Category Archives: Other Collider News

OPERA’s Timing Issue Confirmed? Yes!

[QUICK UPDATE April 2: I’ve now finished an article giving more details of how OPERA, with LVD’s help, solved the mystery.]

[UPDATE March 31 2 a.m.: following study of the slides from a mini-workshop recording the results of investigations by OPERA and LVD, I now have the information to remove all the guesswork from my original post; you’ll see outdated information crossed out and newer and more precise information written in orange.  I’ve also added figures from the talks.]

March 30 5:30 p.m. Two main scientists at OPERA, one leading the OPERA team as a whole and the other leading the neutrino speed measurement, resigned their leadership positions today.  The suggestion from the press is that this is due to personal and scientific conflicts within the OPERA experiment, rather than due directly to the errors made in the neutrino speed experiment; but of course the way the measurement was publicized by OPERA caused serious internal conflicts at the time and are surely part of the issue.    [Oh, and meanwhile, back over at the CERN lab, some good news: collisions at the Large Hadron Collider with 8 TeV of energy per collision were achieved this afternoon.]

The mystery surrounding OPERA, the Gran Sasso experiment which (apparently through a technical problem) measured that neutrinos sent from the CERN lab to the Gran Sasso lab in Italy arrived earlier than expected by 60 nanoseconds, seems to be on the verge of being is resolved.  Statements made by an OPERA scientist in the Italian language press, pointed out to me by commenters (Titus and A.K.), seem to imply that OPERA has more or less confirmed that the problematic fiber optic cable (along with the clock problem, to a lesser extent) was responsible for a 60 nanosecond (billionth-of-a-second) shift in the timing, creating the false result.  We do not yet have official information from OPERA about this, but talks given at a mini-workshop a couple of days ago make clear that this is the case.

The way this was done if I/we understand the Italian correctly is something like is the following  with all details still very uncertain. Continue reading

Taking Stock: Where is the Higgs Search Now?

Today, we got new information at the Moriond conference on the search for the Higgs particle (in particular, Phase 1 of the search, which involves the search for the simplest possible Higgs particle, called the “Standard Model Higgs”) from the Tevatron and the Large Hadron Collider [LHC], the Tevatron’s successor.  With those results in hand, and having had a little time to mull them over, let me give you a short summary.  If you want more details, read today’s earlier post and yesterday’s preparatory post.

Before I do that, let me make a remark.  There is a big difference between healthy skepticism and political denialism.  I get the impression that some people who are writing or reading other blogs misinterpret my caution with regard to experimental results as being somehow a political and unreasonable bias against the Higgs particle being present, either at a mass of 125 GeV/c2 or at all.  That’s ridiculous.  All that is going on is that I simply am not convinced yet by the data.  I’m a careful scientist… period.  And you’ll see that I’m consistent; later in this post I will advise you not to over-react negatively to what ATLAS didn’t see.

What happened today at the Moriond conference?

What did we learn?

The Tevatron experiments see a combined 2.2 standard deviation [2.2 “sigma”] excess in their search, consistent with a Standard Model Higgs particle with a mass anywhere in the range of 115 to 135 GeV/c2.  This is not inconsistent with the Higgs hints that we saw in December from the LHC experiments.  Here I am being perhaps overly careful in not saying, more positively, “it is consistent with the Higgs hints…” only because this measurement is intrinsically too crude to allow us to narrow in on 124-126 GeV, where ATLAS and CMS see their hints.  In short, the Tevatron measurement could, in the end, turn out to indicate a Higgs at a different mass than the one indicated by the current ATLAS and CMS hints.  Anyway, it’s a minor and mostly a semantic point.

The results from ATLAS were a bit of a shock.  In all three processes on which ATLAS reported, CMS has presented results already, and in each case CMS saw a small excess (1 standard deviation [1″sigma”], which is  small indeed.)  But ATLAS reported today that it sees essentially no excess in any of the three, and even a deficit in one of them for low mass.  This has a big effect.

  • First, it allows ATLAS to exclude a Standard Model Higgs all the way up to 122 GeV/c2 (except for a little window 1 GeV/c2 wide centered at 118) and down to 129 GeV/c2.  The only large window left for the Standard Model Higgs particle is 122-129, more or less centered around the hint at 126 GeV/c2 that they saw in December.
  • But second, the significance of the December hint, when combined with the new data that shows no excesses in these three new processes, drops by about a full standard deviation.  That’s a pretty big drop.

