NASA guy Roger Weiss emailed me the link to this video, on Space.com, and I thought I might as well make my comments here.
How to Time Travel?
- But how can you swim upstream in the river of time? Physicists Charles Liu and Michio Kaku have some answers.
Despite being really solid physicists (to whom I could probably listen to talk for hours), Charles Liu and Michio Kaku make a few of those misleading, “popularizer”, statements that I just want to point out. The problem with much of the popular work done in physics is that you only can get the real picture, if you already know what they are talking about. The lay audience for whom popular work is intended, thus misses out on the key concepts. Very important questions like, “whose time are we even talking about”, are not even mentioned in these videos.
Michio Kaku: …Space and time is a fabric – a fabric that, perhaps, you can rip, if you have unimaginable sources of energy.
I object to this “ripping” of spacetime. Time travel, whether you believe the mathematics that supports it are physical or not, is something that exists within spacetime (as does everything else). Travelling backwards in time, just like travelling forwards in time (which I’m doing right now), is just moving to a different part of the spacetime manifold (albeit the causal structure of that manifold, classically, forbids much of the possible movements). The analogy with ripping a piece of fabric is very misleading. Even a wormhole is not a “rip” in spacetime, it is just a topological feature in spacetime. Ripping a piece of fabric, implies that you are holding a piece of fabric and ripping it from the outside. This is completely acausal, when considering the universe. You can not take and rip spacetime, because to do so, within the fabric analogy, would imply the existence of action outside of spacetime. We can not rip or “split open” spacetime – spacetime is just spacetime – nothing exists outside of it or acts on it as a whole (ignoring possible inflationary themed pocket universes (other spacetimes) colliding with our own). It can have an absolutely bizarre shape; it can be curved and warped and have holes (topologically), but can not be “ripped”, because “ripping” implies an “outside” of spacetime.
Charles Liu: …The idea of a wormhole is, very simply, that, you come into that hole from within our universe, temporally exit the universe, exit our spacetime continuum, and return again to our spacetime continuum at some other space location and some other time location.
The idea that a wormhole leads you out of our spacetime, while easier to visualize, is incorrect. Within general relativity, you never leave spacetime, regardless of what you do. The misconception comes from the problem that people seem to have with embedding. Because it’s so difficult (read: impossible) to visualize a curved 4-dimensional manifold representing spacetime, we choose to imagine a 4-dimensional manifold embedded into a 5-dimensional Euclidean space. For an example, see the analogy:
Analogy to a wormhole in a curved 2D space - this image is used in Part 2 of the video
Here, this 2-dimensional image (because it’s a picture) is representing a 2-dimensional spacetime embedded into a flat 3-dimensional space. Travelling through the wormhole looks as if you are leaving the 2-dimensional spacetime, and entering the 3-dimensional flat space. This is not what is actually happening. The 3-dimensional space is just a tool for visualization, there is nothing real about it. The entire universe here is the 2-dimensional spacetime, which includes the wormhole. Embedding diagrams are nice aids, because most of us would rather imagine a curved surface within a larger dimensional flat manifold than just a curved manifold, but they mislead us. Read more about embedding diagrams here. (There are also issues with using 2-dimensional images to help us think about 4-dimensional spaces, but that can be for later). In brief: you are not exiting the universe/spacetime continuum at all when going through a wormhole – it isn’t meaningful to talk about “exiting” something that is the entirety of everything. There is nothing to exit to – our universe (within general relativity) is not actually embedded into anything, it’s just curved.
Michio Kaku: Travelling to the future is easy, our astronauts do it all the time.
Me too? “Interestingly”, one doesn’t need a spaceship to travel to their own future, my chair suffices for me. I honestly not completely sure what he was going for with this… But on to the second, shorter, video.
Can You Time-Travel? (How to Time Travel? Part 2)
The joys, terrors and true possibilities of navigating the Fourth Dimension, with quantum physicist Michio Kaku and astrophysicist Charles Liu.
Michio Kaku: …well, Plutonium does not have energy to drive a time machine. To energize a time machine, to bend time into a pretzel, to punch a hole in the fabric of space and time, would require the energy of a star. One version of a time machine uses what is called, a wormhole. Think of the looking-glass in Alice and Wonderland. That looking-glass is the wormhole.
