# Is it Possible to Directly Measure Energy?

Would it be possible to measure energy directly in joules instead of measuring what effect energy has on something?

Say for example you have a car of known weight traveling a speed v. To get the car's energy we would have to measure its weight and speed, not its energy directly. Same with a calorimeter, we have to measure the change in temperature in the water and then the water's mass in order to get energy instead of directly measuring energy.

I'm wondering if it's possible.

• If it becomes possible then energy would become a fundamental quantity like length, time, temperature etc but is it ? Jun 4, 2021 at 1:05
• $e=mc^2$, which implies that it would take an energy meter with an EXTREME amount of precision to do this job. There are also many different forms of energy. The answer to your question is "no". Jun 4, 2021 at 2:23

I would say it would be impossible to "measure" anything "directly" , unless you get a bit sloppy about the definition of measure and directly. Heisenberg's uncertainty principle in one form or another implies you can't measure anything without disturbing the system. The very concept of "measurement" implies sampling from some realm and converting that sample to another realm so that isn't direct.

Although you can get a measure of a car's speed without directly measuring distance & time, with the old-school speedos that used eddy current torque (but it is still not "fundamental") , so electrodynamics allows a "direct" measure of speed without the distance and time measurement, but the speedo reaction torque slows the car down a tiny amount. Even something subtle like doppler shift requires time=1/F and distance=wavelength.

The best you can hope for is a "more direct" measurement, i.e. one with only one conversion in the process.

As for the car, you could drive it into a perfectly ideal spring attached to an infinite mass, and measure the peak deflection of the spring. The car has the same energy afterwards, but momentum has been reversed,

The other serious show stopper is that the kinetic energy is a function of the reference frame, so there is no absolute value of kinetic energy, after all if the car is the equator the speed is already 400km/hr westward. (even more if you factor in orbital velocity around the sun)

There was a time when Science was known as "Natural Philosophy", measurement angst is part of that Philosophy.

Note that you also can't measure speed directly; you are always measuring distance and time and using a relationship between the two. (There is no "speed meter" that fundamentally measures speed.) The thing you're noticing about energy is not unique to energy; most measurable quantities are not "fundamental," but are derived from fundamental quantities. You may find the following interesting: https://plato.stanford.edu/entries/measurement-science/

• A Doppler radar measures target velocity very well without measuring distance, in fact the range measurement is completely unrelated to it. A Doppler radar measures the pulse to pulse phase variation of the echo signal relative to a local oscillator. Aug 13, 2020 at 9:52
• @hyportnex: I like your comment, but I'm not sure it's absolutely correct. Aren't you still measuring a distance, it's just that the distance you are measuring is the peak-to-peak distance in the radar signal? Aug 13, 2020 at 12:07
• @James the measurement is not not "peak to peak\$ but you would be justified to say that the instantaneous phase is itself a periodic function of distance, so in that sense we do measure distance within a wavelength but otherwise it is a frequency/phase measurement not range. Aug 13, 2020 at 12:25
• @hyportnex Good comment, but the phase measurement is itself a measure of time (time delay relative to the local oscillator), and is not directly a measure of speed. See especially Section 3.1 of the link I gave. Aug 13, 2020 at 15:20
• that is correct, I only wanted to emphasize that velocity measurement does not need distance measurement. Aug 13, 2020 at 15:31