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First Demonstration Of Time Cloaking (technologyreview.com)
143 points by NonEUCitizen on July 14, 2011 | hide | past | web | favorite | 65 comments

These articles always remind me of how prescient Star Trek was when it came to technology, and how we really don't have anything like that these days. The last big SciFi show, Battlestar Galactica, seemed to go out of its way to avoid making any bold tech predictions, and the Star Trek franchise seems to have fizzled out (that movie doesn't count). ST:TOS predicted cloaks just six years after the first laser was ever fired, and 3 years before development began on UNIX. Holograms took until TNG, but they were still a good 20 years early.

I feel like we're getting ripped off. I want radical predictions that I can watch daytime specials on Discovery about in 50 years.

In the postwar era, we [Americans, though I wasn't born, yet] were hopeful about the future. Our science fiction reflected this, but it was just the tip of the iceberg: the whole culture assumed progress and advancement. Immediately after, though, there was a time of fear and pessimism about the future, bringing the concepts of nuclear winter, climate change, resource shortages, and the end of the space age. We call this period "the 70s", and our current culture seems to be far more influenced by that than it is by the previous expectation of momentum and progress.

Also, our basic understanding of physics-as-applied-to-the-limits-of-technology hasn't much changed from 1960 to 2010, unlike the period from 1910 to 1960.

> In the postwar era, we [Americans, though I wasn't born, yet] were hopeful about the future.

Not just y'all.


There's certainly prescient fiction still--post-cyberpunk; Peter Watts, Charlie Stross[1], Greg Egan, Richard K. Morgan...

But the coming technology and culture they're describing is either difficult to show compellingly onscreen, requires an outlier in IQ and education to understand, or is somewhere between disturbing and revolting to people who have no problem with starships and phasers.


Post cyberpunk?

I'm not sure that Star Trek was prescient about what we would go on to invent, so much as it actually inspired a generation of inventors.

I'm sure this is true. After all, the transporter was "invented" when the props for the shuttlecraft weren't ready for the first shoot. Someone said, "well, let's just have them appear on the surface" and presto, you've inspired thousands of people to try to create a transporter.

And it's a well-known fact that the inventor of the cell phone credited ST's communicators as his inspiration. http://www.editinternational.com/read.php?id=4810edf3a83f8

Do you have any ideas or predictions yourself?

Maybe I have a weak imagination, but Star Trek and other sci-fi have probably done anything that can be done with any kind of technology we can come up with. Beyond sci-fi is fantasy land, and sci-fi as a genre has reached that border, beyond that its where Harry Potter and Lord of the Rings take over.

+1 technology has advanced much since then, but nothing spectacular enough to change SF. What we have is basically faster and smaller computers, and network effects due to global connectivity.

There's nothing really new in travel methods (nope, still no flying cars), cities are the same, VR never took off, human-level AI is still far away, we're still fighting for resources (with bullets) and space programs are being dismantled as we speak.

So it makes sense that our current sci-fi is still the same as that of the 80-90's.

>nope, still no flying cars

We have plenty of flying things that ordinary people can afford. Ultralights, powered paragliders, some cheap helicopters, etc. The legal reality of flying things means that they'll never be useful for mass transportation: if you thought drunk driving was bad, wait'll you see drunk flying. Do you want some dumbass crashing into your roof in the middle of the night? Or hell, crashing into you, when you're both 300 feet in the air? Fact is, we've got a nation full of people who can't handle the responsibility of living next to a McDonald's, flight is like a joke.

space programs are being dismantled as we speak.

US is not the whole world.


China is going to launch its first mini-space station soon. Yes, they are a few decades behind US/Russia, but they are moving.

Yes, I sincerely hope they are for real and not just posturing, and that this will start a new space race.

I sincerely hope one day humans will be able to explore space for reasons other than just outrunning the other guy.

I think you're a little restrictive on scifi by referencing only television. There's a lot of very new scifi published as books - television has maybe dropped away, but read something by Charles Stross, for example. I don't think that such stories would have been popular or feasible in the late 80s. Television as a whole has become much less suited to fantastic ideas or new concepts as a reflection of modern tastes - look at the lack of very good documentaries compared to the late 60s and early 70s, for example, and the proliferation of reality TV shows.

