presenting paradox-free quantum time travel?
Time travel is a pretty tough topic to study since any trip backwards in time triggers seemingly endless ways a paradox could unravel the entire timeline. Think of the good, old fashioned grandfather paradox in which a very disgruntled time traveler goes back in time and kills his grandfather as a baby. According to causality as we’re familiar with it, this would mean that our time traveler would effectively cease to exist. No grandfather, no father or mother, therefore, no him. But then who went back in time? And why? Well, some physicists working on the theoretical problems with time travel think that there might be a perfectly plausible way to answer the paradox by using causality itself. Rather than just vanishing into existence, our time traveler out for his grandfather will never be able to carry out his homicidal plans. His gun won’t go off, he won’t find his intended victim, etc. He’s simply not going to be able to kill his grandfather due to the nature of space-time. And while that’s certainly an interesting and workable hypothesis, one wonders how you test it. It’s not like we can build a time machine…
Well, we can’t build a machine that can transport anything other than photons back in time, or rather, put a very small sample of photons in a quantum state we would expect after venturing back in time and seeing how the photons will react. As it seems to turn out, following the rules of general relativity, time and space can from an enclosed loop in which the outcomes for a photon’s position and state are limited. Try to create paradoxes as you change the photons’ quantum states, and these photons will just end back where they began, or refuse to travel back in time at all. Of course this experiment has only been done on paper so far and no one ever saw a photon behave as if it’s been traveling through time, so we really don’t have much in the way of proof for these concepts, but they do have a very appealing logic. If you can’t disrupt the past, you can certainly travel back to it and return with no problem. Try to change it and you’ll fail. It would be somewhat like visiting an interactive and very immersive museum without worrying that you’ll break an exhibit. But of course getting to the past if you’re unfortunate enough to actually have mass, any mass at all, is a major problem. It simply takes too much raw energy to physically traverse the fabric of space and time and hurl yourself back into the past.
Granted, traveling back in time should involve less energy that it takes to power a warp drive, but we would still be talking about energetic outputs on at least a planetary scale. Making a tear in space and time just wide enough for a pretty average 6 foot human to pass through would take roughly the mass of Jupiter crushed to the density of a black hole. No device that we’re even able to imagine could possibly handle this much energy at such incredible densities without a runaway collapse into a fully fledged black hole with tidal forces extreme enough to swallow and disintegrate anything that strays too close. In order to actually build a wormhole out of this gravitational collapse, you’d need to do it twice, making sure the resulting gravitational wells will spin at a certain speed to hurl travelers during a certain time range within the causal loop in which they’ll exist. Sounds complicated? Well, keep in mind that trying to join two black-hole like structures will result in an unstable and extremely energetic tunnel which can pop at any moment, instantly killing any time traveler in it or nearby. And while we might have the chance to send photons, or even electrons which have a very tiny mass, but a mass nonetheless, human time travel is going to be far out of our reach for the foreseeable future and we probably shouldn’t be all that concerned about time traveling paradoxes since they simply won’t apply to us…
See: Lloyd, S., Maccone, L., Garcia-Patron, R., Giovannetti, V., Shikano, Y., (2010). Quantum mechanics of time travel through post-selected teleportation arXiv: 1007.2615v2
[ photo illustration via Flickr ]
Okay. Let’s see how stupid I am. What is time? An arbitrary measure of distance. Which is, as far as we know, linear. With me so far? OK. Time as we know, only moves forward. It’s like dropping a stone. It only goes one way. You can’t stop it in mid fall and will it back up into your palm. You may be able to defy gravity and prevent it from falling, but only if it’s ferrous, and you have a magnet large enough to repel it. If our Big Bang theory is correct, we cannot reverse the effects of motion and expansion. Simply because we cannot generate enough energy. I don’t think you can localize time. In order to go back in time, you would have to reverse the entire known and unknown universe. That’s a pretty tall order. There may quite possibly be a way we don’t understand yet, but that would turn everything we learned from Einstein and Bohr inside out. And, as a matter of logic, to go back in time, you would have to surpass the speed of light. Correct me if I’m wrong, as any object approaches the speed of light, it becomes a singularity. The grandfather paradox makes for good science fiction,but, hey, we got hand-held video phones now. A question to gfish. Do photons have mass?
