Hacker News Reader

Comment by Andrew_nenakhov | Hacker News Reader

I still don't see why this would be a problem. Substitute light with sound and sight with hearing. We can exceed the speed of sound and produce noises that some observer would perceive in the incorrect order (in fact, happens all the time with supersonic jets). And the world doesn't seem to break because of that, no?

View on HN

It's not really about light. It's not so much the case that the speed of light is itself special, so much as it is that light (in a vacuum) travels at a special speed. That speed is effectively that of causality, in the 'speed of the ability to cause' sense.

Trying to make conclusions about that by using simple analogies to light isn't likely to get you anywhere. You're not dealing with the actual thing you're interested in in the first place.

I try to think about the speed of light as a geometric property of the universe, similar to how you can’t draw a straight line between two points on a circle that’s longer than the circle’s diameter.

I don’t know if that’s correct, but the analogy helps me (until I think about it too hard, and then my head hurts).

It’s not a matter of perception or “wrong” order: events that are close enough in time and distant enough in space that it is impossible to be present at both, even travelling at the speed of light — so-called “spacelike separated events” — do not have a single correct order.

Imagine you’re riding on top of a moving train carriage, standing exactly in the middle of its roof. Suddenly you see lightning strike the front end of the carriage, and simultaneously you see it strike the rear end. You know the strikes must actually have been simultaneous, because you’re standing exactly halfway between them: if the flashes arrived at the same time, they must have left at the same time.

However, from the perspective of someone standing at the trackside, you are moving towards the source of the front flash and away from the source of the rear flash. So the rear flash has further to go to reach you than the front flash. So the flashes can only reach you simultaneously if the rear strike happens first.

Who’s right? Do the strikes happen simultaneously or not?

Answer: both observers are right. Simultaneity, and the order of spacelike separated events, are relative, not absolute.

This doesn't at all explain why short wormholes can't exist. The reasoning seems to be heavily about how an observer would see events transpire, but the observer is pretty inconsequential. The events happen. If they seem to come from two directions, or if we see them out of order, who cares?

It's not like we can't see a nova from three different points with the light coming after three different delays due to gravitational lensing.

If we had three black holes in a circle, with some light coming through the middle and the rest lensing around, the middle would seem like a shortcut compared to the lensed light that went around the three, but I fail to see how that, or the wormhole instance, matters at all other than to confuse the observer if they are unaware of the foreshortened distance.

Everything’s probably fine if there’s only one FTL wormhole, and no other method of FTL travel or communication: in that situation, the weird event order is a mere curiosity. “Look at that! In our reference frame, that spaceship left the wormhole before it entered it! Cool!”

But if a hostile observer sees the spaceship emerge from the wormhole before it enters, and they have access to FTL weaponry (or another FTL wormhole to fire missiles through), they could conceivably destroy the ship before it enters the wormhole, which would be a paradox.

(I think, anyway: this is my layman’s understanding.)

But when the hostile observer will arrive to another end of a wormhole, the spaceship that had entered on that end has already exited from the other end.

What I don't understand is this time travel thing. Two events can happen simultaneously here and on Proxima Centaur, and both observers here and there will see that event in another system with 4 year delay. So what? Just like a speed of sound lags behind the speed of light, speed of light will just lag behind the actual events that take place. How is this time travel?

> But when the hostile observer will arrive to another end of a wormhole, the spaceship that had entered on that end has already exited from the other end.

Not necessarily. If the wormhole allows FTL travel, then the spaceship's entry into and exit from the wormhole are by definition spacelike-separated events. Such events have no absolute, universally applicable order. Therefore there is at least one reference frame in which the ship leaves the wormhole before it enters it. Note: it does not just appear to have done so; in the chronology of that reference frame, the ship leaves the wormhole, then sometime later it enters it.