What does it all mean?

I think it basically means, roughly, status quo.  We got some positive information and some negative information today, and none of it is that easy to interpret.  So I think we are roughly where we were before, except that we probably no longer have to worry about any Standard Model Higgs below 122 GeV/c2.  Before today we had a decent hint of a Standard Model-like Higgs particle with a mass around 125 GeV/c2; we still have it.  Let me explain what I mean.

There are easy (relatively!) searches for the Higgs, and there are hard ones.  The easy searches are the ones where the backgrounds are relatively simple and the signal is a narrow peak on a plot.  There are two:

  1. Higgs decaying to  photons
  2. Higgs decaying to two lepton/anti-lepton pairs (often called “four leptons” for short)

Results on these were presented by both ATLAS and CMS back in December.  The hard searches are the ones where the backgrounds are rather complicated and the signal is quite broad, so that a mistake in estimating a background can either create a fake signal or hide a real one.    There are three (mainly) for a lightweight Higgs:

  1. Higgs decaying to a lepton, an anti-lepton, a neutrino and an anti-neutrino
  2. Higgs decaying to a tau lepton/anti-lepton pair
  3. Higgs decaying to a bottom quark/anti-quark pair

These are the three that ATLAS reported on today (where they saw no sign of a Higgs signal), and that CMS presented back in December (and saw a small excess in all three.)  [ATLAS presented a result on the first one in December, but only using part of their data; it showed a small excess at the time, but not now.]  The third process is the main one in which CDF and DZero reported an excess today, though the first one also plays a role in interpreting that excess.

In other words, everything we learned today had to do with the difficult searches — the ones that are hard to perform, hard to interpret, and hard to check.  And everything we learned was 1 or 2 sigma information; not very compelling even statistically.

For this reason,

  • I would not conclude that the new Tevatron results make the 125 GeV Higgs case much stronger
  • I would not conclude that the new ATLAS results make the 125 GeV Higgs case much weaker

For the same reason, when I explained why I was skeptical of the evidence back in December, I told you that in my view the CMS excesses in the difficult searches did not make the case for a 125 GeV Higgs much more compelling.  Since the easy searches at CMS do not show as large excesses as ATLAS’s do, I wasn’t really comfortable with the whole case from CMS.   Their case improved in January, when they added a bit more information from their easy search for two photons.

If, like me, you discount the difficult Higgs searches somewhat relative to the easy Higgs ones, then almost nothing has changed, as far as the current Higgs hints, after today’s up and down information.  The excess in the two easy searches at ATLAS is still there, and there are excesses at CMS at least in the two-photon search.  Even from the beginning, I gave you good reasons to think the ATLAS’s easy-search excesses were a bit larger than they should be, probably due to an upward statistical fluctuation in the background.    Conversely I think now that one should not overstate how bad today’s ATLAS news is for the Higgs hints.  It’s still quite reasonable to think there may be a Standard Model Higgs there at 125 GeV/c2.  There’s some evidence in its favor, and it’s certainly not ruled out at this point. (Whereas now, almost all other masses are.)

So as usual I advise patience and calm and no hyperventilating; the 2012 data will settle the issue.  Either there is a Standard Model Higgs with a mass within a few percent of 125 GeV/c2 , or we’ll soon be fanning out in Phase 2 of the Higgs search, looking for all the other types of Higgs particles that might be out there.

Higgs Results from The First Week of the Moriond Conference

[UPDATE: Tevatron results start a few paragraphs down; LHC results will appear soon]

[2nd UPDATE: ATLAS  new results added: the big unexpected news.   As far as I can tell CMS, which got its results out much earlier in the year, didn’t add anything very new in its talk today.]

[3rd UPDATE: some figures from the talks added]

[4th UPDATE: more understanding of the ATLAS lack of excesses in new channels, and what it does to the overall excess at 125 GeV; reduction in local significance from about 3.5 sigma to about 2.5, and with look-elsewhere effect, now the probability the whole thing is an accident is 10%, not 1%.  Thanks to a comment for pointing out how large the effect was.]

