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== [[Faster-Than-Light Travel]] ==
== [[Faster-Than-Light Travel]] ==


[[FTL Travel]] is one of the bigger thorns in the side of the Hard SF genre. Special Relativity makes it absolutely clear: it is physically impossible to accelerate an object with any kind of mass so that it's moving faster than the speed of light. Even accelerating an object ''to'' the speed of light would require an infinite amount of energy. However, we've also pretty much established that there are no other technological species on any planet in the Solar system other than Earth. If we want to have space adventures involving high-tech aliens, we'll have to travel to other star systems, and the distances involved are so enormous that it would take years to get from one star to another if you were limited to sub-light speeds. Science Fiction writers have had to compromise, and allow ''some'' means of travelling faster-than-light which didn't turn their universe into something totally unrecognizable to a modern reader. Therefore, the ability to move faster-than-light has received more attention in SF than any other fantastic concept as to ways to Limit The Damage of having it around.
[[FTL Travel]] is one of the bigger thorns in the side of the Hard SF genre. Special Relativity makes it absolutely clear: it is physically impossible to accelerate an object with any kind of mass so that it's moving faster than the speed of light. Even accelerating an object ''to'' the speed of light would require an infinite amount of energy. However, we've also pretty much established that there are no other technological species on any planet in the Solar system other than Earth. If we want to have space adventures involving high-tech aliens, we'll have to travel to other star systems, and the distances involved are so enormous that it would take years to get from one star to another if you were limited to sub-light speeds. Science Fiction writers have had to compromise, and allow ''some'' means of travelling faster-than-light which didn't turn their universe into something totally unrecognizable to a modern reader. Therefore, the ability to move faster-than-light has received more attention in SF than any other fantastic concept as to ways to limit the damage of having it around.


The very worst problem with FTL travel (or even just [[FTL Radio]]) is a certain niggling consequence of [[Time Dilation]]. When travelling at any speed, even a brisk walk, relative to somebody else, you'll see his clock move slower than yours -- but he'll see ''your'' clock move slower than ''his''. This way-counterintuitive state of affairs means that some distant events in the universe which are in your future are in the other guy's past, and vice-versa. Without FTL travel, though, this isn't a problem. Einstein and Minkowsky established that for any event that's in Oberver A's future and Observer B's past, no matter how far in Observer A's future the event is, it will always be far enough away that any ''light-speed signals'' from this event would not reach Observer A until the event was also in Observer A's past. When plotted on a space-time graph, the signals from the event would stretch out in spacetime in a "light cone," which guarantees that the signal will not reach any observer in the universe until the event is in that observer's past. To put it another way, let's say that in Observer A's reference frame, Event 1 occurs before Event 2, but in Observer B's reference frame, Event 2 occurs before Event 1. Light cones maintains ''causality'' by ensuring that, if Observer A would find out about Event 1 before Event 2, Observer B ''cannot'' find out about Event 2 before information about Event 1 is theoretically available to him.
The very worst problem with FTL travel (or even just [[FTL Radio]]) is a certain extremely strange consequence of [[Time Dilation]]. When travelling at any speed, even a brisk walk, relative to somebody else, you'll see his clock move slower than yours -- but he'll see ''your'' clock move slower than ''his''. This extremely counter-intuitive state of affairs means that some distant events in the universe which are in your future are in the other guy's past, and vice-versa. Without FTL travel, though, this isn't a problem. Einstein and Minkowsky established that for any event that's in Oberver A's future and Observer B's past, no matter how far in Observer A's future the event is, it will always be far enough away that any ''light-speed signals'' from this event would not reach Observer A until the event was also in Observer A's past. When plotted on a space-time graph, the signals from the event would stretch out in spacetime in a "light cone," which guarantees that the signal will not reach any observer in the universe until the event is in that observer's past. To put it another way, let's say that in Observer A's reference frame, Event 1 occurs before Event 2, but in Observer B's reference frame, Event 2 occurs before Event 1. Light cones maintains ''causality'' by ensuring that, if Observer A would find out about Event 1 before Event 2, Observer B ''cannot'' find out about Event 2 before information about Event 1 is theoretically available to him.


