Time Travel, Parallel Universes (Alternate Realities) and the Greys

Time Travel in Science Fiction

It does not require a physicist to get excited about time travel. Time travel is one of the most popular themes in modern day science fiction genre. “The Time Machine, H. G. Wells, 1895″ is probably the first novel in which an instrument of time travel was described, although the notion of time travel appeared in many earlier stories such as a Japanese legend of a fisherman Urashima Taro or the story of Abimelech (the same person as Ebed-melech the Ethiopian of Jeremiah 38:7) in the pseudepigraphical text of the Old Testament Fourth Baruch. (It is part of the Ethiopian Orthodox Amharic Bible.) Examples of TV shows and Films where the notion of time travel appeared include: Back to the Future franchise (Film), Battlestar Galactica (TV), Butterfly Effect (Film), The Caller (Film), Continuum (TV), Doctor Who (TV), Eureka (TV), The Final Count Down (Film), Frequency (Film), Fringe (TV), Paradox (TV), Sliders (TV, Sliders is not really about time travel but about travel between parallel universes through wormholes), Star Trek franchise (TV, Film), Stargate Atlantis (TV), Stargate SG-1 (TV), Strange Days at Blake Holsey High (TV), Terra Nova (TV), Terminator franchise (Film, TV), Thrill Seekers (Film), Timeline (Film), The Triangle (TV Miniseries), X-Files (TV), etc. The most intriguing idea in time travel stories is that one travels back in time and alter the future. Is that really possible?

What Do Physicists Say About Time Travel?

There isn’t really a physical theory of time travel, so there is no consensus among physicists as to whether time travel is possible. But they mostly agree that traveling back in time is impossible. There is no physical or technical reason for this belief but it is mainly due to a paradox called grandfather paradox. Let us say you went back in time and killed your future grandfather before he ever made your father. Then of course you would not exist in the future. However, you, who traveled back in time, do exist therefore a paradox. In some TV shows or films, when something like this happens you would just disappear by some mysterious force of nature to self-correct herself from an anomaly. Note that grandfather paradox assumes that there is only one timeline which appears to be the case according to our perception. But who knows? J. B. S. Haldane (1892 – 1964) once said “The universe is not only stranger than we imagine, it is stranger than we can imagine.” Time may not be just one dimensional but may be multi-dimensional and if so, there may not be grandfather paradox after all.

On the other hand, Einstein’s theory of relativity allows us to travel forward in time. But it is a boring time travel.

Three Relativistic Effects

Albert Einstein (1879 - 1955)

Albert Einstein (1879 – 1955)

Imagine that you are traveling in a spaceship. According to the theory of relativity, you will experience three effects as the spaceship accelerates.

  • Time Delay

$$\Delta t’=\frac{\Delta t}{\sqrt{1-\frac{v^2}{c^2}}}$$

This means that your clock runs slower as the spaceship accelerates.

  • Contraction of Length

$$\ell=\ell_0\sqrt{1-\frac{v^2}{c^2}}$$

This means the spaceship gets shorter in length as it accelerates.

  • Increase of Mass

$$m=\frac{m_0}{\sqrt{1-\frac{v^2}{c^2}}}$$

This means that the spaceship gets heavier as it accelerates.

The time delay effect allows us to travel forward in time. Let us say you travel to a distant star which is 200 light years away from Earth at the speed of light. When you come back to Earth, you will realize that it is not Earth as you know. It will be future Earth 400 years later. No one that you once knew including all your loved ones will be alive to greet you. But for you only a few hours passed by due to acceleration (departure) and deceleration (turning and arrival). This is a very boring time travel if one sees it as a travel forward in time. In effect of time travel, it is not so different from cryogenic stasis though you are not in sleep while traveling. By the way, if you can travel at the speed of light, you won’t age while you are traveling, so you can literally live forever. Unfortunately, this won’t happen to anyone no matter how advanced our science and technology would become. As shown earlier, as your spaceship accelerates it gets heavier and heavier making it harder to accelerate more. This is not something we can overcome by advancement of technology, but is simply part of nature. The only things that can travel at the speed of light are particles whose rest mass is 0 (massless particles) such as photons.

A Rebellion: Gödel Universe

Kurt Gödel (1906 - 1978)

Kurt Gödel (1906 – 1978)

Inspired by Einstein’s theory of general relativity, Kurt Gödel considered a model of universe that contains closed timelike curves. So, his model allows travel backward in time. Gödel also believed that one may travel back in time, influence past events and alter the future as a consequence. While Gödel universe is a solution of Einstein’s gravitational field equation, most physicists consider the model as unphysical.

Time Machine

It appears that at least at conceptual level the only thing that prevents us from traveling back in time is grandfather paradox or its variants. For now, let us forget about grandfather paradox. Is it then even remotely possible to build a time machine? The answer is affirmative and our time machine is called a wormhole. So what is a wormhole? A wormhole is a tubelike region in space-time which could be used as a shortcut to travel to a distant galaxy (as we have seen in the TV show Stargate) or could be used as a time machine. Wormholes are also solutions of Einstein’s gravitational field equation, but at the moment they are only mathematical solutions and there is no known physical reason as to why they must exist, nor is there known mechanism as to how they can be created. Although it is only a speculation, a wormhole may be created from a massive black hole.

