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byron178
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do they exist in reality or in nature?
bcrowell said:We have no evidence that they exist in our universe. There is also a conjecture, which I think is widely considered plausible, http://en.wikipedia.org/wiki/Chronology_protection_conjecture that a spacetime that doesn't already have CTCs can't acquire them.
byron178 said:so they might not be permitted in the universe?
bcrowell said:Unless (a) the chronology protection conjecture is false, or (b) CTCs naturally existed as a feature of the universe starting from the big bang. There are cosmologies like the Godel metric http://en.wikipedia.org/wiki/Gödel_metric that have CTCs and always have had them. Observations of the CMB anisotropy rule out the Godel metric as a model of our universe, but they don't necessarily rule out all possible cosmologies that have CTCs.
byron178 said:do they exist in reality or in nature?
pervect said:The Kerr Newman metric is probably unlikely to exist in our universe, though - it's basically idealized and inherently unstable. From what I've read it's so unstable that it can't self-consistently describe a single particle falling into the inner horizon - such a particle would acquire infinite blueshift, infinite energy - and distort the geometry into something that wasn't a Kerr Newman metric.
byron178 said:does the kerr metric exist?
PAllen said:The metric exists, it is an exact solution of the field equations. What pervect stated is that it probably doesn't exist *in our universe*.
An exact, symmetric, treatment says if 10 hunters in a circle fire toward the center at the same time, you get a metal ball stationary in the center. You want to try this some time? Pervect is saying the Kerr metric is idealized in a similar sense. Presumably, even less likely than the proposed method of manufacturing ball bearings.
In mathematics "exists" means that there is a solution to an equation. It doesn't mean that it really happens.byron178 said:i don't get it exist but it does not exist in our universe?
byron178 said:also won't quantum gravity rule out all closed timelike curves?
bcrowell said:In mathematics "exists" means that there is a solution to an equation. It doesn't mean that it really happens.
PAllen's example of the hunters exists as a solution of Newton's laws, but it has never happened on our planet.
We don't have a working theory of quantum gravity, so we don't know for sure. For speculation on this point, take a look at the articles listed in #8.
A closed timelike curve is a theoretical concept in physics that refers to a path in spacetime that returns to its starting point. This means that an object following this path would essentially be traveling back in time.
It is currently unknown if closed timelike curves exist in reality. They are only theoretical constructs and have not been observed or proven to exist in our universe.
Closed timelike curves violate the laws of physics, specifically the principle of causality, which states that an effect must always occur after its cause. If closed timelike curves were to exist, it would mean that an effect could precede its cause, resulting in paradoxes and contradictions.
There is currently no known way to create or manipulate closed timelike curves. However, some theories suggest that they could potentially be formed through the manipulation of extremely powerful gravitational fields or through the use of advanced technology in the distant future.
If closed timelike curves were to exist, it would mean that time travel may be possible. However, it would also raise many ethical and philosophical questions, such as the potential for altering the past and the consequences of creating paradoxes. It is an area of physics that is still heavily debated and studied.