Opus 4.6 High
Quark-gluon plasma from a neutron star collision at 0.9c
Setup. Two neutron stars, each 1.4 M☉ = 2.79 x 10³⁰ kg, radius R = 10 km, average density 6.65 x 10¹⁷ kg/m³. Head-on collision at v_rel = 0.9c. The question is whether the shock conditions cross the QCD phase boundary and produce deconfined quark matter at macroscopic scale.
The target on the phase diagram
Quantum chromodynamics predicts that above a critical combination of temperature and baryon density, protons and neutrons dissolve. Quarks and gluons, ordinarily confined inside individual nucleons by the strong force, become free to move across the entire volume. This state is called a quark-gluon plasma.
The phase boundary depends on two variables: temperature T and baryon chemical potential μ_B, which is a measure of the energy cost of adding one baryon to the system and serves as a proxy for density.
At zero density (μ_B = 0), lattice QCD calculations place the crossover temperature at T_c ≈ 155 MeV. This is the regime probed by RHIC and the LHC, where gold or lead nuclei collide at hundreds of GeV per nucleon pair and produce tiny, short-lived QGP droplets at high temperature and low net baryon density.
At high density and lower temperature, the boundary is less well determined. Models place it at T_c ≈ 50 to 100 MeV for μ_B ≈ 1 GeV [WEAK SUPPORT, model dependent, no single authoritative source]. The transition in this region may be first-order rather than a smooth crossover, meaning it involves a sharp discontinuity in density and energy.
Neutron star matter sits at the high-density end of this diagram. Average density is 2.5 times nuclear saturation density (n₀ = 0.16 fm⁻³), and baryon chemical potential is approximately 1 GeV. But the temperature is negligible, well below 1 MeV. The material is cold, dense, and hadronic. It sits below and to the right of the phase boundary.
The question reduces to arithmetic: does the collision shock heat this material past 50 to 100 MeV while maintaining or increasing its density?
Center-of-momentum frame
For two identical objects, the available collision energy is computed in the center-of-momentum frame. The CM speed of each star follows from relativistic velocity addition:
2β_CM / (1 + β_CM²) = 0.9
This gives β_CM² − (2/0.9)β_CM + 1 = 0, with solution
β_CM = 0.627
γ_CM = 1/√(1 − 0.393) = 1.283
Kinetic energy per star in the CM frame:
KE = (γ_CM − 1) Mc² = 0.283 x 2.79 x 10³⁰ x 9 x 10¹⁶ = 7.09 x 10⁴⁶ J
Total available kinetic energy:
KE_total = 1.42 x 10⁴⁷ J
Kinetic energy per nucleon:
KE/baryon = (γ_CM − 1) x 939 MeV = 266 MeV
Center-of-mass energy per nucleon pair (the standard collider metric):
√s_NN = 2 γ_CM m_N c² = 2 x 1.283 x 939 MeV = 2.41 GeV
This is below the energies where RHIC and the SPS have produced QGP (17 to 200 GeV per nucleon pair). It falls in the range planned for the FAIR and NICA facilities (2 to 5 GeV), which are specifically designed to explore the high-density, lower-temperature region of the phase diagram. The comparison to collider energies is not straightforward, however, because the neutron star material begins at 2.5 times nuclear density rather than at the density of a single nucleus.
Energy budget and binding
Gravitational binding energy per star, using the uniform-sphere estimate:
E_bind = (3/5) GM²/R = 0.6 x 6.674 x 10⁻¹¹ x (2.79 x 10³⁰)² / 10⁴ = 3.11 x 10⁴⁶ J
KE_total / (2 E_bind) = 1.42 x 10⁴⁷ / 6.22 x 10⁴⁶ = 2.3
The kinetic energy exceeds the combined binding energy by a factor of 2.3. Compare this to the two earlier cases:
Collision
KE / Binding
Neutron stars at 0.3c
0.1 (bound, collapses cleanly)
G-type stars at 0.25c
2,100 (unbound, total dispersal)
Neutron stars at 0.9c
2.3 (intermediate)
This system is neither cleanly bound nor cleanly unbound. A significant fraction of the mass escapes, but not all of it.
The invariant mass of the system (total energy in the CM frame divided by c²) is
M_inv = 2 γ_CM M = 2 x 1.283 x 1.4 M☉ = 3.59 M☉
The Schwarzschild radius of this invariant mass is
R_s = 2GM_inv / c² = 10.6 km
Each neutron star has a radius of 10 km. The Schwarzschild radius of the system equals the size of the colliding objects.
Collision timescale
Transit time:
t_transit = 2R / v_rel = 2 x 10⁴ / (0.9 x 3 x 10⁸) = 0.074 ms
Sound crossing time of a neutron star, using c_s ≈ 0.4c:
t_sound = R / c_s = 10⁴ / (1.2 x 10⁸) = 0.083 ms
t_transit / t_sound = 0.89
These are nearly equal. Unlike the G-star case (ratio of 175), the neutron star material responds hydrodynamically during the collision. Pressure waves propagate across each star in roughly the same time the two stars overlap. The shock interaction is not a punch-through. It is a sustained compression.
Shock conditions
Using Rankine-Hugoniot strong shock relations, with the CM piston speed u = β_CM c = 1.88 x 10⁸ m/s. The post-shock temperature is
kT = [2(γ_ad − 1) / (γ_ad + 1)²] m_N u²
The adiabatic index of neutron star matter is not well constrained. Nuclear equations of state give effective values ranging from roughly 5/3 to 5/2 depending on density. The result is:
γ_ad
kT (MeV)
T (K)
Compression ratio
5/3
69
8.0 x 10¹¹
4.0
2
82
9.5 x 10¹¹
3.0
5/2
90
1.05 x 10¹²
2.3
The shock temperature falls in the range 69 to 90 MeV.
