Lepton Photon Interactions at High Energies and the Multiverse

This past week the large biennial “Lepton-Photon” (International Symposium on Lepton Photon Interactions at High Energies) conference has been taking place in Ljubljana. These have been going on since 1965, now alternating years with the ICHEP (“Rochester”) conference. It’s been quite a while since the main topic of the conference was lepton-photon interactions, these days it covers the entire field of HEP, with a format of plenary survey talks. Taking a look at the slides will give you an excellent survey of what is going on in HEP, both in experiment and theory.

One newsworthy talk was Mike Lamont’s on the current state of the LHC Run 2. The new LHC run is at 13 TeV collision energy, with 25ns bunch spacing (as opposed to 8 TeV, 50ns in Run 1). No showstopper problems so far, but increasing the luminosity (by increasing the number of bunches) has been a slow process. As a result, Lamont expects only about 3.4 inverse fb luminosity this year, down from early hopes for a number more like 10 inverse fb. The plan is for 30 inverse fb next year.

The summary talk by John Ellis featured undimmed enthusiasm for SUSY. What will be very interesting to see will be the treatment of this subject at the next Lepton-Photon conference.

One subject that was not mentioned at all in Ellis’s talk, and, as far as I could tell, not in any of the other ones either, was the multiverse. The organizers and speakers seem to all realize that there’s no scientific content to this idea worth discussing, so best to ignore it. I’m completely mystified though by the decision to have as public outreach a promotional talk about the multiverse by Alan Guth. Why anybody in HEP thinks it’s a good idea to make pseudo-science the public face of the subject just baffles me. If you want to see one reason why this kind of thing is really a bad idea and doing great damage to the public perception and understanding of the subject, take a look at this.

Away from serious scientific conferences, the multiverse continues to dominate the media’s coverage of fundamental physics. The usually sober publication The Economist features both an article and a video this week promoting multiverse pseudo-science. As usual in such pieces, no skeptical voices are to be heard. Susskind deals straightforwardly with the lack of scientific evidence problem by simply saying things that aren’t true:

This idea of a multiverse is not gratuitous speculation. No, it really comes out of both experiment or observational physics about the universe and the current theories as best we understand them.

He doesn’t explain what the experimental evidence for the multiverse is. The BICEP2 observation of primordial gravitational waves perhaps?

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17 Responses to Lepton Photon Interactions at High Energies and the Multiverse

  1. andrew says:

    Useful links. I still believe in SUSY and that naturalness is important. I concede though that an alternative narrative – that naturalness and SUSY were follies, followed by a generation of physicists but scorned by nature in experiments for years – is gaining traction.

    Does that make you hopeful regarding the perception of ST? Could the dominant narrative – the world’s geniuses following in the footsteps of legendary physicists, uncovering bizarre secrets of the multiverse at elite institutions – come under pressure? Is it plausible that, like SUSY and naturalness, we’ll come to see ST as smart people getting it wrong?

    The thing with narratives is that isn’t events per se but our perception of events that matter. Although LHC says nothing about ST, one perception is that theoretical speculation was wrong and that we need new ideas. Ultimately that might shift our feelings about theoretical physics including ST in the last 30 years or so. Do you think we’re any closer to that point?

    I’m aware that this a contentious view on the development of scientific ideas, but I’m not looking for a discussion on that.

  2. Ilya says:

    I don’t think it’s fair to say that Susskind lies, just from that quote. He doesn’t claim that evidences exists, he just writes “comes out of”, which is more like “is hinted by”. He doesn’t write that it’s not speculation (it is), he says that it’s not *gratuitous* speculation – that’s debatable but not plainly false.

  3. Peter Woit says:

    I don’t think that
    “This idea of a multiverse … really comes out of … experiment or observational physics.”
    can be characterized otherwise than as a statement that is not true. The weasel-word reformulation you suggest (a “hint” can mean any degree of evidence, no matter how negligible) is different.