Honestly, I’m not sure what to make of this discussion. Travelling backwards within one’s own time just requires spacetime to have the time-dimension be a closed loop, not necessarily a pretzel (which has a much more complicated topology than required) – although I assume he picked “pretzel” for poetic license, because it sounds nicer than “ring” or “circle” or something else with the same fundamental group as 
As for the energy required to use a worm hole to travel backwards in time… I honestly am not sure where Kaku is coming up with that, so I can’t comment to the specifics. As far as I know, there are no confirmed estimates to how much energy one would need to use a wormhole for travel back in one’s own time.
The wormhole-looking-glass analogy, I do have a problem with. In Lewis Carroll’s Through the Looking-Glass, Alice passes through a looking-glass, showing her reflection, into an alternate world. That really has nothing to do with what a wormhole is. A wormhole, in general relativity, is just a “short cut” through spacetime. It doesn’t take you to an alternate universe, the laws of physics aren’t changed – it’s just a non-trivial connection between two points in spacetime. The looking-glass that Alice went through didn’t take her to another point in space and time, it took her to an alternate universe.
The way we talk about spacetime, and the tools that we use to help us visualize it, can do a disservice to our understanding of the universe. It’s nice to be able to talk about the universe as a whole, as if we were looking at it from the outside, but to visualize the whole universe – to take into account any theory that requires knowledge of the entirety of spacetime – is to throw out causality (assuming the universe is even reasonably large). This disregard for causality is much more substantial than that allowed for by time travel. To be able to perceive all of spacetime, is to know all future and past events in the entire universe – to be omniscient. So in order to be omniscient, one must be outside of spacetime. But in general relativity, there is nothing outside of spacetime. If we allow for physics to be discussed in a way where omniscience is possible (by imaging that this embedding is something real), we aren’t doing physics (as it currently stands) anymore. If we want wormholes to lead us from our universe into another universe, to preserve causality and timelines, then we really aren’t using the wormholes of general relativity, but imagining some other object. That’s fine, but we should call them as such, and not give properties to a mathematical consequence of general relativity that simply aren’t there.
-S.C. Kavassalis
EDIT: As a minor note, it seems I watched the videos out of order. It doesn’t matter much for continuity though, as they both are fine as stand-alones.
November 15, 2009 at 8:10 pm
I have to admit, I’ve never really liked Kaku’s popularizer voice. Yeah, he does great work as a physicist, and he’s tremendously well-known as a popularizer, but I’ve never thought he did the best job of it.
I think in a way his popularizations are always dissonant with the reality (where The Tao of Physics and its ilk are fully discordant). When I’m reading him the inevitable simplifications and departures from the actual theory sound harsh and grating. As an example on the other side, Tony Zee’s departures feel like they get to the heart of the matter while they gloss over the messy details.
As for “traveling into the future”, he’s probably alluding (badly) to the different rates of elapsed time at different points in a gravity well, or maybe at different speeds. The huge problem here is he’s glossing over the all-important “relative to” clause. If you know what he’s talking about, it makes an odd sort of sense, but if you don’t it will be completely misinterpreted. Similarly, the “punching a hole” language can be interpreted in the sense of changing the topology of the spacetime manifold, but a lay reader will not understand that.
November 15, 2009 at 8:32 pm
I certainly agree with you there. My big problem with most popularizers is that their work only makes good sense for people who already know what they are talking about – but that’s not their audience. They use words and analogies to try to make the lay audience understand, but based on that audience’s background knowledge, they can paint a very faulty picture.
I did assume Kaku’s ‘hole punching’ was referring to changing the topology, but I think there are many better ways to express that, if you’re trying to explain it to someone who doesn’t already have a sense of spacetime. Plus, all that embedding garbage just gets to me in general.
November 16, 2009 at 6:09 pm
I agree completely. Science promoters often become their worst enemies by employing a language that does not convey the truth to their audience, even if it is simple enough to grasp.
I often debate whether a silent scientist is better than a scientist who gives incorrect messages to his audience. I still haven’t decided yet.
November 19, 2009 at 6:55 am
Well here’s my favourite time machine, if anyone wants to do some time travelling here’s how:
http://en.wikipedia.org/wiki/Telescope
Cheers, Paul.
