Page 137: Adieu SG-51

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Absalom
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Re: Page 137: Adieu SG-51

Post by Absalom »

And FTL through shortcuts is directly equivalent to FTL through inflation, by which I mean that it isn't FTL at all.

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icekatze
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Re: Page 137: Adieu SG-51

Post by icekatze »

hi hi
Absalom wrote:And FTL through shortcuts is directly equivalent to FTL through inflation, by which I mean that it isn't FTL at all.
What do you mean by FTL through inflation?

Does FTL through shortcuts mean that some object disappears and is never seen again? That's not very functional as a method of travel between two points.

Does FTL through shortcuts mean that some object goes from point a to point b and takes more time to do so than it would take a beam of light to go from point a to point b without using the shortcut? Because you're right, that isn't FTL at all.

Granted there is plenty of disagreement among scientists about cosmological inflation, but I don't see where you are getting the idea that it allows for FTL travel. Not without negative mass, it doesn't, and it would still violate causality in that case.

Warping spacetime with wormholes implies violating causality.

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RedDwarfIV
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Re: Page 137: Adieu SG-51

Post by RedDwarfIV »

Your average physicist holds Relativity quite strongly. It has been tested again and again with an accuracy of many decimal places. They hold onto Causality even tighter. Without Causality the entire structure of physics crumbles. Causes must preceed effects, or it becomes impossible to make predictions. If it is impossible to make predictions, it would be best to give up physics for a more profitable line of work.
It occurs to me that this is not an argument for why causality must exist. It is an argument for keeping physicists in a job.

It states that relativity has been tested again and again, accurately. But it does not say the same about causality.

Therefore, if we can build FTL drives... then all we have done is put the physicists out of work. So long as effect before cause is limited to people who can't actually do anything with the information, no paradoxes will occur, and the universe carries on as normal.

Just don't build an instantaneous ansible.
If every cloud had a silver lining, there would be a lot more plane crashes.

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icekatze
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Re: Page 137: Adieu SG-51

Post by icekatze »

hi hi
RedDwarfIV wrote:It states that relativity has been tested again and again, accurately. But it does not say the same about causality.
Actually, it does.
Your average physicist holds Relativity quite strongly. It has been tested again and again with an accuracy of many decimal places. They hold onto Causality even tighter.
Emphasis mine. Scientists really have put a significant amount of effort into looking for ways to break causality, especially with quantum mechanics. But even proponents of retrocausal quantum theory recognize that information cannot be sent back in time, even if a future event changes the random quantum state of entangled pairs. (See Bell's Theorem)
RedDwarfIV wrote:Therefore, if we can build FTL drives... then all we have done is put the physicists out of work. So long as effect before cause is limited to people who can't actually do anything with the information, no paradoxes will occur, and the universe carries on as normal.
It is not an argument for why causality must exist, but it isn't an argument for keeping physicists in a job either. It is an argument for why you can't have Causality, Relativity, and FTL travel at the same time, and an argument for why scientific method breaks down without cause and effect.

The reason why scientific method breaks down is because people can do things with that information. If cause can come after effect, then every experiment, every experience you witness, could be due to untestable and invisible forces from the future.

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RedDwarfIV
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Re: Page 137: Adieu SG-51

Post by RedDwarfIV »

I'm sure such tests have been done, the point I was making was that the wording ProjectRho was using either suggested it hadn't, or that physicists getting to keep their jobs was more important.

The highlit part about holding on to causality even tighter was not specific. If they meant they held onto it because of scientific research, then they should have said that, instead of making a joke. The site does intend to be educational, after all.
If every cloud had a silver lining, there would be a lot more plane crashes.

boldilocks
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Re: Page 137: Adieu SG-51

Post by boldilocks »

I've been trying to understand the http://www.physicsmatt.com/blog/2016/8/ ... ime-travel blog post but I think it's going over my head.
If the light of a later action arrives later than the event of an earlier action then causality is violated? Does that mean watching a movie played in reverse violates causality?

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icekatze
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Re: Page 137: Adieu SG-51

Post by icekatze »

hi hi
boldilocks wrote:Does that mean watching a movie played in reverse violates causality?
No. :|

If you played a movie before it was filmed, then you'd be in a violation of causality, but the universe doesn't care what time stamp we put on our recordings of events.

You can set your computer's calendar to 1990, but that's not violating causality. And the light from a series of p-n junction diodes attempting to imitate the light from an earlier event, is not the same thing as the light from that event. It is its own event.

The digitally encoded data at the beginning of a movie does not cause the digitally encoded data at the end of the movie to exist.

boldilocks
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Re: Page 137: Adieu SG-51

Post by boldilocks »

icekatze wrote:hi hi
boldilocks wrote:Does that mean watching a movie played in reverse violates causality?
No. :|

If you played a movie before it was filmed, then you'd be in a violation of causality, but the universe doesn't care what time stamp we put on our recordings of events.

You can set your computer's calendar to 1990, but that's not violating causality. And the light from a series of p-n junction diodes attempting to imitate the light from an earlier event, is not the same thing as the light from that event. It is its own event.

