Figure 1: Emitter voltage measurement
in simplified schematic view
Whether or not we think of it in such terms, any voltage measurement taken with an oscilloscope or voltmeter is, in reality, a differential voltage measurement. A voltage is, by definition, the difference in electrical potential between two points in a circuit. It's impossible to take a voltage measurement with only one voltmeter lead. One lead must be attached to the point of interest while the other must be connected somewhere else as a reference point.
Looking for a good, albeit very expensive, paperweight? Try using an oscilloscope without probes! Probes are often taken for granted but they are one of the most critical elements of the signal chain in any test scenario.