See https://en.wikipedia.org/wiki/Relativity_of_simultaneity

Even if it simultaneity is relative, I still don't quite understand why it is a problem. Every local causality is observed. Take this [1] laborous attempt to explain this issue. They explain that 'you can see the message before it was sent'. NO! You can see the message before light from a distant star system delivers you info about it being sent! Like, a bullet arrives to target's head before he has a chance to hear the gunshot. Never a problem for snipers, shouldn't be a problem for FTL messaging either.

[1]: http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti...

OK, try this one:

Lt Cdr Worf is testing the USS Defiant's impulse engines after a refit. He is currently coasting along in a straight line at a constant 0.6c, and passes Deep Space Nine. We'll call time on board the defiant t and time on board DS9 t', and say that the Defiant flies by DS9 at t = t' = 0.

At t = 10 minutes, Worf decides to reward his efforts with a cup of bahgol, only to discover the replicator is on the blink. After about another 10 minutes of pressing various buttons, he gives up and logs a ticket with the DS9 help desk over the subspace network.

Because the Defiant is travelling in a straight line at a constant speed of reference, it is in an inertial frame reference, and it is just as valid to see DS9 as moving away from the Defiant at 0.6c as the other way round, and this in fact is what we must do to work out what time on DS9 corresponds to "now" on the Defiant. At 0.6c, the Lorentz factor is 1.25: so for every 1.25 minutes that passes on the Defiant, a single minute passes on DS9 -- or put another way, for every minute that passes on the Defiant, only 0.8 minutes pass on DS9. So when Worf submits his ticket at t = 20 minutes, on DS9 it's t' = 16 minutes.

Subspace comms are instantaneous, or so close to it as makes no difference, so O'Brien picks up the ticket very soon after t' = 16 minutes. He replies straight away -- "Have you tried turning it off and on again?" -- and sends the response back over the subspace network.

This is where relativity of simultaneity comes in. DS9 is also in an inertial frame of reference, and in that frame it is the Defiant that is moving away at 0.6c, that has a Lorentz factor of 1.25, and where only 0.8 minutes pass for every minute on DS9. So for O'Brien, t' = 16 minutes is simultaneous with t = 12.8 minutes (not t = 20 minutes): allowing for time for O'Brien to read the ticket and write his response, Worf will receive it somewhere around t = 15 minutes -- while he's still tinkering with the replicator. Somewhat surprised, he tries the solution, it works, and he goes off to enjoy his cuppa, never sending the ticket he's just received a response to.

This is why FTL risks messing with causality.

Thank you for this elaborate answer, but this does not seem right. I am not very well versed in the StarTrek lore, but they use warp drives, which are alcubierre drives, so they expand and fold space to move, thus never actually achieving an FTL speed themselves. If we don't move at relativistic speeds, there would be no time dilation effects, so we'll always have t == t'.

Next off, I feel that this backwards inertial frame of reference is not right. But I'm too lazy to get into actual math so let's put this matter to rest until we get means of FTL travel or information transfer.

They're seeing old light. Even if things are moving at FTL, they still have rational cause-and-effect.

Your weapon might reach the location where the ship had entered the wormhole faster than any telescope there could see light emitted from your star, but the ship will still have moved into the wormhole prior to exiting.

Assuming that to be able to send or receive FTL data implies breakage in causality itself seems pretty preposterous.

Seeing billions of years into the past when looking at distance galaxies doesn't imply that if we had FTL we would be able to affect them billions of years in the past. They have continued to move and emit light in all the billions of years since. It just hasn't reached us yet.

It’s nothing to do with old light. It’s to do with the relativity of simultaneity. Have a look at the Wikipedia article I linked to.

Or read these posts on Ask a Mathematician / Ask a Physicist:

- this one explains the basis of the phenomenon: https://www.askamathematician.com/2010/12/q-according-to-rel...

- this one explains the implications for FTL travel (and why it can result in time-travel): https://www.askamathematician.com/2011/09/q-hyperspace-warp-...

You're assuming an objective reality. Events may not truly occur without an observer.

Reply on news.ycombinator.com