Previous Post in this Debunking Series.
I just watched Looper the movie. It is such a good movie and a great story. But then I’m biased. Anything with Bruce Willis is a great movie. Bruce Willis is getting older, which reminds me so am I!
For those who have not watched Looper I won’t give the story away … Looper is a must watch for science fiction fans. And there were other great movies and episodes about time travel. The three Back to the Future, and the Star Trek episodes, for starters.
That was the good news, and now for the bad news. Time travel is impossible. The mathematics behind time travel is excellent, but the physics is not. In contemporary physics, the mechanism of time travel requires wormholes. You get into a wormhole on one side and you pop out the other side either in the future or in the past, depending on what the wormhole was designed to do.
I did some digging, and found the Polchinski’s billiard ball paradox which is a version of the matricide paradox (travelling back through time before one’s birthday and killing one’s mother, hey what about father?) without the free will component. “A billiard ball sent through a wormhole which sends it back in time. In this scenario, the ball is fired into a wormhole at an angle such that, if it continues along that path, it will exit the wormhole in the past at just the right angle to collide with its earlier self, thereby knocking it off course and preventing it from entering the wormhole in the first place.”
Then Kip Thorne’s students came up another solution “to avoid any inconsistencies, by having the ball emerge from the future at a different angle than the one used to generate the paradox, and deliver its younger self a glancing blow instead of knocking it completely away from the wormhole, a blow which changes its trajectory in just the right way so that it will travel back in time with the angle required to deliver its younger self this glancing blow.”
Add to this second scenario that one collects the older billiard ball in a basket. Of course there are some boundaries driven by conservation of mass and when the wormhole was created, that constraint what is observed. But this then raises some questions. How many balls are there in the basket at the start? How many billiard balls does one observe in the middle of this experiment? Think parallel processing not sequential logic.
And, what can I do?” That is, since cause and effect no longer have a consistent relationship, if the basket fills up with billiard balls before I set off the experiment, can I choose not to set off the experiment?
It is sufficient to stop here to make the case that time travel is not possible.
I’m sure Kip Thorne, his students and many, many others are doing good work to develop theoretical models. I hope these older theoretical wormhole models would evolve to new ‘tunneling’ models that do not allow for inconsistent relationships between cause and effect. And these new ‘tunneling’ models will one day allow us to do interstellar travel using some kind of tunneling technology.
Right now time travel is just too easy to debunk. We are not there, yet.
The next in the Debunking Series.
Benjamin T Solomon is the author & principal investigator of the 12-year study into the theoretical & technological feasibility of gravitation modification, titled An Introduction to Gravity Modification, to achieve interstellar travel in our lifetimes. For more information visit iSETI LLC, Interstellar Space Exploration Technology Initiative.
Solomon is inviting all serious participants to his LinkedIn Group Interstellar Travel & Gravity Modification.
11 Comments so far
I’m hopeful that some future science will allow time travel (and not just to the point in time when the first time machine / wormhole was created, but to any time). Of course that might never be possible; a tragic shame, but that’s reality sometimes.
I’m skeptical, though, that causality is _never_ violated in nature. Certainly we don’t observe it to be violated. But there may be certain conditions in which retrocausation occurs (effects preceding causes), or causal loops form (where effects are the causes of their own causes and vice versa), or even sequences of events where nothing can meaningfully be identified as a cause or effect of anything occurring in those event sequences.
In other words, the rules of cause and effect might be an _approximation_ of reality, albeit an incredibly close approximation that holds in virtually all cases and that as far as we know have never been observed to be violated. It would generalize, I suppose, to a principle of “correlation” rather than causation.
We used to believe the velocity of light (‘c’) was constant, but now we have (1) evidence that the fine structure constant may have changed over time, implying ‘c’ may have changed; (2) some theories of quantum gravity predict spacetime will have a granularity that might cause higher energy photons to travel very very slightly faster in vaccuum than lower energy photons; and (3) M-theorists often suggest that many, if not all, of the “constants” may be “derived” from more fundamental laws (like the geometry of higher dimensions) and could have been different. Now (2) is, as far as I know, still neither verified nor falsified, and (3) is, like most of M-theory, a mathematically elegant theory that is captivating but practically unfalsifiable. But I _believe_ (1) is on more or less solid ground, and that alone might justify reformulating our theories such that ‘c’ is a scalar field rather than a fundamental constant.
