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Old September 20 2012, 05:31 AM   #46
gturner
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Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

newtype_alpha wrote: View Post
gturner wrote: View Post
newtype_alpha wrote: View Post
No, relativistically the speed of light remains constant in all reference frames, even if you're FTL with respect to the receiver. Message transit time is the same either way (you might want to stop accelerating for a while, though, in case the warp field scatters your radio signals). The thing is, your signal would be redshifted by such an insane degree that it might be below the detection threshold of anything that might be able to receive and decode it. 10c is enough of a doppler shift to go from x-rays to ULF radio, and that would have certain complications for message integrity.
I think the redshift is easily calculated because the sender isn't undergoing time dilation.
He is from the receiver's point of view, and so is the receiver from the sender's point of view.

The sender traveling to Alpha Centauri sends 5 months (their perceived time) worth of data, which gets strung out from Earth to Alpha Centauri, so the end of their message arrives 4.2 years later, a factor of 10 (their velocity in C). So a 20 GHz signal arrives at a frequency of 2 GHz.
Which would, once again, have the appearance of time dilation for all the usual reasons: the signals are still arriving at the speed of light, but they are redshifted to the point that time appears to be running at 10% of its normal speed at the transmitting ship. What's interesting is, the ship would still APPEAR to be traveling at about or slightly less than the speed of light (if you were looking at it with a telescope, let's say) despite the fact that it suddenly returns to Earth ten months later with a bunch of photographs and rock samples from Alpha Centauri's dwarf planets; the crew of the ship would actually be able to look out into space and see a reflection of themselves, still in transit, nearly nearly seven years later. So not only the appearance of time dilation, but the appearance of time TRAVEL: the starship is seen arriving at Earth BEFORE it is seen arriving at Alpha Centauri.

Note that the time dilation effect is only observed from Earth's point of view and is again a consequence of the immense distances and the limitations of the speed of light. Time doesn't ACTUALLY dilate, it just seems that way because of the huge differences in their respective reference frames. Likewise, the starship traveling at FTL speeds will almost immediately begin to overtake its own radio transmissions and will eventually look back at Earth and see the planet the way it was years before they actually launched; warping back TO Earth, seems to speed up time in the same way.
I don't think we can call that time dilation, as it doesn't actually involve anything to do with time itself, just purely Newtonian tranmission speeds and velocities.

The same would hold true if we were talking about sound, where an airplane can fly so fast that the doppler shift is dramatic, and can even exceed the speed of sound so that the aircraft arrives before you hear it. Sound and a supersonic aircraft have all the same unusual properties as this warp ship.

If true time dilation were going on, the time on the ship would slow down, so that the crew would only perceive, for example, five days in transit instead of five months, and then get strung out over 4.x years, so their message would get redshifted by a factor of 300 instead of 10 (If I'm doing that right).
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Old September 20 2012, 05:52 AM   #47
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Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

gturner wrote: View Post
I don't think we can call that time dilation, as it doesn't actually involve anything to do with time itself
Strictly speaking, actual time dilation probably doesn't either.

If true time dilation were going on, the time on the ship would slow down, so that the crew would only perceive, for example, five days in transit instead of five months
Actually, this is only true when measured from an observer in a remote reference frame. Say, if you were on Earth and you had a telescope pointed at the window of a ship that was moving past you at 90% the speed of light. In that case, YOU would see that time on the other ship has slowed down considerably relative to your own measurements. The crewman on that ship, however, look out the window with their own telescope and they observe the exact thing about you: from their perspective, YOU'RE the one moving at 90% the speed of light, and therefore YOU'RE the one who's experience time dilation.

Both observations are equally valid. They would HAVE to be, after all, or else a photon emitted from your location wouldn't appear to be traveling at lightspeed in both reference frames, it would be faster in one than the other. Relativity tells us this can NEVER happen, which ironically means that time dilation is only present when the relative velocities REMAIN high; if the fast-moving vessel were to suddenly stop, the dilation vanishes altogether.