November 22, 2009 at 10:58 pm
So for this “hole punching”… how would any of y’all go about describing it to the lay person? At what point does one make concessions of accuracy for gist?
November 23, 2009 at 6:01 pm
I’m honestly not sure. The notion of embedding seems so fundamental to our ability to understand some concepts, that getting around it might be impossible. I can’t think of any real life examples where we don’t imagine surfaces or volumes where they are not embedded into something. That being said, I’m not yet willing to concede that it’s impossible to explain variable topology to a lay person, without losing too much accuracy. I think sometimes, these nice ‘gist’ type pictures do a disservice in the long run, because it’s often really difficult to get the first way you were taught something out of your head (or at least that’s true for me). It’s still very hard for me to imagine creating holes in the spacetime manifold without actually picturing something outside of it (for some reason, I usually picture a pudding or jello type substance that you can mold or pull apart or allow empty pockets to form in, but has a definite orientation – but that is still certainly an imagine that requires embedding). I’d be very excited for a way of picturing spacetime that doesn’t require embedding, but as of yet, I don’t have one.
I wish I knew how to decide where “accuracy” and “gist” should fall and what harm, if honestly any, these incorrect popular conceptions have on people.
Ideally, I’d like to see in popular work, after the analogies that require embedding are given, to mention that the embedding is only for convenience, and does not represent anything physical.
November 23, 2009 at 3:56 am
My thought would be: how could you avoid the embedding version of the story for a first explanation of this stuff to people who know even less about physics and math than I do (enough to sort of understand the definitions at the beginnings of chapters on manifolds, but not enough to get anywhere with the problems).
What’s missing in the popular presentations, I think, is a somewhat organized ’second layer’ of explanations for people who could know more but can’t really manage to slog through real courses, for one reason or another. Perhaps Penrose’ Road to Reality book was an attempt to do something like this, but I think it really requires the internet and blogs like John’s and this one.
November 27, 2009 at 5:08 pm
Read:
http://en.wikipedia.org/wiki/Fixed_point_(mathematics)#Topological_fixed_point_property
and
http://en.wikipedia.org/wiki/Mathematical_singularity
November 30, 2009 at 2:04 pm
Great post! I also agree that Science promoters often become their worst enemies by employing a language that does not convey the truth to their audience, even if it is simple enough to grasp. I am a college sophomore with a dual major in Physics and Mathematics @ University of Canterbury in Christchurch, New Zealand. By the way, i came across these excellent physics flashcards. Its also a great initiative by the FunnelBrain team. Amazing!!!
December 3, 2009 at 11:34 am
Thanks for a great post. I think I agrre with your analysis completely and it’s really good to read an honest post such as this one.
Kaku as a physics popularizer seems to be a charlatan, plain and simple.
Any of his works corroborate this and this one is no exception.
Anyone who feels exhilarated with this kind of science-porn has no place in science and is doing a disservice to the field by dragging the field down to the gutter.
December 11, 2009 at 9:20 pm
This is surely rubbish, but where’s the flaw:
http://www.technologyreview.com/blog/arxiv/24499/
December 11, 2009 at 10:47 pm
Well, I haven’t made it far in the original article, because it says:
“The total momentum [from the vacuum] vanishes…It occurs in all materials except magneto-electrics, which lack both space and time symmetries”.
And I honestly have no idea what magneto-electrics are. I glanced at the abstracts of the three references for that quote, and none of them mentioned that pretty major conclusion. Until I see where that claim (which is the foundation of the paper) come from, I wouldn’t bother with it.
December 11, 2009 at 10:33 pm
Avery, it says:
which is a clear contradiction in terms, since mass is energy.
December 12, 2009 at 5:57 am
Hmm yes, but I was assuming that it was reaction mass that wasn’t needed, not energy mass. From MIT tech review, you’d expect the flaw to be a bit more subtle (even from New Scientist, I think)
December 12, 2009 at 1:45 pm
Don’t put that much faith into the MIT Tech Review arxiv blog – it’s not written by experts in every single field that the papers are on, it’s just a blog. They summarize lots of nonsense papers on the arxiv with nice sounding abstracts, but it doesn’t mean they are full of good content. I’d read the actual paper though for the real gist of it.
December 26, 2009 at 6:12 pm
Nice Blog! Happy New Year!!
IonnKorr, a physicist by Greece