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Flying is impossible, so is timetravel.

  1. Dec 22, 2005 #1
    People in the 1800's though flying was impossible. How can you take metal weighing several tons and fly with it? But look where we are now in only 100 years.

    Well, so now people are saying time travel is impossible. It may be, but it just *may* be possible in the future. Maybe we do not know enough about quantum physics yet.

    I am fasinated by time travel, it may be fantasy, but it may just be possible. I'm aware of the grandfather paradox and other ones, as some say this is proof that you can't, but there may be other loopholes, so tos peak.
  2. jcsd
  3. Dec 22, 2005 #2


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    Achieving flight was only a matter of advancing technology; we observed that other things fly (birds, bees, etc.) long before we attempted it ourselves.

    On the other hand, time travel appears to be prohibited by physical laws, and nothing has ever been observed to travel through time. If our understanding of those laws is correct, no amount of technological progress will allow us to break them.

    - Warren
  4. Dec 22, 2005 #3


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    Scientists in the 1800's were skeptical as to whether flight as we have it today (a plane flying without a bird-like flapping mechanism) was possible, because you cannot generate circulation (and in conjunction with that, fligfht) in an inviscid fluid (although an inviscid fluid may maintain a given circulation). Thus, it all depended on whether the viscosity of air could be sufficient or not to generate the necessary circulation.
    Some scientists thought it might be sufficient, others doubted that.

    As for time travel, I believe the disbelievers have a more solid basis for their skepticism than this type of quantitative guesswork.
  5. Dec 22, 2005 #4
    viscosity if air?
  6. Dec 22, 2005 #5


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    That's actually a pretty big "if". I remember reading a reply from Isaac Asimov to one of his correspondents in a science fiction magazine. The correspondent had said that noone should be dogmatic about something physically "impossible" like superluminal travel, that men once thought the steam engine and flight were impossible.

    Asimov had replied (rather patronisingly I thought) that steam engine and flight were feats of engineering, whereas superluminal travel was forbidden by physical law.

    At the time, I agreed with this sort of dogmatic attitude; now I'm not so sure. Newton's formulation of physical law was considered completely sound till Einstein came along and demolished it at its foundations. We currently believe our understanding to be sound, but there's absolutely no guarantee a genius isn't going to come along tomorrow and make fools out of us all.
  7. Dec 22, 2005 #6


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    But isn't it also true that we are far more capable of testing things where newton and pre-20th century physicists couldn't. Newton couldn't test near the speed of light but we are able to do it and we do notice this whole, E->inf problem and divide by 0 stuff and unless mathematics starts taking a strange turn for hte worse, it's probably not going to happen.

    I bet if any of this stuff DOES happen, it's going to be some really strange concept with stuff we don't even experience or know of. With things like flight, we had hints that it was possible and the idea was not all that far-fetched. The problem here is we don't see birds traveling through time and ive heard we have measured causality to some very precise standards...
  8. Dec 22, 2005 #7


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    Then on what basis does one answer a question like this? If you are suggesting that everything MIGHT be possible, then what's the purpose of asking these types of questions in the first place? If the OP wants an answer based on our current understanding of the physical universe, then that's the kind of responses one will get. If the OP wants to be based on "what might be possible via speculation not based on current understanding", then why bother asking because the question has an automatic answer!

    If we want to play the speculation game, then all bets are off. I can make things up as well as the next quack. What keeps us grounded IS the established physics. Ironically, it is also what keeps us physicists motivated to explore beyond its boundaries.

  9. Dec 22, 2005 #8


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    How so? Relativity is pretty thoroughly tested.

    The "anything is possible" and "people used to think flying was impossible" arguments are common misunderstandings of science, mostly based on factual errors in knowledge, such as....
    No, they weren't. Newton's laws were known to be flawed not long after (immediately after?) they were formulated. Why do you think Einstein (among others) was looking for a new theory? [edit: actually, does anyone know who was first to realize the Mercury issue?]

    In addition, Einstein's theory does not change the accuracy of Newton's theory. If Newton's theory was 1% off in calculating an orbit 200 years ago, it is still 1% off in calculating that orbit today. And the same applies to Relativity: scientists have been able to accelerate objects up to a very high fraction of the speed of light, verifying the theory to an extrordinarily level of precision. Finding a new theory will not retroactively change that experimental data.

    [edit] Also, people think about Newton's gravity, but don't think about what Newton's laws of kinematics have to say about the speed of light. Newton's gravity may have been accurate to within a couple of percent, but scientists were nowhere close to measuring the speed of light at the time, much less accelerating an object to any significant fraction of it.

    [edit2] The best I can find about timing of the discoveries of flaws in Newton's theory was Laplace in the early 1800s. Anyone know of anyone earlier? That gets us 150 years, but with the rate of advancement of our ability to test Newton's theories at the time, that's pretty quick.
    Last edited: Dec 22, 2005
  10. Dec 22, 2005 #9


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    One should also consider the energy and more particularly the energy density required to bring matter to the near speed of light.

    But then one can argue - one might be able to 'warp' space, which usually requires a large mass or some huge energy.

    Now compare that to the conditions under which humans 'live'. :wink:
  11. Dec 22, 2005 #10
    Some people were obviously inspired by seeing birds fly, to make airplanes, but honestly, many people did *not* think it was possible. People also thought the earth was flat. And this was a misunderstanding. Everything can be a misunderstanding.

