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For a static rest-frame electromagnetic stress-energy tensor with no Poynting flux, a Lorentz transformation to a laboratory where it moves along $+x$ gives $u={\gamma }^2(u_0+{\beta }^2{T}_{\ast }^{xx})$ and $S_x={\gamma }^{2}v(u_0+T_{\ast }^{xx})$. Thus energy flux includes stress as well as rest energy density. In an ideal parallel-plate capacitor, $T_{\ast }^{xx}=+u_0$ for motion perpendicular to the electric field, but $T_{\ast }^{xx}=-u_0$ for motion parallel to it. The former has $S_x=2{\gamma }^{2}v{u}_0$; the latter has zero interior flux, with work at the moving plates transferring energy at the boundaries. The field subsystem need not have an integrated four-momentum independent of simultaneity surface; include material support stresses for a closed system.

For complex velocity amplitudes $v_x=v_y=V$ and magnetic amplitudes $b_x=-b_y=B$ in Alfvén-speed units, phase averaging gives the displayed stress. For fixed horizontal velocity energy, equal radial and azimuthal components maximize their positive Reynolds stress. Opposite equal magnetic components maximize the outward tension contribution of the Maxwell stress tensor, since $-B_x{B}_y$ is then positive. These conclusions follow from $2|ab|\le |a{|}^2+|b{|}^2$. The vertical channel transports angular momentum outward and extracts energy from decreasing angular velocity.

For an elastic filament with reference arclength $s$, curvature $c$, axial strain $ϵ$ and excess twist rate $\Omega$, a harmonic energy density is $(K_b{c}^2+S{ϵ}^2+K_t{\Omega }^2)/2$. Bending and torsional rigidities have units of energy times length; the stretch modulus has units of force. Factoring out $k_{B}T$ therefore makes $A,C$ lengths but $B$ an inverse length. Axial strain under tension is approximately $F/S$. Twist is controlled by torque and rotational boundary conditions, not force alone. A three-dimensional thermally fluctuating worm-like chain has persistence length $K_b/(k_{B}T)$.

For electronic $J=1/2$, an extremal total projection has only one product state rather than a two-dimensional spin block. With dimensionless angular-momentum operators and nuclear Zeeman coefficient $g_I$ in Bohr-magneton units, its energy is $AI/2\pm {\mu }_{B}B(g_J/2+g_{I}I)$. Extending both signs of a two-state Breit-Rabi formula square root to each stretched projection would invent an extra state.

The part of elastic energy associated with a change of material length. A simple quadratic constitutive equation assigns energy density $C(\alpha -1{)}^2/2$ per reference arc length, where $C>0$ is stretching stiffness and $\alpha$ is the stretch ratio. Such a small-strain model need not remain realistic as $\alpha \to 0$.

For $\Phi =\mu [{\sum }_i{v}_i^2/2+{\sum }_i{v}_i^{-1}]$ and positive stretch product one, the energy simplifies to $\mu S^2/2$, where $S={\sum }_i{v}_i$. Equal nominal tensile loading adds potential $-\tau S$. The homogeneous diagonal equilibrium equations reduce to $(\mu S-\tau )(v_i-v_j)=0$. Thus the unit triple exists for every load, while the other equilibria form the displayed sum-product family. Since $S\ge 3$, only the unit triple exists for $\tau \le 3\mu$. Above this load, a continuous family is obtained by choosing $v_1=t$ near one and taking $v_2,v_3$ as the two positive roots with sum $\tau /\mu -t$ and product $1/t$.

The 1985 experiment launched $150\mathrm{ps}$ pulses from a mode-locked laser into $1.4\mathrm{km}$ of optical fiber. Dispersion and self-phase modulation produced a broader spectrum and approximately $300\mathrm{ps}$ pulses. A Pockels cell selected a pulse for repeated amplification in neodymium glass, giving approximately $2\mathrm{mJ}$ before the double-pass diffraction grating compressor. Its transmission was 50 percent. The amplified output was reported as $1\mathrm{mJ}$ with a $2\mathrm{ps}$ width limited by the streak camera response. The compressor was adjusted on fiber-output pulses using intensity autocorrelation; that measurement gave $1.5\mathrm{ps}$. Steve Williamson was acknowledged for help developing the concept, and Philippe Bado and Marcel Bouvier for technical support.^[ref]

The crucial comparison is compression before and after amplification. It asks whether enough usable spectral phase structure survives the amplifier, rather than merely whether its output is bright. A response-limited trace cannot establish the true width below the detector's resolution or prove that every higher-order phase coefficient vanished. An intensity autocorrelation is also not a direct unique measurement of the complex electric field. These are limits on the inference, not extra controls attributed to the experiment.