I think this comment is going to be similar to badbass9′s, but with a different spin. Imagine we had a world with one spacial and one temporal dimension. That gives us two dimensions. Great – we can draw a 2D picture of the entire space-time of this world. Let the x axis be space, and the y axis be time. Now each horizontal line in our picture is a snapshop of space at one time.
Let’s imagine a dot that starts at x=0, moves at 1 m/s to the right for 5 seconds. It then teleports back in time to t=0 without moving, continues heading to the right at 1m/s, teleports back in time, etc. Plotting this, it would look like a sawtooth wave on our 2D representation.
The most important thing about this is that the sawtooth wave doesn’t change over “time”. Said another way, the “time” that we use to draw the sawtooth wave – moving our pen over the paper – is different to the “time” in that universe – the y axis in the drawing. Because there is no such thing as “time” in that universe apart from the y-axis in that drawing, the drawing should always be considered static. There is no ‘meta-time’ in which the drawing can change.
This means that regardless of how our ‘dot’ travels through time, and what happens to its subjective timeline, from the POV of an omniscient observer there is only one static picture, which will represent what it represents. The dot can go back in time (like the vertical jump in the sawtooth wave) but it can’t change anything (the picture is static).
The only way to get a time travel where you can change something is to have multiple time dimensions – introduce a meta-time. But we can then do the same thing again – let the meta-time be a 3rd, z, dimension. Now we have a static 3D world with two time dimensions and one space dimension. Things get a little weird, but again there is no paradox.
Either there is no meta-time and you can’t kill your grandfather, or there is a meta-time and you can kill your grandfather, but only in this extra, meta-time, dimension which leaves your original time grandfather free to breed. In either case, no paradox. And you don’t need any quantum mechanics to figure it out.
What is time? An arbitrary measure of distance. Which is, as far as we know, linear.
Not exactly. Time is related to the gravitational wells of different objects and we know that it does get distorted by the immense masses of planets, stars and black holes. Time dilation is actually a big problem for GPS sattelittes and if time is related to the topology of space and space can curve in on itself, so should time.
You may be able to defy gravity and prevent it from falling, but only if it’s ferrous, and you have a magnet large enough to repel it.
Not quite. The rock in your example doesn’t defy gravity, but is overcoming it with a far stronger force, electromagnetism much like a rocket overcomes Earth’s gravity with raw thrust and the pressure from its exhaust.
There may quite possibly be a way we don’t understand yet, but that would turn everything we learned from Einstein and Bohr inside out.
Not at all. In fact, the modern scientific conception of time travel comes from general relativity. My explanation that time wasn’t really linear and we’ve measured how it gets distorted along gravitational wells is simplified Einstein.
Correct me if I’m wrong, as any object approaches the speed of light, it becomes a singularity.
Not really. A signularity is an object that defies the standard laws of physics. When objects with mass travel way too close to the speed of light in a vaccum, their own momentum tears them apart. The increase in their mass at relativistic speeds is given by the formula:
M / √ (1 – v2 / c2) where M is the mass of an object, v is its velocity, and c is the speed of light in a vacuum.
Note that you can exceed the speed of light in water and create an eerie bluish glow known as Cherenkov radiation because water is such a dense medium for photons.
Do photons have mass?
No, they do not. They’re bosons rather than fermions and aren’t particles of anything per se.
The only way to get a time travel where you can change something is to have multiple time dimensions – introduce a meta-time.
And that would be going off into string theory which allows for multiple universes in which you can time travel without paradoxes because you’re never really in your own universe and what would be disruptions to causality in your own cosmos couldn’t affect you because your universe stays as it was. Sure it’s a simple and cool idea, but from an evidence standpoint right now, this is more Sliders than a paper in Science or Physical Review.
