As the LHC supposedly gears up, Harvard physicist Kevin Black, based at CERN, investigates rumors that the particle accelerator may, in fact, soon be shut down—by ripples from the future. From Kevin Black: I came across a bizarre paper recently suggesting that the LHC might be shut down. Not because of the funding cuts that have been threatening particle physics projects around the world, nor because of law suits accusing the LHC of threatening life on Earth. (Not even because we at the LHC have recently been accused of having far too much fun rapping.) No, the paper suggested that future effects caused by the production of particles, such as the Higgs, could ripple backwards in time and prevent the LHC from ever operating. If it hadn't been written by two very well respected and accomplished theoretical physicists, I would have stopped reading at the title alone:"Test of Effect from Future in Large Hadron Collider; A Proposal". To be completely honest, the title reads like titles that occasionally appear in my inbox—“Relativity Principle Untenable," "Quantum Mechanics a Hoax," and other nerdy versions of the emails from the supposed attorney of my long-lost Nigerian uncle who apparently has died and left me millions of dollars, if I can only send him $50,000 so that he can get it to me. But I didn't stop. I read the article. I read it for another reason other than the somewhat awkwardly sounding title and not just because the authors, Holger Nielsen, of the University of Copenhagen, and Masao Ninomiya, of Kyoto University, are somewhat famous. I read it because when I come across such things it tends to remind me of the first time I learned about quantum mechanics. To be honest, if it hadn't come from a professor at a university and a published text book I would have thought that the whole thing was some sort of a scam as well. I mean, really? Sometimes it acts as a wave and sometimes it acts as a particle? The first time I heard about wave/particle duality I was expecting to be asked to send the authors money (perhaps to Nigeria?). So what did the article say? Well, it started out with a reasonable enough point. One of the basic assumptions of classical physics is that time flows in one direction and that when describing a physical system one needs to know the equations of motion and the initial conditions in order to predict the future behavior of a classical system. However, quantum mechanics changes this a bit. Classical mechanics can be formulated in such a way that one sets up an “action” integral. The solution to the physical system can be expressed as the path that minimizes the action integral. It turns out that in quantum mechanics one needs to not simply take one path—but take the sum over all possible paths. For example, if you want to work out how a photon gets from a lightbulb to your eye, you need to take into account not just its straight-line trajectory, but contributions of all possible paths it could have taken, including paths where the photon bounces round the room. It's a bit strange, but it seems to work and 60 years+ of detailed experiments have confirmed this description over and over again to remarkable quantitative precision. The authors of this paper claim to show that other terms can be added to the quantum mechanical action that are consistent with current theory and experiment. However, some of these possible terms include conditions in the future that need to be taken into account and summed over. That is to say, what happens in the future could (according to this paper) affect what happens in the present. Why the LHC? The authors argue that these sorts of time-violating interactions could be associated with whatever new particles we create at the LHC. For example, the production of a large number of Higgs particles in the future could have a backwards-in-time causal effect on the machine that produced them, stopping the machine from ever running. As possible “evidence” for such a backwards-in-time effect, the authors cite the now-canceled Superconducting Super Collider (SSC)—a particle accelerator that was meant to hunt the Higgs and was partially constructed in Texas before Congress pulled the plug on the project. As the authors write in their paper: “Such a cancellation after a huge investment is already in itself an unusual event that should not happen too often. We might take this event as experimental evidence for our model in which an accelerator with the luminosity and beam energy of the SSC will not be built.” It’s as though the Higgs plays the role of the time traveler who goes back to the past and murders his grandfather, thus preventing his own birth. Now, I must admit that this is where I started to get a bit skeptical. The authors go on to suggest that the LHC is also under threat from a possible “miraculous” shutdown caused by the backward influence of particles like the Higgs, which it may create in the future: “Since the LHC has a performance approaching the SSC, it suggests that also the LHC may be in danger of being closed under mysterious circumstances.” Visions of the X-Files’ Fox Mulder launching into some hour-long diatribe about future conditions (and of course government conspiracies) started to appear. But I read on nonetheless, becoming a little bit more amused and a lot more confused as I tried to finish the paper. In order to make it topical they proposed an experiment. Play a game of cards—a kind of particle physics tarot—to determine if future LHC conditions could affect the draw of the cards. The idea being that if the cards were arranged to represent different possible LHC outcomes—discovery of the Higgs particle, discovery of SUSY, failure or cancellation of the LHC, destruction of the world by the creation of mini-black holes, etc—then what actually happens in the future could affect the outcome of the card drawn now. As crazy as it sounds, it’s at least a novel idea. Certainly, time travel and causality violation are topics that physicists, such as FQXi’s Ken Olum, are seriously investigating (see “Charting the River of Time”); so why not look for backwards causation at the LHC? As an experimentalist working at CERN, I’m ready to pull cards out of a hat in the name of science, even at the risk of prognosticating the demise of the LHC. However, I can't get my head around one basic thing. How can this possibly prove or disprove the theory? They went through some argument (which I can't say I completely understood) claiming that a concrete test of this idea could be put in place, but I am not convinced. Even if I pull the “shutdown card” and the LHC is indeed shut down, how will I know that this is proof and not just a strange coincidence? Conventionally, scientific hypotheses are considered “scientific” if and only if they can be falsified by some experiment (at least in principle). That is to say, it may not be technically possible to conduct such an experiment right now but at least in principle such an experiment could be made. That's the part I couldn't get my head around. How could you ever design an experiment that would disprove future causal influence on a current condition? I just can't imagine how one could do that, but I am open to suggestions. Any ideas? So, for now, I will just put down the paper and get back to the grind of my usual days—debugging software and trying to commission the ATLAS muon spectrometer. I will pass this paper off as quirky, but probably not likely to lead to any major discoveries. But then again, one day I might be teaching this experiment out of a text book to incredulous students who think I am trying to sell them the Brooklyn bridge... -- Kevin Black is a postdoc at Harvard University. He works with the ATLAS experiment at CERN, and sincerely hopes that LHC isn’t shut down any time soon.

There has always been an aura of mystical speculation surrounding quantum mechanics and the role of consciousness. The interpretation of wave function collapse (usually referred to as "the measurement problem") seems to lie at the heart of it. So, I'd like to consider the following: The role of the observer making an observation in quantum mechanics is the same as that of a classical observer making an observation of a system that may inhabit one of a number of possibilities. You look at it, and then you know what happened -- until you look at it, you don't know and can only assign probabilities to what may have happened. The weirdness of quantum mechanics is that the possibilities interfere with one another, whereas classically they don't. I'm curious if others here would agree or disagree with this description, and how it might need to be amended or changed?

It has been brought to my attention that the discussion threads for Parts 1, 2 and 3 of "To Be or Not to Be (a Local Realist)" and "On the Origins of Quantum Correlations" are getting increasingly unwieldy and slow, due to the huge numbers of comments. Since there have also been recent further contributions to this discussion on the arXiv, from Richard Gill and from Joy Christian, I am opening up this thread for new discussions about these recent papers. Please be polite and reasonable in your responses. No personal comments are necessary.