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A former member
Post #: 8
I know this isn't the place for philosophy, but I thought this might be of interest.
Two atoms were out for a run at the speed of light; one says to the other
Well that’s just it; they wouldn’t be able to say anything. Atoms communicate by exchanging messenger particles, photons; it is this exchange which is the basis of chemistry, biology, even consciousness. Photons are, among other things, particles of light; as such they travel at the speed of light. You might think of the speed of light, c, in absolute terms, as an inviolable law of nature; or you can just accept it as the speed at which photons happen to travel. Either way, if the two atoms are travelling at the speed of light, it is all a photon can do to keep up; there will be no exchange of photons, there will be no chemical, biological or conscious processes. It would be as though time stood still for any such pair of atoms, the same goes for collections of 6 or 7 billion, billion, billion atoms such as you or me.
Of course, Einstein’s theory of Special relativity demonstrates that atoms can’t travel at the speed of light, but as they accelerate towards it, the paths that messenger photons have to take become longer. Suppose the two atoms were one above the other. If they are at rest a photon can take the shortest route between the two, straight up or straight down. For a similar pair of atoms moving left to right, were a photon to travel vertically, it would miss, because it’s target has moved on. If it is to reach the target atom, a photon has to aim ahead and set off at an angle; it’s speed remains c, but now it has a horizontal as well as a vertical component. The greater the speed of the target, the further ahead the photon must aim and the greater the angle. This increases the horizontal component of the photon’s speed, at a cost to it’s vertical component. When such a system is compared to a stationary one, there are fewer exchanges; less chemistry, less biology, less consciousness.
What is not clear is whether there is any less time. Certainly a body in motion will age less compared to one that is stationary, but does this mean there is a discrete substance that the moving atoms have had less of, or is it simply that less has happened to them? Of all people, it was Plato who first argued that time is no more than the passage of events and although he is one of the ugliest blights on western civilization, on this occasion he may have a point.
Einstein though said that time is not discrete, that it is bound to space creating space-time, the warping of which, it is said, gives rise to gravity. General relativity was given support by the bending of starlight during a total eclipse. The position of stars was a matter of record when viewed at night time from one side of the sun, the eclipse allowed their position to be charted from the other side and the two were found to be different. The photons that made up the starlight experienced a slight deflection towards the sun; in other words, their speed adopted a component that was aimed at the sun. In general, anything that travels through a gravitational field experiences a deflection towards the source. Our two atoms for instance; though as atoms they might be stationary, their components, the electrons and quarks that make them up are not.
Particles travelling through gravitational fields behave as though they were being refracted by a denser medium. Gravity is supposed to be universal; all objects are attracted to all others. Since the universe is expanding it follows that the gravitational field of every object is also expanding and has been doing so for the last 14 billion years, with no apparent effect on the strength of gravity. This gives the impression of a particle as an inexhaustible source of gravitational field that rarefies as it spreads. But to call it gravitational is misleading, because clearly it is a repulsive force which in condensed states causes localised attractive anomalies; better to call it stuff. It is the repulsive nature of stuff that provides the thrust of photons and the energy of atoms and they, like all particles, are just knots and eddies.
A former member
Post #: 27
That is a great contribution. I love the philosophy there, the thought experimentation just like Einstein. Well nearly! I may well post a response but not now.I will leave it to others as I am preparing for a big trip abroad. I hope to keep in touch with this one!

Also I would have been interested in the neuroscience meeting but I have to miss it.
A former member
Post #: 28
Bother, I looked at that posting again and can't resist saying something about special relativity. Following your line of thought, Will, Einstein probably asked himself whether time really does slow down for the traveller relative to the stationary person. On the contrary, might it not seem to the traveller that he was fully conscious, no change in rate of body function, and that it was the stationary person who seemed to be receding at almost light speed with his time slowed down?

And so it is: each of them thinks the other has slowed down their pulse and experience of time. The situation is symmetric, with neither person having a special position of stationarity within the cosmos. That is relativity and the reason for the name. It seems a bit counter- intuitive of course, but it is experimentally observed fact. The latest edition of Scientific American has an article about the latest clock technology which can see this sort of physical effect within a laboratory.

The implication for photons is that the way they get around is far more bizarre than your light ray model, Will. That takes us into quantum physics. A quicksand for philosophy so I will stop right there!
A former member
Post #: 9
I understand that supporting evidence for relativity includes atomic clocks aboard airliners. When compared with similar clocks on the surface, they were found to be slow. If this is so, had they chaperones it may be true that ‘each of them thinks the other has slowed down their pulse and experience of time,’ but if they check their clocks they will have to agree that only one of them is right.
A former member
Post #: 29
Agreed, but you put the clock on an airplane, I didn't. The plane introduces extra features. It has to make a turn to get back to base. At any rate it has to make a curved line of flight to avoid heading into space. Also the flying clock goes higher and experiences slightly weaker gravity. Both these effects complicate the physics and make it asymmetrical. Your two atoms do not experience these asymmetries and so, for them, the principle of special relativity that I mentioned is applicable.
A former member
Post #: 10
If this experiment took place, I can assure you that I didn’t put the clock on the plane. In order to get back to base an aeroplane has only to circumnavigate the world, which, because in our inertial frame we can discount solar and galactic orbits, the expansion of space-time and so on, is flying in a circle. For most of the journey it will be at level flight, equidistant from the strongest local gravitational force. As I understand it, general relativity equates doing so with travelling in a straight line through Euclidean/Newtonian space. I am more confident that time dilation decreases with distance from a gravitational source and that any two atoms are not unique in the asymmetries they experience.
A former member
Post #: 11
Andrew, have a safe and enjoyable trip.
A former member
Post #: 30
Will, you like to provoke a debate and distract me from my packing. Good!

Maybe you misunderstand me. You started with one thought experiment, the two atoms, and I said what Einstein said about that situation - the situation is symmetrical between the two of them. You then switched to a different thought experiment, two clocks with one on the plane and one on the ground, and pointed out that the symmetry did not seem to apply. I agreed with your conclusion but pointed out that the underlying symmetry between the two clocks is confounded by two asymmetrical effects that you perhaps inadvertently introduced with the plane.

That takes us to your latest comment which refers to inertial or 'weightless' motion in the theory of general relativity. That is to do with geodesic paths in curved spacetime. But the plane uses its engines to alter its speed, height and direction and therefore does not follow a geodesic path in spacetime. So that's where the argument and your last two sentences go wrong.
A former member
Post #: 12
Perhaps I do misunderstand you, but it is my understanding that the experiment with the clocks on the aeroplane and on the ground actually happened. I take your point about engines, height and direction; how do you think clocks on Earth would differ from ones on MIR or the moon for that matter?
A former member
Post #: 31
You don't need to ask my opinion about satellite time; the Americans faced the question and answered it when they put up their GPS system. You probably know that when they worked out the equations they found that they had to allow for both the time dilation of special relativity and the gravity effect of general relativity. Good job: if they had neglected either effect then all our SatNavs would go horribly wrong. The answers are all on the web and I have seen them. Try Googling relativity GPS or something.

By the way if Einstein had been asked to advise on GPS we can be quite sure that he would have come up with the correct answer back in 1915 when he had the general principles all worked out. GPS is a great experimental verification of his thinking. That man was so far ahead of his time!

Thanks for your travel wishes, Will, though to be honest with you I'm not sure whether on balance I will have aged more or less than you when I return. I think it could go either way!
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