If you search for ‘wormhole’ you get a lot of hits on this message board.
I’ll link to a previous thread. [thread=199241]old wormhole thread[/thread]
Anyway, the general idea is you creat a wormhole through space. You move one of the mouths of the wormhole into a strong gravitational field, wait a while, and you now have a wormhole with one end in the present and one end in the past.
Now this possibly leads to some paradoxes. One of which being that you throw a ball into one end of the wormhole at the right speed to have it come out the younger end of the wormhole just in time to knock away the first ball.
In the book, The Future of Spacetime one of the authors postulates that the younger ball couldn’t possibly knock the older ball away, because the older ball must have entered the wormhole. (Maybe it only hit with a glancing blow that wasn’t enough to keep the older ball from entering)
The article writer claims that no situation can exist that doesn’t already take into account any event that happened to the wormhole itself and therefore no paradoxes will exist.
I can imagine a mechanism where a sensor set up outside the younger mouth automatically closes a door set up outside the older mouth. If the younger ball is detected, the door slams down keeping the older ball trapped in the ‘present’. If no younger ball is detected than the door stays open.
Is there a scenario that takes into account all the events that do happen that will negate the paradox? I don’t accept that the universe will just happen to wing a meteor at my older ball at just the right time. Let’s make it a somewhat closed system.
Going further, lets assume no such scenario exists, and you end up with two identical balls: I have just violated conservation of matter in the universe. Is this a possible proof of why such a time machine is impossible? (albeit a highly simplified proof): Or is there a way of perhaps borrowing energy from the wormhole in order to create my second ball. Perhaps the wormhole now shrinks in diameter. Are there any current physical theories that would show such an effect? Did I just make an energy to matter converter?
Whew, that post ended up being longer than I planned…
When Stephen Hawking just admitted at a recent scientific conference that he was wrong about black holes, he also noted that it meant he was wrong about wormhole travel.
I was under the impression that wormholes and black holes were not necessarily related.
Overlapping two black holes is one possible way of creating a theoretical black hole.
But wormholes also exist down at the quantum foam, and some way could (possibly) exist for them to be enlarged with ‘negative energy’ and brought up to macroscopic size?
By the way, in the book i mentioned, Hawking’s article included an attempt to prove that time travel was impossible. But now we know he makes the occasional mistake.
Exapno: you’re mixing up two things here. What Hawking was talking about is whether a black hole solution would allow the formation of “baby universes” to resolve the information paradox. Such bridges to other universes would necessarily be one-way trips. This doesn’t preclude the existence of traversible wormhole solutions that connect different portions of our own universe.
As to the OP: You’ve discovered why time travel makes a lot of relativists uneasy. In relativity we think of time as just another direction in spacetime. At every time, the particle has a given position; so as time progresses, these positions trace out a line, called the particle’s worldline, in spacetime. Most physicists think that (modulo some mathematical niceties) the spacetimes that are “physically reasonable” have the property that particle worldlines don’t close on themselves, i.e. if you follow one of these curves forward in time you’ll never be able to double back on yourself & come back to the same point in spacetime.
Wormhole solutions, on the other hand, have “closed timelike curves”: if a particle (like your ball) travels forward in time along one of these curves, it will come back to the same point in space and time. There’s nothing in Einstein’s equation that prohibits this, but it would violate a few things that we think are physically reasonable. Most notably, we usually think that we can set up an experiment in a given state, let go, and see what happens as time passes. However, if we want to solve the mathematical equations of motion in a spacetime with closed timelike curves, we can’t arbitrarily choose how things are behaving at a given time; the equations don’t make sense unless we choose things in such a way that the system evolves back to its initial configuration by the “time” it gets back to the same point in spacetime on its closed timelike curve.
This is probably what the article writer meant when he said “no paradoxes exist”: the mathematics make perfect sense if everything ties itself up in a neat little package (not unlike your “trapdoor” mechanism), but are nonsensical otherwise. Since physicists like their math to be well-behaved (string theorists notwithstanding), they say that the “paradoxes” are not solutions to the equations of motion, and thus aren’t physical.
Whew! Hope that wasn’t too dense. Further elaboration upon request. BTW, which of the authors wrote the essay?
MikeS has said just about all there is to be said about the Grandfather Paradox (which is to say, not much), so I’ll address the conservation of mass problem. Basically, conservation of mass is a local property, not a global one. That is to say, if you have some small region of space, then the amount by which the mass in that region changes must be equal to the amount of mass which passes into or out of that region from elsewhere. This is still valid with wormholes, once you recognize that your regions are allowed to extend into the wormhole itself. But as you’ve observed, in a space with a wormhole, it’s possible for conservation of mass to be locally true everywhere, but not be globally true.
Actually, my trapdoor mechanism is supposed to be an example of a situation which can’t be tied up in a neat package without involving a paradox. I’ll try to remember another example from the book–
The author tried to break his own ball deflection example by covering the ball with a contact explosive. Now, even the slightest collision between the two assures mutual destruction.