Black Holes, White Holes, and Wormholes

When a massive star dies, it collapses due to the influence of its own gravity. This phenomenon is known as gravitational collapse. If the star is very massive, gravitational collapse leads to a singularity in space-time called, a black hole. It is called a black hole because even light is sucked into the black hole making it completely invisible to our naked eyes. According to Einstein’s general theory of relativity, gravitational field bends space-time around it i.e. it not only bends space but also bends time. As a result, there is time delay effect due to the presence of gravitational field: the stronger the gravitational field is the slower your clock runs. When a black hole is created, due to its massive gravitational field, your clock runs slower as you get closer to the black hole and it will eventually stop as you approach its event horizon. (Event horizon is point of no return. Once you pass through even horizon, there is no turning back, no way out of the black hole.) Since the space-time has no edges, the trajectories of particles fell into the event horizon of a black hole must be continued unless particles hit the black hole singularity. This requirement hints us that there may be an exit for these particles, the black hole’s counterpart, called a white hole and that the black hole and its counterpart white hole are connected by a tubelike region called a Lorentzian wormhole (also called a Schwarzchild wormhole or Einstein-Rosen bridge). We do not know yet what happens inside a black hole once objects enter its event horizon. So this is only a speculation but past the singularity time can be bent in either future or past direction. In other words, the white hole may appear somewhere at some time i.e. at an event in the future or at an event in the past. I will explain later but due to causality I claim that time can be bent only in past direction. I also speculate that warping space and time would result fluctuations in the space-time and when two local regions of the space-time get close enough, there can be an attractive force (a reminiscence of Casimir effect) between the two regions causing the creation of a wormhole connecting to a white hole at an event in the past.

A Lorentzian Wormhole

A Lorentzian Wormhole

Causality

Every effect is followed by a cause. This is called causality in physics. For us, the future does not exist yet and so due to causality a wormhole cannot connect to an event in the future. Hence, we cannot travel forward in time through a wormhole. But the creation of a white hole at an event in the past also violates causality since the white hole did not exist at that event in the past. How do we cope with the change of history?

Chronology Protection Conjecture

Regarding time travel there are currently four different conjectures.

  • The radical rewrite conjecture: One can travel forward and back in time and rewrite the history. (All Hell breaks loose case.)
  • Novikov’s consistency conjecture: The Universe is consistent, so whatever temporal transpositions and trips one undertakes, events must conspire in such a way that the overall result is consistent.
  • Hawking’s chronology protection conjecture: The cosmos works in such a way that time travel is completely and utterly forbidden. This conjecture permits spacewarps and wormholes but forbids timewarps and time machines.
  • The boring physics conjecture: There are no wormholes nor spacewarps. There are no timewarps nor time machines.

Protect History at All Cost!

I am personally fond of Novikov’s consistency conjecture. In fact, I require a stronger version of Novikov’s consistency conjecture which does permit timewarps and time machines but does not permit any violation of causality nor any tampering with already recorded history. I speculate that as soon as any tampering with recorded history occurs such as the appearance of a white hole at some time and place in the past caused by a timewarp from the future, a new timeline must be created in order to prevent history from being rewritten. As a consequence, a new reality is created from that moment completely separately from the previously existing reality. This new reality may turn out to be pretty much similar to the previous reality, or it may turn out to be very different. Since timeline is split, whatever you do once you travel back in time through the wormhole, your action would only influence this new reality but not the original reality you are from. There will be no grandfather paradox or its variants and therefore there is nothing that will prevent you from traveling back in time and messing around with the past (new reality) such as killing your future grandfather or even killing your other yourself in this new reality. (Why anyone with right state of mind would do that though?) Suppose that I am right that the creation of a black hole leads to a timewarp into the past. Ever since the universe was created 15 billion years ago, many black holes must have been created. With my consistency conjecture, I can assert that there have already been many parallel universes (different realities) resulted from timewarps and that the universe we are living in (our reality) is very much likely not the one that was created for the first time. In other words, we are living in just one of many alternate realities.

Parallel Universes and Many-Time Physics

The creation of new realities results an emergence of new physics which I call many-time physics. If there are many different realities (parallel universes), the ambient space for the theory of physics should be $\mathbb{R}^{3+q}$ where $q>1$ is the time dimension. This new theory must be consistent with the current theory of physics on each reality i.e. the 4-dimensional space-time $\mathbb{R}^{3+1}$. It would be really interesting to see if different realities can interact with each other.

Tachyons

Tachyons are hypothetical FTL (Faster Than Light) particles whose notion appears a lot in science fiction. But the notion of FTL particles did not really come from science fiction. Tachyons were originally introduced by a Columbia University physicist Gerald Feinberg in 1967 as particles with imaginary rest mass. He thought they were FTL particles but it turned out that such particles do not propagate faster than light. Tachyons still appear in physical theories such as string theory. But they meant to be particles with imaginary rest mass. Here I use tachyons as generic name for FTL particles as in science fiction. These tachyons, if they exist, would not violate the theory of relativity as their initial velocity has already exceeded the speed of light when they were born. But there is no physical reason as to why they must exist or how they can be created. Tachyons are, if they exist, believed to travel back in time. Tachyons could be produced as a byproduct when a new reality is created because of a violation of causality. In fact, they may be the ordinary particles that have survived the singularity and traveled instantly from the future of one reality to the past of another reality through the wormhole. If so, it may be possible that one sends information from the future of one reality to the past of another reality through the wormhole, a message from the future!

