Let's assume that somehow humanity invented a teleporting machine that could momentarily transport a person over very large distances (up to several light years).

As we know, the speed of light is finite, and so the light from all stars and planets gets to us for quite some time. As an example: Sun is about 8 light minutes from the Earth, so if the Sun explodes or whatever, the observers on Earth will know it only after 8 minutes.

So, the question: if a man from Earth teleports momentarily to some planet 1 light year away and looks on the Earth with some really powerful telescope - what will he see? Can he witness all the events (well, whatever can be caught with the telescope) on Earth for the past year prior to teleportation? Does it mean that moving faster than light is actually time-traveling to the past (from the travelers's POV)? And does it mean that because of this "time paradox" an object moving faster than light breaks the timeline and therefore it's impossible?


closed as off-topic by enkryptor, T3 H40, a CVn, Hohmannfan, Green Apr 14 '16 at 17:39

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  • "This question does not appear to be about worldbuilding, within the scope defined in the help center." – enkryptor, T3 H40, a CVn, Hohmannfan, Green
If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ This seems like a solid question to me. It doesn't seem any different than questions like "How do you prove you are from the future" or "Would science really look like magic in the medieval era?" I think it should stay open $\endgroup$ – James Apr 14 '16 at 15:42
  • $\begingroup$ @James it does seem like those two other questions, but to me the first of that question is too broad and the second one is too story based (as it depends on what kind of science they're talking about). This one, in particular, seems a bit unclear to me; yet at the same time I kind of know what he's asking. It also feels a lot like a pure physics question to me so I'm hesitant on what to do here... I do feel like there is a definite lack of research here though. $\endgroup$ – Aify Apr 14 '16 at 16:34
  • $\begingroup$ OP, I recommend watching Brian Greene's Fabric of the Cosmo $\endgroup$ – Aify Apr 14 '16 at 16:39
  • $\begingroup$ Someone could probably do the math, but I suspect you'd need a lens several light years in diameter to see anything useful from a light year away. Possibly, if someone hooked up a camera and beamed a signal to a specific location, via laser you'd be able to collect enough data to forma a meaningful image. Reflected light from the surface will have diverged so much by the time it travels a light year you won't be able to collect enough photons from a given moment in time to form an image beyond a point of light. It's not a matter of technology, but of available photons. $\endgroup$ – Seeds Apr 14 '16 at 16:58
  • $\begingroup$ This whole question depends on what you mean by the word "momentarily", which is very much more complicated than you think it is. If you can define that word rigorously then your question will answer itself, but you'll need a fairly good understanding of special relativity in order to do so. Whether or not two things happening light years apart happen at the "same time" depends on the speed and direction of your motion relative to them. $\endgroup$ – Mike Scott Apr 14 '16 at 17:14

If a man teleports to a planet 1 light-year away, then yes, he would be able to observe events that happened one year ago on earth. He could also pick up all other kinds of EM radiation, such a radio waves (from radio stations). Depending on the method of transportation, he might be required to travel back in time. However, just changing location does not constitute traveling back in time, it's just that his information about what is happening specifically on earth is delayed by one year. Being able to see back in time does not constitute a paradox; we look back in time millions of years every time we look up at the night sky. However, being able to see forward in time would create paradoxes, since it implies that you time traveled to the future and back (the back part is the problem).

  • $\begingroup$ @MichaelKjörling If the man has a light telescope which can pick up what happens on earth from a light-year away, he probably has the technology to also pick up those radio waves :P $\endgroup$ – Ovi Apr 14 '16 at 15:01
  • $\begingroup$ Actually no, he doesn't necessarily. Light is EM radiation in the range about 400 to 700 nm, whereas those radio transmissions use wavelengths in the multiple cm and longer range. This has a huge impact in practice, and the linked answer discusses some of this. It even has some numbers specifically for a 1 ly distance. $\endgroup$ – a CVn Apr 14 '16 at 15:05
  • $\begingroup$ @James Thank you, I've never actually posted before on worldbuilding.SE for some months now I've been reading it multiple times a day because it is so interesting :) $\endgroup$ – Ovi Apr 14 '16 at 16:12
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    $\begingroup$ It depends on your frame of reference. From the frame of reference of someone stationary with respect to the Earth, the teleported person has not gone back in time. But relativity tells us that if the teleportation is faster than light then there are frames of reference in which they have gone back in time. $\endgroup$ – Mike Scott Apr 14 '16 at 17:12

"It depends"

The rules regarding the speed of light and teleportation are quite intermingled, mostly because the speed of light exists and, as of today, teleportation does not. Thus, depending on the rules of your teleportation system, it can do many things.

In general, observing the past is not considered a paradox in Einstein's relativity, or most systems. We have no problem that we can watch a recording of last night's live broadcast, and see the announcers talking about how the even is live. If you travel a light year away, you're really just treating empty space like 1 year's worth of video tape (which, unlike real tape, can't be paused or rewound). To cause a paradox, one must be able to act upon the information in a way which changes the past. Unless the teleporter also has a time travel device, there's no way to go back to a year ago and change the events that are observed. It's too late.

Time travel would occur if you ever actually traveled back in time (as opposed to merely getting far enough away to see the past). If you did that, you'd have to play games with causality, and issues like the speed of light would matter


Mathematically it's possible, Tachyons, only exist at speed above that of light, and start being mass less as they approach speed of light. If you can somehow convert matter into Tacheons and Vice versa, there won't be any kind of Paradox.

  • $\begingroup$ There's no evidence for the actual existence of tachyons. At this point, they're merely a theoretical construct. $\endgroup$ – Mike Scott Apr 14 '16 at 17:13
  • $\begingroup$ Mathematically they do exist, why rule them out? $\endgroup$ – Clarskon Apr 14 '16 at 17:16
  • $\begingroup$ Nothing exists mathematically. We have mathematics that describes such a particle, but that doesn't mean that it exists. We have mathematics that describes lots of things that don't actually exist. $\endgroup$ – Mike Scott Apr 14 '16 at 17:19
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    $\begingroup$ Then they don't exist physically, mathematically they have an origin, but we just dismiss them as the complex conjugate. And we discovered Higgs Boson, and might discover them too. $\endgroup$ – Clarskon Apr 14 '16 at 17:22

Its not a paradox but is does violate causality.

Gravity light mass they are all limited by the speed of light. Normally an effect can only occur when news of its cause reaches it. Since information travels at the speed of light you can only get information of things that are d / c meters away, where d is distance and c is the speed of light, this called the speed of light cone.

If the Sun vanished into thin space earth would continue to feel the pull of its gravity and see light coming from it for 8 minutes when the news of the sun's disappearance would arrive with the disappearance of its gravity and sunlight.

You ship could violate causality and learn things it should not yet know.


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