# If electrons were just positrons moving backwards in time, then shouldn't we see them coming out of black holes?

I have read this question (What would happen if I crossed the shell of a hollow black hole?):

In effect, the formerly spacelike direction inwards becomes timelike, and the singularity is in the future (and just as unavoidable as the future is for us). For someone "inside" the shell, the shell is in the past, and trying to go back to it is as futile as trying to go back to yesterday.

Therefore, time reversal turns electrons into positrons.

If you apply the time reversal operator (which is a purely mathematical concept, not something that actually reverses time), you reverse the direction of the current flow, which is equivalent to reversing the sign of the conserved quantity, thus (roughly speaking) turning the particle into its antiparticle.

As far as I understand, inside a black hole, the spatial and temporal dimensions behave oddly, and we get a world where the singularity is basically a future point in time. Now if positrons (antimatter) are just electrons (normal matter) moving backwards in time, then this could mean that an electron moving towards the singularity (future) is like a positron climbing out of a black hole (towards the past).

But if this were true, then shouldn't we see positrons coming out of black holes? The last answer specifically says that if we apply the time reversal operator to an electron, then it changes the direction (spatial) of that particle and turns it into a positron. Now inside a black hole, the spatial dimension acts like the temporal, and if the positron is moving in the opposite direction (away from the singularity) as the electron was moving (towards the singularity), then you would get a positron climbing out of the black hole.

Question:

1. If electrons were just positrons moving backwards in time, then shouldn't we see them coming out of black holes?
• We do see positrons moving out of a black hole backward in time. They appear to us as electrons moving to the black hole forward in time. Sep 29, 2021 at 8:33
• Why would your time vector switch 180 degrees at the event horizon, rather than (for instance) 90 degrees, changing places with a spatial dimension? Sep 29, 2021 at 13:43
• In quantum mechanics "happening" vs "not happening" can be hard to define. If virtual particles were actually popping into and out of existence, or electrons in the ground state were actually moving in circles, it would shake all matter violently and heat everything up very quickly. (This is in contrast to i.e. a reversible chemical reaction where thermal motions are actually making molecules move back and forth). Similarly, particles "experiencing time backwards" is meaningless because they have no "memory" of what happened to them. Sep 29, 2021 at 19:09
• If I'm not mistaken, this idea of time-flipped particles comes from John Wheeler's one-electron universe postulate. It imagines a particle as a spaghetti-like 4-dimensional object that you see slices of as you move through time; in this picture the positron is a time-reversed electron in the sense that the spaghetti thread has a reversed orientation (it's "going" the other way in spacetime), but your intersection with this thread develops depending on your own time direction, and is not moving out of the black hole. Oct 1, 2021 at 5:20

The short answer to your question is that positrons are not really electrons moving backward in time, and the premise of your argument doesn't work. However, something like what you are saying, is responsible for Hawking radiation.

Slightly longer...

There are a set of words you can give when you do QED in flat space along the lines of "positrons are electrons moving backward in time", but you really shouldn't take these words too seriously. Even ignoring gravity, electrons can be converted into neutrinos and quarks when you include the weak interactions (beta decay, inverse beta decay), so the notion that there is only one electron in the world that is jittering backward and forward in time every time a photon is emitted just doesn't work. The closest you can get is the CPT theorem... since quantum field theory is invariant under reversal of charge, parity, and time, then time reversal (T) is equivalent to reversing charge and parity (CP), which formally exchanges particles with antiparticles. But there's no way to actually implement a T transformation in reality. In controlled conditions, like a particle accelerator, you can set up an experiment done with one set of particles and the same experiment done with the CP-transformed particles to see what happens, and mathematically the results will be the same as if you had applied a T transformation, but at no point has time actually been reversed.

Now, there is something funny going on quantum mechanically with the fact that timelike and spacelike directions are switched beyond the event horizon of a black hole. But since we are sophisticated enough to realize that positrons are not electrons flowing back in time, we know it's not quite as simple as saying positrons will flow out of the event horizon. If you work through the math, you will find the implication of the horizon is that there are modes which have a negative frequency with respect to an observer at infinity. Performing a Bogoliubov transformation, this means that the state that looks like a vacuum to an observer near the event horizon, will look like it has particles to an observer at infinity. This is in fact Hawking radiation, and (in very crude terms) this is the strategy Hawking used to discover Hawking radiation in his original paper on the subject.

If you are really insistent, you can use these words to very crudely describe Hawking radiation: "particle-antiparticle pairs pop into and out of existence in the quantum vacuum, and near the event horizon sometimes a particle will escape the black hole and an anti-particle will fall in, or vice versa." You can loosely map these words onto the positive and negative frequency modes. But, like with "positron = electron moving backward in time" or "Feynman diagrams show trajectories of particles in spacetime," I would treat this more as a colorful analogy, than a rigorous description of what the math really says is happening.