Déjà Vu and Premonition: Messages from an Alternate Reality?

When you walk in the street you saw someone and you can be certain that you have never met the person in your life before. But the person looks so familiar and you cannot shake off the feeling that you somehow know the person. Or, you went to some place you have never been to before but everything looks so familiar and you cannot shake off the feeling that you know the place against the fact. What if it were true? It is just that it didn’t happen in this reality but it did happen in an alternate reality. Déjà vu or premonition that you are experiencing may be the messages you are receiving from the future of a different reality. Even if it were true and you were having a premonition that something bad might happen, you may not have to be serious about that though. What may have already happened in the future of a different reality may not necessarily happen in this reality. The reason for this is that every effect is followed by a cause (causality). Some causes are influenced by the decisions we make and some are random causes. Under the same circumstances, we may likely make the same decision (though I don’t believe it will always be the case), however there is no guarantee that there will be the same random causes at the same time and place in different realities. Otherwise, they wouldn’t be random. Someone like Michel de Nostredame (in French) or Nostradamus (in Latin) as more commonly known might have been tuned to receive messages from the future of another reality. Some events he described in his cryptic quatrains did actually occur but some didn’t including the grand quatrain about the end of the world that supposedly would (or could) have happened in 1999. For whatever reason, it didn’t happen, though it may have actually happened in another reality. How about the Mayan Calendar? Were they wrong? It may be possible that the Mayans were told by some beings (for them Gods) that the end of the world will come in the year 2012 and they believed it (of course because Gods said so). What if those beings the Mayans believed as Gods were time travelers from a distant future? They knew the world will end in 2012 and gave a warning to the Mayans because that is what actually happened in their reality. For some reason we don’t know (and we will perhaps never know), it did not happen in this reality.

J-Rod, a Grey Alien

There is an intriguing story floating around on the Internet about a grey alien called J-Rod. Before I continue, I want to make clear that I have absolutely no intention to get into Alien/UFO conspiracy theories. The reason I am trying to bring up a story about J-Rod is that there are some interesting aspects about this story of J-Rod in connection to my previous discussions. The story about J-Rod goes like this. 6 years after the famous Roswell crashes, there was another UFO crash near Kingman, Arizona in 1953. There were four grey aliens who survived the crash. One of them is J-Rod. This J-Rod worked as a technical adviser to the ultra-secret programs at Area 51. J-Rod does not speak and he communicated with his human collaborators only by telepathy. It turns out that he is not an alien but a human time traveler (or what has become of a human-being) from 50,000 years into the future. The whole race is facing dire medical conditions due to genetic mutations that threaten the entire civilization. (It sounds pretty much similar to the story about Asgards in Stargate or Stargate picked up the story of J-Rod?) They probably tried everything they could to save their dying race and none worked. The final solution they came up with is to come to our present time and harvest genetic materials from us (and even from other animals) to help repair damages in their bodies. (Perhaps this is the reason the greys have been abducting people and doing experiments on them against their will.)

Portals for Time Travel in the Milky Way Galaxy

According to J-Rod, there are naturally formed wormholes lurking in the Milky Way galaxy. I don’t know if they traveled through a naturally formed wormhole or an artificial wormhole or if they actually have ability to create one. Can one actually create an artificial wormhole like the one in Stargate? I don’t know the answer, but I can at least offer a speculation. Albert Einstein thought gravitation and electromagnetism are essentially the same force. Since he fled to America from Hitler’s Nazi regime in Germany, he had devoted the rest of his life to find the unified field theory that combines gravitation and electromagnetism. In spite of his efforts, he failed to find one, or did he? Modern physicists think that Einstein was wrong about his idea of the unified field theory. Regardless of a similarity between gravitation and electromagnetism as both being inverse-square laws, physicists don’t think they are the same force, at least not in this current universe (4 dimensional space-time). However, they believe that gravitation and electromagnetism along with the rest of known forces in the universe, weak and strong forces were the same one force before the Big Bang 15 billion years ago when the universe was 10 (or higher) dimensional space-time (Superstring Theory). But what if Einstein were right? There is a conspiracy theory that claims that Einstein indeed succeeded unifying gravitation and electromagnetism, and this led to the secret US military experiment called the Philadelphia Experiment carried out some time around October 28, 1943. The experiment was an attempt to cloak the U.S. Navy destroyer escort USS Eldridge by warping space using a strong magnetic field. Light would be bent around the ship along the warped space, so that the ship would become completely invisible to the enemy devices. But there was an unintended consequence. A wormhole was created and the ship was instantly teleported from the Naval Shipyard in Philadelphia to Norfolk, Virginia, over 200 miles away through the wormhole. If it were true, one can mimic gravitational field using strong magnetic field. Moreover, one may also create antigravity using electromagnetic field. While a naturally formed wormhole by a black hole is thought to be only one-way traffic, it may be possible to build an artificial wormhole that allows a time travel back and forth between the future and the past. Those folks who came from the future may be able to go back to the future through the artificial wormhole. How about us who are living in the present time? I am not so sure what might happen. There can be two different possible theories as to what might happen. One theory is that their future is not ours, so there will be no violation of causality and we also can travel to their future. Another theory is that although it is not our future, what happened in that reality might likely happen in our reality as well. So, if one travels to their future and comes back to our reality with knowledge from their future which may influence the course of our future, it could may well be a violation of causality. For that the Mother Nature may not allow us to travel to their future.

