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View Full Version : FTL and Causality, Part Two, The Movie.



BigDon
2010-May-13, 06:28 PM
My question is, would only moving "dots and dashs" or "ones and zeros" or anything thats more symbolic (is "information" the word I'm looking for here?) than physical violate causality?

Instant instant messaging between Earth and Mars, or even Sol and A centari seems fairly innocuous. How hard is it to contrive a paradox that way?

EDG
2010-May-13, 06:45 PM
I don't think it makes a difference whether it's information or something physical - you'd still effectively be sending information back in time.

Alastair Reynolds came up with an interesting way around this in his "House of Suns" SF book though. If we have a "spoiler" tag here that could hide the info then I can explain what it was.

EDIT: Spoiler for House of Suns (white text, highlight to see):
There are intergalactic wormholes in the setting created by an ancient precursor race, but a side-effect of the technology is to block all information coming from the other galaxy. At some point in the past when the wormhole is created, the Andromeda galaxy disappears from view as seen from the Milky Way - the gap is known as "the Absence". When the protagonist makes it through the wormhole to Andromeda at the end, he looks back at the Milky Way and finds an Absence in its place too (the galaxy is still there, but hidden from view)

Van Rijn
2010-May-13, 07:19 PM
My question is, would only moving "dots and dashs" or "ones and zeros" or anything thats more symbolic (is "information" the word I'm looking for here?) than physical violate causality?

Instant instant messaging between Earth and Mars, or even Sol and A centari seems fairly innocuous. How hard is it to contrive a paradox that way?


Not that hard. The problem is that there doesn't appear to be any universal clock by which everyone can agree on what is "instant." But if you can get messages going back and forth faster than light, it seems possible to set up situations where you could find out, for instance, tomorrow's stock prices today.

BigDon
2010-May-13, 07:36 PM
Not that hard. The problem is that there doesn't appear to be any universal clock by which everyone can agree on what is "instant." But if you can get a messages going back and forth faster than light, it seems possible to set up situations where you could find out, for instance, tomorrow's stock prices today.

Then sir, you are more clever than I am. Pray tell, can you give a brief example?

Ken G
2010-May-13, 07:50 PM
Then sir, you are more clever than I am. Pray tell, can you give a brief example?The trick with using FTL to get tomorrow's stock prices is you have to say that the FTL signal can be repeated in two different frames, in a symmetrical way. In other words, you keep the principle of relativity, which basically says if you can do something in your frame, I should be able to do it in mine too. So you and I meet and make this plan, then we both speed off in opposite directions at 0.999c. You send and FTL series of dots and dashes to tell me what your stock market says some day. I receive your message, and send it back to you, FTL in my frame in exactly the same way that your message was FTL in your frame (this last bit is important-- if I can only bounce the message back to you in a way that is FTL in your frame the same way your own message was, it doesn't work. You may need to work that part out using signals that are instantaneous in your frame, then with signals that are instantaneous in each of our frames). The point is, if we can really do this, you can get your own message back-- a day earlier. And make a killing.

But the next question to ask is, so what? Quants predict the market in advance all the time, where's the "violation" if you do it this way? Well, after you make your killing, there'd be no reason to send the message that you already received, so there's a certain inconsistency in receiving messages you never sent. But again, my reaction to that is, so what? I receive a message that I do not later send-- where's the "violation" there? I don't think there is, it's just not something that appears to happen, so far as we know. If it started happening, we'd simply have to come to terms with it.

Now, sending a conscious mind back in time this way would be much more fundamentally problematic. You could meet yourself, and then there'd be two of you from that point on, especially if the first one decided not to get into the time machine and become you. Or, you could kill the other you, or your grandfather, and there'd be certain puzzles in constructing a meaningful time series there. But even with all that, we can still say, "so what"? Who said things have to make sense the way we'd like them to? If it happened, we'd just have to come to terms with it. It doesn't seem like the technology is any kind of likelihood, though.

BigDon
2010-May-13, 08:16 PM
Okay I get that, but neither Earth/Mars nor Sol/Alpha Centari are going anywhere near c relative to each other.

And if only information could be sent and everything else has to "crawl" like normal...ohh. Wait a minute... almost had a glimmer of dawn. Lost it.

Any more help?

Ken G
2010-May-13, 08:20 PM
Okay I get that, but neither Earth/Mars nor Sol/Alpha Centari are going anywhere near c relative to each other. Alpha Cen might work, it's far enough away-- I haven't worked out the relative motion of Sol/Alpha Cen, but if they are moving away from each other and we could send "instantaneous" signals in each frame, it's a nice puzzle to figure out how far back in time we could send information. Anyone with a minute to devote/waste, go for it, the time shift is twice v/c times a few years, where v is its speed away from us, assuming it is moving away.

The fact is, the speed of light is pretty darn close to instantaneous communication on most of the scales of our actual experience, so it's only when we need spectacular accuracy (GPS accuracy), or are dealing with huge distances (like Earth to Mars), do we even need to recognize it isn't instantaneous-- and causality problems would require even greater precision, speeds, and/or distances.

undidly
2010-May-13, 11:16 PM
BigDon
I watch with interest as the math people divide by the square root of a negative number and believe the answer.
There is never ever any violation of causality FTL or slower than light.
Your 1s and 0s can go as fast as they want and get there AFTER they left.
Why did Einstein say that information cannot travel FTL?.
His other big mistake maybe?.

pzkpfw
2010-May-14, 12:22 AM
undidly, please keep your ATM opinions out of Q&A threads.

Ken G
2010-May-14, 12:37 AM
Alpha Cen is moving toward us at about 25 km/s, so it's not a good candidate. If you find a star that is, say, 10 light years away, moving at 30 km/s away from us (the Earth's 30 km/s orbit could help there if it was timed right), then if you had an instantaneous transmitter (we've no evidence such a thing is possible), you could send today's stock prices to that star, along with instructions how to build an instantaneous transmitter. An alien there with a sense of humor could then bounce the message back to you with their instantaneous transmitter-- and it would arrive about 4 hours before you sent it (assuming they could translate your message and build the transmitter in a few minutes!). That might be enough time to make a healthy profit on the market. Or, you could just sell your instantaneous transmitter!

It certainly is an interesting take on "Contact": sir, we've decoded the message from the aliens, and it says, "quick, build this device and send these quotes back to us right away. Thanks in advance, and we do mean that."