A better way to illustrate this is to have three or more ships traveling at different speeds to the same destination. One is moving at 90% of C, one at 92%, one at 88%. The middle one observes two spacecraft moving away from it at 2% of the speed of light and gets identical time dilation measurements from both of them. The two on the ends, however, see one ship moving away at 2% and the other moving away at 4%, and the faster of the two has a greater degree of time dilation than the slower. All three measurements will be entirely contradictory, and all three measurements will be correct. But since their OWN internal clocks appear to be unaffected, they will eventually land on a nice planetoid somewhere, put their watches on the table and find out that actually NONE of them have experienced any time dilation at all. They will, of course, look back at their point of origin and think "Well we were moving at nearly lightspeed, so back home, it should only be a few minutes after we left." They will be wrong, of course, but if they whip out a telescope and look, that's EXACTLY what they'll see.

The moral of the story is that relativity can seem to be a very strange thing, until you realize the broader implication of the universe: Just because you see it doesn't mean it really happened.
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Old September 20 2012, 06:24 AM   #48
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Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

I don't think that's quite right.

The faster ship has time pass more slowly. So they all leave at the same time, traveling a light year, but one ship travels at 0.999999 C and the other at 0.5 C.

The faster ship doesn't notice hardly any travel time, so it almost entirely misses the one year to cover the light-year distance, where it stops and waits on the other ship. So when it arrives, it's clock indicates that perhaps a day has passed, even though a year has passed and the second ship is halfway there. A year after that, the second ship arrives. At that point, the second ship has experienced two-years subjective, but the first ship just experienced the one year they spent on the planet and the one day in transit. So their clocks are off by about a year.
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Old September 20 2012, 07:07 AM   #49
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Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

gturner wrote: View Post
I don't think that's quite right.

The faster ship has time pass more slowly. So they all leave at the same time, traveling a light year, but one ship travels at 0.999999 C and the other at 0.5 C.

The faster ship doesn't notice hardly any travel time...
Yes it does. Remember, relative velocity is only meaningful with respect to an arbitrary fixed point in space. If you're moving towards an object that is traveling in the same direction, only 100km/s slower than you, then your velocity relative to this object is 100km/s. If, without accelerating or changing directions, you then measure your velocity relative to a photon torpedo someone just fired at you, you find your relative velocity is now 250,000km/s.

If what you say is true, the amount of time dilation you're experiencing would depend entirely on what you're using as a reference point: there should be no noticeable time dilation while you're chasing the stolen shuttlecraft. There is also no time dilation when someone fires a photon torpedo at you... but the moment you SEE the torpedo moving towards you at near-lightspeed, time suddenly grinds to a halt.

It doesn't work that way, and I shouldn't have to explain why. But just in case: you do not know and CANNOT know how that torpedo happened to be moving towards you at 250,000km/s. Your calculation for Delta-T is therefore true whether the torpedo was fired at you at that velocity, or if it was fired AWAY from you at 200m/s -- say, by that space station you're about to pass on the right -- and you're just moving so incredibly fast that you're about to crash into it.

So when it arrives, it's clock indicates that perhaps a day has passed
No, nearly a year has indeed passed. From your point of view, EVERYONE ELSE seems to have slowed down.

Again, the effect is only manifest while they're moving with respect to each other. When they match velocities again, their world lines reconverge and the clocks are in synch again.

You've got to remember the thing about relativity: all measurements are subjective. Conditions in YOUR reference frame cannot be altered by what anyone else observes, your frame is always true for YOU, so no matter what velocity someone measures when they look at you, in your frame, you're stationary, and time passes normally.
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Old September 20 2012, 03:13 PM   #50
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Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

newtype_alpha wrote: View Post
So when it arrives, it's clock indicates that perhaps a day has passed
No, nearly a year has indeed passed. From your point of view, EVERYONE ELSE seems to have slowed down.
Except that a journey of one light year at .9999c or something to that effect would seem to take a day. In fact you throw enough nines after the decimal point, a person could travel to anywhere in the galaxy and come home in what seems like a day to him. Obviously he will arrive home thousands of years later, but to him everything else in the universe aged millennia in the blink of an eye.