Time traveler with cell phone in 1928?

Time traveler with cell phone in 1928?

The Mayan Calendar and the End of the World in 2012?

According to J-Rod, something catastrophic happened in 2012 in their reality. It appears that there was some sort of solar superstorm at that time and their Earth got hit by the solar superstorm and their genetic mutations were caused by excessive amount of the radiation coming from the sun with the superstorm. Not all of them became like grey aliens.There is another group of humanoids, Nordic-looking humanoids. Both humanoid groups became technologically advanced but Nordic type humanoids became more spiritual. Earth eventually became inhabitable and both humanoid groups migrated to distant planets. Little grey type humanoids ended up on some planet in Zeta Reticuli which is about 39 light years away from Earth. Nordic type humanoids ended up on some planet in Orion’s Belt. This may indicate that those two groups of humanoids did not get along well with each other. It is possible that Mayans knew that the world (as we know it) will end in 2012 because they were told by these extraterrestrial beings who might have been perceived as Gods to the Mayans. In any event, as everybody knows such a catastrophe didn’t happen in 2012 in our reality. As I said earlier, things happened in one reality do not necessarily happen in another reality. What most of us didn’t know back then was that we could have had such a catastrophe but it was miraculously averted: we nearly missed a solar superstorm, one of the strongest ever recorded for the past 150 years, in 2012. When a powerful coronal mass ejection (CME) tore through Earth orbit in July 2012, Earth wasn’t there. However, if the eruption happened a week ago, Earth would’ve been directly hit. It might have been a sheer luck but we were spared. Now the sun became eerily quiet though supposedly it should be at its peak activity and intriguingly the famous sun spots have mostly disappeared. Now that they know the same disastrous catastrophe didn’t happen in this reality and Earth is still nicely habitable planet, the greys might consider mass migration into this reality if they prefer Earth over the planet in Zeta Reticuli. If so, all the nations on Earth might have to give the greys humanitarian (?) amnesty. If not, oh well, they might just take over Earth by force and use us as specimen for their cure. Let us hope that is not going to happen.

Suggested Reading

For those who would like to learn more about Black Holes, Wormholes and Time Travel, I recommend the following two books.

  1. Black Holes and Time Warps: Einstein’s Outrageous Legacy, Kip S. Thorne and Stephen Hawking (Foreword), W. W. Norton & Company, 1995 (for lay people)
  2. Lorentzian Wormholes: From Einstein to Hawking, Matt Visser, AIP Series in Computational and Applied Mathematical Physics, 2008 (for people with a substantial background in mathematics and physics)

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Real Structure, Fundamental Symmetry and Quantum Mechanics

In the standard Hermitian quantum mechanics, the space of states of a quantum mechanical system forms a complex Hilbert space in general. But often the space of states becomes a real vector space. When the space of states is a real vector space, an alternative quantum mechanics may be considered.

Let $\mathbb{V}$ be the complex vector space of states of a quantum mechanical system. Define a map $J: \mathbb{V}\longrightarrow \mathbb{V}$ by
$$J\psi=\bar\psi$$
for every $\psi\in \mathbb{V}$. Then $J$ is an anti-linear involution called a real structure on $\mathbb{V}$. A real structure on a complex vector space is a way to decompose the complex vector space $\mathbb{V}$ into the direct sum of two real vector spaces. Any $\psi\in \mathbb{V}$ may be written as
$$\psi=\psi^++\psi^-,$$
\begin{align*}
\psi^+&:=\frac{1}{2}(\psi+J\psi)=\frac{1}{2}(\psi+\bar\psi)=\psi_{\mathrm{re}},\\
\psi^-&:=\frac{1}{2}(\psi-J\psi)=\frac{1}{2}(\psi-\bar\psi)=i\psi_{\mathrm{im}}.
\end{align*}
$J$ satisfies the properties:
\begin{align*}
J\psi^+&=\psi^+,\\
J\psi^-&=-\psi^-.
\end{align*}
Hence, one obtains a direct sum of vector spaces
$$\mathbb{V}=\mathbb{V}^+\oplus \mathbb{V}^-,$$
where
\begin{align*}
\mathbb{V}^+&=\{\psi\in \mathbb{V}: J\psi=\psi\},\\
\mathbb{V}^-&=\{\psi\in \mathbb{V}: J\psi=-\psi\}.
\end{align*}
Both $\mathbb{V}^+$ and $\mathbb{V}^-$ are real vector spaces. $\mathbb{V}^+$ is isomorphic to $\mathbb{V}^-$ via the isomorphism $\psi\longmapsto i\psi$. So,
$$\dim_{\mathbb{R}}\mathbb{V}^+=\dim_{\mathbb{R}}\mathbb{V}^-=\dim_{\mathbb{C}}\mathbb{V}$$
if $\dim_{\mathbb{C}}\mathbb{V}<\infty$. Let $\mathbb{V}^+=\mathbb{V}_{\mathbb{R}}$. Then $\mathbb{V}^-$ can be written as $i\mathbb{V}_{\mathbb{R}}$, so $\mathbb{V}$ may be viewed as the complexification of $\mathbb{V}_{\mathbb{R}}$
$$\mathbb{V}_{\mathbb{R}}^\mathbb{C}=\mathbb{V}_{\mathbb{R}}\otimes_{\mathbb{R}}\mathbb{C}.$$
The real structure $J$ may be used to define an inner product $\langle\ ,\ \rangle$ on $\mathbb{V}$: For any $\varphi,\psi\in \mathbb{V}$,
\begin{align*}
\langle\varphi,\psi\rangle&=\int (J\varphi)\psi dV\\
&=\int\bar\varphi\psi dV.
\end{align*}
This inner product is of course the standard Hermitian product known as Dirac braket $\langle\ |\ \rangle$ to physicists. Therefore, the real structure gives rise to the Hilbert space structure on $\mathbb{V}$ as in the standard Hermitian quantum mechanics.

