Comments on: Guest Post: Marcelo Gleiser on How do We Know?

By: TSFTRTruth

TSFTRTruth — Tue, 26 May 2009 06:33:32 +0000

Well, i do meditation for investigation. in the end it is said, this is all an illusion. and i must prove it within myself. but of course i have to say, within the air, i dont know sometimes there is this tear in diamension. i cant explain it. our brain or minds could be seeing it, but it could not. The mind sees only what it want to see. it is biased, i am hoping to see what is there and yet it isnt. they say if you want to find it you will. But then again, it is all an illusion. we are made of these tiny tiny molecules. where are we, is my question. can you put your mind on your finger now? Where are we. If we were in Nebula axis now, would earth be a past. who is the past who is the present. there isnt a fixed place of time. when u finish reading this, if you would, it is already a past. it is bad. very bad.

By: Nilanjan Ray

Nilanjan Ray — Fri, 27 Mar 2009 15:27:30 +0000

It seems there is no end to our findings. The more sophisticated our instruments get, the more we find…from atoms to subatomic particles to higgs bosons and more to come, surely. We become more conscious in the whole process. Is there really and to it?..Or is it our consciousness which gives our sense of reality without an end !!!

By: daisyrose

daisyrose — Wed, 25 Mar 2009 17:01:30 +0000

Ha ha – how can you really know that you know anything – You can not ! You start out doing the possible and before you know it you are doing the impossible.

Little capillarial forays into minutia ? Its the big picture that matters —- what ever that it.

Everyone wants to think they live in (are part of) the most interesting times. Its all vanity !

By: Ellipsis

Ellipsis — Wed, 25 Mar 2009 05:21:49 +0000

Hi Marcelo,

Indeed there are certain types of signatures that do not behave like any physics we know that could potentially avoid detection (e.g. events that contained no high-pT tracks or clusters). However, efforts are made to look for, for example, tracks that look like they don’t come from any known charged particle (see e.g.

http://www-cdf.fnal.gov/physics/exotic/r2a/20070208.champ/
http://www-d0.fnal.gov/Run2Physics/WWW/results/prelim/NP/N27/N27.pdf

as well as so-named “global” searches for new physics

http://www-cdf.fnal.gov/physics/exotic/r2a/20080228.vista_sleuth/publicPage.html
http://www-d0.fnal.gov/Run2Physics/WWW/results/prelim/NP/N65/

that do some of what you mention (perhaps as much as is practically possible — unless you have other suggestions!).

By: Marcelo Gleiser

Marcelo Gleiser — Tue, 24 Mar 2009 22:00:59 +0000

First, I’d like to thank the people that took the time to comment on my text. I learned a lot, which was my main goal, together, of course, with inciting some discussion.

People raised both technical and more philosophical issues. First, a general point, very important: even if data filtering is ultimately biased at some level, this is not WRONG; it is INCOMPLETE. It means that what we measure, as I argued, is only a fraction of what is out there. A star, viewed only in the optical is still a star, albeit a fraction of what it is in the full glory of the electromagnetic spectrum. One hundred years back, this optical fraction was taken to be the whole thing. Now we know that’s not the case. Our perception of reality broadened with the development of our tools. This is important to my argument, as captured by Giotis in his comment.

Starting with the more technical points, it is true, as JoAnne mentioned, that at a first passing (and that’s important), triggering does not rely on a specific model. However, it relies on one fundamental assumption (and I thank my friend Michelangelo Mangano, a theorist from CERN and heavily involved with LHC physics for his comments, although any mistakes here are my own): whatever happens during the collision, its final products MUST be Standard Model particles, electrons, muons, photons, jets and missing energy (e.g. neutrinos). After all, we only know how these “objects” interact with the detector. Can we be sure that this will always be the case? One then tracks these objects and records their related events above a minimum threshold of energy to avoid filling the hard drive with useless stuff. One can refine this strategy, but at the cost of introducing theoretical bias. For example, for SUSY searches you can combine the request to have several jets with the request of missing energy or of multiple leptons. Other requests can test other theoretical models. However, there are always events that could be missed, if existent: for example, a very light axion has such a small amount of missing energy that it would never trigger the detector. Also, strange metastable particles or solitonic objects with large mass that could slow down and sit inside the detector for a while; I think JoAnne alluded to this possibility at the end of her comment.

So, from a practical perspective, a lot of thinking has gone into optimizing the triggers so as to cover an enormous number of possible situations. But, as with any experimental set up, there is only so much it can do. Again, this doesn’t mean that physicists are doing something wrong; but it does mean that what we do is always INCOMPLETE. Even as we move onward and perfect the search with more refined filtering and triggers, there is always going to be the possibility that we are missing something. An example is the discovery of the J/psi particle, a bound state of charm and anti-charm quarks. First searches missed it because they weren’t triggering low transverse-momentum muons. Eventually it was found when the search was refined.

We will always be limited by what we can measure. Even as we get more skilled, there is no end in sight. Only improvements. And even as we improve, history tells us that the more we measure, the more there is to measure as new tools open new “vistas” into nature. In this case, what we call reality is based on what we can measure of it. Nothing wrong with this, but it means that our view of the universe is always constrained by our tools. In my opinion, this makes science more human, something I like better than saying that science is the search for some ultimate “truth”.

By: Giotis

Giotis — Tue, 24 Mar 2009 12:05:21 +0000

Since we are part of the physical world and not outside observers of reality, everything we believe automatically acquires a substance and becomes real. At the end any experiment must be interpreted by humans according to their believes and thus its result can have a meaning only in the context of their reality; that is all that counts. There is no absolute measure of reality. We are the reality and its measure.

By: Christian

Christian — Tue, 24 Mar 2009 09:42:07 +0000

To answer the question by Faustus : You cannot always filter things out event by event by comparing with established models. Sometimes single events of “new physics” are identical to Standard Model events – only the statistical distribution of many events differs.

By: Garth

Garth — Tue, 24 Mar 2009 07:57:25 +0000

Astronomers call this the “Selection Effect” – in astronomy assessing what one can ’see’ one always has to be mindful of what might be there but not seen because it is too faint or emitting outside the observed waveband, i.e. the observation is limited and prescribed by the apparatus.

It is important to apply this acknowledgement of ignorance to the theory as well as the apparatus and never be too sure about the ‘Standard Model’.

Garth

By: Dileep

Dileep — Tue, 24 Mar 2009 05:22:06 +0000

Say. With so much filtering, how do you account for statistical biasing while calculating probabilities?
Or is that done at a later stage, the first ones being study of possibilities rather than probabilities of events?

By: Claire C Smith

Claire C Smith — Tue, 24 Mar 2009 00:51:48 +0000

Incredible topic.

Claire