I mention the possibility of a variable ‘c’ to illustrate as yet another example, that the history of science has consistently been about refining good approximations by devising more expansive, accurate, precise, and predictive approximations, and then doing it again (and again).
Which is not to say that I _believe_ there are actual violations of causality (nor do I disbelieve that there are… I’m withholding judgement). It’s certainly difficult (meaning, to me, right now, I can’t, and it might be impossible) to imagine a physics that reimagined causality as an approximation that didn’t always hold in every situation. I do think it’s not harmful to keep an open mind about it though. So causality violations might not necessarily rule out time travel… which is not to say that other considerations may rule it out anyway.
Just a thought.
Marc van Lohuizen, yes it would be nice to be able to build time machines, and go back and forth in time. But I think why time travel was not allowed was because it will result in so much infighting that the humanity would not be able to progress.
Imagine this. Someone, let’s call him Ben, invents a really important breakthrough in technology. Then others, say Albert and Hal, concur and invite Bill to fund this technology. And the world progresses, and the technology becomes a major economic driver and Ben and his colleagues make tons of money off this technology.
Then some unrelated third person, say Gary, gets very upset that Ben and his colleagues are successful, so he recruits Martin to pay for the cost of travelling through time to steal the invention from Ben. It happens. The world lines change, and Gary and Martin are now wealthy, but of course not as much as Ben was because they don’t fully understand this technology. Wrong time line.
When Ben realizes what happened, he talks to Albert and Bill about what should have happened. Bill funds a team to go back and change the past to what it should be. Back to the normal timeline.
Imagine if this went on, back and forth, for every small and big issue in the world. What do you think would happen?
But of course all names are fictional and not in any way related to any know or unknown persons, even if they are from a different time or planet.
Humans like to think of themselves as rational but they are not.
I won’t disagree with you that causality is not never violated in Nature. We haven’t observed it and at the present time we don’t know how to prove or disprove it. The tricky part is that even if one could observe causality violated, we could not be sure that this is ‘an apparent’ violation due to how we measure and observe this causality violation, or due to undiscovered properties of the Universe.
Regarding quantum gravity, Robert Nemiroff and his colleagues (http://www.space.com/17399-gamma-ray-photons-quantum-spacetime.html) looked at gamma-ray burst and found that parts or all of quantum gravity may be invalidated.
So we have to wait and see if the mathematics of quantum gravity could become the real world physics of quantum gravity. This requires more astrophysicists searching through the data to find some more occurrences of these events.
I’m less confident of string theories. Lee Smolin in his book The Trouble with Physics (http://www.thetroublewithphysics.com/), explains that string theory is dependent on particles expanding as their energy increases. But Lorentz-Fitzgerald transformations (of Special Relativity and earlier) require particles to contract.
In my book on gravity modification I have proposed an experiment that could confirm or disprove this. Again we have to wait until someone conducts this experiment.
And at the end of the day you may be right, that causality violations do occur in Nature, and would that keep us awake at night, at the thought of all the new inventions we could come up with!
And a very appropriate title is Loopers to that movie — a name most would gladly put on anyone who thinks it is possible to travel through time. You might enjoy an article in The Daily Mash today — ‘Man Discovers Bin Is Not A Stargate’ — http://www.thedailymash.co.uk/news/science-technology/man-discovers-bin-is-not-a-stargate-2-2012100143206
On a more serious note — it may be technically feasible to send *information* through time — for example Hawking Theories predict particles can flow backward in space-time on the event horizon of black holes — and there are claims out there that such future-scanning technology is already being piloted — although these claims mostly amid… loopers.
Having said that there was an apparent NDA breach at BNL back in 1999/2000 on such developments which caused a bit of a stir in some internet forums — and more recently I acquired a document from LBNL apparently on same said clandestine research at BNL.
People have the right to dissent about titles and as well as choose their own titles.
On a more important note — one should not overlook the concept of traveling at near-luminal speeds to change the angle at which the craft travels through space-time.
For the pilots, it might seem like just a few weeks to get to Alpha Centauri in this case, even if to their colleagues back on Earth it would appear to take them 4.5 years, and another 4.5 years to return. When they return from their apparently short trip, a decade would have passed back on Earth, but to the pilots it would have been a short trip and they would have hardly aged at all. Assuming G-forces could be overcome that is…
Tom Kerwick. Thanks. Time dilation is the major problem with interstellar travel.