Donna Strickland's 2018 lecture describes setting the compressor with the high-repetition oscillator pulses and making the amplified-pulse measurement with Steve Williamson's streak camera. It explicitly notes imperfect chirp matching.^[ref] The first apparatus therefore illustrates the strategy without establishing a universal lossless or exactly phase-matched amplifier.

Donna Strickland leads an ultrafast laser group at Waterloo investigating intense laser interactions and nonlinear optics.^[ref] Gérard Mourou's later work includes coherent combination of laser beams in the XCAN project at École polytechnique.^[ref] Their complementary research settings connect the original short-pulse experiment with the continuing problems of producing useful energy, peak power and average power together.

String tension is energy per unit length. The Regge-slope parameter ${\alpha }^{\prime }$ is conventionally defined by $T=(2\pi {\alpha }^{\prime }{)}^{-1}$.

For $X-X_{\infty }=c/r$, subtracting the planar D3-brane energy leaves $T_3\int |\nabla X{|}^2{d}^{3}x=4\pi T_3{c}^2/r_0$ outside a cutoff. Since $X_{\mathrm{cut}}-X_{\infty }=c/r_0$, its coefficient is the carried string tension. Electric charge quantization gives $c=\pi g_s|N|{\alpha }^{\prime }$ and coefficient $|N|T_{F1}$; magnetic charge gives $c=\pi |N|{\alpha }^{\prime }$ and coefficient $|N|T_{D1}$.

The elastic stiffness satisfies $c_{ijkl}{a}_i{p}_j{a}_k{p}_l>0$ for every nonzero polarization $a$ and propagation vector $p$. It makes the elastic acoustic tensor positive definite, ensuring positive squared wave speeds. Positive strain energy is a sufficient stronger condition in ordinary linear elasticity.

If the differential Zeeman energy $|w|=|{\mu }_{0}B(g_1-g_2)|$ greatly exceeds $|\Delta |$, the two mixed levels are $-\Delta /4\pm |w|/2+O({\Delta }^2/|w|)$ and approach opposite-spin product states. Four energies are generically distinct. Equal signed moments have $w=0$, so that block remains singlet/triplet rather than being unmixed by a stronger field.

The institutional carbon spectral line table explicitly separates vacuum and air wavelengths. Its $193.0906\mathrm{nm}$ C I line is a vacuum wavelength, while its $247.8561\mathrm{nm}$ C I line is an air wavelength. Their differing conventions matter when converting line positions into atomic energy differences.

Consider a long-wave film-rupture model whose strong-Marangoni stress asymptotic growth law isHere $h_0$ is the base film thickness, $\mu$ is dynamic viscosity, $\gamma >0$ is the base surface tension, $V>0$ is the model's destabilizing interaction coefficient with dimensions of energy, and $K=k{h}_0$ is dimensionless wavenumber. The precise relation of $V$ to a microscopic interaction depends on the chosen film model.

Set $y=K^2$. Differentiating $y(1-\Gamma y)$ gives $1-2\Gamma y$, so the unique unstable-band maximum isThe second derivative is negative. Since $\Gamma \gg 1$, the selected $K_m\ll 1$ is consistent with the long-wave assumption. The growth timescale is not itself a rupture time or bubble lifetime: a perturbation starting at amplitude $\delta h_0$ must grow to a specified nonlinear scale, giving an additional logarithmic dependence on the initial disturbance, and the base thickness may evolve before that happens.

An ideal Bjorken scaling model has $F_2(x,Q^2)=f(x)$: varying $Q^2$ at fixed Bjorken x leaves this dimensionless deep-inelastic structure function unchanged. The comparison requires extracting a deep-inelastic structure function, rather than plotting raw event counts. Quantum chromodynamics predicts departures from exact Bjorken scaling; finite hadron masses also limit the idealization.

For an original shape comparison, choose $f(x)=x(1-x{)}^3$ and define $S(Q^2)=F_2(0.3,Q^2)/F_2(0.3,Q_0^2)$ with $Q_0^2=2{\mathrm{GeV}}^2$. The ideal model has $S=1$. To display a contrasting size-dependent suppression, define an illustrative elastic-form-factor shape $G(Q^2)=(1+Q^2/{\Lambda }^2{)}^{-2}$ with the freely chosen ${\Lambda }^2=1{\mathrm{GeV}}^2$. Its normalized squared magnitude isThus $A(5{\mathrm{GeV}}^2)=1/16$, whereas $S(5{\mathrm{GeV}}^2)=1$. The elastic-form-factor shape is a comparison of suppression mechanisms, not an elastic differential scattering cross-section assigned to $x=0.3$: actual elastic scattering from a proton has $W=M$ and $x=1$. Neither curve is measured data or a fit to the SLAC-MIT experiment.