A lot of the tech that we have is invisible to you; it's too commonplace to be called wondrous. You casually dismiss commoditized computing resources and global connectivity...the cultural and societal developments arising from these were not expected 20 years ago.

We do have flying cars, I think. They're called helicopters and are too dangerous to allow everyone to have one and drive wherever they like. They cost about the same as a high-end sports car. Like a high-end sports car, most of us won't own one, you can't just park them on the street and they cost up to about $500k.

We have changed cities hugely! You can buy any food anywhere at anytime regardless of season due to improvements in transport and agriculture technology. Cities are a huge mixture of cultures and nationalities in a way they weren't 20 years ago due to improved communications technology. Public transport is much more reliable - if you want to go somewhere, go to the city website and type in the address; you'll be given a routemap that you can carry on your internet-enabled phone, and in many cities you can track the bus in real time. Use that phone to check where your friends are and meet up spontaneously without worrying about a place's quality, because you checked out a hundred reviews on a location-aware website. How many people could work from coffee shops 20 years ago? Buildings are safer (and smaller), and feature built-in hooks for modern tech to play with. It's not happening fast - but look at how cities 'advanced' in the previous 500 years. You have to temper your expectations, a little.

You say that human-level AI is far away - I say that Google can organise and index things better than I could. Mathematical theorems are solved automatically and markets are balanced by intelligences that operate on the nanosecond and picosecond time scales! It's true that a computer won't chat to you, but wouldn't you prefer to talk to a person? VR is similar - it's a societal disinclination, rather than a technological problem. (You can already make a case that augmented reality is here right now.)

Space programmes are being dismantled, and new ones are being put into place. I think that space exploration is healthier now than it was in the 90s. Private companies are finally looking like being able to launch spacecraft reliably. SpaceX's Dragon module is the most exciting thing to happen since before the Hubble telescope.

All these things are referenced and extrapolated in good, modern scifi - just not on the dying medium that is television. Be more optimistic! :-D

(Aside: Can one can make a case that the most popular form of scifi on TV right now is crime drama? Fancy hand-held analyzers map DNA structure and imaging technologies can reconstruct crime scenes from grainy pictures. Network traces track fugitives and the machines seem to finger the perp. automagically - this is not the case in the real world...)

I agree that a lot has changed since the 80's and most of it is commonplace and 'invisible'. The world has changed a lot, and is about to change even more.

But indeed, the TV format can't really show that effectively. And hey, helicopters already existed in the 80's :-)

>nope, still no flying cars

No, but we do have this wonderful piece of awesome: http://www.gizmag.com/hoverbike/18813/

Ten years from now, robotics, AI, and silicon get good enough that we have a massive robotic revolution, mirroring the Industrial Revolution. Robotics takes the labor out of virtually all production, making capitalism obsolete.

The productivity of Capitalism is replaced with creativity. With our World awash in "stuff" too cheap to matter, corporations shrink and disappear, replaced with small groups mirroring today's open source communities.

That's just off the top of my head. Today's TV and movie sci-fi are far from challenging the limits of anything.

And yeah, this is from a guy with the handle of Capitalist.

Judging from today state of the art AI - creativity would be automated much earlier than dumb physical jobs.

We already have pattern matching and theorem proving software, I think that throwing sheer computing power at hard problem where strict criteria for correctnes are defined would be cheaper than human problem solver.

On the other hand mechanical and interfacing side of robot revolution seems to lag after AI research. Teaching robot to run is hard problem, brain has many layers of hardware for doing this, and it's easier to make new human, than it is to make new robot.

All in all I predict the exact opposite - unskilled workers will have jobs, creativity in many cases would be automated.

Our imagination expands as our possibilities become greater.

Also, there is the 3rd Clarke's law, "Any sufficiently advanced technology is indistinguishable from magic".

Sci-fi can easily get into the realm of fantasy (I've already seen a scientifically explained napalm-breathing dragons in 21st century Earth on TV) and vice versa (I've read a story about designing and building spaceships by stacking many layers of different magic onto a wooden ark).

