Question: How does decoherence explain that atoms can be in the same place at the the same time but not humans, and even though time travel isn\'t banned by the laws of physics, do you think it could realistically happen?
Pete Edwards answered on 21 Jun 2010:
The world of the very small is completely different to the world we live in. The small stuff works to a set of rules called quantum mechanics which tells us that objects can pass through walls and be in many places at once. As you collect atoms together to form larger objects like people cars and planets etc. the rules change and these effects go away.
The idea of time travel is an interesting one. I’ve not made my mind up on this but Steven Hawking has come out recently in favour of it I understand see http://rightways.wordpress.com/2010/05/06/stephen-hawking-predicts-possibility-of-time-travel/
Stephen Curry answered on 21 Jun 2010:
You have taken me down an long road in theoretical physics where I no longer really recognise where I am!
Time travel, particularly travel backwards in time, creates all sorts of paradoxical problems including the well known, ‘what if I went back and killed my grandfather’ conundrum.
it’s one thing to do it with a particle but perhaps quite another to try it with a whole human.
Sorry I can’t be more helpful on this one!
Marieke Navin answered on 22 Jun 2010:
Hi Isabelle. Hmmm, there are a few really deep issues to cover here! Decoherence in atoms is a quantum mechanical process, quantum mechanics being the physics that describes the behaviour of systems on very small scales of length and time – the sub-atomic scale if you like. This is compared to classical mechanics which describes large-scale systems, such as cars on a racing track, and the motion of the planets around the sun for example.
Quantum mechanics predicts that electrons can appear to be in two places at once, however the act of measuring where such an electron is destroys this property and causes the electron to appear in a definite location. The ‘Schroedinger’s Cat’ thought experiment is designed around this principle, you should look it up 🙂
A human being, is a large-scale system of atoms, that is not readily described by quantum mechanics and as such does not display quantum mechanical properties.
Time travel is another issue altogether, and believe it or not has been observed! An atomic clock on a plane travelling at high-altitude will record the passage of time more slowly than an atomic clock at sea level, so, according to the people on the aeroplane, the people at sea-level appear to be in the future! This is a consequence of Einstein’s theory of General Relativity.
Tom Hartley answered on 23 Jun 2010:
Before I answer this – I am not a physicist and I haven’t looked it up – so this is my guess based on my interest in these topics and reading I do in my spare time.
1. My understanding is that quantum mechanics says that particles such as photons and atoms can sometimes be in more than one place at the same time or, more accurately, they can act like waves. The same principles would also apply to big things like people and footballs, but we wouldn’t be able to observe the wave-like behaviour, because for big things the interaction of the different waves is impossible to detect. Another complicating factor is the effect of observation – if by being in a particular place a particle creates a record or is observed, it stops behaving like a wave. It is not clear, to me at least, how this would work with e.g., humans who are effectively recording their experiences all the time. A good book to read on the topic is “In Search of Schroedinger’s Cat” by John Gribben – it deals with all these questions, although I am not sure it comes to a clear conclusion.
2. I don’t know much about time travel, but I hope that it is possible because it would make for some fascinating science (even if it just meant we could find out in more detail about the past).