This morning there are were several talks about the Higgs at the Moriond Electroweak conference.  There will be were talks coming from the Tevatron experiments CDF and DZero; we expected new results on the search for the Higgs particle from each experiment separately, and combined together.  There were also talks from the Large Hadron Collider [LHC] experiments CMS and ATLAS.  It wasn’t widely known how much new we’d see; they don’t have any more data than they had in December, since the LHC has been on winter shut-down since then, but ATLAS especially still hasn’t presented all of the results based on its 2011 data, so they may present new information.  The expectation was that the impact of today’s new results would be incremental; whatever we learned today wouldn’t dramatically change the situation.  The Tevatron results will certainly cause a minor ruckus, though, because there will surely be controversy about them, by their very nature.  I gave you a sense for that yesterday.  They aren’t likely to convince doubters.  But they might provide more pieces of evidence in favor of a lightweight Higgs (though not necessarily at the value of around 125 GeV/c2 currently preferred by ATLAS and CMS; see below.)

There are two things I didn’t explain yesterday that are probably worth knowing about.

First, if you look at Figure 2 in my post from yesterday, you’ll notice that the shape of the Higgs signal at the Tevatron experiments is very broad.  It doesn’t have a nice sharp peak at the mass of the Higgs (115 GeV in the figure).  This is because (as I discussed yesterday) it is hard to measure jets very precisely.  For this reason CDF and DZero will be able to address the question: “is there or is there not a lightweight Higgs-like particle”, but they will not easily be able to address the question “is its mass 115 GeV, 120 GeV, 125 GeV or 130 GeV?” very well.  So we’re really talking about them addressing something only slightly beyond a Yes-No question — and one which requires them to understand their backgrounds really well.  This is to be contrasted with the two-photon and four-lepton results from ATLAS and CMS, which with more data are the only measurements, in my view, that can really hope to establish a signal of a Higgs particle in a completely convincing way.  These are the only measurements that will see something that could not be mimicked by a mis-estimated background.

Second, the key to the CDF and DZero measurements is being able to identify jets that come from a bottom quark or anti-quark — a technique which is called “b-tagging the jets” — because, as I described yesterday, they are looking for Higgs decays to a bottom quark and a bottom antiquark, so they want to keep events that have two b-tagged jets and throw away others.  I have finished a new short article that explains the basic principles are behind b-tagging, so you can get an idea of what the experimenters are actually doing to enhance the Higgs signal and reduce their backgrounds.  Now b-tagging is never perfect; you will miss some jets from bottom quarks, and accidentally pick up some that don’t come from bottom quarks.  But one part of making the Tevatron measurement  involves making their b-tagging techniques better and better.  CDF, at least, has already claimed in public that they’ve done this.

Will update this after information becomes available and when time permits.

UPDATES: New Tevatron Results and New ATLAS Results

New Tevatron Results

Tevatron claims a lightweight Higgs; to be precise, the combination of the two experiments CDF and DZero is incompatible with the absence of a lightweight Higgs at 2.2 standard deviations (or “sigmas”), after the look elsewhere effect.  CDF sees a larger effect than DZero; but the CDF data analysis method seems more aggressive.   But both methods are far too complicated for me to evaluate.

The combination of DZero and CDF results from the Tevatron shows that their observed limit on the Higgs production rate as a function of its mass (solid line) lies about two sigma above the expected limit in the absence of any Higgs (dashed line) indicating an excess of events that appears consistent with a Higgs signal roughly in the 115-135 GeV mass range. By itself this result is not confidence-inspiring, but it does add weight to what we know from ATLAS and CMS at the LHC.

2.2 sigma is not much, and excesses of this size come and go all the time.  We even saw that several times this past year. But you can certainly view today’s result from the Tevatron experiments as another step forward toward a convincing case, when you combine it with what ATLAS and CMS currently see.  At minimum, assuming that the Higgs particle is of Standard Model type (the simplest possible type of Higgs particle), what CDF and DZero claim is certainly consistent with the moderate evidence that ATLAS and CMS are observing.  

There’s more content in that statement than you might think.  For example, if there were two Higgs particles, rather than one, the rate for the process CDF and DZero are measuring could easily be reduced somewhat relative to the Standard Model.  In this case they wouldn’t have found even the hint they’ve got.  (I explained why yesterday, toward the end of the post.)  Meanwhile the process that ATLAS and CMS are measuring might not be reduced in such a scenario, and could even be larger — so it would certainly be possible, if there were a non-Standard-Model-like Higgs at 125 GeV, for ATLAS and CMS to see some evidence, and CDF and DZero to see none.  That has not happened.  If you take the CDF and DZero hint seriously, it points — vaguely — toward a lightweight Standard-Model-like Higgs.  Or more accurately, it does not point away from a lightweight Standard-Model-like Higgs.