The digitally encoded data at the beginning of a movie does not cause the digitally encoded data at the end of the movie to exist.
I guess I'm having problems understanding how you can observe something before it happened.

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Arioch
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Re: Page 137: Adieu SG-51

Post by Arioch »

boldilocks wrote:I guess I'm having problems understanding how you can observe something before it happened.
You can't. That's kind of the point.

boldilocks
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Re: Page 137: Adieu SG-51

Post by boldilocks »

Arioch wrote:
boldilocks wrote:I guess I'm having problems understanding how you can observe something before it happened.
You can't. That's kind of the point.
But for FTL to be impossible you'd have to be able to. So it ends up as "FTL is impossible because if not you'd be able to do this other thing which is impossible."

Victor_D
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Re: Page 137: Adieu SG-51

Post by Victor_D »

boldilocks wrote:
I guess I'm having problems understanding how you can observe something before it happened.
The point is, as I tried to show in my pictures on the preceding page, that there is no such thing as a universal "now" time. It's relative for each observer. That's all fine if you're not allowed to travel or communicate information faster than light. But once you introduce ftl, you can create arrangements where some observers will perceive that effect precedes cause. And that cannot be because no point of view is more valid than any other. The speed of light limit ensures that for all observers in any frame of reference, effect will never precede cause.

If this were not true, then you'd be able to produce the paradox I described in the pictures. It arises because for both observers in that scenario, an FTL ship coming from the other location is going backwards in time.

boldilocks
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Re: Page 137: Adieu SG-51

Post by boldilocks »

Victor_D wrote:
boldilocks wrote:I guess I'm having problems understanding how you can observe something before it happened.
The point is, as I tried to show in my pictures on the preceding page, that there is no such thing as a universal "now" time. It's relative for each observer. That's all fine if you're not allowed to travel or communicate information faster than light. But once you introduce ftl, you can create arrangements where some observers will perceive that effect precedes cause. And that cannot be because no point of view is more valid than any other. The speed of light limit ensures that for all observers in any frame of reference, effect will never precede cause.

If this were not true, then you'd be able to produce the paradox I described in the pictures. It arises because for both observers in that scenario, an FTL ship coming from the other location is going backwards in time.
Which time is it going backwards in, if time is relative for each observer? The time they observe or the time of the people "transmitting"? At the end of the day, the photons it picked up in order to "observe" something would have to have been set in motion for them to be received, right?

Krulle
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Re: Page 137: Adieu SG-51

Post by Krulle »

icekatze wrote:
boldilocks wrote:Does that mean watching a movie played in reverse violates causality?
No. :|

If you played a movie before it was filmed, then you'd be in a violation of causality, but the universe doesn't care what time stamp we put on our recordings of events.
Instant Cassettes, anyone?
(Where did they go?)
Vote for Outsider on TWC: Image
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Arioch
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Re: Page 137: Adieu SG-51

Post by Arioch »

boldilocks wrote:
Arioch wrote:
boldilocks wrote:I guess I'm having problems understanding how you can observe something before it happened.
You can't. That's kind of the point.
But for FTL to be impossible you'd have to be able to. So it ends up as "FTL is impossible because if not you'd be able to do this other thing which is impossible."
Well, what this implies for the real universe is that FTL drive is impossible, which is perhaps a good explanation of why we don't see starships zipping by every day. However, To say "any FTL travel breaks causality" is not really a true statement, because there are instances in the real universe in which phenomena happen faster than light, but the universe is structured so that such phenomena can never transmit meaningful information in violation of causation.

For example, there is an effect of quantum entanglement (see double-slit experiment) in which two entangled particles can be separated by great distances, but performing a measurement on one can force the other into the same state. This would seem to be information exchange at faster than the speed of light, but the universe is structured in such a way that this effect cannot practically be used to transmit information from the future to the past (see this explanation by PBS Spacetime). Another example of real FTL effects is the expansion of spacetime faster than light in the early moments of the Big Bang, and in the potential exponential expansion of spacetime in the future (if the rate of Dark Energy continues to increase exponentially). However, because the effect goes only in one direction, it can't be used to transmit information back in time.

However, when it comes to storytelling, all that really matters is that the system is internally consistent and doesn't have any obvious holes. Going back to what started this whole digression, the rules of a fictional universe can be structured in a way so that FTL travel is limited so that causal violations can't happen. In Outsider, this is done by limiting FTL travel to hyperspace, requiring that travel is not instantaneous, and requiring the endpoints of the jumps to be so far away from each other so that no meaningful information can be transmitted backward in time.

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icekatze
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Re: Page 137: Adieu SG-51

Post by icekatze »

hi hi

I find that it is a really interesting field of research right now, and one where the answers haven't been definitively figured out. "Spooky action at a distance," is still the subject of questions and experiments. There's even some evidence that suggests that contextuality and non-locality are two different parts of the same quantum resource.

I wish time travel was possible though, then someone from a few million years in the future could come back and give us some hard observational evidence on whether the universe really is expanding exponentially or not. :lol:

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