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Precision Science August 25, 2008

Posted by gordonwatts in physics.

You can tell how old a set of tools for a field is by how precise their measurements are. Take the top quark. Its been around since 1996. The latest top quark mass result from both CDF and D0 is 172 +- 1.22 GeV – so we know that to better than 1%.

Some of the most stunning recent discoveries in science have been dark energy and dark matter. Well, I guess I shouldn’t call them discoveries — we don’t know what they are yet — but the fact that something is there is definitely a discovery. But the thing about astrophysics is that it isn’t a precision field.

Perhaps that is changing now – from an article on a new measurement of the Hubble constant done using the Hubble space telescope:

The news was not in Dr. Riess’s value… , but in the precision with which his group claimed to have measured it: an uncertainty of only 4.3 percent.
Only 30 years ago, distinguished astronomers could not agree within a factor of two on the value of Hubble’s constant, leaving every other parameter in cosmology uncertain by at least the same factor and provoking snickers from other fields of science.

Actually, even more recent than that! I remember a rather famous string theorist standing up and claiming “Hey – in cosmology we have finally learned how to use error bars!” And then poking fun at the size of the errors in astro physics.

But that is always the way when you find something new. The top quark, when we discovered it, we basically knew it was there and kind-a knew its mass. We have then spent the last 15 years making the measurement steadily more precise (knowing that mass very well tells us a lot about where to find the Higgs).

Getting down to the 1% level, or the 5% level, even, is a lot of careful work. And, at some level, not as much fun as actually being the first to measure the value. But after verifying that the discover exists, it is the most important thing. That is the beauty of science: all the numbers are connected. The more you know one set of numbers, the better you can predict a second.

Getting the top quark precision down has been 15 years of hard hard work, many graduate student theses, and many post-doc years. But because of that we know a lot more about where to hunt for the Higgs. Doing the same in astrophysics is bound to help with the quest to understand dark energy and dark matter. Can’t wait!

P.S. Can you tell I wrote this on vacation? I’m reading the newspaper!!



1. Lisa Smith - August 26, 2008

This brings it all together then…. since I came to this job, I’ve noticed several references to the Top Quark and to dark matter — in a context that clearly indicates these things exist. When I was an undergraduate I was really into astronomy (and fascinated by astrobiology) and while I was takinga few astronomy classes at UW, I remember reading that quarks were a “cutting edge” line of inquiry, and entirely theoretical. Those who did study it were considered renegades (in this set of reading) and not even to be trusted with how they were using their research funding. Dark matter was also this mysterious thing that could only be proven by what it didn’t reveal…. now I see these things began to credibly exist in 1996! (But my reading and the word of mouth must have been out-of-date, because this was post 1996 – I’m not that old yet ;-). It’s funny in a way… this is something I’ve observed about science – medicine in particular: it really is all relative, and any gospel statement about what is REAL must always stand the test of time… and perception…. and the system of measurement and observation…. as for faith and intuition, those are tests that bear no argument, for there is no fixed point of reference.

2. gordonwatts - August 28, 2008

Hmmmm… So, the quark model has basically been widely heald for almost 40 years now. So I’m not sure what text you were reading, or how dated it was. But if someone was talking about quarks being “not really set” even 20 years ago they were not teaching you modern science. Huge amounts of funding have been spent researching quarks for almost 50 years now. The top quark was only the last of 6 to be discovered. I’d be very curious to know more about this. I hope this is not going on today!

Dark matter & dark energy are pretty recent. We may not know exactly what fraction of the universe is “dark” – but we are pretty darn sure it is very large. This is perhaps one of the hottest areas of research – it is something truely new. No one has any idea what it is or what is going on. But I would say that dark matter and dark energy really arrived on the scene about 7 or 8 years ago.

And your final comment about things standing the test of time — exactly. And even if they do, they are subject to be changed anytime a better theory comes along. Usually, when that happens, the new theory isn’t more “right” as much as “more complete”. This will, we all hope, happen to the Standard Model, for example.

3. gordonwatts - August 28, 2008

Oh, and I meant to add. In some sense quarks are a mathematical construct. No one has every “seen” them. But if you assume they are there and act as the theory predicts, then you can predict just about every measurement we can make. So the evidence for them is very strong. It could be one day we discover that quarks are really made up of super-strings, so there aren’t really “quarks” as much as there are strings of a particular nature. But in that case, the strings will have to behave exactly as our quarks do currently.

4. Lisa Smith - August 29, 2008

You already got my response about the questionable quark statements… and regarding the “test of time” angle… whenever I see shows like Nova: Science Now where they discuss physics, I get this sense that a lot of Einstein’s work is hanging on by a thread these days. Like, some of the laws are so close to having a giant hole blown in them (if they haven’t already), because they just can’t stand up to the challenge of new discoveries forever. I know that is constantly happening by degrees, but I mean in a way that necessitates a total sea change in the accepted views and models for regular everyday people – a historical turning point. (Forgive me if I’m saying anything ignorant – I’m not a physicist!) And though I’m just re-stating the obvious here, the other sweeping truth that science has taught me, is that the more you uncover, the less you know. For me, science leads straight to a place that science can’t touch…

I just realized, upon reading your second comment, it was string theory overall that was targeted in that one article I read. Quarks were part of it, but the mention of strings does jog my memory a bit.

5. anonymous - September 9, 2008

I am very glad that the LHC is now on-line.
Guess what: I have written a matlab program
that can compute n-point and l-loop amplitudes
with as many particles and higher dimension
operators as you like (and form). The time
it takes to compute, would you believe is now
polynomial in npoint and lloop, i.e. npoint^6
* lloop^6 * number of operators, and some
minor complications associated with the number
of particles. Isnt that great! I have mixed
feelings about it. On the one hand you guys
get the dang best program you can imagine, and
on the other hand I get to hate my life (kindof
sortof – anyone want to call off the thugs from
continually screwing me over.)

I figure, and I have already estimated, that the
CERN computer complex can now do in excess of
an oodle number of loops, depending on several

By the way, my front end javascript (not yet written
into the computer) simply says : npoint? lloop?
operator types? particle content? and some other
things such as how you like to arrange the memory
allocation (compress? compress that way or this way?)
which version? you want your momenta or random
generated momenta? how many dimensions? want some patter recognition? scattering amplitude or
inclusive or exclusive? want some beam tips to
enhance cross section? stuff like that. Experimentalists will never have to ask a particle theorist again for theoretical or numerical

I have been working on this program for quite some time,
and now I am finished I believe. (Would you believe I was
able to shrink it from 60000 lines of code to about 3000.
Thats because it is quite much in machine language. )

There is a bad side however, and I am not going to
tell you, but you free to contact your local United
Nations office as to what the bad side is.

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