However, we do have to keep in mind that, as I noted, CDF and DZero can only say the Higgs mass seems as though it might be in the range 115 to 135 GeV; they cannot nail it down better than that, using their methods, for the reasons I explained earlier.  So their result is consistent with a Standard Model Higgs particle  at 125 GeV, which would agree with the hints at ATLAS and CMS, but it is also consistent with one at 120 GeV, which would not agree.   Thus Tevatron bolsters the case for a lightweight Higgs, but would be consistent both with the current hints at LHC and with other parts of the range that the LHC experiments have not yet excluded.  If the current ATLAS and CMS hints went away with more data, the Tevatron results might still be correct, and in that case ATLAS and CMS would start  seeing hints at a different mass.

But given what ATLAS and CMS see: the evidence from December, and the step forward in January with the CMS update in their two-photon data, something around 125 GeV remains the most likely value mass for a Standard Model Higgs.  The issue cannot be considered settled yet, but so far nothing has gotten in the way of this hypothesis.

Now, the inevitable caveats.

First, as with any measurement, these results cannot automatically be assumed to be correct; indeed most small excesses go away when more data is accumulated, either because they are statistical fluctuations or because of errors that get tracked down — but unfortunately we will not get any more data from the now-closed Tevatron to see if that will happen.  The plausibility of Tevatron’s claims needs to be evaluated, and (in contrast to the two photon and four lepton results from ATLAS and CMS, which are relatively straightforward to understand) this won’t be easy or uncontroversial.  The CDF and DZero people did a very fancy analysis with all sorts of clever tricks, which has the advantage that it makes the measurement much more powerful, but the disadvantage of making it obscure to those who didn’t perform it.

One other caveat is that we will have to be a little cautious literally combining results from Tevatron with those from the LHC.  There’s no sense in which [this statement is factually incorrect as stated, as commenters from CDF are pointing out; there are indeed several senses in which it was done blind.  I should have been more precise about what was meant, which was more of a general knowledge of how difficult it is to avoid bias in determining the backgrounds for this measurement.  Let me add that this is not meant to suggest anything about CDF, or DZero, in particular; doing any measurement of this type is extraordinarily difficult, and those who did it deserve applause.  But they’re still human.] the Tevatron result was done `blind’; it was done with full knowledge that LHC already has a hint at 125,  and since the Tevatron is closed and all its data is final, this is Tevatron’s last chance (essentially) to contribute to the Higgs particle search.  Combining experiments is fine if they are truly independent; if they are not, you are at risk of bolstering what you believe because you believe it, rather than because nature says it.

New ATLAS results 

ATLAS has now almost caught up with CMS, in that its searches for Higgs particles decaying to two photons and to two lepton/anti-lepton pairs (or “four leptons” for short) have now been supplemented by (preliminary! i.e., not yet publication-ready) results in searches for Higgs particles decaying to

  • a lepton, anti-lepton, neutrino and anti-neutrino
  • a tau lepton/anti-lepton pair
  • a bottom quark/anti-quark pair (which is what CDF and DZero looked for too)

(The only analysis ATLAS is missing is the one that CMS added in January, separating out events with two photons along with two jets.) In contrast to the CMS experiment, which found small excesses (just 1 sigma) above expectation in each of these three channels, ATLAS does not.  [And I’ve been reminded to point out that the first channel has changed; in December, with 40% of the data analyzed, there was a small excess.] So CDF and DZero’s results from today take us a step forward toward a convincing case, while ATLAS’s result takes us a small step backward.  That’s par for the course in science when you’re squinting to see something that’s barely visible.

In the same search as performed by CDF and DZero, and in the same region where they see an excess, ATLAS sees no excess at all; but ATLAS has less data and is currently less sensitive to this channel than CDF and DZero, so there is no clear contradiction.

But one can’t get too excited about this.  Statistics are still so low in these measurements that it would be easy for this to happen.  And determining the backgrounds in these measurements is tough.  If you make a mistake in a background estimation, you could make a small excess appear where there really isn’t one, or you could make a real excess disappear.  It cuts both ways.