The really interesting thing that sci-fi can give us is this: assume that in described world, a selected type of "magic" is possible (handwaving it into a scientificly-sound concept for the sake of the reading experience) and extrapolate the consequences. What can be done with it, and - more important - how will people behave, how will it transform the society.

Orson Scott Card did something like this with the Internet in "Ender's Game".

>> assume that in described world, a selected type of "magic" is possible (handwaving it into a scientificly-sound concept for the sake of the reading experience) and extrapolate the consequences. What can be done with it, and - more important - how will people behave, how will it transform the society.

That's my definition of good science fiction. Asimov was the master, but I can't find many others like him. I liked OSC too. Do you have any recommendations?

I liked Vernor Vinge's "A Fire Upon a Deep". This book is a goldmine of interesting concepts about alien species and their civilisations, completely different from "standard TV aliens". Also "The Trigger" by Arthur C. Clarke - it's a book about how an accidental discovery causes technological change, which transforms life of societies around the world. One of the best books I've ever read.

The Mote in God's Eye was a great read.

we really don't have anything like that these days

Check out Halting State by Charles Stross and Rainbows End by Vernor Vinge.

Charles Stross is definitely an author to read if you're looking for interesting predictions that have a good chance of coming true.

Sounds right up my alley. Thanks for the suggestion, I'll check it out.

I've often wondered what kind of science fiction the characters of Star Trek read in the 23rd and 24th century. Star Trek characters always seem to be conveniently interested in 20th century culture.

Forget Star Trek and Galactica.

Frank Herbert was the one right. His extrapolation of 20th century is a dystopia and utopia at the same time.

But even he couldn't foresee that we will create no-chamber at the start of 21st century.

It seems like a huge wave of optimism and progressive ideas sprang forth during the '60s, producing all the well-known quirks and odds of that decade.

Then the whole world seemed to sink back into skepticism and the regular blasé mood.

This is as odd as the rampant insanity that took over the world two decades prior to that.

The 20th century was very very strange.

I'm still trying to get my head around this. To the layman, it sounds almost like something trivial (properties of metamaterials) extrapolated into a bit of a sensationalist headline.

So I go grab my good friend Schrödinger. We take his cat, subject of numerous cruel and unusual experiments, and stick it in a box. We tie the health of the cat to a quantum state which could condense either way. If it goes one way, the cat lives. If it goes the other way, the cat dies. Then we wrap the quantum state in these time-cloaking lenses.

We look at the cat, and what? Does the lack of ability to observe the quantum state mean that the condensation of probabilities never happened? Perhaps the cat disappears?

EDIT: Link for those who don't get the reference. http://en.wikipedia.org/wiki/Schr%C3%B6dingers_cat

I haven't read the paper, just the abstract. But the literal answer to your question is: You don't observe the cat. Ergo, it's like the "box" was never even opened.

Now that I've answered that, let's caution you about your deployment of Mr. Cat. First of all, I don't believe you'll need an uncertain quantum object to observe in this particular thought experiment. Life is complicated enough without bringing in more quantum mechanics than you need. Just imagine a brick. It's red. Light is shining on the brick and it looks red. Now, I install my time-bending lenses around the brick, and then at one particular instant the brick changes to be yellow, and then goes back to being red. But you don't see that, you just see a continuously red brick, even though the light was "shining on" the brick the whole time, because our lenses made a "time hole" in the stream of light and the color change happened during the hole.

I should also warn you that our friend Mr. Cat is a bad metaphor in many ways. (He was originally an in-joke, after all, used by people who were debating the apparent absurdity of a new theory.) The problem with the cat is that he's a completely unrealistic creature, and he primes your intuition in all sorts of incorrect ways. In real life you will never stumble across a coherent superposition of two quantum states as dramatically large (trillions of cells!) and dramatically different (alive vs dead!) as this imaginary cat. Real cats interact with their surroundings. A lot. They breathe and meow, they emit organic molecules that smell like cat, they charge up with static electricity (electric fields) and then run around in circles (magnetic fields), and above all they gravitate (which is difficult to cloak). So to make the thought experiment work the cat's imaginary box has to be a cloaking device, a box that permits no interaction with the outside world until it is "opened". And that doesn't exist in real life.

