Artificial Gravity in Space?

Artificial gravity has been portrayed in many sci-fi movies and novels by the use of giant rotating cylinder-shaped spaceships to generate acceleration. According to Einstein, acceleration is equivalent to gravity. However, I have been wondering what happens when a person jumps in a rotating spaceship. Thus, I decided to do a little thought experiment.

Assume we have a giant cylinder-shaped spaceship that rotates at a constant angular velocity w. A person standing inside the spaceship will experience centripetal acceleration directed towards the axis of rotation (or the centre of the spaceship in that particular cross-sectional frame) and the resulting normal force acting on him/her will cause a sensation of gravity. However, if the person jumps in the direction towards the centre, he/she will lift off the surface with 2 velocities components- the tangential velocity and initial velocity towards the center. The resultant velocity can be calculated and he/she will 'fly' in a straight line towards the inner surface of the spaceship at an angle as seen in the video and end up landing at a different spot from where he/she lifted off. This is because once the person is off the surface, there is no longer any resultant force acting on him/her (neglect air resistance), unlike on Earth where gravity continues to act even when you are airborne.

(PS: Does anyone know how to upload flash animation files onto blogger? I had to convert the flash file to .avi format which is much larger in file size)

Applying the same reasoning, objects 'falling' in a rotating spaceship will not 'fall' in a straight vertical line but will end up hitting the ground at some horizontal distance away from the point at which is starts 'falling'. This apparent curved motion of objects is called Coriolis effect. In this respect, artificial gravity is different from real gravity and may lead to many problems living in space.

Prolonged exposure to weightlessness can cause health problems in astronauts such as loss of bone mass, muscle atrophy, dehydration, anemia and weakened immunity, among others. Such adverse effects can be countered by rigorous exercise or by inducing gravity. Currently, a significant amount of time on exercising by astronauts while on space missions. This is not very economical as it results in less time for conducting experiments. In the future when people live in space colonies, we cannot expect everyone to spend so much time exercising. Thus it is of utmost importance to find a way to create artificial gravity that is as close to the real thing as possible. Currently, the only way to create real gravity is to use mass. In order to create 1g, we will need the mass of the Earth, so clearly that is not a viable option for space missions. So for now, we will just have to stick to the idea of large rotating spaceships and put up with large Coriolis forces.

Happy Vesak Day!

Wish everyone a very happy Vesak Day! While we celebrate the birth and enlightenment of Buddha here in Singapore, we should not forget about others who are suffering. Let us pray for the victims of the two recent major natural disasters- the Sichuan earthquake and cyclone Nargis. Amidst our relatively sheltered life in Singapore, which is free from natural disasters, it is easy to forget that there are so many people in other parts of the world whose lives have been wrecked by such calamities and their daily existence have become a ceaseless struggle. It is heartening to know that countries and people all over the world have provided humanitarian aid and monetary donations to the afflicted countries. While we may not be able to help out with the relief efforts, we can donate and pray for the victims. Let them know that the world is still full of compassion.

Do electrons really spin and do quarks really have colour?

Do electrons really spin and are quarks really coloured? Well, the answers to both are no. Why then do scientists come up with such terms? To deliberately obfuscate people? I do not think so. The limitations of human languages in describing the quantum world have made it difficult for us to assign appropriate words to describe properties of subatomic particles. As such, analogies such as spin and colour are used to create a more intuitive understanding of reality. What do scientists mean by the spin of electrons and colours of quarks then?

Before I explain electron spin, imagine a classical spinning top. If you look at the spinning top from the top or bottom, you will see that it is either spinning clockwise or anti-clockwise. If you view it from the side, you will not observe any spin. (Note: Spin is defined to be the rotation of an object about an axis which passes through it.) In order to understand the spin of an electron, we have to move away from the classical notion of spin. The electron is actually not a point-like particle but rather, it is a probability cloud. The spin of an electron is just a name used to describe one of its properties. This electron spin is a result of its intrinsic angular momentum. If you view the electron about any axis in 3-dimensional space, you will always observe either a clockwise or anti-clockwise spin, and the value of the spin is defined to be 1/2. It is difficult to imagine how this can happen, for the quantum world is totally different and exists on different scales from the macroscopic world on which we live in. It is only natural then that we will have trouble visualizing the world of the subatomic, just as a bacterium would have much difficulty in comprehending the human world.

Now let's move on to quarks. Quarks are the constituent particles of hadrons (a class of subatomic particles which interact via the strong force) and come in six different flavours called, 'up', 'down', 'top', 'bottom', 'strange' and 'charm'. These are not names of some newfangled ice-cream flavours but are merely names of different types of quarks. Apart from flavours, quarks also posses another property called colour and come in red, blue and green. A hadron is colour neutral as it consists of three quarks from each of the colours, just like mixing the three primary colours of light produces white light. In a meson, there are two quarks of similar colour but each being the matter and anti-matter counterpart of each other.

Quarks themselves are not coloured in the classical sense in which we see macroscopic objects in different colours. We can see and perceive colour from macroscopic objects because incident light or photons on the objects are absorbed by electrons in the atoms and then re-emitted or transmitted to our eyes. In the simplified model of an atom, bound electrons 'orbit' the nuclei in shells of fixed distances from the nuclei. (Note that electrons are actually probability clouds which flow around the nuclei of atoms and this flow is called a probability current.) Whenever a photon with the right amount of energy (corresponding to the gap between two energy levels of an atom) is incident on a bound electron, it will be absorbed by the electron and cause the latter to become ‘excited’ and jump to a higher orbit. The excited electron subsequently re-emits a photon as it falls back to a lower state. (Note that the excited electron may return to the ground in a single or multi-step process.) Since different atoms and molecules, and thus different objects, have different gaps in energy levels, they reflect different frequencies of visible light, resulting in our perception of colour. Anything smaller than an atom will not possess colour in the classical sense. Hence, it makes no sense to ask how quarks look like.

In trying to understand and describe the quantum world, we are inevitably bogged down by our limited sensory perceptions, preconceived notions of the world and semantic limitations. Nevertheless, we have come very far in building a scientific model of the world since the dawn of civilization. Will we ever be able to fully comprehend the world? Some say that it is impossible for mere human minds which are constrained by the limitations imposed by our physical senses to comprehend the universe. I personally think that Science alone will not be able to fully explain every aspect of reality. However, many people have been wrong countless times in predicting the limits of Science. Thus, only time will tell.