LOVE, n. A temporary insanity curable by marriage or by removal of the patient from the influences under which he incurred the disorder.
[Ambrose Bierce, 1911]

Limerence describes an involuntary cognitive and emotional state of intense romantic desire for another person—in other words, romantic love. Researchers in New York have shown, using functional MRI, that in some human individuals being “in love” with a long-term partner is similar to early-stage romantic love. But perhaps not in the way you might think.

Staying in love—retaining limerence—is not a matter of obsession, craving and euphoria. The euphoria may be useful in emotional bonding at first; in this respect, Ambrose Bierce is on the money. Oscar Wilde seems to agree:

One should always be in love. That is the reason one should never marry.
[Oscar Wilde]

As Bierce and Wilde might have expected, activation in the nucleus accumbens—associated with craving and euphoria from other studies looking at response to cocaine—decreased in line with the number of years married.

So what about long term love? In long-term in-love individuals, activation in a different part of the brain was observed; the ventral tegmental area. This is an area associated with working for rewards, which also is activated in early-stage romantic love. In other words, romantic love can last. This area of the brain reflects positive reward prediction errors (reward unexpected and received).

The researchers go on to suggest that this activation may be a novelty signal; that part of maintaining the romantic love feeling is the maintenance of a novelty response to the partner.

But you and every Agony Aunt knew this already, right? Keep things fresh. Get out of routine. Now confirmed with fMRI.
 
 
Acevedo et al. Program No. 297.10/TT28. Society for Neuroscience annual meeting, 2008.
Acevedo and Aron, Rev. Gen. Psy. 13(1), Mar 2009, 59-65.
Aron et al, J Neurophysiol 94: 327-337, 2005.
Read more here.

Nobel Prize in Physics lecture, 11th December 1952:

“To follow the analogue of mechanical resonance we must now come back to relaxation, which can be seen to act like a friction, and which counteracts the tilt produced by the alternating field.”

Since the net magnetisation of a sample is the vector sum of many protons, and since there is a constant interaction growth rate of the protons with the lattice, we can write

where M₀ is the equilibrium magnetisation, T1 is the longitudinal, or spin-lattice relaxation time, which is the time constant of the exponential recovery of M_z.^†

The transmission of energy to the local environment of a magnetic moment in a magnetic field which mediates this relaxation is primarily due to its thermal “contact” with that environment (fluctuating magnetic fields).

It is possible to perceive why Bloch likened relaxation to friction.

^†Take the general Bloch equation (dM/dt=γMxB, derivation), and rewrite it in terms of parallel and perpendicular components of M to the external, static main magnetic field: dM_z/dt=0 and dM_xy=γ(M_xyxB). From the former we may write the equation in the post above, and from the latter, dM_xy/dt=γ(M_xyxB)-(1/T2)M_xy (or simply dM_xy/dt=-(1/T2)M_xy in the rotating frame).

Magnetic Resonance Imaging (Springer)

“Only a quarter of a century after the invention of MRI one may expect that in the developed countries there will be about one MRI system for every 10⁵ inhabitants.”

MR-Physics, Department of Medical Radiology, University Hospital of Basel

“Some people believe that spins are just a quantum mechanical property of nuclei. However, spins are very small but lovely animals. And they like magnetic fields. We have collected some spins that used to live in our MR scanner, and we put them into our spin zoo.”

See the Spin Zoo, and click on the .gif file links for animation of magnetisation vector evolutions.

Spin Dynamics (Wiley)

“One can speculate how long it would have taken humanity to discover the magnetic field, if naturally occuring pieces of ferromagnetic iron from meteorites did not exist.”

in 1949

“There is nothing that nuclear spins will not do for you, as long as you treat them as human beings.”

Nobel Prize in Physics lecture, 11th December 1952.

“I have not yet lost a feeling of wonder, and of delight, that this delicate motion should reside in all the ordinary things around us, revealing itself only to him who looks for it.”