• +1, but one very minor quibble: In what sense are timelike and spacelike directions switched beyond the horizon? I assume you're referring to the fact that the traditional coordinates switch roles when crossing the horizon, but that's a feature of the coordinate system, not an intrinsic feature of the spacetime. If we really wanted to, we could choose a coordinate system for Minkowski spacetime that has a similar feature ($\partial/\partial t$ is timelike in one region and spacelike in another region, and conversely for $\partial/\partial r$). Sep 29, 2021 at 3:36
• @ChiralAnomaly That's a good point. What I have in mind is that the modes which are positive frequency using the natural time coordinate near the horizon, are a mix of positive and negative frequency modes using coordinates asymptotically far away, which leads to the non-trivial Bogoliubov transformation. The coordinate-independent way to say this is in terms of vacuum states. It's a little late for me to think this through cogently now, but I'll think about a better way to phrase this that makes it less dependent on coordinates. Sep 29, 2021 at 4:29
• @ChiralAnomaly is there an explicit example of such a coordinate system? Sep 29, 2021 at 7:58
• @Ruslan There's a horizon in Rindler coordinates. Sep 29, 2021 at 13:34
• @Ruslan Here's a fun example: start with the usual Minkowski coordinates in which the metric is $dt^2-(dx^2+dy^2+dz^2)$. Let $\theta$ be any smooth function of $t^2+x^2$, and define new coordinates $T,X$ by $$\left(\begin{matrix}T\\X\end{matrix}\right) =\left(\begin{matrix}\cos \theta&\sin \theta\\-\sin \theta&\cos \theta\end{matrix}\right) \left(\begin{matrix}t\\x\end{matrix}\right).$$ You choose the function $\theta$ to make $\partial/\partial T$ be timelike near the origin and spacelike in some places farther from the origin, and conversely for $\partial/\partial X$. Sep 30, 2021 at 0:03

If electrons were just positrons moving backwards in time, then shouldn't we see them coming out of black holes?

If you time reverse an electron falling into a black hole you don’t get a positron coming out of a black hole. Instead you get a positron coming out of a white hole.

You cannot time reverse just the electron, you have to time reverse the black hole too.

• Thank you so much! Can you please elaborate on this "positron coming out of a white hole.", so it is possible for a positron to come out of a white hole, but an electron cannot enter a white hole correct? Sep 29, 2021 at 3:45
• @ArpadSzendrei correct. Nothing can enter a white hole, everything goes out. Nothing can leave a black hole, everything goes in. They are time reverses of each other.
– Dale
Sep 29, 2021 at 3:50
• ... and what is a time-reverse of the Hawking radiation? :) Sep 29, 2021 at 10:00
• @lalala no, there is no evidence for their existence and good theoretical reasons to doubt their existence
– Dale
Sep 29, 2021 at 13:20
• @fraxinus there is no particular name for it. But quantum mechanically particles can go inside a white hole. See arxiv.org/pdf/1711.09625.pdf Oct 1, 2021 at 16:07

When we say “a positron is an electron moving backwards in time,” we’re really talking about symmetries. There are three transformations that we can apply to a field in quantum electrodynamics:

• charge conjugation, $$C$$, reverses the signs of all quantum numbers: electric charge, strangeness, charm, lepton number, etc.

• parity $$P$$, or more descriptively “space inversion,” changes from a right-handed coordinate system to a left-handed coordinate system.

• time reversal, $$T$$, reverses the flow of time.

The “spinors” that are described by the Dirac equation have two negative-charged components and two positive-charged components. Dirac correctly identified these as the electron and positron, even though the latter hadn’t been discovered yet. At high momentum, each same-charged pair separates into a left-handed helicity state and a right-handed helicity state. So to switch the roles of the particle and the antiparticle, it’s not enough just to swap the charges: we have to swap the helicity components as well. When we say “the transformation $$\mathit{CP}$$ changes a particle into an antiparticle,” we are referring to Dirac spinors and their descendants in quantum field theory.

It is a nontrivial result that the combined transformation $$\mathit{CPT}$$ is a symmetry of any model with Lorentz symmetry — which includes special relativity and (locally) general relativity. When we say “a positron is an electron moving backwards in time,” we are talking about the symmetries in our models of these objects. If we predict that an electron would do such-and-such a thing, switched to talking about positrons by applying $$\mathit{CP}$$, and ran time backwards by applying $$T$$, relativity demands that our model would make all of the same predictions.

We know that $$\mathit{CP}$$ is not an exact symmetry of our universe, because our universe has an excess of matter over antimatter. We have reason to believe that all of the $$\mathit{CP}$$ violation can be parameterized as hiding within the short-distance strong and weak interactions; the classical interactions of electromagnetism and general relativity are unchanged if you swap all the charges $$C$$, if you switch coordinate handednesses $$P$$, or if you run the clock backwards $$T$$. Black holes don’t preferentially consume matter particles over antimatter particles; their Hawking radiation shouldn’t preferentially contain one species, either.

A black hole whose Hawking temperature is comparable to the electron mass, $$kT \sim m_e c^2$$, will include electrons and positrons in its Hawking radiation, like any other sufficiently-hot blackbody. But the matter and antimatter particles will be created in equal numbers — apart from the $$\mathrm{CP}$$ violation of the vacuum where they are created.

• Thank you so much! Can you please elaborate on this : "We have reason to believe that all of the CP violation can be parameterized as hiding within the short-distance strong and weak interactions;"? Sep 29, 2021 at 3:50
• @ÁrpádSzendrei As I wrote in the answer, there is neither $C$ nor $P$ violation in electromagnetism or gravity. The weak interaction famously breaks $P$, but approximately conserves $\mathit{CP}$. Violations of $\mathit{CP}$ were first observed in the neutral kaon system; it’s a big field.
– rob
Sep 29, 2021 at 5:13