Yet more dumbass questions about c

bonkey · 22-11-2002 04:54PM #1

c being the speed of light, as opposed to the programming lanugage, the letter, the Hex value, or anything else....

I'm asking these because some kind people suggested to feel free about asking more questions the last time they offered very helpful explanations for my quandries...

Right....here we go...

1) How can c be constant, when time and space are not? In other words...if time and distance are not constants, how do they manage to combine to produce a constant? Does this infer that anything which effects time will also effect distance to an equal, but inverse, degree?

As an example, it has been demonstrated that atomic clocks placed at high-altitude measure time at a different rate to clocks at sea-level. This is caused by gravitational effect. Yet both must measure c equally (if it is to be a constant). Surely, therefore, distance (being speed times time) must also be affected - that a measurement of distance taken at high-altitude would also differ from a measurement taken at sea-level (allowing for the difference in altitude, naturally.

2) Is the speed of light travelling away from an object also a constant? It has been determined that if two bodies are moving relative to each other and one (the source) sends a beam of light at the other (the target), it will strike the target at the same speed as if it had been fired from a body at rest relative to the target - even if the bodies are accelerating relative to one another. This shows that light travelling towards a target is always measured as being a constant - because it is measured at the target.

Does this also imply that the light was travelling away from the source at a constant velocity (regardless of acceleration of the source object etc.).

Cheers.

jc

ozpass · 22-11-2002 11:38PM

I can't answer all of your questions because I'm not clever enough. In fact, i'm probably not clever enough to try answering any of them but i'll give it a go!

With regard to point (1) How can the speed of light be considered a constant:

This is basically a semantic argument. You are right in stating that both 'time' the concept and 'distance' the concept are variable. To make them constant we create a 'definition' of space that is constant and a 'definition' of time that is constant. To this end SI say that a metre is absolutely this much 2 dimensional space and a second is absolutely this much time. It isn't an absolute statement of universal constancy, it's just a frame of reference from which calculations can be performed. Hence:

The speed of light is constant only in that it is constant by definition. Far out maaaan.

This doesn't really matter because any real world scenario with a calculated outcome will vary from the calculated outcome by an amount bound by the laws of physics. As long as you know what laws are applicable to the real world scenario in question you can adjust your calculations based upon 'em. Phew! Brain's straining a bit now. Of course when you're discussing said real-world scenario in quantum terms, Heisenberg's Uncertainty principle fouls things up because you're changing the outcome of your measurements (real not perceived) simply by measuring them. Or something.

I don't quite understand your atomic clock example, I'm afraid, so I can't comment on it.

(2) I think must have it's answer buried within good ol' Einstein's Special Theory of Relativity cos it states:

"The speed of light is independent of the observer"- I'm going to wimp out on you here cos I'm a bit too drunk to fathom this one right now. Do a google on Lorenz, Michelson, Morley and (if you're a real brute for the maths of 4 dimensional space) Minkowski.

/me toddles off to get another can of Stella and relax me brain.....;)

DadaKopf · 24-11-2002 11:09AM

I don't know much about anything to do with physics but I remember coming across something like: the speed of light is not constant. Light loses momentum like tomatoes thrown at the village idiot.

Light isn't constant because of entropy or something.

OSiriS · 24-11-2002 12:40PM

Actually c and the speed of light are 2 different quantities. C is the standardised idealised speed of light through a vacuum. As light doesn't always travel through a vacuum it will not necessarily travel at c.

Nietzschean · 28-11-2002 12:55PM

Originally posted by OSiriS
Actually c and the speed of light are 2 different quantities. C is the standardised idealised speed of light through a vacuum. As light doesn't always travel through a vacuum it will not necessarily travel at c.

to the first part this is what we are told in college...
the speed of light is a constant and all our other values are based around it(i.e. if they can more accurately work out the speed in theory all our other values change and c remains as it is).

to the second, i know measured values for the speed of light vary with the medium but my hypothisis(no one has told me this so well disregard it if you like...)
The speed of light through a medium other than a vacuum is still c, it is the interactions with other objects that cause this aparant change, my theorywould be the speed is really unchanged but the photon's interact with there sorroundings and whatnot causing the change in measured value for c. i.e if they interact with an atom i.e. have to defract around it or that then their route will be longer.

Woden · 28-11-2002 02:44PM

when c is stated it is regarded as the speed of light in vacuo, for arguements of c(t) check out the net, c must be the speed of light in vacuo as this is the fastest speed obtainable check out relativity, the speed of light in any other medium is slower and it is possible for something to move faster than the speed of light in that medium check out also chernenkov radiation.

Bob the Unlucky Octopus · 06-12-2002 05:22AM

The speed of c being defined as constant and unexceedable in a vacuum is a fundamental truth of the universe. In other words, it is one of those scientific Laws which is difficult to justify beyond the observable fact that nothing in our knowledge can exceed it. Given that energy can neither be created nor destroyed, nothing can exceed the speed of light. c is constant in a vacuum because nothing is impeding it- all particles or wave-forms that are in motion will stay in motion until impeded by a force ( or in the case of light, a medium which can refract, reflect or diffract it).

On (2), it depends on the point of view of an observer in free space. If of course you are standing at the light source or at the receiving end you are not an passive observer and the laws of relativity do not function on a similar basis. Just to use the twins paradox as an example- I once asked my physics professor how it worked- after all, the occupants of Earth are moving *away* from the spacecraft at the speed of light, so why shouldn't they age with the same speed as the astronaut? The answer is- *according to the astronaut*- they do. But to an observer in space looking at 2 clocks in respective locations they record time differently. As such, light traveling away from a moving object still travels at c. A beam of light from a supersonic aircraft reflecting off a stationary surface still moves at c. This accounts for the relativity calculations required when triangulating radio transmission on a moving point in three-dimensional space. These calculations are perfectly accurate with c as a constant- therefore it must be time and space that distort.

Hope that clears things up at least partially

Nietzschean · 11-12-2002 11:40PM

yeah occy i asked the same question, the answer currently accepeted is aparently the twin that leaves earth and travels at the speed of light for years and returns doesn't actually hold fully to einsteins postulates in the full form, i.e. he doesn't spend all his time in an inertial frame, he accelerates to his high speed then deaccelerates and so on, those on earth spend all their time in this effectively intertial frame that is why the twin's issue holds.its been proven mathamatically as such so they said....

as for the speed of light in other mediums, i'm still holding to my other theory and it seems to agree with what occy said, must ask what the full explanation is when i get back to college, and was reading there in new scientist that ion's and other charged particles travel faster than light under water, feature they are using in the neutrino detectors(which sound so kewl, telescope at the bottom of the medetarian, and another one under the polar icecap, one in a mine in japan....

)

Yet more dumbass questions about c

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