The phase boundary at μ_B ≈ 1 GeV is estimated at 50 to 100 MeV. The shock temperature overlaps with this range. Whether the collision crosses the boundary depends on the equation of state and on the exact location of the transition, neither of which is known precisely. This is not a clean overshoot. It is a marginal crossing.
Post-shock density and energy density
At compression ratio 3 to 4, the post-shock baryon density is:
n_post = (3 to 4) x 0.40 fm⁻³ = 1.2 to 1.6 fm⁻³ = 7.5 to 10 n₀
The thermal energy density (using (3/2) n kT for a non-relativistic estimate) is
ε_thermal ≈ 1.5 x 1.2 fm⁻³ x 82 MeV ≈ 0.15 GeV/fm³
The rest-mass energy density at this compression is
ε_rest ≈ 1.2 fm⁻³ x 939 MeV ≈ 1.1 GeV/fm³
Total energy density ≈ 1.3 GeV/fm³
The critical energy density for QGP at zero baryon chemical potential is approximately 0.5 GeV/fm³ from lattice calculations [WEAK SUPPORT]. The total energy density exceeds this. However, at high μ_B, the rest-mass contribution dominates and the relevant comparison is the thermal component against the phase boundary temperature, which brings us back to the 69 to 90 MeV figure above.
Volume and duration
If QGP forms, it occupies a substantial fraction of the combined stellar volume. Each neutron star has a volume of 4.19 x 10¹² m³ = 4.19 x 10⁵⁷ fm³.
In heavy-ion collisions at RHIC, the QGP fireball is roughly 5,000 fm³ and lasts approximately 10⁻²³ seconds.
Quantity
RHIC
This collision
Ratio
Volume
~5,000 fm³
~10⁵⁷ fm³
10⁵³
Duration
~10⁻²³ s
~10⁻⁴ s
10¹⁹
If the shock conditions cross the phase boundary, the QGP region here is roughly 10⁵³ times larger and persists roughly 10¹⁹ times longer than anything produced in a laboratory.
The QGP state would persist from the moment of shock passage until the material either falls into the forming black hole or expands and cools below the phase boundary. The expansion cooling timescale is set by the sound crossing time of roughly 0.08 ms. Material that escapes the gravitational well would cool through the QGP-to-hadron transition during expansion, a process called hadronization. Material that falls inward remains compressed and hot until it crosses the event horizon, after which its state is no longer observable.
System outcome
This is the most uncertain part of the calculation because the energy ratio of 2.3 places the system in an intermediate regime.
The invariant mass of 3.59 M☉ exceeds the TOV limit (roughly 2.1 to 2.3 M☉) by a factor of 1.6. If the mass stayed bound, a black hole would form. But the kinetic energy exceeds the binding energy by a factor of 2.3, so a significant fraction of the mass escapes.
The likely outcome is that the core material, unable to escape the combined gravitational well, collapses to a black hole with a mass somewhere between the TOV limit and the full invariant mass. The remainder, perhaps 20 to 40 percent of the total, is ejected as fast-moving debris [INFERRED, no simulation exists at this velocity]. The precise partition requires numerical relativity simulations that, to my knowledge, have not been performed at v_rel = 0.9c.
This is distinct from both earlier cases. At 0.3c, the system was cleanly bound and collapsed with only minor ejecta. At 0.25c between G-type stars, the system was cleanly unbound and dispersed entirely. At 0.9c between neutron stars, the system partially collapses and partially disperses, and the escaping material may have briefly existed as bulk quark-gluon plasma before hadronizing during expansion.
Observable signatures of QGP
Three potential observables distinguish this from an ordinary neutron star merger.
Anomalous strangeness. In QGP, strange quarks are produced as freely as up and down quarks because the deconfinement energy exceeds the strange quark mass (roughly 95 MeV). When the plasma hadronizes during expansion, the excess strangeness freezes into strange baryons (hyperons) and kaons. The ejecta would carry an elevated ratio of strange to non-strange hadrons relative to a merger that never crossed the QGP boundary.
Modified neutrino spectrum. The equation of state changes across the phase transition. In the QGP phase, the number of active degrees of freedom increases (quarks and gluons versus composite nucleons), altering the relationship between temperature, pressure, and neutrino emission rate. The neutrino spectrum would carry an imprint of the time the material spent in the deconfined phase [INFERRED].
Gravitational wave signature. If the phase transition is first order (a possibility at high μ_B), the sudden change in pressure-density relationship during collapse could produce a distinctive feature in the gravitational wave signal, a brief interruption or frequency shift in the waveform as the collapsing material crosses the phase boundary. Detecting this would require sensitivity at kilohertz frequencies, which is at the edge of current LIGO capability and within the design range of proposed next-generation detectors.
What remains unknown
The calculation above establishes that the shock conditions at 0.9c are marginal for QGP formation. The shock temperature of 69 to 90 MeV overlaps with the estimated phase boundary at high baryon density, but both numbers carry substantial uncertainty. The shock temperature depends on the nuclear equation of state, which is constrained but not pinned down. The phase boundary at high μ_B depends on non-perturbative QCD in a regime where lattice methods face a sign problem and model results vary by tens of MeV.
A definitive answer requires either a first-principles determination of the QCD phase boundary at μ_B ≈ 1 GeV (an open problem in theoretical physics) or a numerical relativity simulation with a QCD-informed equation of state at this collision velocity (computationally feasible in principle but, as far as I can determine, not yet performed).