APS News


What Robert Brown Actually Saw

The recent article on Robert Brown’s discovery of Brownian motion (APS News, "This Month in Physics History," August/September 2016) states "Within those grains of pollen he noticed even smaller particles jiggling in seemingly random motions, as if they were alive."

That is incorrect. In a quest to understand plant reproduction, Brown had put some triangular shaped pollen of the plant Clarkia Pulchella into water. He saw through his high-powered one-lens microscope that the pollen burst at the corners. It ejected two kinds of particles, round ones and oblong ones, which then proceeded to undergo what we now call Brownian motion in the water. The fluid in the pollen is too viscous to allow Brownian motion to be observed within the pollen.

Brown pursued these observations with great avidity, which is why the motion is named after him. A major reason is that the round particles he saw appeared to be all the same size. This made him think for a while that, as atoms are the fundamental building blocks of inorganic matter, he had found the fundamental building blocks of organic matter — until he eventually found that all kinds of inorganic matter too, if ground small enough (including a fragment of the sphinx!), would move similarly.

Actually, the round particles are not all the same size, but are smaller than the wavelength of light, and Brown was observing their Airy diffraction disc. This point, as well as identification of the round particles (spherosomes, fat containers) and oblong particles (amyloplasts, starch containers) were made for the first time as part of an investigation into the history, botany and physics of Brown’s discovery, detailed at What Brown Saw and You Can Too.

This site also contains three brief videos of pollen bursting and the resulting Brownian motion, instructions on how to build a microscope like Brown's from a readily available "ball lens," and photographs and analysis of observations made with this microscope, a modern optical microscope and an electron microscope.

A portion of the particularly physics-oriented material on this website was published as "What Brown saw and you can too," by P. Pearle, B. Collett, K. Bart, D. Bilderback, D. Newman and S. Samuels, Am. J. Phys. 78, 1278 (2010).

Phil Pearle
Clinton, NY

Palestine Advanced Physics School

I was delighted to hear about the new Palestinian Advanced Physics School in West Bank, as described in the APS News article by Rachel Gaal. The Palestinians deserve a higher education as much as anyone else. It was a shame that the article had to be political in nature, discussing how the so called "Israeli occupation" is preventing access to students from Gaza. While it may be unfortunate that the Palestinian territories are not simply connected, I don't think that anyone would reasonably advocate for an open border policy at this time. Nobody expects the United States — or any other country for that matter — to have open borders, so I can't see why Israel should be the exception, especially given the tough neighborhood in which it is situated.

Wishing the best of luck to the new school, with hope that their travel complications are soon resolved.

Dov Rhodes
Falls Church, NY

The article by Rachel Gaal entitled "First Palestine Advanced Physics School" in the October issue of APS News, tells of two successful gatherings of Palestinian students to hear about recent developments in physics. Requests by three students from Gaza to cross through Israel to attend one of the conferences were denied by Israel. This prompted David Marsh, one of the organizers of one of the conferences, to write:

"Access to higher education is a human right, and it is deeply regrettable that this right is not respected by the ongoing Israeli occupation."

The censure of Israel for violating the human right to higher education is disturbingly misplaced. Within its region, Israel is the only nation with successful affirmative action policies that have brought diversity and broad access to higher education. Judgments regarding travel to conferences in the Middle East cannot be made in isolation but must be balanced by the complexities of the security situation in the region.

Unfortunately, the students could not simply apply for a visa to travel through Israel to the conference, since Gaza does not have diplomatic relations with Israel. This is because Gaza is committed via Hamas’ founding charter and the organization of its society to the destruction of Israel. Nonetheless, Israel does grant travel permits for travel into Israel. It is hard to know why these were not granted in this case. But it might well have been because citizens of Gaza are taught, starting in kindergarten, that killing Jews is the way to glorious martyrdom. It is regrettable that the right of a nation to defend its citizens becomes for some an abuse of human rights when that nation is Israel.

Azriel Genack
Flushing, NY

APS encourages the redistribution of the materials included in this newspaper provided that attribution to the source is noted and the materials are not truncated or changed.

Editor: David Voss
Staff Science Writer: Rachel Gaal
Contributing Correspondent: Alaina G. Levine
Publication Designer and Production: Nancy Bennett-Karasik