    Also, physicsts have been able to make music submatoimfc particles travel at the speed of light, about 2x as fast. I saw this on the science channel on time travel. They used a laser and encoded music on it, I believe. However, I do realize atoms behave differently than humans, it is cool though.
  12. Dec 22, 2005 #11

    Sorry, as I am only 17 and am not that experineced in physics. But aren't there supposed to be other' theories' to getting to do time traevl *theortically* besides relativeiy? I also heard that general relativiy (or is it special?) DOES allow it.
  13. Dec 22, 2005 #12
    The ancient Egyptians in 200 bc played with model birds that looked like airplanes (replicas have been made of the artifacts and tested. They flew.)

    http://www.catchpenny.org/model.html" [Broken]

    Flying machines were designed by DaVinci in the 1480s (though never built.), in the late 1783 man had achieved lighter than air flight in a balloon and the first manned glider was built in the mid 1800s and flown by a child.

    So in 1800 the idea was pretty well developed that man could someday fly.
    Last edited by a moderator: May 2, 2017
  14. Dec 22, 2005 #13


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    As with such things as evolution today, it was primarily non-scientists who disbelieved. Those in the business of finding things out knew that something was wrong when all of the calculations indicated that a bumblebee can't fly. (They had failed to factor in the wingtip vortices that increase lift.)

    Although I never saw the show, this is either a bad program or a misunderstanding on your part of what was achieved. To start with, music is not made of particles. It's simply information encoded in some manner (in this case, as modulated photons). There's no way that it went faster than light, because photons are light.
  15. Dec 22, 2005 #14
    Time Travel By Internal Holodeck

    Most people have probably heard of a "fashback". This is associated in people's minds with LSD users and Vietnam Veterans, but some people who have temporal lobe seizures also have them. A "flashback" is a sudden hallucination that the person is surrounded by a past environment. This can be mild,in the form of seeming to see unmoving photographs of the past superimposed on their field of vision, to full blown: three dimensional, past environments surrounding them on all sides with accompanying sounds, smells, and tactile qualities. It seems that when sense memories are juiced up by the surge of neuronal activity that constitutes seizure activity the result can be as if we all had a kind of built in holodeck.

    There is a Canadian researcher, Robert Persinger, who has been experimenting for many years with the effects of magnetic fields on the brain, and there's a strong indication that this could be configured to cause a "holodeck-like" experience: full blow hallucinations of being in a different environment. To the extent a person constructs that hallucination with elements of their own memory then something indistinguishable from deliberate travel to the past and back is concievable.

    There's likewise no reason a person couldn't hallucinate being in a future environment. For some, that would probably be a whacky and unrealistic fantasy or science fiction future, but for the more level headed it would be a conservative, plausible picture. In any event, since these pictures are created from the person's own store of sensory data, and in accordance with their ways of thinking, the result would always be particularly persuasive to them. They would feel dead on accurate.

    So, if a Doc Brown shows up with a DeLorean that seems to be able to take you to the past or future, it's a very, very good bet that it would be creating a controlled, deliberate kind of flashback (or "flashforward") by utilizing your own pre-existing capacity to create a "holodeck-like" experience. If that Doc Brown has a penchant for "fooling himself" as Feynman might put it, or if he is a con-man, you might also hear claims that his time machine proves that the laws of physics aren't what we thought.
  16. Dec 22, 2005 #15
    your off a little bit.

    Pre-flight times, people had an intuitive belief that flight was impossible for a machine.....

    For time travel, we have mathematics to describe why it would be very difficult and impracticle (but possible as outlined by kip thorn in a few of his books)
    Last edited: Dec 22, 2005
  17. Dec 22, 2005 #16


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    The difference between flying and time travel is that with flying, people did know that flight was possible by watching birds and insects. What they didn't think was possible was designing/building a device that would allow much heavier humans to fly. The concept of flight was not considered impossible, the idea of humans (or anything else heavier than a bird) flying is what drew the attention of skeptics.

    With time travel, it's not that we observe things traveling through time and just don't know how to make people do it; there's simply no evidence that anything can travel through time, and no indication that anything lingers in some sort of past time. Afterall, are you splitting off and forming a whole new body every fraction of a second that something changes so that your previous state is preserved in the past?
  18. Dec 22, 2005 #17
    What would be weird is if that person could affect the past :-)
  19. Dec 22, 2005 #18
    Depending on the vividness of the experience, they ought to be able to interact with everything there as if it were completely real. When they get "back to the future", though, nothing they did would have changed the authentic present, just like nothing that happens on the holodeck of the enterprise carries over into real life. If it ever does, then you know you're still in the holodeck.
  20. Dec 22, 2005 #19
    One cannot prove absoluteness of our laws of physics, therefore the concept of time travel is neither right nor wrong.

    The only way to make big progress or a quantum leap in science is to ponder on such concepts and not disregard them because some physicist tells you otherwise.
  21. Dec 22, 2005 #20


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    And in both cases, such people were not scientists. That the earth was believed to be flat by scientists (or their predicessors) is another similar myth. Every halfway educated person who lived in about the last 2000 years knew the earth was round. Most notably, an Egyptian (?) named Eratosthenes in 250BC came up with a viable way to measure the circumference of the earth (unfortunately, we don't know how well he did, since no one knows how long the unit of distance he used is). He also measured the distance to the moon and the tilt of the earth's axis wrt to the ecliptic (yeah, that's right: he knew the sky wasn't just some spherical shell). Google him....
    You need to be a little more specific about what you mean by time travel because, quite obviously, we are all traveling through time right now. But if you are looking for a way to go back and meet your parents before you were born.... no.

    I know it's a real kick in the teeth when you first learn about how our current knowledge of science puts limits on our future discoveries, but that really is a reality.
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