In studying a quantum mechanical system, it is not necessary to use complex-valued state functions but one may instead use real-valued state functions. For example, let us consider the Hamiltonian for quantum harmonic oscillator
$$\hat H=-\frac{\hbar^2}{2m}\frac{\partial}{\partial x^2}+\frac{m}{2}\omega^2\hat x^2.$$
The stationary Schrödinger equation $\hat H\psi=E\psi$ determines the eigenstates
$$\psi_n(\xi)=2^{-\frac{n}{2}}\pi^{-\frac{1}{4}}(n!)^{-\frac{1}{2}}e^{-\frac{\xi^2}{2}}H_n(\xi),$$
where $\xi=\sqrt{\frac{m\omega}{\hbar}}x$, and eigenvalues (energies)
$$E_n=\left(n+\frac{1}{2}\right)\hbar\omega$$
for $n=0,1,2,\cdots$. $H_n(x)$ are the Hermite polynomials and they satisfy the parity relation
$$H_n(x)=(-1)^nH_n(-x).$$
The eigenstates $\psi_n(\xi)$ forms a real Hilbert space with the usual Hermitian product (Dirac braket).

This time, let $\mathbb{V}$ be the real vector space of states of a quantum mechanical system. Let $\varphi:\mathbb{R}^3\longrightarrow\mathbb{R}$. $\varphi$ is said to be even under parity if $\varphi(-\mathbf{r})=\varphi(\mathbf{r})$. $\varphi$ is said to be odd under parity if $\varphi(-\mathbf{r})=-\varphi(\mathbf{r})$. Any real-valued function defined on $\mathbb{R}^3$ may be written as the sum of an even function and an odd function. Any $\varphi(\mathbf{r})\in \mathbb{V}$ can be written as
$$\varphi(\mathbf{r})=\frac{1}{2}[\varphi(\mathbf{r})+\varphi(-\mathbf{r})]+\frac{1}{2}[\varphi(\mathbf{r})-\varphi(-\mathbf{r})].$$
$\frac{1}{2}[\varphi(\mathbf{r})+\varphi(-\mathbf{r})]$ is an even function and $\frac{1}{2}[\varphi(\mathbf{r})-\varphi(-\mathbf{r})]$ is an odd function. Define a map $J: \mathbb{V}\longrightarrow \mathbb{V}$ by
$$J\varphi(\mathbf{r})=\varphi(-\mathbf{r})$$
for every $\varphi(\mathbf{r})\in \mathbb{V}$ i.e. $J$ is the parity. Then $J$ is a linear involution. Any $\varphi(\mathbf{r})\in \mathbb{V}$ may be written as
$$\varphi(\mathbf{r})=\varphi^+(\mathbf{r})+\varphi^-(\mathbf{r}),$$
where
\begin{align*}
\varphi^+(\mathbf{r})&=\frac{1}{2}[\varphi(\mathbf{r})+J\varphi(\mathbf{r})]=\frac{1}{2}[\varphi(\mathbf{r})+\varphi(-\mathbf{r})],\\
\varphi^-(\mathbf{r})&=\frac{1}{2}[\varphi(\mathbf{r})-J\varphi(\mathbf{r})]=\frac{1}{2}[\varphi(\mathbf{r})-\varphi(-\mathbf{r})].
\end{align*}
We have
\begin{align*}
J\varphi^+(\mathbf{r})&=\varphi^+(\mathbf{r}),\\
J\varphi^-(\mathbf{r})&=-\varphi^-(\mathbf{r}).
\end{align*}
The map $J$ is called the fundamental symmetry (see [1] for example). One gets a direct sum of vector spaces
$$\mathbb{V}=\mathbb{V}^+\oplus \mathbb{V}^-,$$
where
\begin{align*}
\mathbb{V}^+&=\{\varphi\in \mathbb{V}: J\varphi=\varphi\},\\
\mathbb{V}^-&=\{\varphi\in \mathbb{V}: J\varphi=-\varphi\}.
\end{align*}
If $\varphi(\mathbf{r})$ is even, then $\nabla\cdot\varphi(\mathbf{r})$ is odd and if $\varphi(\mathbf{r})$ is odd, then $\nabla\cdot\varphi(\mathbf{r})$ is even. If we say two functions $\varphi_1(\mathbf{r})$ and $\varphi_2(\mathbf{r})$ are identical if $\varphi_1-\varphi_2$ is constant, then $\mathbb{V}^+$ is isomorphic to $\mathbb{V}^-$ via the linear map $\nabla\cdot\varphi(\mathbf{r})$.

