# Wormhole time travel theory **URL:** \ **Category:** Factual Questions\ **Created:** [August 4, 2004, 3:27pm UTC](https://boards.straightdope.com/t/wormhole-time-travel-theory/258298 "2004-08-04T15:27:01Z")\ **Posts on this page:** 9\ **Page:** 2
**Author:** ![BrainGlutton](https://avatars.discourse-cdn.com/v4/letter/b/82dd89/32.png) [@BrainGlutton](https://boards.straightdope.com/u/BrainGlutton)\ **Post date:** [August 5, 2004, 11:43pm UTC](https://boards.straightdope.com/t/wormhole-time-travel-theory/258298/21 "2004-08-05T23:43:10Z")
> [@Chronos](#): > > There’s every reason to believe that the mouth of a wormhole would react to gravitational fields in the same manner as anything else. Things react to the presence of gravity fields because they have mass. Do wormholes, or the mouths of wormholes, have mass? ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [August 5, 2004, 11:52pm UTC](https://boards.straightdope.com/t/wormhole-time-travel-theory/258298/22 "2004-08-05T23:52:14Z")
> [@Thaumaturge](#): > > 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. Ok, I give. What happens if C continues to slow down? To the point of, say… 99.9%? ---
**Author:** ![Thaumaturge](https://avatars.discourse-cdn.com/v4/letter/t/858c86/32.png) [@Thaumaturge](https://boards.straightdope.com/u/Thaumaturge)\ **Post date:** [August 6, 2004, 12:12am UTC](https://boards.straightdope.com/t/wormhole-time-travel-theory/258298/23 "2004-08-06T00:12:07Z")
Then the expansion appears to accelerate even more. It takes longer for the signals to get to us, so it appears like the galaxies are farther away than they ‘should’ be…ergo they appear to be accelerating. ( since D=rt…if the rate, light speed goes down, and we assume it is constant, then we falsely assign a larger D than actually exists.) ---
**Author:** ![Folly](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/folly/32/3219_2.png) [@Folly](https://boards.straightdope.com/u/Folly)\ **Post date:** [August 6, 2004, 3:21pm UTC](https://boards.straightdope.com/t/wormhole-time-travel-theory/258298/24 "2004-08-06T15:21:05Z")
Isn’t negative energy one of the theories put forth to explain the acceleration of the expanding universe? If the acceleration is really just an optical illusion, then that is that much less proof that negative energy exists I suppose. I believe quantum mechanics predicts wormholes down at the ‘quantum foam’ level. But I don’t remember where I was reading about it. So to get a macroscopic wormhole, you’d need to somehow _enlarge_ the wormhole by an insane amount. Nobody’s tackled my paradox yet! Of course, you need to make a lot of assumptions before you get to the conditions that create the paradox. ---
**Author:** ![emarkp](https://avatars.discourse-cdn.com/v4/letter/e/3be4f8/32.png) [@emarkp](https://boards.straightdope.com/u/emarkp)\ **Post date:** [August 6, 2004, 4:43pm UTC](https://boards.straightdope.com/t/wormhole-time-travel-theory/258298/25 "2004-08-06T16:43:17Z")
> [@BrainGlutton](#): > > Things react to the presence of gravity fields because they have mass. False. Light has no mass, but its path is altered by gravity. Basic general relativity. ---
**Author:** ![emarkp](https://avatars.discourse-cdn.com/v4/letter/e/3be4f8/32.png) [@emarkp](https://boards.straightdope.com/u/emarkp)\ **Post date:** [August 6, 2004, 5:02pm UTC](https://boards.straightdope.com/t/wormhole-time-travel-theory/258298/26 "2004-08-06T17:02:25Z")
> [@spingears](#): > > 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. This (the link) is some of the goofiest stuff and worst science I’ve ever seen. A quick look at the data in the article would suggest that the speed dropoff is faster as you go back in history and then slows down in the last century. That’s just plain bizarre, and there’s a more simple explanation: the measurements were wrong. A similar graph exists for the charge on an electron. After it was first measured, later measurements got higher and higher at a slower rate until we reached the accepted value of today. Was the charge on the electron increasing? No, it was simply erroneous. When a new measurement was made, those who published were aware of the previously measured value, and simply didn’t publish the highest values they measured assuming they were wrong. ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [August 6, 2004, 7:15pm UTC](https://boards.straightdope.com/t/wormhole-time-travel-theory/258298/27 "2004-08-06T19:15:38Z")
> [@emarkp](#): > > False. Light has no mass, but its path is altered by gravity. Basic general relativity. False. Light has no “rest mass”, but it has a momentum 4-vector. Mass-energy is an artifact of the coordinate system in use and only the tangent covector (a geometric object) enters the equations. _That_ is basic GR. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 6, 2004, 7:34pm UTC](https://boards.straightdope.com/t/wormhole-time-travel-theory/258298/28 "2004-08-06T19:34:13Z")
Wormholes probably do have mass, but that’s not necessary for them to be affected by gravity. _Everything_ is affected by gravity. Negative energy (at least, of the sort necessary to stabilize a wormhole) has never been observed, and it’s not even actually predicted by theory like magnetic monopoles are (as my advisor puts it, it’s certain that monopoles exist, but there may be only a very small number of them, like zero). Discussions of negative energy are more along the lines of “Well, we don’t know if this stuff exists or can exist, but if it does, this is how it would behave”. It’s not the same as the stuff that’s driving the accelerating expansion of the Universe: That’s dark energy, which is certainly weird, but not quite _that_ weird. Dark energy has a negative (and very strong) pressure, but still a positive energy density. Gravity is actually generated both by energy density and by pressure, though the latter is usually relatively insignificant, so the negative pressure can cause gravitational effects. As for speculation that the speed of light varies, it’s very difficult to quantify that. By comparison with what do you measure the speed? The usual standard of speed is c, but by that standard, of course, the speed of light has not and cannot change. In order to say that the speed of light is changing, you would need to construct some other speed from constants of physics, and what you actually end up saying is something along the lines of “the ratio of Planck’s mass to the mass of the proton has changed”. But even then, is it Planck’s mass that’s changing, or the mass of the proton? And even if it’s Planck’s mass, does that mean that c is changing, or that hbar or G is changing? ---
**Author:** ![Loopydude](https://avatars.discourse-cdn.com/v4/letter/l/e56c9b/32.png) [@Loopydude](https://boards.straightdope.com/u/Loopydude)\ **Post date:** [August 6, 2004, 8:31pm UTC](https://boards.straightdope.com/t/wormhole-time-travel-theory/258298/29 "2004-08-06T20:31:31Z")
I seem to remember reading that some astronomical observations suggested the fine structure constant had changed slightly from some number of billions of years ago to the present. Since the easiest way to vary the fine structure constant is to vary c, many VLS theorists took this to be evidence their theories were correct. However, subsequent observations of the same astronomical phenomena that first brought the constancy of alpha into question appear to have refuted the earlier observations. There’s apparently no good observed evidence presently that c has changed. I’ve read the OP’s billiard ball scenerio over and over and over and over. And over. I think I get it. What I don’t get is how it resolves the grandfather paradox. Sure, it works for certain very specific arrangements of billiard balls; but how the heck does it work if I go back in time and kill my grandfather? [Previous page](https://boards.straightdope.com/t/wormhole-time-travel-theory/258298.md?page=1)