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Exceptions To The Rule: The Speed Of Light (And Gravity?) - by John Prytz

Within Relativity Theory, if there is anything that is not intuitiveit is the fact that in the entire Universe, it is the speed of light (andgravity?) alone that is absolute or fixed, not something like space beingabsolute or time being absolute. It’s not intuitive in that all the other not-lightbits and pieces that are in motion can be added or subtracted. The lone exceptionto that universal rule that velocities can be mathematically combined is thespeed of light (and gravity?).


Repeat for emphasis: the anomaly here is that in any otherscenario, anything that is not-light and in motion, velocities can be added andsubtracted. Repeat again: with that one speed of light exception, velocitiesmay be added or subtracted. If you are on a treadmill that’s moving left at 5MPH, and you’re on it walking to the right at 5 MPH, to an external observeryou are waking yet standing still. You’re much more likely to hit a home run ifthe wind is blowing towards the outfield fences; planes fly faster with atailwind than with a headwind; you swim in the river faster going with the flowthan against the current. If you are in a train that is moving at say 100km/hour and you throw a ball at 10 km/hour in the direction at which the trainis moving, to an observer outside the train, your ball is traveling at 110km/hour. If you throw the ball towards the rear of the train, an outsideobserver will measure the ball as moving at 90 km/hour. If on the other hand,you shine a flashlight in the train, an outside observer will see the velocityof the resulting light beam moving at the speed of light – not the speed oflight PLUS the velocity of the train, or the speed of light MINUS the velocityof the train, but at the speed of light! That’s nuts, but it’s scientificallynuts and been proven again and again in any experiment you care to devise. Ihave no issue with experimentally verified results. That doesn’t mean I don’thave issues with the anomaly.


The velocity of light is a constant to an external observerno matter what. Why that should be no one knows, but it is so. However, my takeon this can of worms, which as a consequence requires somewhatcounter-intuitively that both time and length have to be flexible, is oneshould always be a bit suspect when it comes to the lone ranger, the loneranger being the exception to the rule*. I don’t tend to like exceptions to therule. There’s something weird afoot here. Mother Nature is trying to tell ussomething we haven’t figured out yet.


All that said, the speed of light isn’t really a constant ifyou take into consideration the differing mediums that light can travelthrough. However, the speed of light is probably also a constant withinwhatever other medium it happens to be in. The speed of light in air at STP(standard temperature and pressure) is less than the speed of light in therelative vacuum of outer space; even less traveling through pure water and lessagain traveling through clear glass. One would presume that just as the speedof light in space is constant for observers regardless if they are traveling ina high velocity spaceship or as an astronaut ‘stationary’ on the InternationalSpace Station, the speed of light in water should be a constant for the crew ina submarine that’s underway thus in motion or a diver standing on theseabed.  


That makes me sort of wonder, theoretically, if you couldget an underwater submarine to go faster than the speed of light in water (oran aircraft to outpace the speed of light in air), though in either case thatwould be less than the speed of light in a vacuum, could you then claim, albeitwith qualifiers (in water; in air), that you actually traveled faster than thespeed of light, if only for the bragging rights? 


Apart from light (photons), presumably gravity, orgravity-waves, also propagate at the speed of light and thus a gravity-wavewould have a constant speed regardless of any frame of reference for anyobserver. But I also presume that necessitates finding the hypotheticalgraviton particle (to mirror the photon) and that, for the moment, resides inthe unknown basket.


That brings up yet another puzzlement. Is gravity justgeometry as per General Relativity or is gravity a force transmitted by a forceparticle, the theoretical graviton, and thus more akin to electromagnetism’sphotons (infinite extension; obeys the inverse square relationship) apart fromlacking EM’s negative counterpoint – in the case of gravity, that’s antigravity.


Speaking of gravity (gravitons) and light (photons), if gravitycan have an effect on light (i.e. – the gravitational lens), then it should bea two-way street and light should be able to have an observable affect ongravity, but I can’t recall ever reading about that aspect of the relationship. 


As per the speed of light being medium dependent; one speedin a vacuum, another in water, well that makes for an interesting question whenconsidering gravity. If a gravity-wave is approaching Earth, will it have oneconstant velocity in the vacuum of space for all observers, then a differingvelocity as it passes through our atmosphere, then another value as it passesthrough the ocean and yet others as it hits the various density layers insidethe planet?    


As is often the case, there tends to be more questions thananswers!


*Not that the differing rules for the speed of lightvis-à-vis other not-light velocities is the only exception to the rule inmodern physics. Quantum tunneling is one of those exceptions to the rule ofcausality. With respect to the standard model of cosmology and the Big Bang,first there was nothing; then there was something. That means the Big Bangevent created both matter and energy out of less than thin air. That’s also afree lunch and one of those exceptions to the rule of physical law usuallyexpressed as the conservation of matter and energy. All these exceptions to thenorm suggest that some of the final chapters in modern physics haven’t yet beenwritten.

Science librarian; retired.

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