“Time dilation is the major problem with interstellar travel.”
I would say the opposite — it could the major solution to interstellar travel. A ship being piloted at near luminal speed to a star system 1,000 light years away might only require a few months time **for the crew involved*. They would hardly age at all…
Tom Kerwick. Yes you are right, but our life experiences color our world views. You see time dilation as a strength/opportunity and I, a weakness/threat. Let me explain.
The presence of time dilation shows that Lorentz-Fitzgerald transformations are present. Therefore we are limited to subluminal velocities. Therefore, galactic and intergalactic space exploration will take millions of years.
Imagine if we could figure out how to ‘bypass’ Lorentz-Fitzgerald transformations. That is why I suggested that there is the need for new ‘tunneling’ theories.
Benjamin T. Solomon:
I don’t see exceeding subluminal velocities as being particularly important to interstellar or even intergalactic travel.
Time dilation and the “light speed limit” are both things that, if one were to anthropomorphize the Universe, suggest that it is quite a friend to the _individual_, but not to large groups / societies.
Time dilation means that, given enough energy and means to survive awesome G-forces from rapid acceleration, the crew of a spaceship can go star and even galaxy hopping, seeing so very much of the Universe, with much of the (subjective) time spent enjoying / colonizing / strip mining / studying the destinations and very little spent actually travelling.
The “light speed limit”, of course, means that those embarking on such a trip are effectively saying good bye to everything they leave behind. For some, this will be seen as a downside. I see it as an incredible positive. Small communities will be able to go in peace, untroubled by the rather horrific prospect of the spread of some ever-expanding interstellar society, a cancerous social plague of arbitrary laws and homogenity. As it is, on Earth our nation states are inefficiently and cripplingly large, and are the single greatest threat to individual freedom. In a Universe where superluminal travel was impossible, the sorts of atrocities and restrictions on individual liberties that large societies cause would, gratefully, at most be limited to individual planetary systems. This seems merciful.
All of that also only really applies if we remain human. If we’re talking about superluminal interstellar travel, then we may as well also talk about indefinite life extension and eliminating aging, and mind upload technology. Both of these have a solid theoretical foundation, which no one can say about superluminal travel; the only issues with them are practical, not theoretical. If we didn’t age, then barring accidents we could take that time dilated space voyage, come back years / centuries / millenia later and catch up with those left behind. With mind uploading and mind emulation, the whole species could travel together living in a virtual reality, all on a massive server farm. Alternatively, it’d allow us to leave a copy of ourselves behind so that no one has to miss us and we can pursue goals at home, and take copies of those we’d miss or need; when we come home, both copies of us integrate our data and personal experiences into a single person. Admittedly this might sound very “sci-fi”, but again, if we accept a position of scientific physicalism when it comes to consciousness and mind (and I see no reason not to do so), these are things that we absolutely _will_ eventually be able to do (possibly as early as 50 years from now or sooner)… and again, no one knows for sure if we will ever be able to effectively travel faster than light.
Or, bringing it back to the original topic of the post, how _any_ ways around the cosmic “light speed limit” and effective superluminal speed wouldn’t be a form of backwards-in-time time travel.
Benjamin T. Solomon wrote: “It is sufficient to stop here to make the case that time travel is not possible.”
Although many physicists would love to agree with you, they can’t. Despite a tremendous amount of effort to prove the contrary, the laws of physics as they stand today do not forbid time travel. [It’s only extremely impractical — like reversing the rotation of the galaxy.]
The case that you’ve considered can be dealt with by imposing the principles classical consistency or quantum consistency to eliminate paradoxes.
Moreover, there are reasonable theories of gravity (of which Einstein’s General Relativity is a special case) that permit wormholes (i.e. time machines) in the absence of (fantastically impractical) “exotic” matter (e.g. Gauss-Bonnet and Lovelock gravity).
Source: The Physics of Stargates — Parallel Universes, Time Travel and the Enigma of Wormhole Physics by Enrico Rodrigo.
Thanks David, great comments.
I’ll answer in my next blog post, why time travel should not be possible even though the mathematics say it should be possible. And can we test this?