BBC on wormholes

We get a mention in this nice BBC article. Here is the latest version (PDF) of the talk Roman Buniy will give on Tuesday at a conference at Vanderbilt.

Wormhole 'no use' for time travel
By Paul Rincon
BBC News science reporter

Artist's impression of a wormhole Image: SPL
Wormholes contort the fabric of the Universe
For budding time travellers, the future (or should that be the past?) is starting to look bleak.

Hypothetical tunnels called wormholes once looked like the best bet for constructing a real time machine.

These cosmic shortcuts, which link one point in the Universe to another, are favoured by science fiction writers as a means both of explaining time travel and of circumventing the limitations imposed by the speed of light.

The concept of wormholes will be familiar to anyone who has watched the TV programmes Farscape, Stargate SG1 and Star Trek: Deep Space Nine.

The opening sequence of the BBC's new Doctor Who series shows the Tardis hurtling through a "vortex" that suspiciously resembles a wormhole - although the Doctor's preferred method of travel is not explained in detail.

But the idea of building these so-called traversable wormholes is looking increasingly shaky, according to two new scientific analyses.

Remote connection

A common analogy used to visualise these phenomena involves marking two holes at opposite ends of a sheet of paper, to represent distant points in the Universe. One can then bend the paper over so that the two remote points are positioned on top of each other.

[The wormholes] you would like to build - the predictable ones where you can say Mr Spock will land in New York at 2pm on this day - those look like they will fall apart
Stephen Hsu, University of Oregon
If it were possible to contort space-time in this way, a person might step through a wormhole and emerge at a remote time or distant location.

The person would pass through a region of the wormhole called the throat, which flares out on either side.

According to one idea, a wormhole could be kept open by filling its throat, or the region around it, with an ingredient called exotic matter.

This is strange stuff indeed, and explaining it requires scientists to look beyond the laws of classical physics to the world of quantum mechanics.

Exotic matter is repelled, rather than attracted, by gravity and is said to have negative energy - meaning it has even less than empty space.

Law breaker

But according to a new study by Stephen Hsu and Roman Buniy, of the University of Oregon, US, this method of building a traversable wormhole may be fatally flawed. In a paper published on the arXiv pre-print server, the authors looked at a kind of wormhole in which the space-time "tube" shows only weak deviations from the laws of classical physics.

These "semi-classical" wormholes are the most desirable type for time travel because they potentially allow travellers to predict where and when they would emerge.

The Tardis (BBC)
The concept is a favourite of science fiction writers
Wormholes entirely governed by the laws of quantum mechanics, on the other hand, would likely transport their payloads to an undesired time and place.

Calculations by the Oregon researchers show a wormhole that combines exotic matter with semi-classical space-time would be fundamentally unstable.

This result relies in part on a previous paper in which Hsu and Buniy argued that systems which violate a physical principle known as the null energy condition become unstable.

"We aren't saying you can't build a wormhole. But the ones you would like to build - the predictable ones where you can say Mr Spock will land in New York at 2pm on this day - those look like they will fall apart," Dr Hsu said.

Tight squeeze

A separate study by Chris Fewster, of the University of York, UK, and Thomas Roman, of Central Connecticut State University, US, takes a different approach to tackling the question of wormholes.

Amongst other things, their analysis deals with the proposal that wormhole throats could be kept open using arbitrarily small amounts of exotic matter.

Fewster and Roman calculated that, even if it were possible to build such a wormhole, its throat would probably be too small for time travel.

It might - in theory - be possible to carefully fine-tune the geometry of the wormhole so that the wormhole throat became big enough for a person to fit through, says Fewster.

But building a wormhole with a throat radius big enough to just fit a proton would require fine-tuning to within one part in 10 to the power of 30. A human-sized wormhole would require fine-tuning to within one part in 10 to the power of 60.

"Frankly no engineer is going to be able to do that," said the York researcher.

The authors are currently preparing a manuscript for publication.

Supporting view

However, there is still support for the idea of traversable wormholes in the scientific community. One physicist told BBC News they could see problems with Hsu's and Buniy's conclusions.

"Violations of the null energy condition are known to occur in a number of situations. And their argument would prohibit any violation of it," they commented.