But actually there is a really important result coming out of ATLAS today; it is the deficit of events in the search for the Higgs decaying to a tau lepton/anti-lepton pairs.  For a putative Higgs below 120 GeV, ATLAS sees even fewer tau lepton/anti-lepton events than it expected from pure background — in other words, the background appears to have fluctuated low.  But this means there is not likely to be a Standard Model-like Higgs signal there, because the likelihood that the background plus a Higgs signal would have fluctuated very low is small.  [UPDATE: actually, looking again, I think I am somewhat overstating the importance of this deficit in taus compared to the lack of excess in the other two channels, which is also important. To be quantitative about this would require more information.  In any case, the conclusion is the same.]    And this allows ATLAS to exclude new regions in the mass range for the Standard Model Higgs, at 95% confidence!

This is very important!  One of the things that I have complained about with regard to those who’ve overplayed the December Higgs hints is that you can’t really say that the evidence for a Higgs around 125 GeV is good if you can’t start excluding both above and below that mass.  Well, ATLAS has started to do that.  Granted, it isn’t 99% exclusion, and since this is the Higgs we’re talking about, we need high standards.  But at 95% confidence, ATLAS now excludes, for a Standard Model Higgs, 110-117.5, 118.5-122.5, and 129-539 GeV.  Said better, if there is a Standard Model Higgs in nature, ATLAS alone restricts it (to 95% confidence only, however) to the range 117.5 – 118.5 GeV or 122 – 129 GeV.

ATLAS, just from its own data alone, excludes (pink-shaded regions) the Standard Model Higgs particle at 95% confidence (but not yet at 99%) across the entire allowed range except around 118 GeV and between 122 and 129 GeV, where the two-photon and four-lepton searches provide some positive evidence. What is shown is how large a Higgs signal can be excluded, in units of the Standard Model expectation, as a function of the Higgs mass. Anywhere the solid line dips below the dotted line marked "1" is a place where the Standard Model is 95% excluded. The red dotted line indicates how well this experiment would perform, on average, if there were no Standard Model Higgs signal.

The window is closing.  Not only has ATLAS completely excluded the old hints of a Standard Model Higgs at 115 from the LEP collider, it seems it has probably excluded CMS’s hint around 120, which was the next best option for the Higgs after 125.  And as far as I can tell, this is coming mainly from the tau lepton/anti-lepton measurement As I said above in an update, I think it is really a mix of all three channels… hard to be quantitative about that without talking to the experts.

So if the Standard Model Higgs is what nature has to offer us, we’re probably down to a tiny little slice around 118 GeV for which there’s no evidence, and a window that has 125 GeV smack in the middle of it, where the evidence, though not much stronger today, if we include both Tevatron and ATLAS, than it was yesterday, is certainly no weaker.

UPDATE: Well, it’s been pointed out to me by the first commenter that the last statement is misleading, because it doesn’t emphasize how the ATLAS excess at 126 GeV has decreased substantially  in significance. Somehow I thought originally that the decrease was marginal. But it isn’t.

The statistics numbers as I think I have them now: What was previously about 3.5 sigma local significance for the Higgs-like peak at 125 GeV is now down to 2.5, and what seemed only 1% likely in December to be a fluctuation is now 10% likely.

There is an issue, however, with combining many measurements.  Of course the two-photon and four-lepton results from ATLAS are the same as before, and they are just as significant; nothing changed.  But the other three measurements came in low, and that pulls the significance of the combination down.  However, I must remind you again how difficult the last three measurements are.  I would trust the first two before the last three.  So I think we should be careful not to overinterpret this change.   When you combine what you trust most with what you trust least, you reduce your confidence in what you have.

That said, it also indicates why one should be very cautious with small amounts of data.

Comparison of the December ATLAS results (left), combining all measurements that were available at the time, with the March 2012 ATLAS results (right). I've lined them up as best I could, given the scales were slightly different. What is shown is how large a Higgs signal can be excluded, in units of the Standard Model expectation, as a function of the Higgs mass. Anywhere the solid line dips below the dotted line marked "1" is a place where the Standard Model is 95% excluded. Compared to December, there is much more excluded and the height of the peak at 126 GeV is noticeably lower.

