The American Programmer

Science fiction books spin fascinating tales of travels to the future, or to the past. The hero "goes" to a time in the past and sees things that happened. The hero is tempted to change the course of history, but doesn't. All reality is safe.

Or sometimes the hero changes the course of history. The hero "kills" his or her grandparent and so is never born and can't be there killing the grandparent. This sort of contradiction is often called a paradox. It is no paradox. It is impossible. It can't happen.

The hero "travels" to the future and sees things that "are going to" happen. He or she sees John F. Kennedy alive and well, at a golf course. Old and gray, but still kicking. That "means" that the hero has a new mission: go back and prevent Kennedy's murder. The hero rushes back to Nov 22, 1963 and knocks the gun out of Oswald's hand. Then rushes back again to the future to see John F. Kennedy get a hole in one.

I like science fiction. I enjoy science fiction. I like imagining "what if" scenarios. Wouldn't it be wonderful if we could go back and stop Hitler before he got to power? How many of us would love to go back to the morning of September 11, 2001 and work in Airport Security, just for that morning.

Fascinating, but impossible. I'll be very clear: "Time Travel" is impossible. It can't happen. It will never happen. It never happened. Time is not something you can travel in.

Why are you so doggone sure, and why are scientists saying "well, maybe, we don't know, the equations don't rule it out, the equations allow it...". I'm glad you asked. I'm quite sure because events follow one another in a cause-effect sequence. The cause precedes the effect. This can not be proven through experiments. It is proven philosophically, however.
Most scientists rely entirely on experimental methods. If they can't do an experiment that proves a theory, they don't accept the theory; and if they can't do an experiment that disproves the theory, they don't reject the theory.

About the equations. Equations are created in an effort to explain reality. Equations are not reality. Equations do not tell reality what to do. If the equations do not represent reality correctly, the equations are wrong.

What's this "Cause and Effect" stuff? Just the way that everything works in the Universe. An object hits another object and gives it some of its energy of motion. My cue stick hits the cue ball and sets it moving. It hits the colored ball which then goes off and stops near the pocket.
If you want to say that it's possible for the colored ball to come back and touch the cue ball which then comes back and hits my cue stick, go right ahead. However we have nothing further to say, because your understanding of reality and mine are completely different.

Time is the assigning of numbers to the motion of bodies. The Earth goes around the Sun and we call it a year. The Moon goes around the Earth and we call it a month. The Earth spins on its axis and we call it a day. Chop it up into 24 hours, then each hour into 60 minutes, then each minute into 60 seconds. Motion is what happens. Time is how we understand and deal with this motion. There is no objective reality "Time". There is motion. Objects move. Time is the idea that we form of this motion as a succession of events.
We need time so that you and your coworkers can all be able to meet at 9 AM in the East Conference room on Tuesday.

Suppose you want to go back to 2007, Dec 31, 11:59 PM, Times Square, facing the big ball. Can't do it because Matter cannot be created or destroyed. We learned this in Physics class. This means that it's impossible for some matter to just "appear" all of a sudden. Even if this matter is the "time traveler".
You don't believe it and go to Times Square with your "Time Machine" and turn the dial to 2007, Dec 31, 11:59 PM. "Well I want to go back and see the ball drop. I missed it on TV". Sorry. You can't just disappear from now. That would destroy your matter in the now.

Well, let's suppose that that doesn't make any difference (just for the argument) and you can suddenly make a chunk of matter (you) disappear now, and reappear in Times Square in 2007. You go to Times Square with your strange looking time-machine contraption. New Yorkers just walk around you.
You suddenly do go back to 2007, Dec 31, 11:59 PM, Times Square, facing the big ball.
Huge explosion. Times Square in 2007 is destroyed, you die a painful death, New York City is levelled, the ground is flattened all the way to Stamford. Your poor mangled body tried to displace a whole lot of air in a real big hurry. Two objects can't occupy the same space at the same time (remember physics class?). That air and you try to take up the same space. A lot of pain as the little air molecules get into the same place as your body's molecules.
You didn't hear it, but there was a big pop when you left "now". A really big pop. All the air molecules surrounding your body are looking at a vacuum. Nature abhors a vacuum (physics class), and the air rushes in to fill the space where you were. Times Square "now" is shaken. All the windows are broken. All the people milling around the area are knocked flat. Everybody for a mile around is permanently deaf.