"If that's true, then don't worry about Hawking radiation from a black hole; the entire black hole vacuum becomes unstable."

The underlying physics was not in doubt, the researcher argued. The real challenge was in explaining how to engineer wormholes big enough to be of practical use.

Cambridge astrophysicist Stephen Hawking is amongst those researchers who have pondered the question of wormholes.

In the 1980s, he argued that something fundamental in the laws of physics would prevent wormholes being used for time travel. This idea forms the basis of Hawking's Chronology Protection Conjecture.

Shooting Through the Wormhole

I spent almost 12 hours today shooting for an episode of the Science Channel's Through the Wormhole. The producer told me our efforts would result in a 6 minute segment that will air towards the end of the summer. See earlier post for some discussion of the actual science in the episode.

In the episode I drive the VW bug below through a tunnel (the Caldecott tunnel; don't ask how many takes we did!), to simulate what it would be like to go through a wormhole :-)

Through the wormhole: DIY

I'm in the Science Channel show Through the Wormhole later this week (first air date is Weds. 7/20/2011), talking about faster than light travel via wormhole.

I blogged about shooting the episode here. I haven't seen the show, other than the excerpt below. It's kind of cool to hear Morgan Freeman say my name :-)

Through the wormhole: Q&A

Below are some of the follow up questions I answered for viewers of Through the Wormhole. I was very impressed with the quality of the show after watching the episode I had taped.

We know that time and space are relative, is 'information'?
Information can be defined in relative terms, as in what is known to a particular observer. There are also notions of absolute information, as in what is the total amount of information (bits or qubits) required to specify the exact quantum state of a system, or even of the entire universe. (See here, here and here.)


How is our scientific knowledge going to progress in the coming centuries? Right now, there are certain scientific properties and equations that only a handful of genius humans can grasp. Are we to evolve better brains?
Unfortunately, the conceptual frontiers of modern science are only comprehensible to a small fraction of humans. Perhaps genetic engineering in the future will raise the average intelligence level, but there will always be outliers in the intelligence distribution and they will likely be the ones driving scientific progress. It is possible that someday machine intelligence will surpass that of all humans and our entire species will become spectators to scientific progress. (See here and here.)

Is reality a perception of one's own mind? Or does it exist at all? No one knows the answer to this deep philosophical question. Most scientists make the assumption that our senses and devices convey information about a "real" universe of which we are a part. Our job is to understand how that universe works. (See here.)

Replica Wormholes and Quantum Hair

Replica Wormholes and Quantum Hair 

https://arxiv.org/abs/2404.02331

Xavier Calmet, Stephen D.H. Hsu 

We discuss recent applications of Euclidean path integrals to the black hole information problem. In calculations with replica wormholes as the next-to-leading order correction to the Gibbons-Hawking saddlepoint, the radiation density matrix approaches a pure state at late times, following the Page curve. We compare unitary evaporation of black holes (in real time), mediated by calculable quantum hair effects, with the replica wormhole results. Both replica wormhole and quantum hair approaches imply that radiation states are macroscopic superpositions of spacetime backgrounds, invalidating firewall and monogamy of entanglement constructions. Importantly, identification of modes inside the horizon with radiation modes (i.e., large scale nonlocality across the horizon) is not required to provide a physical picture of unitary evaporation. Radiation modes can encode the interior information while still remaining independent degrees of freedom.

Wormholes dominate the Gibbons-Hawking saddlepoint of the Euclidean path integral after the Page time. This is because wormholes can connect the interiors of any two black holes i,j. At late times the number of such pairs grows as the dimensionality of the radiation Hilbert space squared. 

The wormholes connect BHs with macroscopically different recoil trajectories. This means the radiation approaches a pure state that is a macroscopic superposition - very similar to what our quantum hair expressions indicate.

Morgan Freeman on physics and physicists

Freeman is the host of the Science Channel show Through the Wormhole. (Thanks to a dude at PIMCO for sending me this video :-)




I think I'm the only physicist on the show who actually went through a wormhole -- in a VW bug, no less :-)

Through the wormhole

Tomorrow a team from the Science Channel show Through the Wormhole (Morgan Freeman is the narrator) is flying up to Berkeley to interview me for an upcoming episode. They found me because of a paper I wrote with my former postdoc Roman Buniy (see below). Roman is a brilliant guy who twice placed first in the Ukrainian physics Olympiad. He made the beautiful figures below.

