Your Mom’s A Blog

I have undertaken, by popular demand (of 1 constituting about 10% of my readership) to address the subject of time travel, mostly how the current laws of physics make Terminator Salvation quite impossible (aside from the idea that we could inadvertently create sentience in a computer system.)

Time travel can be quite confusing, so I’ll begin by putting things in their correct context. A physicist does not like the term time travel because travel is, by definition, something you do in spatial dimensions. You may travel through a room, but we do not travel through time. Time is a part of the universe in which we live and, like space, is also relative. This means that there is no such thing as an absolute standard of time. In other words, no physical law states that two observers must measure time at the same rate. This is part of the Twins Paradox. Basically if you gave a pair of twins the most accurate atomic clocks known to man and then put one twin on top of Mt. Everest (highest point on Earth) for the rest of his life and the other twin at the bottom of the Marianas Trench (lowest point on Earth) set both clocks for 50 years to the second, the Everest clock would expire first, relative to an observer at sea level.

All this is to say that neither time nor space are constants, so in some sense if you get in a car and drive anywhere (thanks to time dialation) you have “traveled” forward in time, i.e. you have arrived at a point in time that did not take the whole time to get to from the point of a relatively stationary observer. With me so far? Okay. Time travel is actually exploiting a closed timelike curve, or a loop in space-time. Because space and time are inter-connected it is (theoretically) possible to perturb space and time into a loop. Though there is only speculation about how this might be accomplished, a loop in space-time would allow something to return to a previous point in time that exists on the loop. Attaining this goal requires something science calls exotic material or matter which has the quality of negative average energy density. At the moment exotic matter is just an idea; there is currently no proof of its existence. (If it did exist it would also be integral to ½ of a Star Trek-like warp drive, but that is another story.) For now I’ll ignore the more popular go-faster-than-light time travel theories because they don’t work well for literally getting a tangible object to some place corresponding with a time in the past.

So let us assume that such a thing as a time machine exists that is capable of both creating and exploiting a closed timelike curve. Usually this sort of thing appears in science fiction, particularly in the film series Terminator. So the question becomes, can a person, or machine go back in time and change things? The answer to this question is most often no. The reason is that it violates causality. Causality is the idea that every physical event has a cause. We may not know or fully understand the cause, or even be able to predict a cause’s effect but all effects have a cause. For this reason, the probability of an effect having a cause in the future is zero unless the cause is not a cause, i.e., is produces no effects. Theoretically a person could go back in time and observe some historic event, but not interfere. A person could go back in time and change something, but only so long as that thing did not have any effect on history. This means a person could save some of a species of plant, animal etc., and replace it with a facsimile, and then bring the thing back to the future to begin having effects there. This idea is called the Novikov Self-Consistency Principle. Basically, no event in the past can be predicated on an event in the future which might be predicated on that same event in the past.

What does this ultimately mean? It means that John Conner could not have sent his own father back in time to conceive him with his mother. It also means that SkyNet could not possibly hope to improve the future by changing the past. Doing so is the definition of a space-time paradox which is either prevented from occurring by the laws of physics or drastically bad for the universe because it would cause the laws of physics to breakdown. The only other places I know where the laws of physics as we know them do not apply are inside the event horizons of black holes. This doesn’t mean that a space-time paradox would create a black hole; it does mean that in said paradox the universe as we know it breaks down and the ability to comprehend as we do goes along with it.

On the off chance that there is a bigger geek than me reading this going “Hey! What about the many-worlds theory of quantum mechanics?” Novikov assumes these are inaccessible, and if you have some evidence to suggest that they are, I’m all ears Doc Brown. 1.21 JIGAWATTS!

I had this crazy idea that I think will really bake your noodle.  I’ve been reading about quantum mechanics lately.  One of the things about quantum mechanics is that it is almost completely counterintuitive.  In out macro world generally things are just that thing, or so the appear.  Your desk is a desk, your car keys are your car keys.  Rarely, however, is your bowling ball a bowling ball and some radio waves.  This is the case in the quantum physics.  Photons are particles and waves, electrons are particles and waves.  Some people may read this and think, “That’s stupid.”  I totally agree, but experimentation has more or less proved it to be the case.

So here are a few quantum mechanical principles I need to quickly introduce before going on with my noodle bake.  I already talked about particle wave duality; there is also entanglement, superposition and multiple dimensions.

Entanglement is when two particles are connected irrespective of space.  In short two particles are created.  When they are created they have certain properties, one such property is called spin.  No matter how far apart they are the spin of one particle is linked to the spin of its entangled particle.  If we have control of one particle and do something to change its state the other entangled particle immediately changes.  Even if these particles are hundreds, thousands, millions of miles separated, the change happens immediately.  This means that what ever device causes these two particles to be entangled can move faster than light.  Einstein just got p0wnd.

Superposition is when one particle exists wholly in one place and wholly in another at the same time.  What!  Yeah, build yourself an interferometer you can see it in action.  Or give IBM a call, their quantum computer works on superposition.

Multiple dimensions are, as one might suspect, dimensions besides the four in which we live.  There is some speculation that may soon be proved by the particle accelerator at CERN that there could be 10 or 11 dimensions.  Physicists suspect that gravitons, if they exist escape very quickly into other dimensions, thus the comparatively low effect of gravitons on matter.

Here comes the switch up.  What is a ghost?  As a quick disclaimer I’m not factoring in any explanations that can’t be tested with current science.  I don’t want to step on anyone’s toes or give preference to any particular explanation; this is just a random idea I had and decided to blog about it.

So dig on the following scenario.  The human eye is little more than a fancy photon detector, more than that it can only pick up photons at a very specific wavelength.  So, if a human sees a ghost, they are detecting either photons being reflected from the ghost, or photons being emitted from the ghost.  Because ghosts are reported to be visible in the dark, let us assume that the ghost is emitting photons.  In order to make this really scientific we need multiple independent observers, and if one of those observers where something like a video camera, without imagination, emotion or agenda, more the better.

So here is the theory.  Ghosts could be energy sources that are passing back through our dimension from another dimension.  I’m making a fairly big leap here but assume some person walked through a room some time in the past.  As they walked through the room light was absorbed and excited some atoms in the individual’s body.  These excited atoms re-emitted some of this radiation as photons.  Typically this wouldn’t happen, but for whatever reason these photons escaped into another dimension.  These dimensions are strange things that can warp and loop back on one another.  Anywho, sometime in the future these particles come back into our dimension and are absorbed by our eyes and camera.  Because they appear to take the form of a human who is probably dead, we freak out.  Or maybe the individual emits an entangled pair of photons, one of which escapes into another dimension and the other stays in the current dimension.  Because time is different in these other dimensions some effect on the photon in the past causes a change that forces the photon in the future back into our dimension and…  instant ghost.  OR…  Ghosts are superpositions of photons coming of a person in the past that aren’t located in two places at the same time, but instead are located in two times at the same place.

I don’t know, what do you think?