And, therefore, importing the cat into this thought experiment is a particularly bad move, because it's redundant. If you have Shroedinger's imaginary box you don't need an additional set of imaginary time-cloaking lenses. The box is already cloaking the cat, for a fairly long period of time.

Always be careful: Thinking about completely impossible things is fun for a while, but don't make a habit of it, or you'll become the sort of insufferable spaced-out quantum-mechanics groupie who says things like gosh, maybe when I turn around and stop believing in this tree it will cease to exist. Of course not. Tree-sized objects do not physically disappear from your universe, not without the sound of a chainsaw.

Real cats interact with their surroundings. A lot. They breathe and meow, they emit organic molecules that smell like cat, they charge up with static electricity (electric fields) and then run around in circles (magnetic fields)

I actually laughed out loud; thank you for this lucid description. Fantastic way to start my morning.

Put an LED behind the temporal cloak. Look at it from the outside. You see an LED. Look at it through the temporal cloak. You see an LED. Now turn the cloak on and blink the LED (very fast). If you're looking through the cloak, you won't see the blink happen. If you're looking from the outside, you will.

Throw the box away; it's not helpful here. What you see when you look through the time lens is a live cat, then either a) nothing happens or b) it suddenly turns into a dead cat. What you don't see is the cat's death, if it occurs. Look at the graph in the article; light moves from left to right through one spatial dimension, and from bottom to top along the temporal dimension. You see the before and after, but not the in-between moment.

Instead of a cat, consider a situation where you could 'use' this time lens, if it operated over human-scale time periods. I sneak the time lens into your party, disguising it as a beer keg. At an appropriate moment, I trigger it; then I rush over to your wall safe, open it and extract the diamonds stored within, close the safe, and return to my starting position. From your point of view, everything is as it was before. Unless you were looking at the clock - in which case you would see it jump forward by two minutes! You realize someone at the party has been using a time lens, and raise the alarm. Suspicion soon falls upon me, and I shamefacedly produce the diamonds from my pocket.

But just as you and your friends are about to lay hands on me, I vanish! I triggered the time lens again, and slipped out between you, unobserved.

Of course, I can't see what's going on with the time lens either, in all likelihood; perhaps I need some sort of ninja skills to move blindly through the temporal hiatus.

Unless you were looking at the clock - in which case you would see it jump forward by two minutes!

wait, where is this clock?

Above the mantelpiece, where else? Seriously, I just threw that in because most people have a clock (or several) and ISTM that you would experience a forward jump in time if this hypothetical device could work as described. Not having read the paper yet, I don't know if it would only work for people observing things from a particular perspective, or whether it would just create a bubble of 'extra' time for anything within its field, during which observations from outside would be impossible. Of course, this might make it impossible to escape as well - I took some dramatic liberties with my scenario to explore the possibilities, although I left out some of the wilder ones that came to mind.

Schrodinger's cat is a flawed thought experiment. The cat is not in a super-position of live-dead just because the decay (or absence of a decay) hasn't been observed by a human. Observation just means "interaction". If one particle interacts with another, it has been "observed".

Schrodinger's cat is a flawed thought experiment. Observation just means "interaction"

The way I learned it, Bohr would say "the atom is in a superposition of states until it is observed". Schrödinger used the cat thought experiment to show the absurdity of Bohr's position: "... and then, when I look at the cat, it is forced to take a stand, poof it goes 'alive', or 'dead'"

As far as I know, we still don't understand what, exactly, constitutes an "observation". Quoth Bell (the guy from Bell's experiment):

... current interest [in questions of foundations of quantum mechanics] is small. The typical physicist feels that they have long been answered, and that he will fully understand just how if he ever can spare twenty minutes to think about it.


Jaynes is also worth reading, in spite of his non-mainstream position: http://bayes.wustl.edu/etj/articles/prob.in.qm.ps.gz

The thing is, you don't actually need to postulate that the cat goes "poof" at the point of observation. Suppose this didn't happen. Then the state of the world before observation is this:

    |cat dead, observer unsure> + |cat alive, observer unsure>
After observation:

    |cat dead, observer sad> + |cat alive, observer happy>
(Normalization dropped.) This isn't weird. What would be weird is if these two states could interfere with each other.

