PRL:物理学家创造“负质量”（附原文）

诸平

据物理学家组织网（Phys.org）2017年4月17日报道,美国华盛顿州立大学（Washington State University）的物理学家创造了一种具有负质量流体,听起来有点奇怪！推动它,与我们知晓的世界上每一个物理对象截然不同,它并不在其推动的方向上加速前进，反而在其反方向加速即向后加速运动。

Experimental TOF images of the effectively 1Dexpanding SOC BEC for expansion times of 0, 10, and 14 ms.

华盛顿州立大学的物理与天文学助理教授、同时也是华盛顿大学的助理教授迈克尔·福布斯（Michael Forbes）说,要在实验室条件下创建这种现象非常罕见,但是它可以用于探索宇宙的一些更具挑战性的概念。相关研究结果2017年4月17日已经在《物理评论快报》（Physical Review Letters）杂志网站上发表，而且是被列入“编辑建议（Editor's Suggestion）”之列。 其实此项研究结果是由华盛顿州立大学（Washington State University）物理与天文学系的研究人员与日本冲绳科学技术大学院大学（OIST Graduate University）、上海大学物理系张永平教授（通讯作者之一）、华盛顿大学（University of Washington）物理系的研究人员合作完成。

就像一个电荷既可以是正的也可以是负的一样，假设物质可以有负质量。在这些方面很少有人认为,我们的日常世界只能看到艾萨克·牛顿（Isaac Newton）第二运动定律正的方面, 其力等于物体的质量乘以其加速度即F = ma。换句话说,如果你推动一个物体,它将会在你推动的方向上加速运动。在力的方向上质量将会加速。福布斯说：“这就是我们用来做大多数事情时的情况, 但是，并非所有情况都是如此。具有负质量的东西，如果你推动它，它会向着你加速运动。”这真是让人不可思议。更多信息请注意浏览原文：

M.A. Khamehchi, Khalid Hossain, M.E. Mossman, Yongping Zhang, Th. Busch, Michael McNeil Forbes, P. Engels. Negative-Mass Hydrodynamics in a Spin-Orbit–Coupled Bose-Einstein Condensate(点击可以免费下载原文). Physical Review Letters，2017,118: 155301 – Published 10 April 2017. DOI:10.1103/PhysRevLett.118.155301

负质量的条件及其他

Conditions for negative mass

He and his colleagues created the conditions for negative mass by cooling rubidium atoms to just a hair above absolute zero, creating what is known as a Bose-Einstein condensate. In this state, predicted by Satyendra Nath Bose and Albert Einstein, particles move extremely slowly and, following the principles of quantum mechanics, behave like waves. They also synchronize and move in unison as what is known as a superfluid, which flows without losing energy.

Led by Peter Engels, WSU professor of physics and astronomy, researchers on the sixth floor of Webster Hall created these conditions by using lasers to slow the particles, making them colder, and allowing hot, high energy particles to escape like steam, cooling the material further.

The lasers trapped the atoms as if they were in a bowl measuring less than a hundred microns across. At this point, the rubidium superfluid has regular mass. Breaking the bowl will allow the rubidium to rush out, expanding as the rubidium in the center pushes outward.

To create negative mass, the researchers applied a second set of lasers that kicked the atoms back and forth and changed the way they spin. Now when the rubidium rushes out fast enough, if behaves as if it has negative mass."Once you push, it accelerates backwards," said Forbes, who acted as a theorist analyzing the system. "It looks like the rubidium hits an invisible wall."

Avoiding underlying defects

The technique used by the WSU researchers avoids some of the underlying defects encountered in previous attempts to understand negative mass.

"What's a first here is the exquisite control we have over the nature of this negative mass, without any other complications" said Forbes. Their research clarifies, in terms of negative mass, similar behavior seen in other systems.This heightened control gives researchers a new tool to engineer experiments to study analogous physics in astrophysics, like neutron stars, and cosmological phenomena like black holes and dark energy, where experiments are impossible."It provides another environment to study a fundamental phenomenon that is very peculiar," Forbes said.