The fundamental symmetry may be used to define an inner product $\langle\ ,\ \rangle$ on $\mathbb{V}$ as follows: For any $\varphi(\mathbf{r}),\psi(\mathbf{r})\in \mathbb{V}$,
\begin{align*}
\langle\varphi(\mathbf{r}),\psi(\mathbf{r})\rangle&=\int (J\varphi(\mathbf{r}))\psi(\mathbf{r})dV\\
&=\int\varphi(-\mathbf{r})\psi(\mathbf{r})dV.
\end{align*}
For any $\varphi^+(\mathbf{r})\in \mathbb{V}^+$, $\varphi^-(\mathbf{r})\in \mathbb{V}^-$, we have
\begin{align*}
\langle\varphi^+(\mathbf{r}),\varphi^+(\mathbf{r}\rangle&=\int[\varphi^+(\mathbf{r})]^2dV\geq 0,\\
\langle\varphi^-(\mathbf{r}),\varphi^-(\mathbf{r})\rangle&=-\int[\varphi^-(\mathbf{r})]^2dV\leq 0
\end{align*}
We may assume that each $\varphi(\mathbf{r})$ is separable i.e. $\varphi(\mathbf{r})=X(x)Y(y)Z(z)$. ($\varphi(\mathbf{r})$ is a solution of the stationary Schrödinger equation $\hat H\varphi=E\varphi$ and this is a condition that can be imposed when we solve the equation.) Furthermore, we may assume that each of $X(x)$, $Y(y)$ and $Z(z)$ is either even or odd. Since $\varphi(-\mathbf{r})=X(-x)Y(-y)Z(-z)$, if $\varphi(-\mathbf{r})$ is odd, then at least one of $X(x)$, $Y(y)$, $Z(z)$ must be odd. For any $\varphi^+(\mathbf{r})\in \mathbb{V}^+$, $\varphi^-(\mathbf{r})\in \mathbb{V}^-$, $\varphi^+(\mathbf{r})\varphi^-(\mathbf{r})$ is odd. Let us write $\varphi^+(\mathbf{r})\varphi^-(\mathbf{r})=X(x)Y(y)Z(z)$ and say $X(x)$ is odd. Then
\begin{align*}
\langle\varphi^+(\mathbf{r}),\varphi^-(\mathbf{r}\rangle&=\int \varphi^+(\mathbf{r})\varphi^-(\mathbf{r})dV\\
&=\int_{-\infty}^\infty X(x)dx\int_{-\infty}^\infty Y(y)dy\int_{-\infty}^\infty Z(z)dz\\
&=0.
\end{align*}
Hence, $\varphi^+(\mathbf{r})$ and $\varphi^-(\mathbf{r})$ are orthogonal for all $\varphi^+(\mathbf{r})\in \mathbb{V}^+$, $\varphi^-(\mathbf{r})\in \mathbb{V}^-$, and so the direct sum $\mathbb{V}^+\oplus \mathbb{V}^-$ can be replaced by the orthogonal sum $\mathbb{V}^+\dotplus \mathbb{V}^-$. Note that $\langle\ ,\ \rangle$ is indefinte, so the fundamental symmetry gives rise to a Krein space structure rather than a Hilbert space structure on $\mathbb{V}$. Here, we are using a particular fundamental symmetry which is the parity, however our discussion may be generalised in terms of an arbitrary fundamental symmetry $J$. Although it may seem redundant, physically the inner product needs to be redefined as
\begin{align*}
\langle\varphi(\mathbf{r}),\psi(\mathbf{r})\rangle&=\int\overline{J\varphi(\mathbf{r})}\psi(\mathbf{r})dV\\
&=\int\bar\varphi(\mathbf{r})\psi(\mathbf{r})dV
\end{align*}
for any $\varphi(\mathbf{r}),\psi(\mathbf{r})\in \mathbb{V}$. The reason is that in quantum mechanics $\langle\ ,\ \rangle$ often interacts with operators that may be complex.

In the above example of quantum harmonic oscillator, let $\psi_n^+(\xi)=\psi_{2n}(\xi)$ and $\psi^-_{2n+1}(\xi)$ for $n=0,1,2,\cdots$. Then we have $\langle\psi^+_m,\psi^+_n\rangle=\delta_{mn}$, $\langle\psi^-_m,\psi^-_n\rangle=-\delta_{mn}$, and $\langle\psi^+_m,\psi^-_n\rangle=0$. Let $\mathbb{V}^+$ and $\mathbb{V}^-$ be spaces spanned by the orthogonal bases $\mathcal{B}^+=\{\psi^+_n(\xi): n=0,1,2,\cdots\}$ and $\mathcal{B}^-=\{\psi^-_n(\xi): n=0,1,2,\cdots\}$, respectively. Then $\mathbb{V}=\mathbb{V}^+\dotplus \mathbb{V}^-$ is a Krein space.