While nowhere near light speed, the GPS satellites are precise enough and moving fast enough that their data needs to be adjusted for the fact the clocks on a satellite are moving slower than the clocks on the surface of the Earth.
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Old September 20 2012, 04:28 PM   #51
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Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

I curse the twins experiment for the amount of confusion it's caused with relativity. :bitch:

Twins works in general relativity, because acceleration is absolute. However, in special relativity which is what [B]newtype[b] is discussing, each twin sees themselves aging at the normal rate, and their other twin as aging slowly. Special Relativity involves two objects moving in completely different frames of reference that never meet - ie they don't accelerate. In General Relativity, the frames of reference meet, which causes it to be a lot more complicated, so that's where the Twin thought experiment comes into play.

Not that any of this applies to a warp drive because the ship isn't accelerating, decelerating, or moving. The spacetime around it is, and that's a separate kettle of fish entirely.
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Old September 20 2012, 05:14 PM   #52
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Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

Mars wrote: View Post
Sounds like a novel I read by Travis Taylor "Warp Speed" 10 times the speed of light is "Warp 2" right?
I remember reading some article when TMP came out, and it said Warp 6 is 216 times the speed of light - Which means Warp 1 is 36x.
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Old September 20 2012, 06:10 PM   #53
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Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

The Green Mushroom wrote: View Post
newtype_alpha wrote: View Post
So when it arrives, it's clock indicates that perhaps a day has passed
No, nearly a year has indeed passed. From your point of view, EVERYONE ELSE seems to have slowed down.
Except that a journey of one light year at .9999c or something to that effect would seem to take a day.
Again, only when measured from someone else's perspective. Because you are stationary with respect to your own reference frame, your subjective travel time is unchanged.

While nowhere near light speed, the GPS satellites are precise enough and moving fast enough that their data needs to be adjusted for the fact the clocks on a satellite are moving slower than the clocks on the surface of the Earth.
No they don't. That's a common but oddly pervasive myth.


Manticore wrote: View Post
I curse the twins experiment for the amount of confusion it's caused with relativity. :bitch:

Twins works in general relativity, because acceleration is absolute. However, in special relativity which is what [B]newtype[b] is discussing, each twin sees themselves aging at the normal rate, and their other twin as aging slowly. Special Relativity involves two objects moving in completely different frames of reference that never meet - ie they don't accelerate. In General Relativity, the frames of reference meet, which causes it to be a lot more complicated, so that's where the Twin thought experiment comes into play.
And even then it only works because General Relativity allows for a universal reference frame in the context of the timelike curves of gravity and energy. In the end, the Twins Paradox doesn't demonstrate anything other than GR's inapplicability to local reference frames, in the same way Schroedinger's Cat demonstrates the inexplicability of quantum mechanics to macroscopic objects.

Which is ironic, because:
Not that any of this applies to a warp drive because the ship isn't accelerating, decelerating, or moving. The spacetime around it is, and that's a separate kettle of fish entirely.
Actually, the ship IS accelerating, but it's doing it in a non-inertial reference frame. This is ironic because it means that General Relativity should apply locally, even if it is not really valid beyond the space ship and its enclosing warp field.
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Old September 20 2012, 06:22 PM   #54
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Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

newtype_alpha wrote:
So when it arrives, it's clock indicates that perhaps a day has passed
No, nearly a year has indeed passed. From your point of view, EVERYONE ELSE seems to have slowed down.
I don't think that's how it works, at least according to Einstein and Hawking who talk about a starship traveller returning and meeting his great grandchildren. Travelling at high speed is like getting very close to a black hole. Hawking discusses that as a method of prolonging your life so that you can watch the universe evolve over a human lifetime, and that travelling at high warp is a more practical way of doing the same thing.