I did some work a few years back which argues that interference between these states is virtually impossible - they live too far away from each other in configuration space.


Ultimately the argument goes all the way back to David Bohm in the 50's. The novelty of what I did is cooking up a plausible experiment for which the calculations are tractable.

Cool. How do you deal with the double slit experiments if there is no interference? Aren't you back to a fully deterministic worldview, from the observer's perspective?

There is interference - my model is merely the many body schrodinger equation, after reduction to a two-particle NLS.

The key observation is that a measurement apparatus involves N particles, each moving a distance O(1). Thus, the distance (in configuration space) between both wavepackets (i.e., |cat dead, observer sad> and |cat alive, observer happy>) is O(sqrt(N)).

Because the two wavepackets are so far apart, they can't interfere. This is true with or without the observer, since the cat still has O(10^23) particles.

On the other hand, if there were no observer, and the system were merely an electron, the distance between wavepackets would be O(1) and interference would be possible.

> If it goes one way, the cat lives. If > it goes the other way, the cat dies.

No, that is wrong. The cat does not live or die. The cat is both alive and dead at the same time. This is why so many people have problems understanding quantum mechanics -- they don't seem to get the implications of a state being a probability. Schrödinger came up with his cat in an attempt to make the reality clear to other people, and yet many intelligent people misinterpret what Schrödinger's point was. It's important to get this right: the cat is both alive and dead. Both. If the scenario wasn't so extremely counter-intuitive, then there would not have been a need to invent it in the first place.

No, that is wrong. The cat is not both alive and dead at the same time. The Schrödinger's cat thought experiment was invented to show the absurdity of the collapse of quantum states being caused by a conscious observer. The collapse of states happens far before the cat dies or survives.

Or if you accept the many-worlds interpretation, there is no collapse. There's one blob of amplitude corresponding to a configuration with a dead cat and you observing a dead cat, and another blob of amplitude corresponding to a configuration with a live cat and you observing a live cat. Excellent explanation with more detail here: http://www.quora.com/Why-does-observation-collapse-probabili....

There's still an event beyond which there appears to be no superposition, whether that's because the two blobs of amplitude have separated enough or whether it's because there's only one left, and I think the word collapse serves well enough.

Schrodinger and his cat have nothing to do with this.

Just got my mind blown for the rest of the day...

Anyway, I'm reading it over and over again, and I'm still not sure if I understand this well. Can someone confirm that, and elaborate on that a little bit?

And I also didn't realize that we've gone so far with "traditional" cloaking devices. I thought we're still at microwave and near-infrared level.

EDIT. Abstract of the original paper makes this a little bit easier to understand for me.

EDIT 2. And here we have:

- http://www.technologyreview.com/blog/arxiv/23519/ - illusion cloak

- http://arxivblog.com/?p=698 - cloaking at a distance

Could someone tell me a practical application of this?

If someone made a box, with time lenses around it, and someone went inside the box, what would happen?

This is essentially a concept for a device that interrupts the observation of the space it protects by probes meant to glean temporal information. My understanding is that nothing spectacular would happen to the contents of the device, but to a probe aimed at the contents. That probe would be manipulated around the space, preventing it from making inferences about what would be occurring in the space.

As for what exactly constitute a temporal probe, I am not certain. Some form of laser meant to be manipulated by the occurrence of an event perhaps.

This is actually a spatiotemporal cloak, not purely temporal. The probe is simply the laser (the green lines in the article picture).

no need for a box. You put a camera after the second lens, and, if "things happen" between the lenses, the camera will fail to record what happens while the cloak is on. Not because the light has moved around the events, but because it has been split in two parts, a fast-moving and a slow-moving, leaving a short gap in time in which neither the fast or slow parts can interact with the events.

probable application: Magicians can make their tricks completely undetectable using a cloak while they do them (assuming they can do them in sub-microsecond time).

What I miss in this article is what it implies that time is cloaked. What does is it mean and where can it be used for?

I'll hope that other brighter members of our community correct me if I'm wrong, but here's a stab at it.