The “grandfather paradox” reminds me of Schrödinger’s Cat, in that everyone seems to treat the metaphor as if it were the actual paradox. And thus “solving” the metaphor, “solves” the paradox.(*)
Killing your grandfather isn’t required to cause a paradox, appearing in the past is itself sufficient. Hence it doesn’t matter if “the gun won’t go off”. A person sized object appeared, without cause. That’s the paradox. It doesn’t matter if you step on a butterfly, your very existence is an information/causation paradox.
Which means, of course, that it doesn’t require a person, just a photon. (Or rather, the photon’s information. If you can conspire with quantum mechanics to keep the information from regressing, there’s no paradox. Same as FTL.) So the only way the universe can prevent “changes” to the past is to prevent time-travel itself.
* If anyone cares: What people get wrong (especially the woo brigade) with Schrödinger’s Cat is treating it as an example of what quantum mechanics actually predicts. It’s not. That was Schrödinger’s point. Spooky entangled quantum states don’t extend to macro objects like cats (or vials of poison or detectors). Schrödinger was asking why not? When does it break down, and why? And there’s a whole class of experiments exploring the fringe between quantum and classical (endearingly called Schrödinger’s Kittens.)
Will,
Not related to the article, but just something I heard about a few years back: You don’t need a time dimension to define your hypothetical universe.
Imagine you have a universe with just two particles. How do you define time? Or a coordinate space? The only possible measurement is the distance between the particles. Not even change-in-distance-over-time, since there’s no external way of measuring time or absolute distance.
Add another particle. Now you have three distances, AB, AC, BC. That’s it. So you set one distance, say AB, as your “metric”, and measure the other two distances relative to that. Your entire universe can now be mapped on a two dimensional graph. Every instant, the entire universe maps to a single point on your graph, “movement” maps to a line. A fixed line. Hence no time.
Four particles maps to five dimensions, five particles to 9D, and so on. It gets messy fast, but no matter how many particles, you don’t need a time dimension, just relative distance. So you can map our entire universe as a fixed line, from the (0,0,0,0,0,0…) point (the big bang) to wherever it is now.
Add in quantum uncertainty, and you have a probability of each distance instead of a clear measurement, smearing your neat line. You end up with a solid block spanning the entire graph, with varying “densities” within the block reflecting quantum probabilities. A time dimension is not required, conventional space dimensions are not required. Mind bending, huh?
If the entire universe would need to be time-reversed to travel back in time, one thing pops out as being a real spoiler- the mechanism that activates the time-machine. As soon as the device turns on, time reverses and the device is no longer turned on. However short that interval is would be the limit of any travel back in time.
I’m neither a physicist nor mathematician so forgive my musings from a position of ignorance but I’ve often wondered if our perception of time is actually a perception of the expansion of the universe, as referenced against some other external metric (per Paul). If I understand correctly, time slows as mass increases, and one can thus considered expansion to slow around massive objects, and essentially stop at infinite mass – and time there also stops.
I think also of the classic image of the grid, with marbles sinking in it to represent matter bending space-time and wonder if its perhaps the wrong way round. That matter is actually an external intrusion into what we see as our universe, like a pin prick poking into an ever expanding sheet, and where-ever it happens the sheet temporarily slows expansion, held back by the intrusion. Externally to the universe perhaps the “matter” moves in waves, and motion in our universe is peaks of these waves poking in and out of our universe.
What’s difficult to get my head around is that the idea of expansion requires change over time whereas I’m saying it is time, so it’s expansion over something else, or perhaps it is change over time, but what we are perceiving at a sub-cellular level is the expansion itself.
As such, travel back in time isn’t really an option is it?
Their is a way to avoid the paradox to traveling back in time. You could go back in time and be invisible thus being able to avoid the grandfather theory nothing would be changed, you would just be an observer. This would be the way that nature would resolve all of the paradoxes for going back in time. Of course their is no paradox for future travel.
hmmm
The EPR photons can be described by 0.5cos2(fi).
hmm,hmm
How do I write down when ONE photon goes through two polarisers?
Same way.
The EPR par is actually one photon, but going backwards in time on half-way.