What you're talking about, possibly, is that the 0.999c traveller would see light from the people he left behind as slowing down (because the light from them is barely catching up to him). Once he arrives, the light from them that's arriving was that sent barely after he left, so they sseem like they're stuck a year in the past. But they're not, the traveller is just a light-year away, observing their activities from a year behind. His year is actually missing. It didn't happen (as if he'd been very close to the event horizon of a black hole). The people he's observing will eventually catch up to him traveling at 0.5C, and he'll realize that the apparent one-year lag (where he thought he was in-sync with their timeline) was an illusion created by being 1 light-year away.

Or, more simply, if he travels one light-year at 0.9999C and looks back, the light he's seeing is from shortly after he'd left. To him only a day has transpired, and only about a day on the home planet seems to have transpired, as far as he can tell from light that's a year out of date. If he turns around and goes back at 0.99999C, two days have transpired to him, but two years have transpired on the home planet he returns to.

That's why Einstein and Hawking talk about time-dilation, or getting very close to a black hole, as a form of time travel.
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Old September 20 2012, 06:33 PM   #55
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Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

gturner wrote: View Post
newtype_alpha wrote:
So when it arrives, it's clock indicates that perhaps a day has passed
No, nearly a year has indeed passed. From your point of view, EVERYONE ELSE seems to have slowed down.
I don't think that's how it works, at least according to Einstein and Hawking who talk about a starship traveller returning and meeting his great grandchildren. Travelling at high speed is like getting very close to a black hole. Hawking discusses that as a method of prolonging your life so that you can watch the universe evolve over a human lifetime, and that travelling at high warp is a more practical way of doing the same thing.
Again, you're talking about the Twin Paradox, which is only applicable to General Relativity. Newtype is talking about Special Relativity. :bitch:

So fricken annoying to see the two so constantly conflated, which is why I despise the twin paradox.
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Old September 20 2012, 06:51 PM   #56
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Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

gturner wrote: View Post
newtype_alpha wrote:
So when it arrives, it's clock indicates that perhaps a day has passed
No, nearly a year has indeed passed. From your point of view, EVERYONE ELSE seems to have slowed down.
I don't think that's how it works, at least according to Einstein and Hawking who talk about a starship traveller returning and meeting his great grandchildren.
I can't speak for Einstein or Hawking, but the MATH doesn't lie, and the underlying logic is pretty clear on this matter. If we're flying towards each other, we will both look at each other and we will be able to say the same three things: "You're moving towards me, you have more energy than me, and time is moving slower for you."

Now how could that be possible? "You're moving towards me" is simple enough: we're both moving towards each other, so we'd both be correct. But how could you have more kinetic energy than me AND I have more kinetic energy than you? That's the thing about relativity: kinetic energy is a relative value between the two of us, and in MY reference frame I have no kinetic energy and you have alot, while in your frame you have no energy and I have a lot. If we collide head on, however, the same amount of energy will be released regardless of whose frame you're using.

And the same is true of time. I see that your clock is running slower and you see that MY clock is running slower. But if we suddenly come to a stop right next to each other we see that our clocks are running at the same speed and apparently always have... but that jerk over there doing warp 7 in a school zone is picking his nose in ultra-slow motion (and he sees us very slowly giving him the finger and wonders "Wow, look at the time dilation on those too!")

You have to keep it in your head that velocity, energy and time can only be measured subjectively. In relativity, the only thing that doesn't change between any two frames is the speed of light.

Hawking discusses that as a method of prolonging your life so that you can watch the universe evolve over a human lifetime
And like most things Hawking says publicly, this is vastly oversimplified for the sake of people he considers to be simpletons.

