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A question on speed and time

  1. Apr 27, 2014 #1
    Okay, this question may sound wierd and confusing, but I'll try to make it clear. It is something I thought of while studying relativety and time. So, say an alien spaceship is travelling at a speed of 100km/s towards us, which is 3 light years away. So, according to the speed of light, it needs to take 3 years for us to actually see the image of the ship traveling towards us (through telescope or whatever). However, after moving 10 minutes or so, the ship suddenly speeds up until it surpasses the speed of light (I am not saying it is possible, only if)..then, let's say it now only needs 1 year to travel the 3 light year distance. Then, when arriving upon Earth we see the ship. But 2 years later, the light from it's previous motion (when at only 100km/s) reaches us. But according to Time, what we see 2 years ago is the ACTUAL or PRESENT state of the ship(itself), and then 2 years later, we see the PREVIOUS state of the ship. By first seeing the present then seeing the past, could this be an explanation why time could be reversed? I mean if the aliens on board were 40 years old and when arriving on Earth in a speed faster than light, it'll only be 41 years. However, the light which were seen on Earth 2 years later, these aliens were only 40 years old. Could this be possible and explainable? (Not talking about if matter could develop a speed faster than light or other phsysical conditions...only about time and speed.)
  2. jcsd
  3. Apr 27, 2014 #2
    The ship can't break the speed of light. What you ask is impossible and thus cannot be answered. This is not just a lame excuse to avoid answering your question, we literally don't know the answer since it's impossible. It's like asking "What happens when an unstoppable force meets an immovable object?" It just can't be answered.
  4. Apr 27, 2014 #3
    Well I understand that this really sounds impossible to humans, but how would we know it's not possible for a far more advanced alien civilization to master this techinique. I mean there are billions of unknown galaxies out there and I certainly believe we are not the most advanced. :) Anyways, thanks for pointing out, but like what I said, maybe someday we'll find something that travels faster than the speed of light~~ and that's not the whole point of the question :)
  5. Apr 27, 2014 #4
    It's not an engineering problem. Going at 99% of the speed of light (relative to somebody "stationary") is an engineering problem. Perhaps a sufficiently advanced civilization can do that.

    But going faster than the speed of light is not an engineering problem. It is a physical impossibility. Even the most advanced civilizations cannot do it. Experiments and theories show this.
  6. Apr 27, 2014 #5
    Time going backwards is one of the reasons why moving faster than the speed of light (FTL) is not possible. The explanation you came up with for why time would go backwards is not correct though. That would just give the appearance that time went backwards. FLT leads to actual backwards time evolution, not just apparent backwards time. Making it actually impossible not just apparently impossible.
  7. Apr 28, 2014 #6


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    Coming to terms with the c speed limit is one of those 'rites of passage' that everyone has to go through. You first need to accept it (grudgingly, probably) then, when you learn more about it, you will see why it is an impossibility. It's a limit like Absolute Zero Temperature. You can get very near but never beyond.

    Compared with c, the 'sound barrier' is a piece of cake because it's not 'that' sort of limit.
  8. Apr 28, 2014 #7


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    The problem with your question is that if at some time in the future we are able to travel faster than light- or if, somewhere in the galaxy, aliens are able to travel faster than light, then all of our physics is wrong. So essentially you are asking "If all of our physics is wrong, what would our physics say about this situation", a pointless question.
  9. Apr 29, 2014 #8
    Since the Speed Limit in our Universe is a fundamental constant upon which everything depends, to break it would mean no longer being in This Universe. Some wonder if what would pass for FTL travel might be possible by folding space so that the distance is reduced but that is not the same as increasing speed, just the duration of the trip.

    There was a really good Sci Fi short story many years ago, I think by Heinlein, about a future in which scientific research was strictly regulated and the protagonist belongs to a police force who investigates possible "crimes". He discovers that the mad scientist he was recently assigned has manufactured a perfect prism as well as a device to stream a beam of light exactly one photon in diameter at said prism. He soon realizes that the only purpose possible is to attempt to slow a photon below C, so he begins to wonder about constants.

    He then reasons that Pi is such a constant and hasn't it always been 3.14159? He finds that some early civilizations thought it was 3.0 and realizes that "even a child with a circle and a length of string can see that's not accurate at all", so why would they settle on 3.0? Then comes the day the experiment is ready to start and the policeman has become so fascinated with the concept he can't make himself stop it. He has to see it to it's conclusion. The result is priceless and fun.

    I wish I could remember the name of the story but it was something like "The Case of the Incredible Dr. Wu". Anyone know it?
  10. Apr 29, 2014 #9
    Imagine a mirror in a distance of 1.5 light years and a ship starting at a distance of 1 LY traveling to Earth with almost the speed of light. The ship will arrive one year later and its mirror image at the original position after three years. This is the same result as in your thought experiment: You see the present of the ship first and the past later but there is no reason to speculate about reversion of time.
  11. Apr 30, 2014 #10
    OK I think it is fair to assume that like primitives stuck with drums and smoke signals who haven't a clue about radio signaling, we are unaware of technologies beyond ours, and for exactly the same reason - Time. They really aren't all that dissimilar since drums, smoke, and radio signalling are all accomplished in packets. The only difference is the media.

    Quantum Theory states that it is not impossible that in the next instant you might find yourself on the surface of Jupiter, but the odds are unimaginably... well, astronomical, that you won't. The odds are equally astronomical that if indeed some vastly advanced civilization does manage to effectively achieve FTL travel, they did not achieve it simply by "more power". Mass changes with speed and a turning point is reached where no additional means of producing power to accelerate (not even somehow harnessing a supernova) can overcome the increase in mass.

    Therefore, the paradox which you describe cannot occur, even IF some means of effective FTL does exist. It cannot, by definition, be the sort of means that would give rise to such a paradox or the Universe would be an unrecognizably different place.
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