When the state functions of a quantum mechanical system are real-valued, the state functions may form a real Hilbert space or a Krein space depending on the choice between the inner products we discussed above. Hence, one may expect to have a quantum theory alternative, in which case a Krein space arises, to the standard Hermitian quantum mechanics. This alternative quantum theory is PT-Symmetric Quantum Mechanics which was first introduced by Carl M. Bender. PT-symmetric quantum mechanics has been promoted as a new quantum theory which admits a certain type of complex non-Hermitian Hamiltonians, the so-called PT-symmetric Hamiltonians. I recently showed that PT-symmetric quantum mechanics is indeed a Hermitian quantum mechanics and that those complex non-Hermitian (they are actually Hermitian) Hamiltonians are not really physical. (They would result the violation of unitarity.) After all, once disregarding those unphysical complex Hamiltonians, this so-called PT-symmetric quantum mechanics is not that much different at all from the standard Hermitian quantum mechanics. I will write more about this some other time. In the meantime, interesting readers may be referred to my recent papers [2], [3] on the issue.

Update 1: To clarify, here I meant a PT-symmetric Hamiltonian being Hermitian by it being self-adjoint with respect to the indefinite Hermitian inner product above. That’s why I said “PT-symmetric quantum mechanics is a Hermitian quantum mechanics.”

Update 2: My stance on PT-symmetric quantum mechanics has changed as I have a much better understanding of what’s going on. I considered complex PT-symmetric Hamiltonians were unphysical due to physicists’ insistance on using definite inner product because they want to interpret $|\psi|^2$ as probability like the case in ordinary quantum mechanics. But if we can relax about the meaning of $|\psi|^2$ as not necessarily probability but something that needs to be preserved under time evolution, the issue of unitarity violation can be resolved and complex PT-symmetric Hamiltonians become physical. It is possible that the interpretation of $|\psi|^2$ being probability is valid only in the ordinary quantum mechanics and if there is a new formulation of quantum mechanics, perhaps it may require a new set of rules and interpretations.

References:

[1] János Bognár, Indefinite Inner Product Spaces, Springer-Verlag 1974
[2] Sungwook Lee, $PT$-symmetric quantum mechanics is a Hermitian quantum mechanics, arXiv:1312.7738 [quant-ph]
[3] Sungwook Lee, On Finite $J$-Hermitian Quantum Mechanics, arXiv:1401.5149 [quant-ph]

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Doing Quantum Physics with Split-Complex Numbers

Theory of Photons (Electromagnetism) and Wave-Particle Duality of Light

Earlier investigations on the nature of light show that, light must be described by electromagnetic waves or by particles (wave-particle duality). de Broglie hypothesized that what is true for photons should be valid for any particle. We may assign a particle with mass $m$, propagating uniformly with velocity $v$ through field-free space, an energy $E$ and momentum $\mathbf{p}$. In the wave picture, the same particle may be described by a frequency $\omega$ and a wave vector $\mathbf{k}$.  We require that these quantities satisfy the equations
$$E=\hbar\omega,\ \mathbf{p}=\hbar\mathbf{k}.$$
In fact, these equations are satisfied by the photon and that a photon is described by the plane wave
$$\psi(\mathbf{r},t)=A\exp[i(\mathbf{k}\cdot\mathbf{r}-\omega t)].$$
Following de Broglie, to every free particle, a plane wave shown above is assigned. So, naturally quantum mechanics was formulated in terms of complex numbers.

The Trouble with Quantum Mechanics

Quantum mechanics as it was formulated appears to be seamless and it has had decades of success with its great advancement as theory of matter and also with its so many wonderful applications in science and engineering. Nevertheless, there are still some fundamental issues with current quantum mechanics. One issue is about antiparticles. In non-relativistic quantum mechanics, there is no clear indication that antiparticles exist. In relativistic quantum mechanics, antiparticles are considered as ones that have negative energies (due to P.A.M. Dirac) or equivalently ones that travel back in time. (Since wave functions themselves are not observables, those particles travel back in time do not violate causality.) But then this may imply that antiparticles have negative rest masses. So to me, considering antiparticles as particles that have negative energies appears to be unnatural and unphysical. Another issue is about the path integral. In quantum mechanics, the amplitude of a particle to propagate from a point $q_I$ to a point $q_F$ in time $T$ is given by
\begin{equation}\label{eq:pathint}\langle q_F|e^{-\frac{i}{\hbar}\hat HT}|q_I\rangle=\int Dq(t)e^{\frac{i}{\hbar}\int_0^TdtL(\dot q,q)}\end{equation}
where $L(\dot q,q)$ is the Lagrangian
$$L(\dot q,q)=\frac{m}{2}\dot q^2-V(q)$$
and $Dq(t)$ is the Feynman measure given by
$$\int Dq(t):=\lim_{N\to\infty}\left(\frac{-im\hbar}{2\pi\delta t}\right)^{\frac{N}{2}}\left(\prod_{k=1}^{N-1}\int dq_k\right)$$
with $\delta t=\frac{T}{N}$. This path integral, while it makes perfect sense physically, does not converge due to the oscillatory factor appeared as the integrand. What Physicists do about this problem is to take the Wick rotation $t\mapsto it$ which turns Minkowski spacetime to Euclidean spacetime. Accordingly, the path integral turns into Euclidean path integral
$$\langle q_F|e^{-\frac{i}{\hbar}\hat HT}|q_I\rangle=\int Dq(t)e^{-\frac{1}{\hbar}\int_0^TdtL(\dot q,q)}.$$
The integrand becomes a decaying exponential whose maximum value occurs at the minimum of the Euclidean action. Most physicists appear to be satisfied with this resolution, however to me it is troublesome that the path integral \eqref{eq:pathint} cannot be calculated in actual spacetime and that it must be calculated in Euclidean spacetime which is not physical spacetime. Besides, most Euclidean solutions are approximations and there is no guarantee that these solutions will be stable when they are brought to Minkowski spacetime. Furthermore, analytic continuation via Wick rotation works when the spacetime is flat. So Euclideanization will have a problem when the spacetime is curved i.e. gravitation is considered.