What you're talking about, possibly, is that the 0.999c traveller would see light from the people he left behind as slowing down
Nope. The light still arrives at the same speed in all reference frames; if you're moving away from me at .99C, you still receive a photon from me at exactly the speed of light. That alone is the reason you observe time dilation at my position. I have a device that emits one pulse every second. You receive the first pulse, but by the time the second one fires you're just under 300,000km farther away from me. In your reference frame, the second pulse is ALSO coming at you at the speed of light, but it takes one second longer to reach you than the first one did. That means that from your point of view, my clock is running slow.

But guess what? You've got an atomic clock on your ship too. It's also emitting one pulse every second. You're moving away from ME at .99C, so from my point of view, and for the exact same reason, your clock is running a little bit slower than mine.

I don't know what Einstein or Hawking quotes you have in mind, but Einstein was pretty clear about the concept of curved spacetime. When you're traveling at relativistic velocities, you're in a reference frame this is LOCALLY flat, but is curved to outside observers. You should think of it like standing on a very tiny planet -- say, the size of a beach ball -- and you bend over and look at a person standing on the other side of the planet. You say to this person, "Hey dude, you're upside down." He looks at you and he says, "No, YOU'RE upside down." You're both correct, incidentally, because you're standing in a curved spacetime (General Relativity, I know, but the concept still applies). It works the same way in time dilation: I look at the starship and say "Your clock is running slow." The starship looks back at me, standing on Earth, and it will say the exact same thing about me, because velocity -- like time -- is relative, and in your own reference frame your OWN velocity is always zero.
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Old September 20 2012, 06:59 PM   #57
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Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

Manticore wrote: View Post
Again, you're talking about the Twin Paradox, which is only applicable to General Relativity. Newtype is talking about Special Relativity. :bitch:

So fricken annoying to see the two so constantly conflated, which is why I despise the twin paradox.
No, the twin paradox comes from special relativity, which had no trouble explaining that the twins wouldn't be the same age, just trouble explaining why A should be older B instead of B older than A, if there wasn't a preferred reference frame, which special relativity said there wasn't. That's why it was called a paradox.

As I recall, Einstein had largely resolved it around 1910, before general relativity, by distinguishing btween the non-accelerated reference frame and the accelerated one, which initially would also seem to be relative (which one is accelerating? Wouldn't it depend on your point of view?), but only one twin feels like he's pulling some serious G's somewhere along the way, leading to the importance of the non-accelerated reference frame's proper time, etc.
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Old September 20 2012, 07:12 PM   #58
gturner
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Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

newtype_alpha wrote: View Post
I can't speak for Einstein or Hawking, but the MATH doesn't lie, and the underlying logic is pretty clear on this matter. If we're flying towards each other, we will both look at each other and we will be able to say the same three things: "You're moving towards me, you have more energy than me, and time is moving slower for you."
I think that's backwards. If we're moving toward each other we're both blue shifted, not red shifted.
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Old September 20 2012, 07:13 PM   #59
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Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

gturner wrote: View Post
Manticore wrote: View Post
Again, you're talking about the Twin Paradox, which is only applicable to General Relativity. Newtype is talking about Special Relativity. :bitch:

So fricken annoying to see the two so constantly conflated, which is why I despise the twin paradox.
No, the twin paradox comes from special relativity, which had no trouble explaining that the twins wouldn't be the same age, just trouble explaining why A should be older B instead of B older than A, if there wasn't a preferred reference frame, which special relativity said there wasn't. That's why it was called a paradox.

As I recall, Einstein had largely resolved it around 1910, before general relativity, by distinguishing btween the non-accelerated reference frame and the accelerated one, which initially would also seem to be relative (which one is accelerating? Wouldn't it depend on your point of view?), but only one twin feels like he's pulling some serious G's somewhere along the way, leading to the importance of the non-accelerated reference frame's proper time, etc.
But for the twins paradox to occur, one has to change your inertial frame of reference, which is the domain of general relativity. Otherwise, one twin sees the other as aging more slowly, and vice versa.
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Old September 20 2012, 07:16 PM   #60
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Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

I see.

And by that I mean that I don't understand.
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