Why Time Travel Into the Past is Impossible

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I always have heard that time travel is currently impossible because, to do so, one would have to travel faster than the speed of light?

Only if you are using that method. Wormholes present a different theoretical technique because they cause a rift in space-time that allows you to travel to another point in space-time without needing to go there at a specific speed. As to the reason why faster than light travel is not possible, it is because "M" or mass in E=MC^2 becomes infinite at "C" (speed of light or "Constant"), and mass cannot be infinite. ("E"= Energy, if anyone is curious)

What is a wormhole as compared to space-time? A oft-used illustration is a piece of paper. Think of the paper as space-time. Now, for you to move your finger from one point on the sheet of paper to another takes a amount of time relative to the speed at which you can move your finger. However, if you were to fold the paper and make the two points touch, then the transit time becomes instantaneous. This is what a wormhole is. It essentially folds space-time. Now, this would actually be a faster than light method of travel, but it isn't. Remember that Einstein was referring to speed within a closed system, not absolute speed. After all, if you had two objects in space moving away from a common point at 0.6 C then they would be moving away from each other at 1.2 C , or 20% faster than the speed of light. This is not how it is measured though as there are an infinite number of things moving away from or towards each other faster than light in the universe at any given time. Instead, it is a relative measure, which is why it requires the closed system.

The Anime Steins;Gate has a good theory of time travel. Instead of transferring people across time which could create a paradox instead transfer information across time. The show uses text messages to pass the information back in time. The past receiver of the information would then read it and act upon the information accordingly. This however has it's limitations.

1. It uses a lot of energy.
2. It can only send a small about of information 36 bytes
3. The person receiving the information in the past can choose not to follow the advice
4. The information can only be sent to a Pager or Mobile so it cannot have any affect before that time.
5. When it gets sent the world reverts back to the time sent and all memories of the event before this are forgotten. (You forget you sent and the reason for sending the information)

It uses the time line system. This means at the point in time the information was sent back the time line splits and heads of in a different direction. If there is a difference of over 1% then a new time line is created.

They also work out a way of moving people back in time. Instead of sending information they use black holes to compress memories into 36 bytes and sends them back instead.

In the PBS popular science show NOVA episode 2 "The Illusion of Time" in the series The Fabric of the Cosmos, physicist and author Brian Greene explains some aspects of General Relativity pertaining to this thread.

Apparently, according to Einstein's equations which seem to be supported by real-life measurements, if an alien were sitting ten billion (the number he used, probably what was needed to make the results come out right for this example) light years away from you but not moving relative to you, you and that alien share a common now moment. Your now is his now, and vice versa. But if that alien got on a bicycle and started moving away from you (not very fast, being on a bicycle) then his now would be two hundred years in your past.

That's weird enough, but then if that alien were to turn the bicycle around and start pedaling towards you instead of away from you, his now would be two hundred years in your future. Our past or our future could be somebody else's "now", and since their now is just as valid as ours (is it?) then apparently the past is not gone and the future is already here. Of course there might be more than one possible past and more than one possible future, but apparently they all exist outside of time (or in time but all times are "now"). Trippy stuff!

I used to worry that some of my best guitar playing was never recorded, but if this relativity stuff is true, then they were recorded and not only that, those moments STILL exist just as real as the present moment! If so, then maybe someday we might be able to at least view (if not influence) so-called "past" events. I want to see where the bullet came from that killed the Red Baron. I have a bit less interest in the one that killed JFK, although I realize his death had a greater impact on more people than the Red Baron's death. Christ on a cross anyone? We could settle once and for all whether or not Christ existed.

On a related note is something Alan Watts once said. The past does not cause the present. The present causes the past. It trails behind the present moment like the wake of a ship, fading away the further back one looks from the present, but the wake of the ship does not push the ship. Opinions on this, anyone?

Larry Niven's First Law of Time Travel: If your plenum permits travel into the past, and events can be changed, time travel will never be invented.

People will change the past. Others will seek to undo those changes by changing something else. Eventually, things will be so fracked up that someone will decide the only way to fix it all is to kill the inventor of the time machine before he can invent it, thus wiping out all the changes made by the machine's invention.

And if someone else invents it later, the same cycle occurs...

Backward time travel contradicts the flow from cause to effect.

Or to put it another way, determinate cause to effect and backward time travel contradict each other.

ruveyn

then again cause and effect have already been broken experimentally.

Even so, when effects come from causes they never precede the causes in time.

ruveyn

yes they do,

a measurement can be affected by something that happens in the future, experimentally.

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