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诺奖得主Wilczek:破解物相的秘密

撰文 | Frank Wilczek
 
翻译 | 胡风 梁丁当
 
 
中文版
 
从液体和气体,到核物质与夸克相,研究正在揭示物质在极端条件下的相变行为。
 
大型粒子加速器所产生的超高温火球、高压锅中的水蒸汽和冰箱中的冰,它们有什么共同之处?那些致力于探索中子星内部、宇宙初期或其他极端条件下的物质性质的科学家会告诉你:有很多。
 
生活用水的经验告诉我们,同一种物质可以具有不同的相——液态水、水蒸汽和冰。又比如,氧气是我们新陈代谢的能源,而液态氧和固态氧则可用作火箭燃料。然而,物质具有不同的相这件事是非常奇怪的。早在19世纪初,科学家们就对此展开了深入的研究。但直到20世纪后期,与实际吻合的理论才最终出现。1982年,物理学家肯尼斯 · 威尔逊(Kenneth Wilson)因其在物相方面的理论工作获得了诺贝尔奖。
 
通过将水置于不同的温度和压强下,我们可以绘制出一幅相图,来刻画水在不同条件下的形态。加热炉子里的水使它沸腾时,水温达到沸点后便不再升高,这时热量全部被水吸收,使其从液体转变为气体,这部分热量叫做潜热。水具有稳定不变的沸点这个性质对烹饪非常有益,它可以帮助我们得到可预见的结果。另一方面,我们可以用高压锅来调节沸点。
 
当压强升高,沸点也相应升高,而潜热减小。最终,在217.25个大气压下,潜热完全消失。在这个临界点,水从液体连续地变成气体,液相与气相的区别不再明显。在压强升到略低于临界值时,水沸腾的温度达到极限值:373.946℃。
 
当压强和温度同时达到临界值时,有趣的现象发生了。液相和气相的区别完全消失了。水的密度涨落很大,仿佛不知道应该如何组织自己的结构。二氧化碳和许多其他物质也具有类似的行为。威尔逊的重大成就就是解释了为什么这些不同的物质会产生相同的现象。根据他的普适理论,涨落有其自身的规律,几乎与物质的具体性质无关。
 
现在,在日内瓦附近的大型强子对撞机和纽约布鲁克海文的相对论性重离子对撞机中,科学家们比以往任何时候都更努力地尝试突破这个普适性适用的极限。在对撞机中,核物质类似于液相,而夸克-胶子等离子体类似于气相。研究人员让拥有不同能量的金原子核相互碰撞,产生具有不同温度与密度的火球,然后绘制出相图。
 
20世纪90年代初,克里希纳 · 拉贾戈帕尔(Krishna Rajagopal)和我预测,在核物质和夸克之间,也存在一个大约为10万亿度的临界温度。在此处,核物质和夸克相之间的区别会消失。我们提出,通过观测接近临界温度与压强的火球的涨落,可以证实这个临界点的存在。
 
布鲁克海文的实验团队已经观察到了相关迹象。为了确定(或者否定)这点,现在他们正在分析一个更庞大的数据集。如果一切顺利,他们将证明,那些深邃的理论虽然最初只是为了理解物理化学与热学工程领域中脚踏实地的问题而构建的,但在远离这个出发点的地方,这些理论依然大有用武之地。
 
英文版
 
New research sheds light on the behavior of liquids and gases under extreme conditions.
 
What do superhot fireballs produced at big particle accelerators have in common with the steam in your pressure cooker and the ice in your refrigerator? The answer is quite a lot, according to scientists trying to understand the nature of matter in neutron stars, the early universe and other extreme conditions.
 
Everyday experience with water in our kitchens exposes us to how the same material has different phases-liquid water, gaseous steam and solid ice. Similarly, we breathe in gaseous oxygen to power our metabolism, while we use liquid and solid oxygen as rocket fuel.
 
Yet the fact that matter has different phases is profoundly strange. Science has studied it deeply since the 19th century, but it wasn't until the late 20th century that theory caught up to reality fully. In 1982, physicist Kenneth Wilson won the Nobel Prize for his theoretical work on phases of matter.
 
By subjecting water to varying temperature and pressure, we can map out a "phase diagram" that depicts the form it takes under different conditions. When you bring water to a boil on the stove by applying heat, the temperature pauses in its rise at the boiling point, and the heat instead goes into changing liquid into gas; this is called latent heat.
 
The stable boiling temperature of water is quite useful in cooking, as it helps us to get predictable results. We can also tweak the boiling point using a pressure cooker. At higher pressures, the boiling temperature increases while the latent heat shrinks.
 
Eventually, at exactly 217.25 times atmospheric pressure, the latent heat vanishes altogether. At that point the water passes from liquid to gas continuously; those phases are no longer distinct. Just below the critical pressure, the water boils at a limiting temperature of 373.946 degrees Celsius, or about 705 degrees Fahrenheit.
 
Very interesting behavior occurs near the critical juxtaposition of pressure and temperature, where liquid and gas become indistinguishable. The density of water fluctuates wildly, as if it isn't quite sure how it should organize itself. Carbon dioxide and many other substances behave similarly. Explaining the reasons for that universality in the behavior of many different substances was Kenneth Wilson's big achievement. According to his general theory, the fluctuations have a life of their own, almost independent of the details of the underlying matter.
 
Now, at the Large Hadron Collider near Geneva and the Relativistic Heavy Ion Collider in Brookhaven, N.Y., scientists are pushing universality harder than it's ever been pushed before. In the colliders, nuclear matter is the liquid-like phase, while the gas-like phase is quark-gluon plasma. Researchers collide nuclei of gold atoms at different energies and produce fireballs with a range of temperatures and densities, then map out the phase diagram.
 
In the early 1990s, Krishna Rajagopal and I predicted that there would be a critical point-around 10 trillion degrees-where the distinction between nuclear and quark matter fades away. That point's existence could be confirmed, we proposed, by observing fluctuations in the fireballs that have nearly the critical temperature and pressure.
 
Experimenters at Brookhaven have reported hints of just such behavior. They are now analyzing a much bigger data set to nail it down (or not). If all goes well, they'll have demonstrated that deep ideas invented to understand problems in physical chemistry and thermal engineering continue to be useful far beyond their down-to-earth origins.
 
Frank Wilczek是麻省理工学院Herman Feshbach物理学教授,也是中国上海李政道研究所的创始所长。在生活中,他一直视史蒂芬·温伯格为令人敬佩的朋友,偶尔也是他的竞争对手。
 
本文经授权转载自微信公众号“蔻享学术”。



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