We’ve heard the story; a time traveler goes back in time, killing his grandfather. The upshot is that the time traveler ceases to exist. If the time traveler doesn’t exist, how could he have traveled back in time to kill his grandfather?
This logical paradox is known as the “Grandfather Paradox,” and although it makes for a great science fiction storyline — or a seriously creepy Futurama “Grandma Paradox” adaptation — it is a perplexing conundrum that has physicists scratching their heads.
If it is possible to travel back in time, wouldn’t that cause a tangle in time? If, in the future, something is sent to a date in the past, shouldn’t we already see it? How does the Universe prevent such paradoxes from occurring? If it doesn’t, how can we exist at all?
Enter the Large Hadron Collider (LHC), a particle accelerator that might (might!) become mankind’s first time machine,* thereby helping us find out if we can kill our grandfathers in the past and still exist (or something like that).
“Time machine” is a very loose term in this case, as you couldn’t actually use it to transport yourself through time (although there is a wormhole-LHC-time traveling theory that disagrees with this point), but the LHC might (might!) generate a type of Higgs particle that cuts through time like a hot knife through butter, and its decay particles appear in our universe before its own creation event.
This theory has been formulated by two Vanderbilt University theoretical physicists, Tom Weiler and Chui Man Ho. Stating the obvious, Weiler said that the theory “is a long shot,” but it “doesn’t violate any laws of physics.”
We’ve heard about the devious nature of time-traveling Higgs particles before, but this time the Higgs isn’t traveling back in time to sabotage the LHC, a second Higgs particle — called the “Higgs singlet” — might be generated at the same time as the creation of a “normal” Higgs boson.
According to the physicists’ calculations, the Higgs singlet may be able to travel in a fifth dimension (a dimension beyond our four-dimensional universe). But for this theory to hold true, our universe needs to abide by the laws of “M-theory,” a theory that requires there to be 10 or 11 dimensions (basically an extension of string theory).
In M-theory, our Universe is only one of many universes that can be envisaged as layers of an onion skin, each layer being a different universe. The skin that represents our Universe is known as a “brane” and it is stacked atop other branes as part of the “bulk.”
In the bulk, some forces, such as gravity, are predicted to permeate from one brane to the next. The details of M-theory are complex, and as yet unconfirmed, but the high-energy collisions inside the LHC may produce artifacts (such as short-lived micro-black holes) that reveal the presence of these predicted extra dimensions.
So, assuming M-theory describes the real nature of our Universe, how could we detect a Higgs singlet? If this particle only travels in a fifth dimension, time in our Universe isn’t of consequence to that particle, so it could be created by the LHC in the fifth dimension, and when it decays, its “decay particles” (i.e. everyday particles that the Higgs singlet will create after it dies) will be detected at an arbitrary time.
This arbitrary time could be in the past, before the particle was even generated, or even in the future. Therefore, if physicists see particles spontaneously pop into existence before an LHC collision even occurs, that could be indicative of the Higgs singlet decay particles appearing in our universe. Simple!
But what of the Grandfather Paradox? If a Higgs singlet can pop into existence before it is created, wouldn’t that cause all kinds of kinks in the space-time continuum?
“One of the attractive things about this approach to time travel is that it avoids all the big paradoxes,” Weiler said. “Because time travel is limited to these special particles, it is not possible for a man to travel back in time and murder one of his parents before he himself is born, for example. However, if scientists could control the production of Higgs singlets, they might be able to send messages to the past or future.”
So, the upshot is that a person couldn’t go back in time to kill his or her grandfather (because only Higgs singlets have time-traveling abilities). But, if an assassin could manipulate a signal consisting of time-traveling Higgs singlets, they could send a message to someone else to kill their (or someone else’s) grandfather!
It may not be a violation of physics per se, but I wonder how the universe would react to the “Grandfather Paradox By Proxy”?
I, for one, wouldn’t want to find out.
*Actually, as space correspondent Irene Klotz reminded me, this isn’t totally correct. Technically, the Hubble Space Telescope is mankind’s first “time machine.” Hubble can see light from the furthest most reaches of our Universe, receiving light that has taken billions of years to reach us. Hubble is basically looking at stars and galaxies as they were many billions of years ago. But, I’d argue that by simply looking up on a clear night with your own eyes, seeing stars that may only be a few dozen light-years away means you are seeing those stars as they were a few dozen years ago. So, your eyes are time machines too. So there.
Image: A simulation of how a Higgs event might look inside the ATLAS detector (CERN/LHC/ATLAS)