$i=\sqrt{-1}$ is the Problem!

I believe that this issue with the path integral has its origin in the way our current quantum mechanics was built. It was built upon complex numbers! And the reason complex numbers entered in the formulation of quantum mechanics is that the founders used the theory of photons (electromagnetism) to build the theory of matter, i.e. theory that explains the physics of other particles, in particular of massive particles such as electrons. However, it may be that complex numbers are really for light (photons) and they are not meant for describing other particles, especially massive particles.

Complex Numbers are Really for Light

Maxwell’s equations in vacuum are:
\begin{align*}
\nabla\cdot\mathbf{B}&=0,\ \nabla\times\mathbf{E}+\frac{\partial\mathbf{B}}{\partial t}=0\\
\nabla\cdot\mathbf{E}&=0,\ \nabla\times\mathbf{B}-\frac{\partial\mathbf{E}}{\partial t}=0
\end{align*}
The transformation
$$\mathbf{B}\mapsto\mathbf{E},\ \mathbf{E}\mapsto -\mathbf{B}$$
takes the first pair of equations to the second and vice versa. This symmetry is called Electric-Magnetic Duality. The duality hints that the electric and magnetic fields are part of a unified whole, the electromagnetic field. Let us introduce a complex-valued vector field
$$\mathcal{E}=\mathbf{E}+i\mathbf{B}.$$
Then the duality amounts to the transformation
$$\mathcal{E}\mapsto -i\mathcal{E}$$
and the vacuum Maxwell’s equations boil down to two equations for $\mathcal{E}$:
$$\nabla\cdot\mathcal{E}=0,\ \nabla\times\mathcal{E}=i\frac{\partial\mathcal{E}}{\partial t}.$$
Let $\mathbf{k}$ be a vector in $\mathbb{R}^3$ and let $\omega=|\mathbf{k}|$. Fix $\mathbf{E}\in\mathbb{C}^3$ with $\mathbf{E}\cdot\mathbf{k}=0$ and $\mathbf{E}\times\mathbf{k}=i\omega\mathbf{E}$. Then the plane wave
$$\mathcal{E}(\mathbf{r},t)=\mathbf{E}\exp[i(\mathbf{k}\cdot\mathbf{r}-\omega t)]$$
satisfies the vacuum Maxwell’s equations. That is, complex-valued plane wave functions may describe light (electromagnetic waves).

Here is another example that shows complex numbers are for light. The light cone in Minkowski spacetime $\mathbb{R}^{3+1}$ is the hyperquadric
$$\mathbb{N}^3=\{(t,x,y,z)\in\mathbb{R}^{3+1}: t^2-x^2-y^2-z^2=0\}.$$
Let $\mathbb{N}^3_+$ and $\mathbb{N}^3_-$ denotes the future and the past light cones, respectively. The multiplicative group $\mathbb{R}^+$ acts on $\mathbb{N}^3_+$ and $\mathbb{N}^3_-$ respectively by scalar multiplication. The orbit spaces $\mathbb{N}^3_+/\mathbb{R}^+$ and $\mathbb{N}^3_-/\mathbb{R}^+$ are identified with the two-sphere $S^2$. Physically this means that for an observer at the origin (the event), light rays through his eye correspond to null lines through the origin and the past null directions constitute the field of vision of the observer which is the two-sphere $S^2$. This two-sphere is called Celestial Sphere in astronomy. In mathematics, the two-sphere $S^2$ is the extended complex plane $\mathbb{C}\cup\{\infty\}$ called the Riemann sphere.

In the beginning, God might have said:

“Let there be complex numbers!”

Wave Functions are Real?

In current quantum physics, a wave functions itself is not considered as a physical reality but rather a manifestation of something that is both particle and wave. What if we assume that wave functions are real, say they represent actual waves in spacetime? If so, there can be wave functions that are split-complex-valued. I assert that split-complex numbers might have been the right choice for the theory of matter, theory of massless particles other than photons and particularly that of massive particles. It turns out that quantum mechanics can be completely rebuilt based upon split-complex numbers. This new quantum mechanics, split-Hermitian quantum mechanics, exhibits distinct features. In split-Hermitian quantum mechanics, antiparticles arise naturally. Remarkably, the path integral may be calculated in Minkowski spacetime without turning it into Euclidean path integral via Wick rotation. Furthermore, the path integral also can be transformed to the one looks just like Euclidean path integral, but it is still defined in Minkowski spacetime. Hence, what physicists have believe to be Euclidean path integral is a form of path integral defined in actual spacetime.

Baryon Asymmetry and Twin Universes

Split-Hermitian quantum mechanics may also offer an explanation on baryon asymmetry, i.e. an explanation as to why there aren’t as many antiparticles as particles in the universe (which is a fortunate thing for us). Although it is highly speculative at this juncture, split-Hermitian quantum mechanics appears to hint that Big Bang may have created twin (not identical though) universes, one with signature $(- + + +)$ made mostly of matter (our universe) and the other with signature $(+ – + +)$ made mostly of antimatter.

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