Doing this kind of show isn't exactly a win-win: I would guess the volume of crackpot mail I receive could go up by an order of magnitude :-(

Semi-classical wormholes and time machines are unstable

hep-th/0504003

Abstract: We show that Lorentzian (traversable) wormholes and time machines with semi-classical spacetimes are unstable due to their violation of the null energy condition (NEC). Semi-classicality of the energy-momentum tensor in a given quantum state (required for semi-classicality of the spacetime) implies localization of its wavefunction in phase space, leading to evolution according to the classical equations of motion. Previous results related to violation of the NEC then require that the configuration is unstable to small perturbations.

Here are some slides on the subject. Click for larger version. For more background on why it is difficult to construct effective field theories which lead to (stable) violation of the null energy condition (NEC), see Phys. Rev. D 74, 063518 (2006).

Wormholes, NEC and all that

The BBC and New Scientist articles seem to have generated a lot of interest in this topic. Odd how my colleagues can hear me loudly discussing this stuff for six months with my postdoc and grad student, but only after the BBC decides to write about it do they want to know more :-)

The original papers are listed below. Both have been revised since posting on arxiv.org - if you want a more up to date version please contact me.

http://arxiv.org/abs/hep-th/0504003 (wormholes)
http://arxiv.org/abs/hep-th/0502203 (instability and NEC)

There is a longer version of the instability analysis forthcoming, by Buniy, Hsu and Murray.

Let me make some comments here for physics readers:

1) Our original interest was in dark energy. The observational data suggest (although not strongly - see comments) that w = p/rho < -1, which violates various energy conditions. We wanted to understand how easy or hard it is to build models with w < -1. With some collaborators at Caltech, I had obtained a result in classical scalar models that w < -1 implies instability. We wanted to generalize this result.

2) Our strongest results are in the contexts of classical field theory (including both gauge and scalar fields) and perfect fluids. There is a quantum loophole involving renormalization that allows for small violations of the NEC (well-known examples are the Casimir effect and black hole spacetimes).

3) When applying this to wormholes, we are considering the exotic (NEC-violating) matter necessary to stabilize the wormhole. This matter must have large energy-momentum tensor T_mn. We focus on wormholes which have nearly-classical spacetimes (the other type is less useful for Sci Fi). We show that this condition is strong enough to require that the exotic matter evolves semi-classically - i.e., it is subject to our results in classical field theory.

4) Some readers (esp. from the relativity community) have misinterpreted our results as claiming that the Casimir or black hole vacuum is unstable, but this is not the case (see point (2) above). In the wormhole case, the key point is that semi-classical wormholes cannot result from exotic matter which violates the NEC via quantum effects.

New paper

This paper follows up on our earlier work on the null energy condition (NEC). Imagine you've built a device which "warps" spacetime enough to create a wormhole or time machine. General theorems show that somewhere the matter in your device has to violate the NEC. In our earlier work we showed that any classical system which does so is unstable to small perturbations. In this paper, we show that semi-classicality of the device spacetime is a strong enough condition to require semi-classicality of the matter fields from which it is constructed. In other words, a device which warps space in a deterministic (nearly classical) manner is subject to the earlier results and is unstable.

We leave open the possibility of intrinsically quantum (or "fuzzy" devices) whose spacetime is strongly fluctuating. However, these might not be the most safe or realiable means of transportation! A wormhole or time machine cannot be both predictable and stable.


Semi-classical wormholes and time machines are unstable
Authors: Roman V. Buniy, Stephen D.H. Hsu

We show that Lorentzian (traversable) wormholes and time machines with semi-classical spacetimes are unstable due to their violation of the null energy condition (NEC). Semi-classicality of the energy-momentum tensor in a given quantum state (required for semi-classicality of the spacetime) implies localization of its wavefunction in phase space, leading to evolution according to the classical equations of motion. Previous results related to violation of the NEC then require that the configuration is unstable to small perturbations.

http://arxiv.org/abs/hep-th/0504003

Infinity and Solipsism, Physicists and Science Fiction

The excerpt below is from Roger Zelazny's Creatures of Light and Darkness (1969), an experimental novel which is somewhat obscure, even to fans of Zelazny. 