We all learn about causality. Things don't just happen without being there being some kind of reversible action. If an ice cube melts, that's a time reversible aspect of physics. Basically if you could run time backwards like a video in reverse, you would see the ice unmelt.

In another instance, imagine a block of ice in the middle of a star. There's no way that a chunk of ice can just appear in a star. Now, that's not to say its impossible for such a chunk to be there. But you can trace back a line of actions that led to it being there, ranging from the debris of the spacecraft that contained it to the launch pad on a planet orbiting nearby, to all of the patterns of matter and information encompassing the civilization that built and launched it in the first place.

So if I'm understanding this correctly, the temporal cloak effectively allows stuff to happen inside it that isn't detectable from the outside. To stretch my last analogy a bit, imagine you put such a temporal cloak around the entire civilization leading up to where the ice ends up in the star. To an outside observer, say some aliens doing a research study on our star, all of a sudden they would detect the block of ice in the star but not have any idea where it came from.

Nice Anathem reference :)

The problem with your analogy is that the debris from the spaceship, etc. would still be visible to an observer after the fact. A better analogy would be this:

The aliens point a camera at our entire civilization. Ignoring the fact that this cloak works on the order of monumentally smaller scales than civilizations and the time it takes to put an ice chunk in a star, we use the cloak while we put an ice chunk in a star. The aliens would see our civilization jump from a state with no ice chunk in a star to one with an ice chunk in a star. Assuming that their measuring devices are up to the task, they would still be able to find all the spaceship debris, etc.

So basically, time cloaking enables the possibility of Cuil[0] realities over 2.

[0] http://cuiltheory.wikidot.com/what-is-cuil-theory

the abstract is clearer http://arxiv.org/abs/1107.2062.

here's the relevant bit:

Alternatively, it may be desirable to cloak the occurrence of an event over a finite time period, and the idea of temporal cloaking was proposed in which the dispersion of the material is manipulated in time to produce a "time hole" in the probe beam to hide the occurrence of the event from the observer. This approach is based on accelerating and slowing down the front and rear parts, respectively, of the probe beam to create a well controlled temporal gap in which the event occurs so the probe beam is not modified in any way by the event.

I found the MIT Tech Review's summary confusing. I didn't understand the article it was based on fully, but the idea seems to be covered in the following paragraph:

Time-space duality represents the analogy between diff raction and dispersion that arises from the mathematical equivalence between the equations describing the diff raction of a beam of light and the one-dimensional temporal propagation of a pulse through a dispersive medium [17,18]. Similar to a spatial lens that imparts a quadratic phase in space, a time-lens can be implemented that produces a quadratic phase shift in time [19{21]. This time-lens can, for example, magnify [22] or compress [23] signals in time and has an equivalent of the lens law. Time-lenses can be created with an electro-optic modulator [18] or via a parametric nonlinear optical process such as four-wave mixing (FWM) with a chirped pump wave [19,20]. In the latter case the signal wave is converted to an idler wave with a linear frequency shift in time (i.e., a quadratic phase in time) [21,22].

I understand this to mean that the passage of time in observed space can be slowed down relative to an observer, rather as happens when observing a body falling into a black hole.

Is this similar to the idea of the light cone? http://en.wikipedia.org/wiki/Light_cone

That is, it's impossible for information from this "cloaked" time to reach us, because it's outside of our light cone?

Not really, it's not a relativistic phenomenon.

Fantastic its like that movie I saw in which a guy can freeze time whenever he likes and then gets the reality going by cracking his knuckles.

Old. Shane Carruth showed me this back in 2004.

It's a nice way to put people in prison.

There's a better article in the guardian http://www.guardian.co.uk/science/blog/2011/jul/13/harry-pot... . They have an interesting intuitive description:

He used one set of lenses to prise open a gap in a beam of light, by slowing down long wavelengths, such as red, and speeding up short wavelengths, such as blue. With a second set of lenses, he then closed the gap, so at the end of the experiment, the light beam looked exactly as it did at the start.

I'm not sure that i understand it correctly, the "cloak" occurs because there is no light during the event as it has either moved on or is lagging behind. It's the recombination at the second lens that does the trick, giving a false sense of continuity to the observer after the lens.

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