Here's an example: Suppose Observer A is standing on Earth, and Observer B is in a space ship, coasting in a straight line at 86.60254% of the speed of light. This gives him a gamma (γ) factor of exactly 2. When the space ship passes by Earth, both Oberver A and Observer B synchronize their clocks at 5:00 PM. In Observer A's frame of reference, when his clock reads 7:00 PM, Observer B's clock will read 6:00 PM. However, in Observer B's frame of reference, when his clock reads 7:00 PM, Observer A's clock will read 6:00 PM. At 6:20 PM on Observer A's clock, an event happens on Earth -- the winning State Lottery numbers are announced. At 7 PM on Observer A's clock, this event is 40 minutes in Observer A's past; but at 7 PM on Observer B's clock, this event is still 20 minutes in Observer B's future. It's not just that Observer B ''perceives'' it to be in the future, it really ''is'' in the future, it really hasn't happened yet. What prevents Observer B from knowing about the event before it happens in his reference frame is that it takes ''time'' for any information about the event to reach him. At 6:20 PM in Observer A's reference frame, Observer B's clock would only read 5:40 PM, but Observer B would be 69.282 light-minutes away from Earth; if Observer A radioed the winning lottery numbers to Observer B at this moment, they'd take 521 minutes in Observer A's reference frame to reach Observer B's space ship, at which point Observer B's clock would read 9:40 PM and the event would be 4 hours in Observer B's past. Even if Observer B magically reversed his velocity at 5:40 PM on his clock, so that he was headed ''toward'' Earth at 0.866''c'' instead of away from it from that moment onward, the radio signal would still take 37.128 minutes in Observer A's frame of reference to reach the space ship.
Here's an example: Suppose Observer A is standing on Earth, and Observer B is in a space ship, coasting in a straight line at 86.60254% of the speed of light. This gives him a gamma (γ) factor of exactly 2. When the space ship passes by Earth, both Oberver A and Observer B synchronize their clocks at 5:00 PM. In Observer A's frame of reference, when his clock reads 7:00 PM, Observer B's clock will read 6:00 PM. However, in Observer B's frame of reference, when his clock reads 7:00 PM, Observer A's clock will read 6:00 PM. At 6:20 PM on Observer A's clock, an event happens on Earth -- the winning State Lottery numbers are announced. At 7 PM on Observer A's clock, this event is 40 minutes in Observer A's past; but at 7 PM on Observer B's clock, this event is still 20 minutes in Observer B's future. It's not just that Observer B ''perceives'' it to be in the future, it really ''is'' in the future, it really hasn't happened yet. What prevents Observer B from knowing about the event before it happens in his reference frame is that it takes ''time'' for any information about the event to reach him. At 6:20 PM in Observer A's reference frame, Observer B's clock would only read 5:40 PM, but Observer B would be 69.282 light-minutes away from Earth; if Observer A radioed the winning lottery numbers to Observer B at this moment, they'd take 521 minutes in Observer A's reference frame to reach Observer B's space ship, at which point Observer B's clock would read 9:40 PM and the event would be 4 hours in Observer B's past. Even if Observer B magically reversed his velocity at 5:40 PM on his clock, so that he was headed ''toward'' Earth at 0.866''c'' instead of away from it from that moment onward, the radio signal would still take 37.128 minutes in Observer A's frame of reference to reach the space ship.
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But by going faster than light, even just [[FTL Radio]], you ''can'' receive information about events that are in your own future. You can perceive Event 2, which was caused by Event 1, before Event 1 actually occurs in your reference frame. In our lottery-winning scenario above, suppose Observer A and Observer B had a [[Subspace Ansible]] that allowed instant communication no matter how far apart they were. Observer A could send the winning lottery numbers to Observer B's space ship at 6:20 PM on Oberver A's clock. With instantaneous communication, the numbers would arrive on board the space ship at 5:40 PM on Observer B's clock. If Observer B then sent the same numbers ''back to Observer A'' over the same subspace ansible, they'd arrive on Earth at 5:20 PM on Observer A's clock. Observer A would have the winning lottery numbers ''an hour before they were announced.''
But by going faster than light, even just [[FTL Radio]], you ''can'' receive information about events that are in your own future. You can perceive Event 2, which was caused by Event 1, before Event 1 actually occurs in your reference frame. In our lottery-winning scenario above, suppose Observer A and Observer B had a [[Subspace Ansible]] that allowed instant communication no matter how far apart they were. Observer A could send the winning lottery numbers to Observer B's space ship at 6:20 PM on Oberver A's clock. With instantaneous communication, the numbers would arrive on board the space ship at 5:40 PM on Observer B's clock. If Observer B then sent the same numbers ''back to Observer A'' over the same subspace ansible, they'd arrive on Earth at 5:20 PM on Observer A's clock. Observer A would have the winning lottery numbers ''an hour before they were announced.''


In other words, '''[[Time Travel]]'''.
[[Translation: "Yes"|In other words]], '''[[Time Travel]]'''.


How do veteran SF writers handle the time travel consequences of FTL travel? Most of them don't. They simply [[Hand Wave|sweep it under the rug]] and hope no one will notice. Those authors who do address it often end up with bizarre universes where wars are fought before they've even started, and characters can [[Grandfather Paradox|shoot their own grandfathers]].
How do veteran SF writers handle the time travel consequences of FTL travel? Most of them don't. They simply [[Hand Wave|sweep it under the rug]] and hope no one will notice. Those authors who do address it often end up with bizarre universes where wars are fought before they've even started, and characters can [[Grandfather Paradox|shoot their own grandfathers]].
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