Positing infinity, the rest is easy. 

The Prince Who Was A Thousand is ... a teleportationist, among other things ... the only one of his kind. He can transport himself, in no time at all, to any place that he can visualize. And he has a very vivid imagination. 

Granting that any place you can think of exists somewhere in infinity, if the Prince can think of it too, he is able to visit it. Now, a few theorists claim that the Prince’s visualizing a place and willing himself into it is actually an act of creation. No one knew about the place before, and if the Prince can find it, then perhaps what he really did was make it happen. However, positing infinity, the rest is easy.

This contains already the central idea that is expressed more fully in Nine Princes in Amber and subsequent books in that series.

While traveling (shifting) between Shadows, [the prince] can alter reality or create a new reality by choosing which elements of which Shadows to keep or add, and which to subtract.

Creatures of Light and Darkness also has obvious similarities to Lord of Light, which many regard as Zelazny's best book and even one of the greatest science fiction novels ever written. Both have been among my favorites since I read them as a kid.

Infinity, probability measures, and solipsism have received serious analysis by theoretical physicists: see, e.g.,  Boltzmann brains. (Which is less improbable: the existence of the universe around you, or the existence of a single brain whose memory records encode that universe?) Perhaps this means theorists have too much time on their hands, due to lack of experimental progress in fundamental physics. 

Science fiction is popular amongst physicists, but I've always been surprised that the level of interest isn't even higher. Two examples I know well: the late Sidney Coleman and my collaborator Bob Scherrer at Vanderbilt were/are scholars and creators of the genre. See these stories by Bob, and Greg Benford's Remembing Sid: 

... Sid and some others created a fannish publishing house, Advent Publishers, in 1956. He was a teenager when he helped publish Advent’s first book, Damon Knight’s In Search of Wonder. ... 

[Sid] loved SF whereas Einstein deplored it. Lest SF distort pure science and give people the false illusion of scientific understanding, Einstein recommended complete abstinence from any type of science fiction. “I never think of the future. It comes soon enough,” he said.

While I've never written science fiction, occasionally my research comes close -- it has at times addressed questions of the form: 

Do the Laws of Nature as we know them allow ... 

... time travel?

... the construction of a stable wormhole? 

... the creation of a baby universe in the laboratory? 

... the creator of a baby universe to send a message to those inside? 

... genetic engineering of superhumans?

This research might be considered as the ultimate in hard SF ;-) 

Wikipedia: Hard science fiction is a category of science fiction characterized by concern for scientific accuracy and logic.

Note Added: Bob Scherrer writes: In my experience, about 1/3 of research physicists are SF fans, about 1/3 have absolutely no interest in SF, and the remaining 1/3 were avid readers of science fiction in middle school/early high school but then "outgrew" it.

Here is a recent story by Bob which I really enjoyed -- based on many worlds quantum mechanics :-) 

It was ranked #2 in the 2019 Analog Magazine reader poll!

Note Added 2: Kazuo Ishiguro (2017 Nobel Prize in Literature) has been evolving into an SF/fantasy writer over time. And why not? For where else can one work with genuinely new ideas? See Never Let Me Go (clones), The Buried Giant (post-Arthurian England), and his latest book Klara and the Sun.

NYTimes: ... we slowly discover (and those wishing to avoid spoilers should now skip to the start of the next paragraph), the cause of Josie’s mysterious illness is a gene-editing surgery to enhance her intellectual faculties. The procedure carries high risks as well as potential high rewards — the main one being membership in a professional superelite. Those who forgo or simply can’t afford it are essentially consigning themselves to economic serfdom.