    Note that I was careful to say that Susskind was saying something untrue, but did not say that he was lying, which would imply that he was intentionally saying something he knew to be untrue. Quite likely he believes what he said. I don’t think multiverse-maniacs are liars. I think that they have for various reasons convinced themselves of bad arguments, which they unfortunately are very actively trying to promote to their colleagues and the public.

  4. Cosmonut says:

    I’ve read your criticisms of multiverse mania for a while now and I agree with you.

    But is there any push back within the scientific community against this rubbish ?

    Or is the pro-multiverse camp winning by virtue of constant repetition ?

    It seems every time I read a popular article about fundamental physics in the media, its the same old bilge about string theory, inflation and multiverse – or even worse, the “Tegmarkian hierarchy of multiverses”.

    No genuine new understanding of fundamental physics and cosmology over the last 10 years or so, as far as I can tell.

  5. phil fogle says:

    I agree that Susskind is (probably) sincere but (likely) delusional. It’s not at all uncommon for good scientists to ‘believe’ in some of their speculations – subjectively confusing ‘sufficient’ for ‘necessary’.

    The Spectator review is in another category (parallel universe?) all together, and is an utter disgrace. But it’s the entertainment industry, on the level of mesmerism and radium therapies, and will always be there as a background pollutant.

  6. verruckte says:

    The reason that the multiverse narrative keeps getting pushed is that it’s what the public wants to hear. Media picks up on it, because it sounds mystical and exciting. Saying “no, nothing new here yet, we are still just stuck with the standard model” is NOT mystical and exciting. It sells no books, attracts no eyeballs to advertising, and is basically just boring.
    The intoxication of popular interest and the financial opportunities thereof are tempting. The fact that the subject matter is not able to be tested is *not* a disadvantage, it is a huge advantage, for the simple reason that no one can ever actually show you are wrong. This plus the mantle of authority given by being an ACTUAL SCIENTIST are the perfect recipe to sell books and headlines and expensive talks. I believe, as Peter seems to, that most proponents have probably convinced themselves first of all. The capacity of humans to adjust reality to match desires can not be overestimated.

  7. Shantanu says:

    Peter, I think the problem lies with the organizers and also the audience.
    Hope at least some brave member asked some provocative question.

    Peter: an OT question prompted by the recent passing away of Bekenstein. When you were an undergrad student at Harvard, were you impressed by Hawking’s (and related work by others) 1974 work on the black holes have a temperature and radiate? Was this extensively discussed in the hallways at Harvard?

    Or is it that in Harvard at that time, no one cared about GR and you don’t recall any discussions of this

  8. Peter Woit says:

    I’ve never met Bekenstein. When I was at Harvard (1975-79), the focus was on the SM, few people were thinking about quantum gravity. At Princeton (1979-84) there was a lot of interest in quantum gravity, especially in Hawking’s semiclassical Euclidean methods, which were closely related to the semiclassical gauge theory methods a lot of people there had been working on. Malcolm Perry, Hawking’s student, was a young faculty member and Hawking visited and gave lectures.

  9. Dave Miller in Sacramento says:


    Alan Guth is, of course, one of the inventors of inflation: I knew Alan, very slightly, when he did his initial work on inflation at SLAC back around 1980. So, doesn’t the inflationary multiverse predate the “landscape scenario” by a couple decades or so? Is there any reason for the two to be connected?

    I know there is an ongoing confusion among the “many-worlds” interpretation of quantum mechanics, the landscape scenario, and the inflationary multiverse. Is there any “accepted” connection among these, or is it just a free-for-all open to anyone’s speculation?

    By the way, I would interpret Lenny’s “comes out of” as just meaning that Lenny thinks the ideas are suggested or motivated by observation or experiment, not that there is any firm connection.

    All the best,


  10. Dave Miller in Sacramento says:


    To elaborate on your question to Peter: I was an undergrad at Caltech when Hawking gave a talk on his recent work on black-hole evaporation — I think it was in ’74: I had enough sense to keep my copy of the typewritten paper that was handed out to the audience (although I looked at the paper a few months ago and still cannot understand his notation). I do recall that we all understood enough to grasp that Hawking was saying that black holes evaporate and therefore do not last forever; however, even when I took Kip Thorne’s class on GR a year or so later, I do not recall any discussion of Hawking evaporation in class (Kip was obsessed with accretion disks around black holes at the time — we students were not so obsessed).

