I understand that originally QM was a successful theory that has been experimentally proven, and was non-relativistic.

Now its successor was said to be QFT. Later on, it incorporated effects of SR, and was called relativistic QFT.

I have read these questions:

What is the difference between Quantum Physics, Quantum Theory, Quantum Mechanics, and Quantum Field Theory?

What are the main differences between these three quantum theories: QM, QFT & QG?

Formalism Of Quantum Field Theory vs Quantum Mechanics

And it made me curious. This does not say whether the main difference between the original QM and relativistic QFT is relativity itself or not. I do not find anything on this main difference.

Luboš Motl's answer says:

Quantum field theories are a subset of quantum mechanical theories. So they obey all postulates of quantum mechanics, they have Hilbert space, linear Hermitian operators i.e. observables, obey the superposition principles, calculate probabilities from squared absolute values of complex amplitudes, and so on.

 

Quantum field theories have other operators (observables). The existence of the "momentum basis" or "position basis" is a particular property of a class of (non-relativistic) models of quantum mechanics; this existence is not belonging among the general postulates of quantum mechanics and these theories (with a fixed number of particles with positions or momenta) don't describe our Universe accurately.

And this:

Differences between principles of QM and QFT

Robin Ekman's answer:

Quantum field theory is quantum mechanics applied to Lorentz covariant causal systems. That is, quantum field theory is simply quantum mechanics plus special relativity.

Question:

1. If I would like to explain to everyday people, what the main difference is between the original QM and relativistic QFT, what would the answer be?

2. Is it simply just that one is non-relativistic, and the other deals with relativistic effects?

I understand that originally QM was a successful theory that has been experimentally proven, and was non-relativistic.

Now its successor was said to be QFT. Later on, it incorporated effects of SR, and was called relativistic QFT.

I have read these questions:

What is the difference between Quantum Physics, Quantum Theory, Quantum Mechanics, and Quantum Field Theory?

What are the main differences between these three quantum theories: QM, QFT & QG?

Formalism Of Quantum Field Theory vs Quantum Mechanics

And it made me curious. This does not say whether the main difference between the original QM and relativistic QFT is relativity itself or not. I do not find anything on this main difference.

Luboš Motl's answer says:

Quantum field theories are a subset of quantum mechanical theories. So they obey all postulates of quantum mechanics, they have Hilbert space, linear Hermitian operators i.e. observables, obey the superposition principles, calculate probabilities from squared absolute values of complex amplitudes, and so on.

Quantum field theories have other operators (observables). The existence of the "momentum basis" or "position basis" is a particular property of a class of (non-relativistic) models of quantum mechanics; this existence is not belonging among the general postulates of quantum mechanics and these theories (with a fixed number of particles with positions or momenta) don't describe our Universe accurately.

And this:

Differences between principles of QM and QFT

Robin Ekman's answer:

Quantum field theory is quantum mechanics applied to Lorentz covariant causal systems. That is, quantum field theory is simply quantum mechanics plus special relativity.

Question:

1. If I would like to explain to everyday people, what the main difference is between the original QM and relativistic QFT, what would the answer be?

2. Is it simply just that one is non-relativistic, and the other deals with relativistic effects?