Awaiting Higgs News from the Tevatron Experiments

The search for the Higgs particle has been dominated recently by the new kids on the block, the ATLAS and CMS experiments at the Large Hadron Collider [LHC], who benefit from the LHC’s record high energy per collision. But at its predecessor, the now-closed Tevatron, the CDF and DZero experiments still have a few tricks up their sleeves. Though the energy per collision in recent years at the Tevatron was 3.5 times smaller than was the LHC’s  in 2011,  CDF and DZero have twice as much data as do ATLAS and CMS right now. And there’s one more thing going for them. In contrast to the LHC, where protons collide with protons, at the Tevatron protons collided with antiprotons. That gives the Tevatron a little edge in one particular search mode for the Higgs. It won’t be enough to beat the LHC at the game for which it was designed, but it’s enough that the Tevatron experiments can at least play. And we’ll see results from the two experiments tomorrow (Wednesday) — with a preview already publicly available, as you’ll see below. Continue reading

Welcome 2012

Well, 2011 was certainly an interesting and exciting year for particle physics. And 2012 promises to be even better.

LHC and the Higgs search

At the Large Hadron Collider [LHC], the accelerator physics team did a fantastic job of assuring the collider worked effectively, and provided significantly more proton-proton collisions than were originally expected. Meanwhile the experimental teams found clever ways to dig more information out of the collision data than was initially anticipated. And thanks to this, the search for the Higgs particle (or Higgs particles, or whatever replaces the Higgs particle) is  most of the way through Phase 1 — the search for the simplest possible form of Higgs particle, known as the “Standard Model Higgs”. We started 2011 knowing that the mass of the Standard Model Higgs particle might lie almost anywhere between 115 GeV/c2 and 800 GeV/c2 (where GeV, a measure of energy, is described here, and c is the speed of light, as in E = m c2.) The exception was a narrow gap around 160-170, excluded by the Tevatron experiments.  We ended the year, thanks to the great work at the ATLAS and CMS experiments, with the Standard Model Higgs excluded everywhere except above 600 GeV/c2 (where it is disfavored for other reasons) and in a window between 115 and about 128 GeV/c2. Even more exciting, there is a serious hint of a Higgs particle signal at around 124–126 GeV/c2. Continue reading

Two-Photon Events Discrepant, But What’s the Cause?

Matt Strassler 11/1/11.   No rest for the weary: yet another discrepancy. This one is somewhat different from the small multi-lepton excess at CMS of a couple of weeks ago (tenuous, but in a very interesting and plausible place) and from the OPERA faster-than-light neutrino claim (not so tenuous, but not so plausible either). Now we have a discrepancy involving collisions that produce two low-energy photons [particles of light]. The effect is seen in four experiments, not one: both ATLAS and CMS at the Large Hadron Collider (LHC), and also CDF and DZero at the Tevatron collider. It’s too large to be a statistical fluke (the excess is not small and it shows up in four experiments). Nor does it look like an experimental mistake (since it shows up in four experiments). Might it be a sign of a new phenomenon not predicted by the Standard Model (the equations that describe the known particles and forces, plus the simplest possible Higgs particle)? Maybe… Can’t rule it out, though there’s not enough information in the experiments’ public documents for a serious evaluation of that possibility. But in any case, my preliminary impression is that it’s most likely something else: either a problem with the theoretical calculation of what the Standard Model predicts, or a problem with the way this theoretical calculation was used by the experiments.

Now why would I come to that conclusion? To read more, click here.

The Tevatron Comes to an End

[If you are a layperson interested in the faster-than-light neutrino claim, and you haven’t yet looked at my recent “open-space’’ post and the list of excellent questions laypeople have asked in the comments, you definitely should.  And ask your own if you want. That post also gives an organized list of links to my main posts on the neutrino experiment.]

Today is a sad day in American particle physics.  It is the day that Fermilab will shut down the Tevatron, once the world’s leading particle accelerator, which discovered and measured the mass and other properties of the top quark (the sixth discovered, and by far the heaviest), tested the Standard Model of particle physics in very great detail (confirming that everything I wrote in this post about the known particles is correct to the available precision of the experiment), and looked hard for the Higgs particle before being overtaken by the Large Hadron Collider.   Continue reading

What Have We Here?

Well, every now and again an experiment reports a result that forces scientists to go back to a long-established principle, to check whether it needs revision, extension or adjustment, or perhaps even replacement. Most times it eventually turns out that the experiment is wrong, though often in some subtle and non-obvious way, and the principle survives. But of course there are the rare occasions when it is the other way round. So a scientist must go into such a situation with an open, though skeptical, mind.

Is there a more famous principle from 20th century physics than Einstein’s principle that nature has a speed limit? We call this the “speed of light”.

We call it that. However, let’s be a bit careful. Continue reading