The experimental interpretation required more than a flat curve. The angular dependence and small longitudinal response supported spin one-half partons, expressed at leading order by the Callan-Gross relation $F_2=2x{F}_1$. Identification with fractionally charged quarks also drew on later electron, muon and neutrino comparisons. A scaling plot alone does not determine the individual electric charges of constituents.^[ref]

The leading parton model formula already shows the ambiguity: $F_2=x{\sum }_f{Q}_f^2(q_f+{\bar{q}}_f)$ combines squared electric charges with unknown parton distribution functions. One measured function cannot independently fix all these quantities. Confinement also explains why an inclusive final hadron state need not contain an isolated outgoing quark.

A subatomic particle is a constituent or excitation studied at the level below ordinary atomic structure. The category includes elementary particles, such as the electron, and composite hadrons, such as the proton and neutron.

Calling something a particle does not require a tiny classical solid sphere. Quantum mechanics describes localized detections and wave-like propagation, while high-energy scattering probes whether internal structure is resolved. A particle detector usually infers a particle from transferred energy, emitted light or a trajectory reconstructed from many interactions. The measured track width and the particle's intrinsic spatial structure are therefore different quantities.

At ${\mathbf{K}}_{+}$ and positive energy, a normalized eigenvector for $\mathbf{q}=|\mathbf{q}|(\cos \varphi ,\sin \varphi )$ is $(1,e^{i\varphi })/\sqrt{2}$. Its Pauli matrix expectation values give the displayed direction. This pseudospin records the relative amplitudes and phase on the two sublattices. Real spin supplies another label; confusing these two labels would double-count states and misinterpret the two-band Hamiltonian.

Subtracting the normal-state grand potential fixes the reference constant in the uniform BCS pairing-field effective action. Here ${\omega }_n=(2n+1)\pi /\beta$ has energy units and $E_k^2={\xi }_k^2+|\Delta {|}^2$. The ratio cancels a field-independent determinant phase and produces a convergent high-frequency sum under the same momentum regulator. Differentiation gives the BCS gap equation. For a fixed attractive pairing shell, this is a finite regulated model; an unrestricted continuum contact interaction additionally needs ultraviolet renormalization. Its weak-coupling condensation density $-{\nu }_F{\Delta }^2/2$ uses a single-spin density of states per volume.

The sudden approximation treats a rapid, finite change of a Hamiltonian operator as leaving the quantum state approximately unchanged during the switch. The energy measurement can nevertheless change immediately: the same quantum state has different probability amplitudes in the new eigenbasis. Compute their inner products with the new energy eigenstates, then apply the Born rule. The two-level sudden quench below makes this distinction quantitative without assuming the change is small.

An ideal finite step has exact quantum state continuity at a finite Hamiltonian step; a finite-duration implementation has a small evolution error under the bounded-model conditions below. An impulsive quantum kick is different: its Hamiltonian operator grows as its duration shrinks, retaining a finite integrated action and a finite change of quantum state. A sudden approximation therefore concerns short evolution during switching, not conservation of energy during externally driven change. The subsequent unitary time evolution can be substantial.^[ref]

The sudden-decay approximation replaces a dominant nonrelativistic species $X$ by radiation near $H=\Gamma$, neglecting expansion during the conversion. Use natural units, reduced Planck mass $M=(8\pi G{)}^{-1/2}$ and relic mass $m$. Define the pre-decay yield $Y=n_X/s_{-}$, where $n_X$ is its number density and $s_{-}=(2{\pi }^2/45)g_{\ast ,s}{T}_{-}^3$ is the residual bath's entropy density. Relic domination givesAfter conversion and prompt thermalization, conservation of energy giveswhich defines the reheating temperature. The entropy count $g_{\ast ,s}$ and energy count $g_{\ast ,\rho }$ have different definitions and need not be identical. The yield in the first equation is defined before the newly produced entropy dilutes it.

If the relic energy greatly exceeds the residual radiation energy, the post-decay temperature can greatly exceed $T_{-}$. The related entropy production by decay of a dominant relic follows by comparing the two equilibrium baths at the same scale factor. The assumptions are relic domination, nonrelativistic relics and rapid conversion/thermalization; a gradual decay or incomplete equilibrium requires coupled evolution instead.

A superbubble is an expanding cavity of hot gas driven by the combined energy input of stellar winds and multiple supernova explosions. Constant mechanical power and negligible radiative losses lead to a radius proportional to $t^{3/5}$ in a uniform ambient medium.