WSJ: ... Automation has created a kind of technological apartheid state, which is reinforced by a dangerous “genetic editing” procedure that separates “lifted,” intellectually enhanced children from the abandoned masses of the “unlifted.” Josie is lifted, but the procedure is the cause of her illness, which is often terminal. Her oldest friend and love interest, Rick, is unlifted and so has few prospects despite his obvious brilliance. Her absentee father is an engineer who was outsourced by machines and has since joined a Community, one of the closed groups formed by those lacking social rank. In a conversational aside it is suggested that the Communities have self-sorted along racial lines and are heavily armed.

New Yorker on epigenetics

This is a fairly balanced account of recent progress in epigenetics (at least the part I excerpt below). But see here for negative reactions.

New Yorker: ... But, if epigenetic information can be transmitted through sperm and eggs, an organism would seem to have a direct conduit to the heritable features of its progeny. Such a system would act as a wormhole for evolution—a shortcut through the glum cycles of mutation and natural selection.

My visit with Allis had ended on a cautionary note. “Much about the transmission of epigenetic information across generations is unknown, and we should be careful before making up theories about the kind of information or memory that is transmitted,” he told me. By bypassing the traditional logic of genetics and evolution, epigenetics can arouse fantasies about warp-speeding heredity: you can make your children taller by straining your neck harder. Such myths abound and proliferate, often dangerously. A child’s autism, the result of genetic mutation, gets attributed to the emotional trauma of his great-grandparents. Mothers are being asked to minimize anxiety during their pregnancy, lest they taint their descendants with anxiety-ridden genes. Lamarck is being rehabilitated into the new Darwin.

These fantasies should invite skepticism. Environmental information can certainly be etched on the genome. But such epigenetic scratch marks are rarely, if ever, carried forward across generations. A man who loses a leg in an accident bears the imprint of that accident in his cells, wounds, and scars, but he does not bear children with shortened legs. A hundred and forty generations of circumcision have not made the procedure any shorter. ...

How you got here

Below are the top keyword searches that led people to this blog in the past 24 hours. Apparently my post on the time travel movie Primer is popular. Other topics of interest: China, hedge funds, globalization, financial bubbles and the occasional wormhole...

Num Perc. Search Term
10 25.00% primer movie
3 7.50% china climbing
2 5.00% single-digit millionaires
2 5.00% why not hyperinflation
2 5.00% primer/movie
1 2.50% emerging markets debt processing
1 2.50% analyzing hedge fund returns
1 2.50% information processing in brain
1 2.50% advantages and disadvantages of globalization
1 2.50% ltcm today
1 2.50% horvitz cleveland
1 2.50% daniel kahneman and amos tversky
1 2.50% jeremy grantham 2005 interview
1 2.50% john d gartner the hypomanic hedge
1 2.50% cds implied volatility
1 2.50% asness bubble logic
1 2.50% tennessee candidate issues
1 2.50% china-japan relations
1 2.50% all about wormholes
1 2.50% cdx index
1 2.50% china explaining high savings
1 2.50% price to rent ratios
1 2.50% sony vs samsung tv
1 2.50% global real interest rates
1 2.50% why long bonds
1 2.50% price to rent ratio

Null energy condition

New paper! A bit too technical to discuss here. But for Sci-Fi fans, the following tidbit: wormholes and time machines generally require violation of the null energy condition. Our paper shows, among other things, that none of the particles or fields of the type generally considered in particle physics will allow construction of a (stable) wormhole or time machine.

For those familiar with the Einstein equations, they are of the form:

{Curvature tensor R, fn of spacetime geometry} = {matter energy-momentum tensor T}

The set of possible spacetime geometries (could there be wormholes? loops in time?) satisfying the Einstein equations is unconstrained without some information about the right hand side of the equation. Traditionally in general relativity, one assumes that the matter fields satisfy certain energy conditions, for example the null energy condition, which requires that the tensor T, when contracted with any null (lightlike) four-vector n, is positive: Tnn > 0. We show that when this condition is violated, the system is necessarily unstable to small perturbations.


Instabilities and the null energy condition
Authors: Roman V. Buniy, Stephen D.H. Hsu

We show that violation of the null energy condition implies instability in a broad class of models, including all gauge theories with scalar and fermionic matter as well as any perfect fluid. When applied to the dark energy, our results imply that $w = p / \rho$ is unlikely to be less than -1.

http://arxiv.org/abs/hep-th/0502203