Newton’s Third Law

Newton's Third Law of Motion describes the phenomenon of when one object exerts a force on another, the second exerts an equal and opposite force on the first. It is a law of physics, so it cannot literally be applied to love. But my intuition points toward an interesting question in psychology and systems theory:

If falling in love creates convergence between two people, what simultaneously undergoes divergence?

I looked at research on romantic attachment, attention, pair-bonding, hormones, and responses to alternative partners. There isn't one scientifically established "equal and opposite reaction." But there are several measurable things that appear to move away, weaken, or narrow as two people become strongly bonded.

My strongest hypothesis: two people move closer, while the rest of the field moves farther away

Imagine two points in a field:

A → ← B

As A and B become psychologically closer, something interesting happens to the relative distance between the couple and everyone else:

Others ← ← [ A ↔ B ] → → Others

They don't necessarily abandon anyone. Rather, the relationship becomes a new center of gravity. Attention, time, emotional priority, and decision-making become disproportionately concentrated inside the pair.

Research offers a fascinating example. People in committed relationships can show reduced attention toward attractive alternatives and may psychologically devalue potential alternative partners—a process researchers have studied as a possible relationship-maintenance mechanism. In other words, as one person becomes increasingly salient, competing possibilities can become less salient. (PMC)

So, if we wanted to create a provisional "law of love," I might write:

As attachment concentrates, alternatives recede.

But I think it goes considerably deeper.

What may "move apart" when two people fall in love

1. The self and its former independence.
   The boundary between "my life" and "your life" becomes more permeable. Your schedules, plans, memories, possessions, social networks, and imagined futures begin overlapping. The two independent systems become a partially interdependent system. The opposite movement is therefore a movement away from complete autonomy.

2. Attention and the rest of the world.
   Romantic love strongly engages motivational and reward systems. The beloved acquires unusually high psychological importance. Research describes romantic love partly through neural systems involved in motivation toward a highly valued reward—in this case, proximity and intimacy with a particular person. (PMC)

   If attention is finite, then:

   more attention here → less attention available elsewhere.

   This may be one reason the beginning of love can feel as though the rest of the world has become quieter.

3. The person and alternative mates.
   This is perhaps the clearest empirical analogue to your Newtonian idea. As commitment to one partner strengthens, the psychological pull of alternatives can diminish. Researchers have documented processes including inattention to attractive alternatives and "derogation of alternatives." (PMC)

4. The present self and the possible selves that will never exist.
   This is my hypothesis rather than an established scientific law, but I think it's profound. Choosing one person eventually means not choosing thousands of possible lives. If I build a life with A, the hypothetical futures I might have built with B, C, or D disappear. Love therefore creates something while simultaneously closing possibility space.

5. Two individual futures converge; alternative futures diverge.

   Before love:

   A ↗ ↖ ↑ ↘ → many possible futures
   B ↗ ← ↓ ↘ → many possible futures

   After pair-bonding:

   A → [shared future] ← B

   The number of imaginable futures may actually shrink while the depth of investment in one future increases.

This suggests another possible principle:

Love exchanges breadth for depth.

You stop being equally available to every possible future because you are constructing one particular future more deeply.

There's an even stranger biological possibility

Early romantic love isn't simply biological "calm." It appears to be a period of substantial physiological reorganization. Studies have found changes involving cortisol, testosterone, nerve growth factor, oxytocin, serotonin-related systems, and reward circuitry, although findings vary by study and stage of relationship and should not be reduced to a single "love chemical." Some early changes also appear to normalize as relationships mature. (PubMed)

So another way of describing the opposite movement might be:

external distance decreases → internal equilibrium is temporarily disrupted.

You become closer to someone outside you, while something inside you becomes less settled.

That is fascinating because falling in love often feels simultaneously like:

"I have found home."

and

"My entire internal world has been disturbed."

The embrace research is interesting here too: in one experimental study, women who embraced their romantic partners before an acute stress task subsequently showed a lower cortisol response than controls. Once a bond exists, proximity to the bonded person can therefore become part of the body's regulation of the outside world. (PMC)

My broader hypothesis: love doesn't produce an opposite force—it produces a boundary

I think this may be the deepest answer to your question.

When two separate entities become a unit, a new inside and outside appear.

Before:

A | world | B

After:

[ A + B ] | world

The movement that creates "us" simultaneously creates "not us."

That principle appears everywhere in nature. A cell membrane creates an inside by separating it from an outside. A family forms through unusually strong bonds among certain individuals, thereby creating a distinction between family and non-family. A nation creates citizens and, simultaneously, noncitizens. A friendship circle creates members and outsiders.

So perhaps the "opposite reaction" to union isn't separation between the lovers.

It is differentiation between the newly formed unit and everything outside it.

And this gives us a rather beautiful hypothetical law:

Every act of attachment is simultaneously an act of distinction. The closer two things become to one another, the more clearly a boundary can emerge between their shared system and everything outside it.

That would explain why healthy love has a paradoxical architecture. Two people come closer, alternatives recede. Two lives combine, possible lives disappear. Two identities overlap, absolute independence decreases. Attention concentrates, the background becomes quieter. A new "we" emerges, and therefore, for the first time, there is a "not-we."

But there's an important second phase. In mature, healthy love, I suspect the system can reverse the narrowing effect: the secure bond becomes a base from which both people expand outward again—toward friendships, children, work, creativity, community, and service. Then the geometry becomes not:

world ← A ❤️ B → world

but:

A → B → world
world ← A ← B

The couple doesn't consume the two individuals. It becomes a stable center from which both can reach farther.

So if I were developing your idea into a broader theory, my hypothesis would be: early love concentrates; mature love radiates. First, two systems reorganize around each other. Then, if the bond becomes secure rather than possessive, the energy created by that convergence can flow outward. That distinction—between love that contracts the world and love that creates a center strong enough to expand into the world—may be one of the most important differences between attachment and genuinely generative love.