Does a speeding observer feel things as hotter or colder?

When you move relativistically past a hot object, does it appear hotter or colder? Three competing formulas — due to Planck-Einstein, Ott, and Landsberg — have disagreed for decades. Analyzing photon gas, ideal gas, and electron gas using the energy-momentum tensor, the authors find that effective temperature consistently rises with velocity, siding with Ott. The catch: the exact factor depends on the material, meaning temperature is observer-dependent and only fully consistent when treated as a four-vector quantity.