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An important question, but a conclusion that does not follow. To explore this, we're going to need to talk a bit about the philosophy of science - that is, the purpose and methodology thereof. Please bear with me; this could take a bit.

In a realm of pure logic, one in which the truths of all premises were known beforehand, science would not exist. In such a world, reaching a conclusion would be as easy as logically assembling the premises. Mathematics essentially dwells in such a realm, because we create it in such a way; it certainly holds strong ties to nature, but it exists in a realm removed. However, for most things in our universe and our lives, we do not have such confidence. Many things remain unknown, many premises undiscovered or unverified. In such a world as ours, logic alone is insufficient to understand reality - it is a powerful tool, but we need something to ground it to reality itself.

That is where empiricism comes in. Empiricism is, essentially the idea that we can test. Aside from a few axiomatic principles we all hold - things like “there exists a universe” - it makes one crucial assumption: the way the universe works is consistent, and because of that, we can test it and draw conclusions from our tests and observations.

Science makes use of empiricism, turning it from a philosophy about the origin of knowledge into a methodology for creating predictive models of the world around us. The major methodology for this is rather simple: falsifying hypotheses. We make an observation, form a testable hypothesis about it, and preform an experiment to attempt to prove our hypothesis incorrect; if it is proven so, it is discarded or updated, and further testing is preformed. If it is not disproved, we try different lines, further attempting to disprove it. If, after quite a bit of testing, it is not disproved - and as it gathers supporting evidence - it may be accepted as true.

When we test a single aspect enough that we are certain it works the same way each time under specific conditions, we call it a law. It is important to note that a hypothesis does not become a law, but rather a law is based on the results of repeated and varied hypothesis testing. An example of a law would be the law of gravitational attraction on earth, which states that objects accelerate towards the earth at a rate of ~9.8 m/s^2, modified slightly for altitude.

While we're talking terms, its important to note the most misused term, and yet the most important one: theory. While a lot of us use the word theory in our daily lives to mean a guess or a suggestion, that's much closer to a hypothesis (if its testable). A scientific theory has a rather specific, and different definition: a scientific theory is a model that explains a number of laws and which is capable of making falsifiable predictions; it must be supported by the evidence at hand and not contradicted. A theory is, by definition, the highest class of scientific knowledge - the theory of relativity, for example, explains numerous physical laws dealing with motion, matter, and time, including the one mentioned above, and it makes predictions - one of which is used to allow GPS to work.

Now, I'm sure you're wondering, “Where does consensus” come into all this? A good question, and rest assured, we're getting there. But first, there's one important thing to realize: science is not the art of finding the truth, but the art of becoming ever less wrong.

Let me explain this using an example: the shape of the earth.

At first, for a time, some peoples of earth believed the earth to be flat. This was actually much less common than often imagined - the earth was known to be round since the Greeks or earlier. But at any rate, some peoples considered it flat for a time; this was a working model they generated based on the evidence they had available to them - typically on long rolling plains. Now, is this idea wrong? Yes of course! However, it remains useful in the context in which it was derived; treating the earth as flat works on the small scale, and today architects do it all the time.

Now, quite a long time ago, the earth was found to be round - originally by observations made of shadows in wells which spoke of different angles by which the sun shone onto different latitudes. This new model - the round earth - allows for more predictive power; in addition to the above, it lets us chart travel routes in ways that would not be sensible if the earth were flat, but which save distance due to it being round. But this model is wrong too.

Still later, we have discovered that the earth bulges out at the middle - it is an oblate spheroid. The bulging is a matter of it's rotation as it so happens, but what this means is the earth is thicker around the middle than a perfect sphere would be. Using this new model, more detailed predictions still can be made, and provide use in air and sea travel, as well as minute measurement. But still, this is wrong - or not quite right.

Most recently, we have found that the southern hemisphere of the earth actually bulges collectively - if slightly - more than the northern hemisphere, meaning the earth is an oblate spheroid with a slight pear-shaped nature. Now, you'll note that this change is rather miniscule compared to some of the others, but it still becomes important for the most careful measurement, such as detailed GPS tracking.

Remember, each of these models was based on the evidence they had available to them, and each is still useful in its own context. None the less, as new evidence emerges, our models are revised and improved upon, and in that regard we become ever less wrong.

Now, to finally answer your question, why do we have a scientific consensus? Well, it's really just a fancy way of saying “essentially all of us are pretty sure, based on our present evidence that *this* is the way it is”. The scientific consensus is our *present* working model, the best one we have. This comes with the natural precaution: we realize we may be wrong. The reason we call it a consensus is because while we can be pretty damn sure about most things, we're not done. There is yet more to learn and discover in essentially every field, and we must never fall prey to the mistaken belief that we know all there is to know.

Science, of course, doesn't *require* a consensus but rather it *acquires* one. It's not something that we take to a vote; science is not democratic. Instead, the consensus is the group of us as a whole saying “Yes, we're very sure about *this*, given our present data”.

By the same token, you mention the speed of light. As it so happens, the speed you mention is also what the scientific consensus holds it to be - not because, once again, we needed to vote on it, but because that's what we're pretty sure it is. However, there are those who have proposed models of the universe in which the speed of light may change with time; you can view them here. http://en.wikipedia.org/wiki/Variable_speed_of_light

None of these holds the scientific consensus, because we're not sure about these; we're still looking for more data, still discussing their merits, still testing. One of them might be right, but we're not prepared to say that that is so, because while some are internally consistent models, they do not yet have enough support.

So no, I'm afraid you're incorrect; merely because a field has a consensus does not mean it's not “true science” - rather the opposite; that a consensus is present is a strong indication that we have the right idea; it means that a lot of other scientists also come to the same conclusions based on the evidence.

Does that answer your question?