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This should be a very basic question. In introductory QFT books, often one of the first things we see is the following claim: for every Lorentz transformation $\Lambda$, we can associate an unitary operator $U(\Lambda)$ such that: $$ U(\Lambda)^{-1} \varphi(x) U(\Lambda)= \varphi(\Lambda^{-1}x)$$ And we also demand it to be a homomorphism, $U(A)U(B)=U(AB)$.

Where of course, $\varphi$ is an operator-valued quantum field.

I want to know what guarantees the existence of mappings $U$ which satisfy these conditions.

It seems like it ought be possible to choose an operator field $\varphi$ such that no such set of operators $U(\Lambda)$ exist. Is the existence of $U$ given by some requirement on quantum fields, or am I missing something else?

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Your last sentence answers your question.

We observe Lorentz symmetry in the laws of nature. Therefore, we demand that the building blocks of our theory transform in definite representations of the Lorentz (or rather Poincaré) group.

Would you allow fields that are not representations of the Lorentz group, it would become extremely hard to construct a theory that looks Lorentz invariant.

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  • $\begingroup$ So is it correct to say candidates for operator fields that correspond physically to quantum fields depend on the representation of SO(3;1) that we choose? Then does the size of this set of (candidates for) quantum fields also depend on the choice of representation? $\endgroup$ – zzz May 20 '14 at 22:44
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    $\begingroup$ Yes and yes. There is a powerful way of rewriting the Lorentz algebra in order to constructively determine all irreducible representations of it. The choice of representations is limited by the fact that we don't know how to write down a non-trivially interacting theory for spins $S \geq 2$. For the lower spins, all representations exist in theory, most notably Supergravity requires them all to exist. $\endgroup$ – Neuneck May 21 '14 at 10:27
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I claim that

In any relativistic quantum theory, Lorentz transformations of states must be realized as a projective, unitary representation of the Lorentz group acting on the Hilbert space of the theory.

Here's the logic:

  1. In any relativistic theory, the observations of spacetime events of inertial observers are related by Lorentz transformations.

  2. We ask ourselves how the observations of quantum states, namely elements of some Hilbert space $\mathcal H$ that models a certain quantum system, of different inertial observers are related. In mathematical terms, for each Lorentz transformation $\Lambda$, we would like to associate a function $f_\Lambda:\mathcal H\to\mathcal H$ such that if $|\psi\rangle$ is the state measured by one inertial observer, then $f_\Lambda |\psi\rangle$ is the state measured by the inertial observer whose spacetime observations are related to the first by the Lorentz transformation $\Lambda$.

  3. We notice that whatever $f_\Lambda$ is, it should preserve quantum-mechanical transition probabilities. In other words, for each $\Lambda$, $f_\Lambda$ should be a symmetry in the general quantum mechanical sense defined by the preservation of transition probabilities.

  4. We recall that Wigner's Theorem guarantees that any such symmetry can be represented by a unitary or anti-unitary operator up to phase.

  5. We argue no $f_\Lambda$ can be anti-unitary (I actually have forgotten the conventional argument for this, perhaps you can try to fill in this detail before I do). So we use the notation $f_\Lambda = U(\Lambda)$ instead to emphasis this.

  6. We consider three inertial observers $A$, $B$, and $C$. We let $\Lambda_{ij}$ be the Lorentz transformation that connects the spacetime measurements of observer $j$ to those of $i$, so, for example, $\Lambda_{AB}$ is the Lorentz transformation connecting the spacetime observation of observer $B$ to those of $A$.

  7. We note that the state measurements of observers $A$ and $C$ are related by $U(\Lambda_{AC})$. Moreover, we expect that if we transform the state measurements of observer $A$ to those of $B$ with $U(\Lambda_{AB})$, and then transform those measurements to those of $C$ with $U(\Lambda_{BC})$, then we should get the same answer up to phase (aka a physically equivalent state), namely \begin{align} U(\Lambda_{AC}) = c(\Lambda_{BC}, \Lambda_{AB})U(\Lambda_{BC})U(\Lambda_{AB}). \end{align} where the "up to phase" part comes from the fact that states that differ by a phase are physically equivalent in quantum mechanics. But now we note that $\Lambda_{AC} = \Lambda_{BC}\Lambda_{AB}$, so we get the homomorphism property up to phase \begin{align} U(\Lambda_{BC}\Lambda_{AB}) = c(\Lambda_{BC}, \Lambda_{AB})U(\Lambda_{BC})U(\Lambda_{AB}) \end{align} and we're therefore done.

  8. Related post (to better understand the "projective" part): Idea of Covering Group

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  • $\begingroup$ Thanks but I think you're missing the point. Your argument shows that if one were to implement Lorentz transformations in Hilbert space they ought to use Unitary operators. My question is concerned with whether we can define such unitary representations for any given field theory, or how does the existence of such representations restrict the definition of the field theory. $\endgroup$ – zzz May 21 '14 at 2:44
  • $\begingroup$ Also, I applaud you for the motivation in points 1-6, but we can reach the homomorphic property in 7 by simply demanding the map to the Unitary operators implementing the Lorentz transformations to be a representation of the Lorentz group. To me this demand is well-motivated already. $\endgroup$ – zzz May 21 '14 at 2:52
  • $\begingroup$ in fact we can further demand that it's an isomorphic representation on well-motivated grounds- namely that each Lorentz transformation has a unique action (one to one), and since it's a representation the image of the map is closed under subsequent application of the unitaries, hence we can restrict the target to this set and call the map onto. $\endgroup$ – zzz May 21 '14 at 2:53
  • $\begingroup$ @bechira I see, yes I misread the question. I disagree about simply demanding the map be a representation; I personally feel that the homomorphism property is ill-motivated from a physical perspective without argumentation as in points 1-6. $\endgroup$ – joshphysics May 21 '14 at 17:00
  • $\begingroup$ To complete joshpysics' answer, I would recommend to take a look at the QFT book by Weinberg, which contains a lot of details on this subject. In particular, chapters 2, 4, and 5 contain relevant information about unitary transformations and representations. $\endgroup$ – Stan Oct 22 '14 at 8:36

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