4 ms·

The light cone only matters if you actually move. The accepted idioms from relativity don't apply here. I think we are just not talking about the same "warp" d

by w0000t 11y ago

The light cone only matters if you actually move. The accepted idioms from relativity don't apply here. I think we are just not talking about the same "warp" drive. The version that doesn't violate causality works like this. You are not allowed to accelerate( using normal rockets ) to near c velocity and then warp to a stop to some distant location. This would cause problems. Instead you will end up there with all of your accumulated velocity before the warp. Then you have to come to a stop the same way you accelerated. In other words, warp will change your location but not your velocity in spacetime.

drdeca 11y ago
I am aware that the ship does not accelerate in space. No, I don't think we are talking about different warp drives, but either way it doesn't matter, (unless the one you are talking about has weird additional restrictions on things like what directions you can go or you can only use it once and only in a straight line, or something like that). What I'm saying is the "how you get there" doesn't matter. If you pick a point in spacetime, the future light cone of that point does not depend on your reference frame, and neither does the past light cone. And, no, I'm fairly sure that the light cone is still a thing even if you do not accelerate. So, pick a point X in space time, and a point Y which in some inertial frame of reference is "after" that point, but not inside the future light cone of X, such that your ftl device allows point X to influence point Y. Then there is some other frame of reference where point X is still not inside point Y's past or future light cones, but now is "after" point Y. Then, the same ftl device would allow point Y to influence point X, or other points in X's past light cone. (to simplify, assume that the ftl device does not have a maximum "speed" This implies that it can impact any future point. I'm pretty sure that the same line of reasoning with the two points still applies when ftl device is limited to a "speed" of twice c, but I am not well versed in relativity, so I am treating the "speed" of the ftl device as unlimited. I am putting "speed" in quotes to indicate that it isn't a literal speed, so much as a ratio of (I think) distance in some reference frame compared to time in the reference frame, within which the ftl device can reach.) Note that I have not at any point in this talked about a path between X and Y, or anything like that. So, you can't get around this by noting that the ftl device never actually goes faster than c in the space it is in, because none of the things deal with the ftl device moving, just influencing things outside of its light cone.
- w0000t 11y ago
  You are clinging to light cones because that is what you know about relativity, but you don't understand it intuitively. I didn't get any new information from you last comment, just what has already been said. Warp and light cones clearly don't work. Does that mean that warp couldn't work at least in theory, no. Learn more about relativity( it is really interesting ), otherwise you will always be stuck in your frame of mind.
  - drdeca 11y ago
    Ok, heres another explanation which does not refer to light cones. Take some ftl causation (i.e. two points in spacetime such that one is a causal influence on the other, and such that the spacetime interval between them is spacelike). Take the one which has a causal influence on the other to be first. Then, by relativity of simultaneity for things which are separated by a spacelike spacetime interval, there is another frame of reference in which the point which is causally influenced by the other point occurs before the point which causally influences the other point. That is, A influences B, and B is before A. Which is retrocausality. Therefore, because any two points in mostly flat spacetime (so like, no black holes or anything) which have spacelike separation can be taken to be before or after eachother, if one has a causal influence on the other, there is a frame of reference in which that would be retrocausality. edit: also, do you mean that I don't intuitively understand relativity, or don't intuitively understand light cones? I don't understand where you are getting the idea that "warp and light cones clearly don't work". Light cones are still used in general relativity (at least locally). Shifting things based on frame of reference still works in general relativity. Do you have anything to back up your claim that light cones "don't work" with "warp"? At the very least a good search term? edit2: further, consider this quote by alcubierre: ' Miguel Alcubierre briefly discusses some of these issues in a series of lecture slides posted online,[31] where he writes "beware: in relativity, any method to travel faster than light can in principle be used to travel back in time (a time machine)." ' [31] http://ccrg.rit.edu/files/FasterThanLight.pdf http://ccrg.rit.edu/files/FasterThanLight.pdf So, you can see, Alcubierre (who, iirc, first showed the negative energy density method of ftl mentioned in the article), also states that it could be used for retrocausality (though there's a caveat based on the “chronology protection conjecture"). So, I think my arguments are fairly well supported.

by w0000t 11y ago

drdeca 11y ago

I am aware that the ship does not accelerate in space. No, I don't think we are talking about different warp drives, but either way it doesn't matter, (unless the one you are talking about has weird additional restrictions on things like what directions you can go or you can only use it once and only in a straight line, or something like that). What I'm saying is the "how you get there" doesn't matter. If you pick a point in spacetime, the future light cone of that point does not depend on your reference frame, and neither does the past light cone. And, no, I'm fairly sure that the light cone is still a thing even if you do not accelerate. So, pick a point X in space time, and a point Y which in some inertial frame of reference is "after" that point, but not inside the future light cone of X, such that your ftl device allows point X to influence point Y. Then there is some other frame of reference where point X is still not inside point Y's past or future light cones, but now is "after" point Y. Then, the same ftl device would allow point Y to influence point X, or other points in X's past light cone. (to simplify, assume that the ftl device does not have a maximum "speed" This implies that it can impact any future point. I'm pretty sure that the same line of reasoning with the two points still applies when ftl device is limited to a "speed" of twice c, but I am not well versed in relativity, so I am treating the "speed" of the ftl device as unlimited. I am putting "speed" in quotes to indicate that it isn't a literal speed, so much as a ratio of (I think) distance in some reference frame compared to time in the reference frame, within which the ftl device can reach.) Note that I have not at any point in this talked about a path between X and Y, or anything like that. So, you can't get around this by noting that the ftl device never actually goes faster than c in the space it is in, because none of the things deal with the ftl device moving, just influencing things outside of its light cone.

w0000t 11y ago
You are clinging to light cones because that is what you know about relativity, but you don't understand it intuitively. I didn't get any new information from you last comment, just what has already been said. Warp and light cones clearly don't work. Does that mean that warp couldn't work at least in theory, no. Learn more about relativity( it is really interesting ), otherwise you will always be stuck in your frame of mind.
- drdeca 11y ago
  Ok, heres another explanation which does not refer to light cones. Take some ftl causation (i.e. two points in spacetime such that one is a causal influence on the other, and such that the spacetime interval between them is spacelike). Take the one which has a causal influence on the other to be first. Then, by relativity of simultaneity for things which are separated by a spacelike spacetime interval, there is another frame of reference in which the point which is causally influenced by the other point occurs before the point which causally influences the other point. That is, A influences B, and B is before A. Which is retrocausality. Therefore, because any two points in mostly flat spacetime (so like, no black holes or anything) which have spacelike separation can be taken to be before or after eachother, if one has a causal influence on the other, there is a frame of reference in which that would be retrocausality. edit: also, do you mean that I don't intuitively understand relativity, or don't intuitively understand light cones? I don't understand where you are getting the idea that "warp and light cones clearly don't work". Light cones are still used in general relativity (at least locally). Shifting things based on frame of reference still works in general relativity. Do you have anything to back up your claim that light cones "don't work" with "warp"? At the very least a good search term? edit2: further, consider this quote by alcubierre: ' Miguel Alcubierre briefly discusses some of these issues in a series of lecture slides posted online,[31] where he writes "beware: in relativity, any method to travel faster than light can in principle be used to travel back in time (a time machine)." ' [31] http://ccrg.rit.edu/files/FasterThanLight.pdf http://ccrg.rit.edu/files/FasterThanLight.pdf So, you can see, Alcubierre (who, iirc, first showed the negative energy density method of ftl mentioned in the article), also states that it could be used for retrocausality (though there's a caveat based on the “chronology protection conjecture"). So, I think my arguments are fairly well supported.