Temperature Converter (Physics)

Absolute zero (0 K = −273.15 °C) is the minimum possible temperature, corresponding to the state of minimum thermal motion. At 0 K, a classical ideal gas would have zero kinetic energy. Quantum mechanically, particles retain zero-point energy (ℏω/2 per mode) due to the uncertainty principle, so 0 K is approached asymptotically. The third law of thermodynamics states that absolute zero can be approached but never reached in a finite number of steps.

In laboratory settings, temperatures below 100 picokelvin (10⁻¹⁰ K) have been achieved in ultracold atomic experiments. The MIT group achieved 450 pK (4.5 × 10⁻¹⁰ K) in 2003 using laser cooling and evaporative cooling of sodium atoms. The cosmic microwave background radiation sets the minimum background temperature at 2.725 K; space is slightly above absolute zero due to this radiation.

In plasma physics, particle energies span a wide range and expressing temperature in electron volts (using kT in eV) is more natural than millions of Kelvin. The conversion is simply T(eV) = kB × T(K) = 8.617 × 10⁻⁵ eV/K × T(K). A plasma at 10 keV is at about 116 million K — a common temperature for tokamak fusion experiments. The convention plasma T = 1 keV means kT = 1 keV, or T ≈ 11.6 million K.

The Rankine scale is an absolute temperature scale using Fahrenheit-sized degrees. 0 °R = absolute zero = 0 K = −459.67 °F. T(°R) = T(°F) + 459.67 = T(K) × 9/5 = T(°C) × 9/5 + 491.67. It is used in engineering thermodynamics in the US, particularly in steam tables and gas dynamics calculations where absolute temperature is needed but Fahrenheit-sized degrees are preferred.

Before 2019, the Kelvin was defined by the triple point of water (273.16 K exactly). The 2019 SI redefinition instead fixes kB = 1.380649 × 10⁻²³ J/K exactly. As a result, the triple point of water is now 273.16 K with a small experimental uncertainty (~4 × 10⁻⁷ K). The change was made for fundamental consistency but has negligible practical effect on temperature measurements.

The Big Freeze scenario predicts the universe will cool toward absolute zero over timescales of 10¹⁰⁰ years or more. Currently, the cosmic microwave background (CMB) temperature is 2.725 K, cooling as T_CMB ∝ 1/(1+z) where z is redshift. In 1 trillion years, T_CMB ≈ 0.003 K. Eventually, after all stars die and black holes evaporate via Hawking radiation, the universe temperature will approach 10⁻³⁰ K.

Wien's displacement law states that the peak wavelength of blackbody radiation is λ_max = b/T, where b = 2.897771955 × 10⁻³ m·K. At room temperature (300 K), λ_max = 9.66 μm (infrared). At solar surface temperature (5778 K), λ_max = 501 nm (green light). This law explains why hot objects glow red, then white, then blue-white as temperature increases.

Yes — spectroscopically. By measuring the ratio of intensities in spectral lines (Boltzmann distribution of excited states), the temperature of distant stars, plasmas, or gases can be determined without physical contact. The color (blackbody spectrum) of a star gives its surface temperature. In laboratories, laser spectroscopy of Doppler-broadened spectral lines measures temperature from the velocity distribution width.

The Debye temperature θD is the temperature above which all phonon modes in a solid are thermally excited, and the heat capacity approaches the classical Dulong-Petit value (3R per mole). Below θD, quantum effects suppress the heat capacity. θD ranges from ~100 K for lead to ~400 K for aluminum to ~2230 K for diamond. It is calculated as θD = ℏω_max/kB where ω_max is the maximum phonon frequency.