physics is gravity. Astronomers find that their calculations & observations don't quite match up, though.An undetected form of matter, called dark matter, was theorized to fix this. Recent evidence supports dark matter.Other work indicates that there might exist a dark energy, as well.Current estimates are that the universe is 70% dark energy, 25% dark matter, and only 5% of the universe is visible matter or energy!
In attempts to solve the measurement problem in quantum physics (see above), physicists frequently run into the problem of consciousness. Though most physicists try to sidestep the issue, it seems that there is a link between the conscious choice of experiment and the outcome of the experiment.Some physicists, most notably Roger Penrose, believe that current physics cannot explain consciousness, and that consciousness itself has a link to the strange quantum realm.
Recent evidence shows that were the universe just slightly different, it wouldn't exist long enough for any life to develop. The odds of a universe that we can exist in are very small, based on chance.The controversial Anthropic Principle states that the universe can only exist such that carbon-based life can arise.The Anthropic Principle, while intriguing, is more a philosophical theory than a physical one. Still, the Anthropic Principle poses an intriguing intellectual puzzle.