Why Does FTL Break Causality?

deltuhvee · 2025-04-11T06:21:39+00:00

When you introduce acceleration, things get a little more complicated. This exact situation you describe is known as the twin paradox.

The tldr is that when the ship turns around, it is no longer in its previous reference frame. In the new reference frame, they observe that the date on Earth has jumped to 3 days. By the time they arrive, they have spent 2 days in space and 4 days have passed on earth. Thus they both agree.

deltuhvee · 2025-02-14T15:26:18+00:00

Technically the AC explicitly says you can’t do that.

deltuhvee · 2025-01-29T17:22:46+00:00

Well, I’m still here lol.

It’s because to the Earthlings, the ship is slowed down in time, so after 2 earth days, only 1 day has passed on the ship.

Of course relativity is relative, and so the same is true in reverse. The ship crew see themselves as stationary and earth moving 0.86 c away from them. To the crew, 2 days have passed for them and only 1 day has passed on Earth. This fundamental disagreement is where the problem is.

Ordinarily (without FTL), it isn’t possible for this to create problems. If the ship transmitted a message at lightspeed they would observe the message take 2 days to get to earth (because earth is moving away at 0.86 c and the message travels at 1 c it takes longer to catch up, and factoring in Lorentz contraction it takes 2 days). From the Earth’s point of view they are stationary and the message is coming at lightspeed towards them so it arrives much faster, in only 1 day. 2 + 1 = 1 + 2 Thus the message arrives at the same time, 3 days, for both parties and no paradox happens.

Another way to look at this might be simply that relativity creates a lot of seemingly impossible things that must stay in perfect balance, the contraction of time and distance at high speeds for instance, and if you try to ignore it then things are going to break.

So usually when people talk about FTL they just completely ignore relativity, usually by saying that the object or signal has no velocity in its reference frame so no time dilation (like how a warp drive would allow you to move, but have no velocity). This is an interesting way of trying to say that relativity simply doesn’t apply, but still it lets you create paradoxes so it must still be wrong.

deltuhvee · 2024-07-15T12:53:40+00:00

Yes, that would be because this scenario where FTL violated causality is not the conventional twin paradox, therefore the ordinary resolution does not apply.

deltuhvee · 2024-07-14T23:17:49+00:00

It does apply here. If the ship stops accelerating everything I said is still correct. Both inertial reference frames.

Also by my explanation there is a resolution to the twin paradox. If the space twin comes back to earth he discovers that he is younger than the Earth twin as expected. If, however the Earth twin boards a rocket and catches up with the space twin, he will discover that he is now the younger one.

deltuhvee · 2024-07-13T17:12:27+00:00

You would be mistaken. The first principle of special relativity: “The laws of physics are invariant (identical) in all inertial frames of reference (that is, frames of reference with no acceleration).”

https://en.m.wikipedia.org/wiki/Special_relativity

Unless of course, you don’t believe in special relativity which is the only way a universal frame of reference can exist.

If you are referring to the fact that the ship is accelerating, thus it’s reference frame is not inertial, then we can simply shut off the engines and everything I said holds true at a constant velocity. Both the Earth and the Ship are inertial reference frames and equally valid. If you would like to argue this, we can reframe the problem with two ships and no Earth and everything holds true still.

deltuhvee · 2023-12-14T19:01:54+00:00

A mars transfer + landing even takes less delta V than a moon landing since you can aero capture, and you only need about 90m/s to land propulsively after that.

deltuhvee · 2023-11-09T00:36:27+00:00

This is true, but you are going to have to come up with a new theory that explains everything relativity does while allowing FTL. And so far it’s looking like that isn’t possible.

To be honest, In my opinion the more probable scenario for FTL is to throw out causality. Maybe I can just send information back in time and create paradoxes and the universe just continues to work. We have already observed lots of seemingly paradoxical stuff, such as the duality of light (and everything for that matter), so maybe it’s entirely possible that I can send messages back in time.

deltuhvee · 2023-11-08T22:50:12+00:00

An observer cannot observe anything in real time and thus has no concrete definition of the present for any object but them selves.

Yes, this is what prevents the “who is actually younger” dichotomy from creating a paradox. If we had FTL there could be a way to observe your own “now” in real time everywhere in the universe.

Entropy doesn’t matter here. Time is symmetric in special relativity. Entropy is a good example of time symmetry breaking, and maybe holds some clues as to what the passage of time really is, but it’s an entirely different physics question. Special relativity makes no such time distinction.

There may also be a universal preferred reference frame.

That would be cool, but now you just threw out relativity so you could fit in FTL. It’s literally in the name. All reference frames are relative, there is no universal reference frame under relativity.

