Newton showed that many phenomena could become one.
The falling stone and the orbiting Moon, the motion of a projectile and the movement of the planets, could be understood through a common relation: universal gravitation.
For more than two centuries, this was one of the great achievements of scientific thought.
But there was a difficulty.
The Newtonian framework did not merely describe particular phenomena. It carried with it a picture of the world: space as a fixed arena, time as a universal measure, motion as change of position within that arena.
These concepts were so deeply embedded in the framework that they scarcely appeared to be concepts at all.
They appeared to be reality.
Then Einstein changed the question.
Not simply:
What laws govern things moving through space and time?
But:
What if space and time themselves belong to the relational structure that physics must explain?
At that point, the world does not merely require a new law.
It must be reconceived.
When the framework becomes visible
This is one of the most difficult forms of conceptual change.
As long as a conceptual framework works, we tend not to notice it.
We notice the objects within it.
We notice the phenomena it describes.
We notice anomalies when predictions fail.
But the background concepts through which the phenomena become intelligible can remain invisible.
Newtonian physics made space and time extraordinarily successful conceptual resources.
Space provided the arena in which positions could be specified.
Time provided a common measure against which changes could be compared.
The framework was so productive that its assumptions became almost indistinguishable from the world itself.
Einstein's achievement was partly to make those assumptions available for questioning.
The problem was already there
Relativity did not emerge from nowhere.
By the end of the nineteenth century, physics contained tensions that the Newtonian framework could not comfortably absorb.
Electromagnetism described light in ways that did not fit neatly into the classical picture of motion.
The speed of light appeared to have a peculiar status.
The question of how measurements of space and time should relate between observers became increasingly difficult.
Yet none of this logically dictated Einstein's theory.
This matters.
We can look backwards and see that something had to give.
But several possibilities might have been pursued.
Perhaps the equations could be modified.
Perhaps an invisible medium could be retained.
Perhaps transformations of measurement could be treated as mathematical conveniences without altering our conception of space and time.
Perhaps something else entirely.
The conceptual crisis created pressure.
It did not supply the solution.
This is precisely where Einstein differs from the convergence represented by Darwin.
The problem may be strongly constrained while the reconception remains open.
The observer enters the relation
Special relativity makes the issue especially clear.
Suppose two observers move relative to one another.
In Newtonian physics, we can imagine that there is a single time against which both descriptions can ultimately be compared.
Einstein removes that assumption.
Measurements of time and space are not independent of the observer's state of motion.
But this does not mean that reality becomes subjective.
Quite the opposite.
The theory replaces an absolute background with a deeper relational structure.
Different observers may disagree about distances and elapsed times while agreeing about the invariant relations connecting their measurements.
The important thing is no longer an absolute quantity possessed independently of every relation.
It is the structure that remains invariant across transformations of perspective.
This is a profound conceptual shift.
The observer is not simply someone looking at a pre-existing world from a particular location.
The observer's relation to the world becomes part of the physical description.
The world is not a stage
This is why relativity is more than a correction to Newtonian mechanics.
Newton had given physics a magnificent framework for describing bodies moving through space and time.
Einstein asks us to stop treating space and time as the stage on which the drama occurs.
They are part of the drama.
Space and time are not two substances waiting to be distorted. Nor are they simply containers within which relations occur.
They are aspects of the relational structure through which events can be ordered and compared.
Once this possibility becomes available, concepts that had seemed fundamental acquire a different status.
Space can no longer be understood independently of the relations among events.
Time can no longer be understood independently of the physical conditions under which intervals are measured.
The framework itself has entered the field of inquiry.
From special to general
General relativity takes the reconception further.
Gravity had been one of the great successes of Newtonian unification.
But Einstein asks whether gravity should really be understood as a force acting between bodies within an absolute spatial and temporal framework.
The answer is extraordinary.
The geometry through which distances and times are related is itself affected by the distribution of matter and energy.
This is often described as the curvature of spacetime.
The phrase is useful, but it can obscure the conceptual revolution.
The important point is not that some mysterious substance called spacetime gets bent.
It is that the relational structure through which spatial and temporal intervals are defined is not fixed independently of the physical world.
Gravity is therefore no longer simply something that happens within a pre-existing geometry.
The geometry participates in the gravitational relation.
The stage has become part of the action.
A new concept changes old concepts
This illustrates a feature of conceptual evolution that we have not yet encountered quite so sharply.
A new concept does not merely add itself to the existing conceptual inventory.
It can change the meaning and role of concepts that were already there.
After Einstein, space is not quite the Newtonian space.
Time is not quite the Newtonian time.
Simultaneity is not quite the Newtonian simultaneity.
Even motion acquires a different conceptual setting.
This is why conceptual change cannot always be represented as a growing collection of discoveries.
Sometimes the change is structural.
The elements remain recognisable, but their relations change.
And when the relations change, the elements themselves can acquire new significance.
Reconceiving is not abandoning
It would be easy to describe this as the overthrow of Newton.
That is too simple.
Newtonian mechanics remains extraordinarily effective in the domain where relativistic effects are negligible.
The older conceptual structure becomes a limiting case of the newer one.
This is another important feature of conceptual evolution.
A new conceptual framework does not necessarily destroy the possibilities generated by the old one.
It can preserve them within a larger relational structure.
What changes is their status.
The Newtonian world is no longer the final framework of physical possibility.
It becomes an approximation available under particular conditions.
A previous conceptual world can therefore survive as a special case of a new one.
The possibility of reconception
We can now see why Einstein occupies a distinctive position in our sequence.
Darwin:
An idea is in the air.
Watson, Crick, Franklin and others:
Evidence constrains a structure.
Copernicus:
The centre moves.
Kepler:
The circle breaks.
Galileo:
The question changes.
Newton:
The many become one.
Einstein:
The world must be reconceived.
Each step changes the field in which subsequent possibilities can arise.
But Einstein's transformation is distinctive because the framework itself becomes available for revision.
The assumptions that had organised the questions become part of the questions.
That may be one of the deepest forms of conceptual innovation.
The furrow turns back upon itself
We began this series by asking how an idea can become possible before anyone has thought it.
Einstein suggests that the answer cannot be limited to the accumulation of information.
A conceptual possibility may require us to recognise that the very distinctions through which we organise information are no longer adequate.
The furrow does not simply guide the plough forward.
At some point, the plough may reach a place where the furrow itself becomes the object of attention.
The conceptual landscape can become reflexive.
We can ask not merely what lies within the field of possibility, but what structure makes that field possible in the first place.
And once that happens, conceptual evolution can become extraordinarily discontinuous.
The new possibility may not be a better answer within the old world.
It may be a different world in which the old question takes on a different meaning.
But what if there is no new world?
Einstein's achievement might therefore seem like the culmination of the sequence.
The framework fails.
A deeper framework is found.
The world is reconceived.
But the history of quantum mechanics will make this reassuring pattern much harder to sustain.
There, the problem was not simply that an old conceptual framework needed to be replaced by one new and coherent alternative.
Several radically different possibilities emerged.
Wave and particle.
Continuity and discreteness.
Determinacy and probability.
Matrix mechanics and wave mechanics.
Different conceptual paths could describe the same emerging domain, while none initially possessed an obvious claim to conceptual supremacy.
The problem was no longer simply how to reconceive the world.
It was how a conceptual ecology behaves when several reconceptions become possible at once.
That is where the furrow begins to branch.
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