Faster than light travel. Does this make sense?

Factual Questions
1

Explain the faults in my thinking.

On a summer day you see a man roofing a house. If you are about 100m away, you may notice that the sound of the hammer does not coincide with the swing of the roofer’s arm. That is you see the arm in the up position as you hear the sound of the hammer striking the nail. Obviously, this is because light travels faster than sound. So by the time the sound reaches you, the hammer is already up.

So by this logic, if the roofer was a John Henry type, and could swing the hammer faster than the speed of light, you would see his arm rise before he lifted it for the swing, then hear the sound of the hammer striking way after the swing, actually several arm motions later.

Suppose instead of it being a roofer, you were observing Mr. Jones, your neighborhood jogger. Let’s say Mr. Jones bought a great pair of running shoes and was sprinting to the end of the block at faster than light speed. Would you see Mr. Jones at the end of the block before he actually got there, or would you see him running along even though he was actually at the end of the block?

For another example, let’s say Ms. Wallace wanted to go to Busch Gardens in Tampa, Florida from Shreveport, Louisiana. Ms. Wallace just bought the brand new Camaro and had some work done on it so she can drive faster than the speed of light. She plugged in her GPS, started the car and drove off at light speed plus .5. After she had only gone a mile she realized that she forgot her sunscreen, and must turn back. Does this create a paradox since her destination was changed after she left? (Assuming she could hook one helluva U-Turn and get back without slowing down.)

My logic tells me that each of our travelers would arrive before they left, thus producing a form of time travel. Mr. Jones and Ms. Wallace would be able to travel into the future relative to an observer, but would not be in the future relative to them. That is, Mr. Jones would still feel the time elapse as he ran, but the observer would see him cross the finish line faster than he ran, right? Or am I backwards? Either way, the physical form of Mr. Jones would have to be in two places at the same time, relative to an observer. If Mr. Jones got tired during his run, and slowed down to a pace of about 4 mph, would an observer see both forms of Mr. Jones catching each other?

Where does my logic fail?

SSG Schwartz

2

Relativity is missing.

3

In assuming faster-than-light neighbors. You are actually correct in your conclusion that FTL travel is equivalent to time travel, though you need a bit more sophisticated argument to show it.

4

Congratulations, you’ve just described the Picard Maneuver.

5

Can I ask for some clarification here? The OP said “my logic tells me that each of these travelers would arrive before they left.” It seems to me that this would be true from the standpoint of the traveler but not from the standpoint of the observer. For example, if I left Earth now traveling at exactly the speed of light and heading for the sun, I would get there in about six minutes (as measured from the viewpoint of a stationary observer). If I were traveling at a little faster than the speed of light, I would get there in a little less than six minutes… right? To get there before I left, wouldn’t I have to be traveling at an infinite speed?

6

In order to construct a scenario that results in time travel, you need to include frames of reference that are moving in relation to each other. Classic Relativity in the Real World that doesn’t have FTL shows that if you pick the right frame, you can alter the apparent order events happen in the different frames. Then if you allow FTL travel, all you have to do is make an FTL jump from the frame in which A happens before B to the frame where B happens before A, and there’s your time travel.

7

How do you figure? No matter how fast the roofer is swinging the arm, the light reflecting off his arm travels at the speed of light, and you will see his arm in a particular position 100m/c seconds after it has been in that position, never before.

Again, I don’t see what Mr. Jones’s speed has to do with the speed of light reflecting off of him and traveling to you, striking your eye. Even if it were possible even in theory even in a thought experiment for Jones to be running at faster than the speed of light (which, incidentally, it is not), the light that will bounce off him and travel to you will still be traveling at c.

8

There’s also the other Picard maneuver (currently the number 1 definition at Urban Dictionary).

