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From a regular human's point of view, there's only two degrees of existence:
- Your sensory experiences and your thoughts, which definitely exist
- Everything else, which only exists as abstract concepts that your brain learns by observing correlations between sensory experiences in an attempt to predict the future.
For example, the concept you have of "your mom" is just the observation that if you walk around to certain places, correlating to some memories you have and some sounds that you might hear about where she is, you'll get back some very consistent sensory experiences in the familiar shape and voice of your mom. But of course you can't be sure about your mom's degree of real existence in any way, it could all be an illusion or a simulation which could end at any time. But as long as it doesn't, the concept retains its predictive power and you can regard it as real in the context given by your senses.
If you accept this, you can see that all physics concepts are real to exactly the same degree, that is: not a lot. They're just concepts that make it easy to predict the things that you observe through your senses like the motion and temperature of bodies, the numbers that appear on measuring instruments, and so on.