His solution to this (because remember you have to take the outcome into account when the initial conditions are imagined) is to imagine a tiny piece of the ball coming out through younger mouth. This tiny piece touches the main ball, causes an explosion that pushes a ball fragment into the older mouth. Aha! tidy package indeed.
I can’t however think of a tidy package for my trapdoor mechanism. How can my initial conditions take into account what’s going to happen, when my mechanism is designed to thwart them? so to speak…
I don’t have the book on me since it’s at home. All I remember is that the author wasn’t Hawking or Kip Thorne.
Chronos:
You mean I can’t define the universe to be my system and say mass must be conserved? Is this because ‘universe’ is not a very well defined term?
I’m also having trouble with my tenses. Big surprise, this is a time travel question after all.
Earlier I wrote this.
this should be:
I can imagine a mechanism where a sensor set up outside the younger mouth automatically cloes a door set up outside the older mouth (that is, the mouth further along in the future). If the (older ball from the future) is detected, the door slams down keeping the (younger) ball trapped in the ‘present’. If no older ball is detected then the door stays open.
One of the fundamental assumptions of physics is that the speed of light is a constant.
A recent investigation by an Australian graduated student revealed that the speed of light has decreased approximately 7% over the last 300 years. This, if true and confirmed, will have profound implications for physics and consequently Sci.-Fi.
Well, to go into it a bit deeper, integrating the divergence of the stress-energy tensor over any measurable subset of the spacetime manifold will give zero. This means that if the universe is Minkowski 4-space (as in SR), then the value of “mass-energy” at any two slices all the way across space (say, at two different values of t for a given observer) will be the same, since on the “earlier” surface it’s the amount of stress-energy entering the 4-slab between the surfaces and on the “later” surface it’s the amount leaving.
The problem is that you’re trying to invoke this on a manifold with a relatively complicated topology, and you need to be very careful about where the boundaries of your region are.
To elaborate a bit: It is possible to maintain conservation of energy even in the presence of wormholes. What might happen (keep in mind that this is all speculative) is that the wormhole mouths have a topological “mass” associated with their curvature, similar to the way a black hole’s mass is associated with the spacetime curvature of the region. Then sending a ball through the wormhole might increase the mass of the entry mouth and decrease the mass of the exit mouth, locally conserving energy.
Heh… Well there was some handwaving over that part. Wormholes are theoretical structures that some advanced civilization might be able to create and keep open using ‘negative energy’.
I’m not sure if there is even a way to describe a method with which you’d be able to move the mouth around.
Thanks for all the answers so far guys. They’ve been interesting.
How to make a wormhole is a very good question, and if you find an answer, please get back to me. But moving one is straightforward: There’s every reason to believe that the mouth of a wormhole would react to gravitational fields in the same manner as anything else. So you move a wormhole by tugging a planet along in front of it.
It’s because the Universe isn’t local. To be more precise, I should have said that, for any point in the Universe, there exists a small region (exactly how small is small enough might depend on the point) around that point within which conservation of mass holds. If you have a “simple” universe, this can be extended to be true over any arbitrarily large finite region (but still not the whole universe). But a universe containing a traversable wormhole is not “simple” in this sense.
In that case, I guess it would be hard to park the wormhole near a neutron star in order to get the time dilation effect. Not without one mother of an anchor.
Or a very strong cable. Hey! Maybe I could fly it like a Kite! … nah.
Maybe you can take one of the mouths and just accelerate it up to close the speed of light for awhile.
And I’m starting to see why introducing a wormhole into the system starts making concepts like ‘conservation of mass’ hard to keep hold of.
Some physicist somewhere ( might even have been Hawking) pointed out that vacuum energy fluctuations near the mouths of a time-traveling wormhole would constructively interfere with each other and destroy the hole. So making such a hole is impossible.
I’ve read Dr. Joao Magueijo’s book on varying light speed, and simply by the Occam’s razor test it works far better to explain the horizon problem than inflation does.( It does the same job without having to throw in extra weird scalar feilds that no one has ever or is likely to ever see) Add in the quantization of galaxy redshifts and anomalous Pioneer redshifts and I think it’s highly likely that the speed of light isn’t as constant as we once thought.
Oh-forgot to add on the speed of light hijack- a slowing speed of light would make the universal rate of expansion appear to speed up, and would remove the need for mysterious ‘dark energy’ making up most of the mass of the universe. The expansion would be an optical illiusion caused by the changing ratio between the speed of the galaxies and the speed of the information the galaxies send us. And another mysterious never seen or likely to ever be seen feild gets explained away with one simple and beautiful theory.
Maybe I shouldn’t even be in this thread . . . I haven’t studies physics since high school . . . but I am a hard-SF fan . . . so I’ll try to keep up. If I could handle Robert Forward I can handle this.
Okay, what exactly is this “negative energy”? Is there any proof it exists, or is it just something some equations predict, like magnetic monopoles?
Also – never mind creating wormholes, is there any evidence for the existence of naturally occurring wormholes? Has a wormhole ever been observed? Or is this something else that, like the monopole, is predicted by theory?
