
  1) First of all, let us consider a particle of light, also known as a photon. One of the interesting properties of photons is that they have momentum and yet have no mass. This was established in the 1850s by James Clerk Maxwell. However, if we recall our basic physics, we know that momentum is made up of two components: mass and velocity. How can a photon have momentum and yet not have a mass? Einstein’s great insight was that the energy of a photon must be equivalent to a quantity of mass   and hence could be related to the momentum.
  
  2) Einstein’s thought experiment runs as follows. First, imagine a stationary box floating in deep space. Inside the box, a photon is emitted and travels from the left towards the right. Since the momentum of the system must be conserved, the box must recoils to the left as the photon is emitted. At some later time, the photon collides with the other side of the box, transferring all of its momentum to the box. The total momentum of the system is conserved, so the impact causes the box to stop moving.
  
  3) Unfortunately, there is a problem. Since no external forces are acting on this system, the centre of mass must stay in the same location. However, the box has moved. How can the movement of the box be reconciled with the centre of mass of the system remaining fixed?
  
  4) Einstein resolved this apparent contradiction by proposing that there must be a ‘mass equivalent’ to the energy of the photon. In other words, the energy of the photon must be equivalent to a mass moving from left to right in the box. Furthermore, the mass must be large enough so that the system centre of mass remains stationary.


My questions:
1) I'm not able to grasp the concept of centre of mass in paragraph (3).
2) What's the center of mass of the system of the box and photon?
3) If no external forces are acting on the system, does the location of the center of mass remains the fixed? Then what does it mean by the location of center of mass being fixed if the box has moved?
4) What's the relation between the mass being large enough and the center of mass to remain stationary in paragraph 4?

Ice cubes have three distinct cooling effects:


The cube, initially at sub-zero temperature, absorbs some heat to reach fusion point (0⁰C).
The cube absorbs more heat to switch phase: it takes some energy to turn 1 kg of ice at 0⁰C into 1 kg of liquid water at 0⁰C.
The water absorbs some heat to become warmer than 0⁰C.


The three effects occur more or less successively, although not necessarily simultaneously throughout the ice cube. But the idea remain the same.

For ice, the bulk of the cooling comes from the melting. Let's put some figures on it. Heat capacity of ice is 2.06 kJ·kg-1·K-1, meaning it takes 2.06 kJ to transform 1 kg of ice at -12⁰C into 1 kg of ice at -11⁰C. For liquid water, that's 4.217 kJ·kg-1·K-1. The latent heat, i.e. energy used for turning ice into liquid water (at constant temperature) is 333 kJ·kg-1. Imagine that you have some beverage at room temperature, which you want to lower to 8⁰C with ice cubes. The ice cubes come from the freezer and are initially at -18⁰C. The three cooling effects amount to, per kg of ice:


Raising ice temperature to 0⁰C: 18×2.06 = 37.08 kJ.
Melting the ice: 333 kJ.
Raising the water temperature to 8⁰C: 8×4.217 = 33.736 kJ.


So, in this example, the melting contributes to about 82% of the cooling.

Non-melting stones work only on heat capacity. So they are effective only insofar as a material with high heat capacity is used -- but, in practice, water (and ice) have a quite high heat capacity, higher than stones, so the cooling effect of such stones is necessarily quite reduced compared to ice cubes. On the other hand, since there is no dilution effect, you can put a lot of stones in your glass.

Reusable ice cubes are actually much better at cooling things, because they do melt -- but they do so internally, in a sealed envelope, thus not spilling into your drink. Since they use latent heat of phase transitions, they are as good as true ice cubes. Although they do lack in aesthetics.



But what business have you torturing perfectly fine Whisky with unnatural coolness ?
