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  • richardmitnick 7:11 am on November 27, 2017 Permalink | Reply
    Tags: , Time Travel,   

    From Ethan Siegel: “How Traveling Back In Time Could Really, Physically Be Possible” 

    From Ethan Siegel

    Nov 21, 2017

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    The idea of traveling back in time has long fascinated humans, such as in Back To The Future’s Delorean DMC-12. After decades of research, we may have hit upon a solution that’s physically possible. Image credit: Ed g2s of Wikimedia Commons.

    And you don’t even need a Delorean at 88 MPH.

    It’s one of the greatest tropes in movies, literature, and television shows: the idea that we could travel back in time to alter the past. From the time turner in Harry Potter to Back To The Future to Groundhog Day, traveling back in time provides us with the possibility of righting wrongs in our own past. To most people, it’s an idea that’s relegated to the realm of fiction, as every law of physics indicates that motion forward through time is an absolute necessity. Philosophically, there’s also a famous paradox that seems to indicate the absurdity of such a possibility: if traveling backwards through time were possible, you’d be able to go back and kill your grandfather before your parents were ever conceived, rendering your own existence impossible. For a long time, there seemed to be no way to go back. But thanks to some very interesting properties of space and time in Einstein’s General Relativity, traveling back in time may be possible after all.

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    An illustration of the early Universe as consisting of quantum foam, where quantum fluctuations are large, varied, and important on the smallest of scales. Positive and negative energy fluctuations can create minuscule, quantum wormholes. Image credit: NASA/CXC/M.Weiss.

    The place to start is with the physical idea of a wormhole. In our known Universe, we have tiny, minuscule quantum fluctuations in the fabric of spacetime on the smallest of scales. These include energy fluctuations in both the positive and negative directions, often very close by one another. A very strong, dense, positive energy fluctuation would create curved space in one particular fashion, while a strong, dense, negative energy fluctuation would curve space in exactly the opposite fashion. If you connected these two curvature regions together, you could — for a brief instant — arrive at the notion of a quantum wormhole. If the wormhole lasted for long enough, you could even potentially transport a particle through it, allowing it to instantly disappear from one location in spacetime and reappear in another.

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    Exact mathematical plot of a Lorentzian wormhole. If one end of a wormhole is built out of positive mass/energy, while the other is built of negative mass/energy, the wormhole can become traversible. Image credit: Wikimedia Commons user Kes47.

    If we want to scale that up, however, to allow something like a human being through, that’s going to take some work. While every known particle in our Universe has positive energy and either positive or zero mass, it’s eminently possible to have negative mass/energy particles in the framework of General Relativity. Sure, we haven’t discovered any yet, but according to all the rules of theoretical physics, there’s nothing forbidding it.

    If this negative mass/energy matter exists, then creating both a supermassive black hole and the negative mass/energy counterpart to it, while then connecting them, should allow for a traversible wormhole. No matter how far apart you took these two connected objects from one another, if they had enough mass/energy — of both the positive and negative kind — this instantaneous connection would remain. All of that is great for instantaneous travel through space. But what about time? Here’s where the laws of special relativity come in.

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    A “light clock” will appear to run different for observers moving at different relative speeds, but this is due to the constancy of the speed of light. Einstein’s law of special relativity governs how these time and distance transformations take place, but it means that the stationary and the moving parties age at different rates. Image credit: John D. Norton, via http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/Special_relativity_clocks_rods/.

    If you travel close to the speed of light, you experience a phenomenon known as time dilation. Your motion through space and your motion through time are related by the speed of light: the greater your motion through space, the less your motion through time. Imagine you had a destination that was 40 light years away, and you were able to travel at incredibly high speeds: over 99.9% the speed of light. If you got into a spaceship and traveled very close to the speed of light towards that star, then stopped, turned around, and returned back to Earth, you’d find something odd.

    Due to time dilation and length contraction, you might reach your destination in only a year, and then come back in just another year. But back on Earth, 82 years would have passed. Everyone you know would have aged tremendously. This is the standard way time travel physically works: it takes you into the future, with the amount of travel forward in time dependent only on your motion through space.

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    Is time travel possible? With a large enough wormhole, such as one created by a supermassive black hole connected to its negative mass/energy counterpart, it just might be. Image credit: Wikimedia Commons user Kjordand.

    But if you construct a wormhole like we just described, the story changes. Imaging one end of the wormhole remains close to motionless, such as remaining close to Earth, while the other one goes off on a relativistic journey close to the speed of light. You then enter the rapidly-moving end of the wormhole after it’s been in motion for perhaps a year. What happens?