Keep supposing. Sometimes it's necessary to prove something is wrong because it leads to absurdity. I hope your "Time Machine" was real good. Because Times Square in 2007 was not in the same place as it is now. What? you say! Yes. The Earth has moved. It's gone for a bit of a ride around the Sun. Its spin on its axis would put it in a different place. The Sun has been orbiting around the center of the galaxy, and the whole galaxy has been rotating around the center of the Universe. Your "Time Machine" had better be a space ship too, because it had better take you to wherever the Earth was in 2007. Take oxygen.
If your "Time Machine" doesn't take you exactly where you want to be, get ready for more pain, as you suddenly reappear in 2007, inside of a speeding yellow taxi! That's got to hurt!

Much of the popular confusion regarding the possibility of "Time Travel" comes from the idea that time and space are interchangeable. The term "spacetime" goes a long way in contributing to this error.

Time is sometimes considered as a "fourth dimension". Dimension means measurement, so it's understandable that one might think this way.

The idea of "parallel universes" is brought in to explain how it might be possible to go back into the past and change something. You would change something and then there would spring forth a new universe. Now you have two "universes". The one that we know, and the new one that you created by changing something. Carl Sagan said that the Universe is all that there is, that there ever was, and that there will ever be. There is only one Universe. If a person is able to accept the idea of multiple universes, then it is no surprise that that person can accept the idea of "Time Travel", and probably lives in an alternate universe too.

Check to see if any new books have been published on Time Travel: Books on Time Travel at Amazon.com

Time Traveler: A Scientist's Personal Mission to Make Time Travel a Reality By Ronald L. Mallett A college professor has great hopes that one day a "Time Machine" can be built, and that there can be "Time Travel". Inspect/Order Time Traveler: A Scientist's Personal Mission to Make Time Travel a Reality

The Science Before Science: A Guide to Thinking in the 21st Century By Anthony Rizzi One of the few books that talks about "Time Travel" and disproves it. The book devotes a small amount of space to "Time Travel", and shows why it can't happen. Inspect/Order The Science Before Science: A Guide to Thinking in the 21st Century

Physics of the Impossible: A Scientific Exploration into the World of Phasers, Force Fields, Teleportation, and Time Travel By Michio Kaku A very interesting, well-written, and readable book on many scientific questions such as invisibility, teleportation, time travel, parallel universes and perpetual motion machines. He explains his 3 types of "impossibilities". Class 1: we don't know how to do it, but when we advance some more, we should be able to do it. Teleportation, psychokinesis, and invisibility are placed in this category. Class 2: it may or may not be possible, and if it can be done, it would be millions of years in the future. This includes time travel, hyperspace travel and travel through wormholes. Class 3: won't happen and can't happen. Really impossible because they violate the known laws of physics. No civilization, no matter how advanced, will be able to do it. My comments. If teleportation means beaming a live human several miles away, it should be in category 2. If invisibility means that a live human being can be made truly invisible (not just hard to see), this should be in category 2. If invisibility means "good enough invisibility - good enough to fool radar, cameras, and eyes at a distance, this should stay in in category 1. If time travel means that a live human being can "go back" to be present and interact with people and events that once existed, this should be in category 3. If time travel means going really fast in a space ship and coming back when all your friends are long gone, this should be in category 1. If time travel means unusual events on the atomic level that seem to have an effect that precedes the cause, this should be in category 1. Inspect/Order Physics of the Impossible

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