Water is the source of life, and the most important liquid on earth. Yet despite years of research since ancient time, it is embarrassing to say that we still know very little about water. In fact, the more we look, the more mysteries we find. Difficulties often stem from our poor understanding of water from its structure at the molecular level. In this talk, instead of describing specific research accomplishments, we shall discuss a selective set of mysteries and unsolved problems on water and ice. Some of them are extremely important because our future life depends on whether they could be solved or not. We shall emphasize on what we need to know from research to solve such problems, and theoretical and experimental techniques we need to develop for such research. The field definitely needs more concerned scientists interested in contributing to the benefit of the mankind.

水是生命之源，也是地球上最重要的液體。儘管自古以來對水的研究已歷時長久，但令人困窘的是我們對水仍然所知甚淺。事實上，我們對水愈深入去看，愈覺深奧難解。難題根源於我們對水分子層級的結構不甚了解。 在本演講中，我們將討論一組選出的有關水和冰的深奧和未解的問題，而非描述特定的研究成果。其中的一些問題極為重要，因為這些問題的解決與否，攸關我們的未來生活。我們將著重在我們需要從研究中知道什麼，可用來解決這些問題，以及我們進行這類研究所需要的理論和實驗技巧。這個研究領域定然需要更多科學家的關注，合力為人類的福祉做出貢獻。

The invention of lasers more than 50 years ago has dramatically changed our modern life. Lasers have become indispensable in essentially all areas of science and technology. It is even more remarkable that after so many years of advances, the field of lasers or optical science is still experiencing exciting new development. Fascinating discoveries of new phenomena, effects, and applications have continuously attracted tremendous interest of the scientific world. Here, we shall discuss a few selected topics of interest based on simple physics. We start with a discussion of photons, which are usually defined as the quantized units of light energy. This definition is however not rather vague, and it is important that we know it better in order to understand physics at low light intensities. A few examples will be presented. Interactions of single photons with single atoms or ions have provided a testing ground for some most fundamental quantum physics rules. (Nobel Prize, 2012) Atoms can be cooled by absorption and emission of photons. (Nobel Prize, 1997) Laser cooling of atoms has led to the creation of new phases of matter. (Nobel Prize, 2001) Lasers can have extraordinarily pure frequency, high intensity, and short pulsewidth, The spectrum of light is directly correlated with the time variation of the light (by Fourier transform). Lasers can generate a well-defined periodic series of pulses with a corresponding set of utmost sharp spectral lines in the frequency space, known as a frequency comb. Such spectral lines allow precision spectroscopic studies of the highest resolution. (Nobel Prize, 2005) Intense femtosecond pulses can be used to generate attosecond pulses. They can also be used to generate output through harmonic generation extending into the X-ray regime. The strongly amplified laser pulses can have such a high field that it can accelerate electrons to relativistic velocity in half a cycle. High-field and high-energy laser interaction with matter is a new exciting field. The newly available X-ray lasers have provided a brand new tool for studies of materials. Obviously, after more than 50 years, laser science is still going strong.

五十多年前發明的雷射已經戲劇化地改變我們的現代生活。在科學和技術的所有領域中，雷射已經變成不可或缺。更令人注目的是，在這麼多年的發展後，雷射或光學依然持續地有令人興奮的新進展。新現象、效應、和應用的炫目發現仍然吸引科學界的極大興趣。在本演講中，我們將討論一些奠基於簡單物理的有趣主題。 我們先從光子的討論開始。光子通常被定義為光能量的量子化單位。無論如何，這個定義不能說是相當模糊，為了理解弱光下的物理，對光子的較佳詮釋是相當重要的。我們將提供一些例子來說明。單一光子和單一原子或離子之間的交互作用已經為一些最基本的量子物理規律，提供了測試場地（2012年諾貝爾物理獎）。原子可以藉由吸收和放射光子而冷卻（1997年諾貝爾物理獎）。以雷射冷卻原子可以導致產生物質的新相態（2001年諾貝爾物理獎）。 雷射可以有極單純的頻率、高強度、和短脈寬。光的頻譜直接關聯於光強的時間變化（利用傅立葉轉換）。雷射可以產生一連串非常整齊的脈衝，在頻譜上對應有極為細窄的光譜線，稱為頻梳。這種光譜線可作為最高解析率的精密光譜研究（2005年諾貝爾物理獎）。強光的飛秒脈衝可用於產生更快的阿秒脈衝。它們也可利用諧振生成的方式，將輸出的頻率延伸進入X光的範疇。強烈放大的雷射脈衝產生的強電磁場，可將電子在半週期內加速到相對論性的速率。強電磁場和高能量的雷射和物質之間的交互作用是一個新的熱門研究領域。新近製出的X光雷射已經成為研究物質的嶄新工具。顯然，在五十多年過後，雷射科學依然是蓬勃地在發展中。