1) The original twin paradox is formulated to resolve a seeming contradiction in special relativity, not general relativity. The important point is that special relativity does not violate common sense, without even having to worry about gravity. However,

2) The equivalence principle doesn’t imply that both twins feel absolutely identical forces. If we take gravity into account, the stationary twin feels a constant force of gravity, while the traveling twin necessarily feels a time-varying force of gravity (in one direction on the way out, and in the opposite direction to go back. The resolution of the paradox is still that the two twins have different experiences — namely different forces — and therefore there is no problem with arguing that they can both agree on who has aged more.

That’s an interesting idea that I hadn’t heard before. Personally, I think there are two problems with such an interpretation: 1. It assumes that a medium is absolutely necessary for wave propagation. 2. It helps not at all with the other ‘mediumless’ wave: ordinary matter. All matter has wavelike properties, and those waves are not associated with E/M waves.

I think I’ve got a real-world example to further elucidate these points, but I’ll leave it for a separate post, after I’ve fleshed out the details…

I mean, it’s Electro-Magnetic waves. One induces the other, so the magnetic wave serves as a medium for the electric one and vice versa. Or is that too far-fetched?

As you pointed out, in the current state of knowledge, we don’t necessarily need an aethereal substance to describe physical phenomenae already explained by the theory of relativity. This is absolutely true.

Many different “aether” theories have been proposed. The formulation of “aether” by Paul Dirac is not mechanical; it is statistical. In essence, what he proposed was that vacuum, or aether, must be subject to quantum mechanical laws. Thus “perfect vacuum” as we know it becomes an ideal unattainable state. This formulation of vacuum (aether) is consistent with relativity within his “New theory of electrodynamics” [Proc. Roy. Soc. London, (1951),(1952),(1954)]. Of course, this is just a theory, and any theory must be confirmed by experimental facts. Obviously, when trying to rigorously describe what vacuum is, one may reach the limit of knowledge.

The ‘catch’ in your sentence is in the word, ‘intuitively’. If I were to summarize the greatest contribution that Einstein’s relativity made to the philosophy of science, it was to demolish the role of intuition. Scientists had spent the hundred years before Einstein running around in circles trying to quantify the properties of the aether, and made very little progress. Einstein came along and said, in essence, “Why are you so convinced there is an aether at all?” Most scientists thought about it a bit and realized that it was their everyday intuition that had convinced them the aether existed.

The problem with ‘intuition’ is that it is based on an extremely limited terrestrial worldview. We live in a quiet corner of a galaxy, in a relatively weak gravitational field, moving at relatively slow speeds compared to anything around us. Our eyes can only observe an almost negligibly small range of the electromagnetic spectrum, and we cannot readily observe objects smaller than a tenth of a millimeter. It is an extremely big reach to assume that all phenomena in the universe must be easily reconcilable with everyday experience.

That isn’t to say that intuition isn’t useful: it can be a useful guide at times in understanding phenomena, but should always be backed up with direct experimental evidence. Einstein made researchers of his time realize that their notions of an aether were based purely on intuition. (Ironically, the more one understands Einstein’s theory of relativity, the more intuitive it gets. I can hardly imagine any other theory of relativity now.)

You wrote: “From my understanding, the latter fact is true for water waves, sound, and many more physical phenomenae; except for electromagnetic waves which we assume can travel trough “free space”, or in other words, “vacuum”.”

This is, in fact, why scientists of the early 1800s postulated an aether in the first place! This was what was known as the ‘mechanical theory of light propagation’. Since that time, though, we have another important group of waves which propagate in vacuum: all matter in the universe has wavelike properties. If we assume these wavelike properties are also due to an ‘aether’, we either need a second aether for matter or we need our original aether to support both light and matter waves. In either case, we end up making an even more complicated aether model and don’t gain any scientific insight.

Who knows? It may turn out, in some distant future research, that we find that light propagation is based in some sort of mysterious substance. The problem right now is that any aether model would involve aetherial matter which behaves completely unlike any other matter in existence. Once your ‘mechanical theory of light propagation’ starts to look completely unlike any other form of mechanical wave propagation, the reasonable conclusion is that nothing is gained by holding that mechanical point of view.

Thanks for the comment! Yes, I seem to have finally figured out how to make ResearchBlogging work with WordPress…

In case you were wondering, ResearchBlogging did pick up your post, for I found it via their RSS feed.