    Well, a year isn’t the same for everyone, particularly if they’re moving through time and space differently! If we talk about the same speeds as we did earlier, the “in motion” end of the wormhole would have aged 40 years, but the “at rest” end would only have aged by 1 year. Step into the relativistic end of the wormhole, and you arrive back on Earth only one year after the wormhole was created, while you yourself may have had 40 years of time to pass.

    If, 40 years ago, someone had created such a pair of entangled wormholes and sent them off on this journey, it would be possible to step into one of them today, in 2017, and wind up back in time at the mouth of the other one… back in 1978. The only issue is that you yourself couldn’t also have been at that location back in 1978; you needed to be with the other end of the wormhole, or traveling through space to try and catch up with it.

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    Warp travel, as envisioned for NASA. If you created a wormhole between two points in space, with one mouth moving relativistically relative to the other, observers at either traversible end would have aged by vastly different amounts. Image credit: NASA / Digital art by Les Bossinas (Cortez III Service Corp.), 1998.

    Satisfyingly, we discover that this form of time travel also forbids the grandfather paradox! Even if the wormhole were created before your parents were conceived, there’s no way for you to exist at the other end of the wormhole early enough to go back and find your grandfather prior to that critical moment. The best you can do is to put your newborn father and mother on a ship to catch the other end of the wormhole, have them live, age, conceive you, and then send yourself back through the wormhole. You’ll be able to meet your grandfather when he’s still very young — perhaps even younger than you are now — but it will still, by necessity, occur at a moment in time after your parents were born.

    A great many unusual things become possible in the Universe if negative mass/energy is real, abundant, and controllable, but traveling backwards in time might be the wildest one we’ve ever imagined. Owing to the oddities of both special and general relativity, time travel to the past might not be forbidden after all!

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 11:22 am on July 26, 2015 Permalink | Reply
    Tags: , , , , Time Travel   

    From RT: “Time-traveling photons connect general relativity to quantum mechanics” 

    RT Logo

    RT

    23 Jun, 2014
    No Writer Credit

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    Space-time structure exhibiting closed paths in space (horizontal) and time (vertical). A quantum particle travels through a wormhole back in time and returns to the same location in space and time. (Photo credit: Martin Ringbauer)

    Scientists have simulated time travel by using particles of light acting as quantum particles sent away and then brought back to their original space-time location. This is a huge step toward marrying two of the most irreconcilable theories in physics.

    Since traveling all the way to a black hole to see if an object you’re holding would bend, break or put itself back together in inexplicable ways is a bit of a trek, scientists have decided to find a point of convergence between general relativity and quantum mechanics in lab conditions, and they achieved success.

    Australian researchers from the UQ’s School of Mathematics and Physics wanted to plug the holes in the discrepancies that exist between two of our most commonly accepted physics theories, which is no easy task: on the one hand, you have Einstein’s theory of general relativity, which predicts the behavior of massive objects like planets and galaxies; but on the other, you have something whose laws completely clash with Einstein’s – and that is the theory of quantum mechanics, which describes our world at the molecular level. And this is where things get interesting: we still have no concrete idea of all the principles of movement and interaction that underpin this theory.

    Natural laws of space and time simply break down there.

    The light particles used in the study are known as photons, and in this University of Queensland study, they stood in for actual quantum particles for the purpose of finding out how they behaved while moving through space and time.

    The team simulated the behavior of a single photon that travels back in time through a wormhole and meets its older self – an identical photon. “We used single photons to do this but the time-travel was simulated by using a second photon to play the part of the past incarnation of the time traveling photon,” said UQ Physics Professor Tim Ralph asquotedby The Speaker.

    The findings were published in the journal Nature Communications and gained support from the country’s key institutions on quantum physics.

    Some of the biggest examples of why the two approaches can’t be reconciled concern the so-called space-time loop. Einstein suggested that you can travel back in time and return to the starting point in space and time. This presented a problem, known commonly as the ‘grandparents paradox,’ theorized by Kurt Godel in 1949: if you were to travel back in time and prevent your grandparents from meeting, and in so doing prevent your own birth, the classical laws of physics would prevent you from being born.

    But Tim Ralph has reminded that in 1991, such situations could be avoided by harnessing quantum mechanics’ flexible laws: “The properties of quantum particles are ‘fuzzy’ or uncertain to start with, so this gives them enough wiggle room to avoid inconsistent time travel situations,” he said.

    There are still ways in which science hasn’t tested the meeting points between general relativity and quantum mechanics – such as when relativity is tested under extreme conditions, where its laws visibly seem to bend, just like near the event horizon of a black hole.

    But since it’s not really easy to approach one, the UQ scientists were content with testing out these points of convergence on photons.

    “Our study provides insights into where and how nature might behave differently from what our theories predict,” Professor Ralph said.

    See the full article here.

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

     
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