    At SLAC around 1980, I remember a casual discussion with a post-doc, Subhash Gupta, about the issue: we were specifically interested in the point that, prima facie, Hawking evaporation means the event horizon never actually forms. Nothing came of that discussion and no one else at SLAC seemed interested. (If Subhash and I had only known, we could have ignited the discussion about the “firewall” thirty years earlier!)

    Hope this gives you a bit more info as to how it was viewed back then.


  11. Peter Woit says:

    I think claiming that the multiverse is “suggested or motivated by observation or experiment” is also an untrue statement.

    A small number of people (Susskind among them) have been known to speculate about a connection between MWI and the inflationary multiverse. I’ve yet to see a comprehensible explanation of how that is supposed to work. Popular press articles often conflate the two, as far as I can tell purely because the writer doesn’t understand what they’re writing about.

    Multiverse fans often claim that “typical” inflationary models imply eternal inflation and thus a multiverse (while not mentioning that in such models physics will be the same in all universes). Others claim that there are plenty of inflationary models that don’t imply eternal inflation. The big problem with inflation right now (and I think this is a point Steinhardt emphasizes) is that you can get pretty much whatever you want out some model in the framework, making it of dubious testability.

  12. Dave Miller in Sacramento says:


    You wrote:
    >Multiverse fans often claim that “typical” inflationary models imply eternal inflation and thus a multiverse (while not mentioning that in such models physics will be the same in all universes).

    I recall that Alan claimed he had a proof that inflation that was eternal in the past direction was not possible, though I myself could never understood the details: I think I ran across a reference to it in Alan’s book.

    In fact, this paper ( http://arxiv.org/pdf/hep-th/0702178.pdf ) seems to make that point (from the abstract: “Although inflation is generically eternal into the future, it is not eternal into the past: it can be proven under reasonable assumptions that the inflating region must be incomplete in past directions, so some physics other than inflation is needed to describe the past boundary of the inflating region”), though I do not believe this is where I first saw it.

    The paper also does have a brief discussion of the landscape (p.9-11), but it is frustratingly vague: Alan indicates a bit of skepticism, but, while he suggests that you can get different physics in different universes (at least different cosmological constants), no details are given of how that could work.

    Anyone know where anyone has actually talked about that in any detail? I fear I am still confused.

    It will be interesting to hear what Alan has to say in his talk.


  13. Shantanu says:

    Thanks, Dave. However Caltech had a strong GR group then (in fact still does).
    and 70s was the golden age of GR. As Peter mentioned, very few people in Harvard were interested in quantum gravity.

  14. Chris W. says:

    Speaking of Hawking and black hole evaporation, he is in the news again with another (?) take on the problem:

    The [Wall Street Journal] notes that Gerard ‘t Hooft of Utrecht University, winner of the 1999 Nobel Prize in physics who was present at the conference, had presented a similar idea in 1996.

    He said the problem was that his math did not add up.

    “I claim he is now where I was 20 years ago,” he told the Journal. “If he announces this as a new idea, I won’t be thrilled.”

  15. Peter Woit says:

    Chris W.,
    I’m not convinced that what the world needs is more unclear speculative claims about the black hole information paradox, even if they’re coming from Hawking. I have no idea what’s going on here, urge those interested to try Sabine Hossenfelder, who wrote about the talk here:

    Personally I’ll wait for the paper with Perry promised for next month and see if I can make any sense of that. Maybe before then the experts on this will have come to some consensus on whether it’s worth paying attention to.

  16. Chris W. says:

    I’m not convinced of that either, which is partly why I excerpted the comment from ‘t Hooft. Pushing out preliminary conclusions and arguments to make a conference deadline doesn’t help.

  17. Curious Mayhem says:

    Even if primordial gravitational waves are detected (which I think will happen soon), it’s no evidence for the multiverse. The multiverse theory is constructed in such a way as to not require such evidence, or any evidence for that matter.


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