9

Ah, but does he tug down the hem FTL? :slight_smile:

10

Going by the Picard Maneuver example (the ship, not the shirt), I would think that, given a distant ship that suddenly accelerated to FTL, headed toward you, and stopped, it would appear like this to you, the observer:

  • it would suddenly blink into existence directly in front of you, while the distant “double” of the ship remained in place

  • then, you would see THREE ships: the one that stopped in front of you, the distant double from before the ship moved, and one between that would appear to be rushing away from you, backwards (as photons that the ship encountered en route to you are reflected back at you; those that started out nearest you would reach you earlier than those that started out farther away, despite the order in which the traveling ship encountered them)

  • eventually (depending on distance of starting point and speed above C) the intermediate, backwards-traveling ship would reach the distant double (which, again, based on starting distance and speed, may appear to have begun moving toward you) and merge with it, and both ships would disappear as the last reflected photon reaches you, leaving only the nearby ship.

11

Back when I first saw that “Picard Maneuver” episode of TNG, I was a bit disappointed in the writers that according to them, it took almost 300 years of FTL being available before someone discovered something so very basic. Oddities involving the apparent positions of objects travelling at FTL speeds should have been demonstrated practically from day 1 of warp experiments.

12

Things with mass can only approach the speed of light, they cant reach it nor can they go faster than it. They would begin to aquire an infinite mass. If youre talking FTL youre talking fiction, so purple elephants magically appearing works just as well as all the things youve written. Read up on some relativity.

13

I totally agree with toadspittle’s description in Post #10. The big differences between that description and the OP are that toadspittle’s story works only for objects moving towards the observer, and that it is merely an illusion, not a true time-travel function.

The OP tries to describe this which are seen before they actually occur, and that just doesn’t make any sense to me. Toadspittle describes things which are seen out of sequence, and that makes perfect sense.

14

Thanks for all the responses so far. I never really understood physics, or, for that matter Star Trek.

SSG Schwartz

15

I think the problem would be emerging from warp precisely enough to be useful - you have to be close enough to get in a shot, and not past your opponent.

Oddly enough an old PLATO game called Michelin, later Empire, had a bug which allowed the Picard maneuver - in 1975. One type of ship, the Orions, was weak but fast. The plasma panel updated every so often, and with the right speed and direction, an Orion player could move faster than the update, being on top of you without seemingly to have occupied the intermediate space. I’m not sure of how this was done - I played Federation ships.

ETA: I vaguely remember this same maneuver in a Blish adaptation of a Star Trek story (if not in Spock Must Die.) I remember the line “one second of warp 1” (or less) which is kind of what you’d do.

16

Yike. Than makes my head spin (slower than light speed.)

17

Even though objects can’t travel faster than light, you can create superluminal visual effects that mimic the behavior you’re describing.

For example, aim a laser beam at a spinning mirror (like the set-up used in a supermarket scanner). Then position the rotating beam so it sweeps across a wall. If the wall is far enough away, the spot will travel across the wall faster than the speed of light.

(This doesn’t violate relativity because the spot isn’t actually moving. It’s just an illusion of movement caused by the rotating beam.)

If you stand near the wall, so the spot is sweeping toward you, what you’ll actually see is the spot sweeping AWAY from you. The spot is moving so fast that light from later positions arrives at your eye before the light from earlier positions. Essentially you’ll see the spot moving backwards in time.

18

Before I run out to the hardware store for supplies, Hamster King, could you quantify “far enough away”? :slight_smile:

19

It depends on how fast your mirror is spinning. If it’s spinning at ~12 revolutions per second, you should be able to set up the laser here in L.A. and the wall in New York City.

(Of course the curvature of the earth gets in the way … .) :smiley:

20

That depends on how fast your mirror can rotate. It looks like there are Dremel tools which can hit 35000 RPM, so we’ll use that as a standard. That comes out to about 3,700 radians per second, which means that the beam will be rotating at twice that speed (due to angle of reflection equaling angle of incidence), or about 7,300 radians per second. The speed of light is 3*10[sup]8[/sup] m/s, so to exceed the speed of light, you’d need a screen distance of 3.0e8/7.3e3 meters, or about 41 kilometers.