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Warp Drive May Be More Feasible Than Thought, Scientists Say

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Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#151

Post by dclowd9901 »

nessus42 wrote: post #35

You can't have faster than light travel without violations of causality. And while the equations of General Relatively allow for universes where causality is routinely violated, the probability that we are in such a universe seems vanishingly small. E.g., just because you can violate causality does not mean that you can go back in time and kill your grandfather. If you were to try, something would stop you, as a univ…

It's not that the universe might "literally" prevent you from performing an action. It's just that our universe doesn't allow it in the first place. The coherence of the universe is a biproduct of its inherent order.

I guess what I mean, without sounding too pseudo-scientific, is I believe the way it works is that, if, say, you go back in time and kill your grandfather, then what is actually happening is you are killing the person you thought was your grandfather, but they were not, in fact, your grandfather to begin with.

In fact, all you would know going back is that your grandfather was murdered by someone. Let's say you believe it was you, and you have a change of heart. Decide it's not in you to kill someone, even to prove a scientific point. That doesn't mean he doesn't get killed. Nothing's changed. It's simply that someone else killed him.

The answer to the question of Schroedinger's cat isn't that the cat is dead or alive, it's that it is.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#152

Post by nessus42 »

stryker wrote: post #142
nessus42 wrote: post #140

Earlier quoted context omitted.

You're simply closing your ears and exclaiming "special relativity" which only applies locally (if you want an appeal to authority, I talked about this with a former physicist who worked with Stephen Hawking). If you come up with a mathematical "proof" and I can use its conclusion to prove that 1 + 1 = 3, then I have shown your proof to be incorrect via contradiction, no matter how hard you continue to argue for it.…

I agree with your assessment, but you're still missing my point. Following your analogy, what I'm saying is not that 1+1=3, but that we're not really using the plus operator here. I think we both agree that wormholes do not violate causality. By creating a wormhole, you are connecting two points in space with a shortcut. As a result, traveling through a wormhole is not even FTL -- there are two paths to the destinati…

Wormholes most certainly do allow you to violate causality!

How about I just point you at the FTL FAQ?

http://www.physicsguy.com/ftl/html/FTL_part4.html

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#153

Post by dhughes »

nessus42 wrote: post #115
sentientbicycle wrote: post #112

Earlier quoted context omitted.

I'm sorry, but I just can't find any sort of citation stating that the Alcubierre drive violates any of these principles. I also can't find any mention of paradoxes incurred by the drive and there is only one mention of where the meta-materials themselves would have to violate the light speed limit. Here is how I understand it (and correct me if I'm wrong), but it is not travelling in a vacuum, it is crunching that v…

Did you read the part above where I wrote this: It is an irrefutable fact of Special Relativity that if you can transmit any information from point A to point B faster than light would travel from point A to point B in a vacuum, then there are inertial frames of reference in which the information arrived at point B BEFORE it left point A. Is there a reason why I can write this irrefutable fact a dozen times and peopl…

> Is there a reason why I can write this irrefutable fact a dozen times and people continue to ignore it like it was never stated?

Because the tone of your replies are condescending.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#154

Post by gus_massa »

jonmrodriguez wrote: post #77
nessus42 wrote: post #35

You can't have faster than light travel without violations of causality. And while the equations of General Relatively allow for universes where causality is routinely violated, the probability that we are in such a universe seems vanishingly small. E.g., just because you can violate causality does not mean that you can go back in time and kill your grandfather. If you were to try, something would stop you, as a univ…

Since QED and relativity aren't consistent, and QED says that can photons travel at speeds less than and greater than c, isn't it possible that relativity is just probabilistically true but not "enforced" to be true?

QED is consistent with Special Relativity. The standard introduction to QED starts with the successful attempt of Dirac to create a "covariant" version of the Quantum Mechanics that is consistent with Special Relativity (in that process he explained the particle spin and predicted the positrons).

Then it's necessary to make the theory more complicated to explain some experiments, but the final QED theory is consistent with Special Relativity (or "covariant" in the jargon). The extensions that involve the weak and strong forces are also covariant and they form the Standard Model (with the slightly hypothetical Higgs boson).

Any reasonable extension of the Standard Model is "covariant". And it would be very difficult to make the physic community consider seriously a no "covariant" theory (nothing is impossible).

Nobody knows the quantum version of General Relativity, so perhaps there is a very tiny loophole, but I would not be very hopeful about it.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#155

Post by nessus42 »

nessus42 wrote: post #48
geuis wrote: post #44

Earlier quoted context omitted.

We aren't talking about time travel. So many people confuse the idea of FTL with time travel. General Relativity talks about frames of reference. Suppose you had a base on Mars and sent a radio signal to it from Earth. It would take between 4.3 to 21 minutes to reach Mars, depending on its distance from Earth at the time. If you simultaneously jumped in your 10x warp drive and set out, you would arrive at Mars in 25.…

You are not correct. If anything with information travels faster than the speed of light, then there are frames of reference in which that information arrived at the destination before it left the origin. Furthermore, unless your FTL drive is limited in ways that would make it utterly useless, any FTL drive can be used to make "closed time-like loops", which would allow travelers to meet up with their own past selves…

I want to make a point of clarification about something I wrote above. I wrote:

> any FTL drive can be used to make "closed time-like loops", which would allow travelers to meet up with their own past selves.

That should have read "closed causal curves", or perhaps "closed space-like curves". I made the same typo on another occasion or two in this thread.

"Closed time-like curves" do also exist, at least theoretically, and they also let you visit your our own past history, but they do not involve FTL travel. Instead, they are artifacts of General Relativity, in which causality is violated without traveling faster than the speed of light. They typically involve traveling around certain kinds of very massive objects. Another kind involves traveling in a very very large circle at close to the speed of light.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#156

Post by chii »

stickfigure wrote: post #101
chii wrote: post #86

Earlier quoted context omitted.

Theoretically, it isn't possible to stop an advanced alien race that has mastered ftl travel from being able to destroy earth. There is this concept called relativistic bombardment - your "artillary" is set to travel so fast, that the light that comes off it only arrives moments before the actual shell, and thus, there is little to no warning. The only way to stop this sort of attack is a shield that is impenatrable.…

If my (admittedly rough) understanding of relativity is correct, there's a problem with this weapon: From the recipient's perspective, the shell would never arrive. Basically, as the shell accelerates to C relative to the target, time in the target's reference frame expands relative to time in the shell's reference frame. A shell traveling at C would instantaneously travel from origin to destination (from the shell's…

> would take infinite time (from the target's perspective).

this doesn't sound right - the shell does arrive, and arrives exactly X light years after being launched, as measured by the alien. Its just that from the perspective of the people in the shell, heaps of time has passed on the outside.

But my knowledge of relativity is cursory at best, so im not sure if i m right.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#157

Post by stickfigure »

chii wrote: post #156
stickfigure wrote: post #101

Earlier quoted context omitted.

If my (admittedly rough) understanding of relativity is correct, there's a problem with this weapon: From the recipient's perspective, the shell would never arrive. Basically, as the shell accelerates to C relative to the target, time in the target's reference frame expands relative to time in the shell's reference frame. A shell traveling at C would instantaneously travel from origin to destination (from the shell's…

> would take infinite time (from the target's perspective). this doesn't sound right - the shell does arrive, and arrives exactly X light years after being launched, as measured by the alien. Its just that from the perspective of the people in the shell, heaps of time has passed on the outside. But my knowledge of relativity is cursory at best, so im not sure if i m right.

heaps of time has passed on the outside

As I understand it, as relative speed goes to C, "heaps" goes asymptotically to infinity. Thus my point.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#158

Post by darkestkhan »

nessus42 wrote: post #48
geuis wrote: post #44

Earlier quoted context omitted.

We aren't talking about time travel. So many people confuse the idea of FTL with time travel. General Relativity talks about frames of reference. Suppose you had a base on Mars and sent a radio signal to it from Earth. It would take between 4.3 to 21 minutes to reach Mars, depending on its distance from Earth at the time. If you simultaneously jumped in your 10x warp drive and set out, you would arrive at Mars in 25.…

You are not correct. If anything with information travels faster than the speed of light, then there are frames of reference in which that information arrived at the destination before it left the origin. Furthermore, unless your FTL drive is limited in ways that would make it utterly useless, any FTL drive can be used to make "closed time-like loops", which would allow travelers to meet up with their own past selves…

Wake up. Quantum entanglement is at least 10000 x c, and somehow it doesn't brake causality. And information first leaves the origin and gets to you before you know that it left its origin. But no causality is broken. Everything is still happening in proper order.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#159

Post by Dylan16807 »

nessus42 wrote: post #150
Dylan16807 wrote: post #146

Earlier quoted context omitted.

Sure, B sees things happen in the wrong order, but I will maintain that causality is not actually violated until an effect actually disturbs a cause. In other words, if B sends that message back to A, you have violated causality. If B just sits there and watches events happen out of order, then the causality is obscured but not broken.

> Sure, B sees things happen in the wrong order, but I will maintain that causality is not actually violated until an effect actually disturbs a cause. You maintain incorrectly. All inertial frames are equally good in Special Relativity. If there is any inertial frame that observes events elsewhere in the universe where an effect precedes a cause, then causality has been violated. Furthermore, whenever Special Relati…

All inertial frames are equally valid. All inertial frames see that the effect does not munge the cause, even if it happens out of order.

I know that 'causality' is a bad term for me to use here; I'm open to suggestions. But my point is that cause and effect are still intact. Cause, with effect not inside its light cone in the past, sends information toward effect. Effect, with cause not in its light cone in the future, receives information from cause. I don't care what order those happen in, as long as the thread of causation is intact and one-way.

Yes there are 'ways' to create proper violations, but you can avoid them if you intentionally choose to.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#160

Post by cygx »

cygx wrote: post #132
jxcole wrote: post #84

Earlier quoted context omitted.

A lot of people responding to this thread do not understand relativity. The OP is correct, the fundamental problem is causality, relativity and FTL do not mix. The common way of saying it is that you can have two of the three: FTL, causality, or relativity. You cannot have all three. Here is a very nice explanation of why: http://thebestforumever.com/community/threads/tachyon-pistol... Also, I've never understood how…

Tachyons do not necessarily violate causality if you actually take the time to derive the laws of tachyonic kinematics consistent with special relativity. Sure, tachyons will be strange (different observes won't agree on direction of travel or charge of a tachyonic particle - if we are even able to localize the particle at all; there will be frames of reference where a tachyon won't have energy but will still have mo…

This answer doesn't deserve its downvote: The problems with tachyons were largely resolved in the 80s, based on work by Stückelberg and Feynman, Schwartz, Recami and others.

The community just lost interest in them because our current models of particle physics prevents bare tachyons:

QFT does predict real tachyonic particles. However, they are solutions of unstable vacua, which transit to a tachyon-free phase via symmetry breaking.

Stringtheory predicted tachyons as well, but got rid of them again when superstringtheory came around.

Tachyons are not gone completely, though: Virtual particles can be off-shell and thus tachyonic and feature in the description of self-energy.

Tachyons can also be useful to explain EPR-type quantum 'paradoxa' without having to give up reality.

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