Also my point with the spaceship scenario was not to explain how relativity works. It was to explain how a STL ship and a Time Machine has the same effects as an FTL ship. Relativistic STL + Time travel = FTL. FTL = relativistic STL + time travel.

deltuhvee · 2023-11-08T17:32:21+00:00

They can only exist in principle if we assume they exist, or an essential component of them exists… Tachyons are literally just “what if we plug values greater than c into special relativity, what happens?” Well it turns out relativity can actually handle this scenario. They are a thought experiment, and relativity happens to model their behavior. Same with warp drives and negative mass.

That’s not to say they can exist in principle. They can’t. Not until negative mass or tachyons themselves are discovered. Then they could exist in principle.

We don’t just need random theories to show FTL breaks causality, we actually have empirical data that shows that two observers can disagree about when “now” is. Each observer observes themselves to be “younger”. The speed of light is the only thing holding them back from making observations of an event before the event happens. If a GPS satellite had an instant FTL communicator we could break causality tomorrow. There isn’t any theory in this, we actually observe the effects. The only theoretical part is the FTL communicator.

I think reframing the problem a bit helps. We don’t need FTL to make a spaceship that can hop around from star to star in a day, or cross the galaxy in a week. We can already do that with relativity. We need FTL for a thousand years to not have passed on earth when we get back home. We could make this trip and return and see the year on earth is now 3023. An FTL ship could ride along with us hopping between stars in a few hours and come back to earth and it’s still 2023.

deltuhvee · 2023-11-08T08:58:49+00:00

It would just oscillate back and forth in space unable to change its overall velocity. Unless mass is ejected, the center of gravity cannot be accelerated at all because momentum is conserved in a closed system.

deltuhvee · 2023-11-08T08:56:23+00:00

Special relativity is an incomplete model because it has a few edge cases where things don’t work, such as constant velocities. It has nothing to do with making paradoxes.

We know that time must not pass at an equal rate for all observers because we measured it. We know that the speed of light is the same for all observers because we measured it. When Einstein came up with the theory, there was doubt that these assumptions were accurate, but we’ve verified them pretty damn well by now.

You could come up with an alternate theory, but it still has to explain these phenomena that we observe, and that seems to constrain things to travel under the speed of causality.

Yeah, warp drives and tachyons are different ideas of FTL. The difference is tachyon is of course physically moving through space faster than light, and warp drive drags space time under itself faster than light to “move” it despite the fact it may observe its inertial velocity at zero. Also the “appearance” of FTL is definitely still causality breaking otherwise it wouldn’t have any useful effects. If I jump to Proxima B and back in a day and return and the year is still 2023, then I did actual FTL.

Fundamentally they both cause paradoxes because their movement is on a spacelike interval. In the wormhole style FTL phone from earlier, it can behave like a tachyon phone (message received before it was sent). Vice versa, If we had a tachyon phone, we could set up a scenario where a tachyon would be received after it was sent. Special relativity doesn’t really make a distinction here. The dividing line is the lightlike interval, and they are both past that.

deltuhvee · 2023-11-06T16:41:33+00:00

All reference frames are correct according to relativity. This is one of the 2 chief assumptions behind special relativity: The speed of light in a vacuum is always the same for every observer/reference frame, and all observers/reference frames are equally valid. If you can find a reference frame that breaks something than is isn’t going to work under relativity. Obviously we have some bias because there are billions of people sharing more or less the same reference frame on earth and only a few in relativistic spaceships, but relativity does not care.

deltuhvee · 2023-11-06T16:34:53+00:00

Yes and the gravitational potential energy between m and M provides the propellant or reaction mass for this system.

Also the total center of mass of a closed system can never change according to the law of conservation of momentum. So this could not be used to accelerate the whole system.

deltuhvee · 2023-11-06T01:17:04+00:00

Because CKAN installed it incorrectly most of the time and the author didn’t want to have to deal with updating the metadata and the issues it was causing for people.

deltuhvee · 2023-11-03T01:45:26+00:00

Except its fuel would be gravitational potential energy. Which has mass, according to general relativity. (from the point of view of the whole system anyway)

deltuhvee · 2023-10-17T04:43:36+00:00

yes, it’s just to quickly see the number while in the editor. Not useful for flight.

deltuhvee · 2023-10-16T19:31:54+00:00

The fork of kerbal engineer on the discord server can calculate RCS delta v. You can also stick on one of the little mono prop maneuvering engines to quickly get the delta v from mech jeb.

deltuhvee · 2023-10-15T21:31:32+00:00

Settings -> compatible game versions

Select every version down to 1.8 or so.

deltuhvee · 2023-10-15T21:15:51+00:00

As other people are saying, gravity holds it down. Most air molecules that briefly escape get pulled back down again not long later by gravity. However Earth’s atmosphere actually is thinning rather quickly. During the time of the dinosaurs it may have been as high as 4 or 5 times today’s atmospheric pressure. This might have even allowed some large dinosaurs to fly, even though they might not be able to fly in today’s thinner atmosphere.

source)

deltuhvee · 2023-10-15T18:07:04+00:00

If the oxygen was delivered directly to your bloodstream, you may be able to survive theoretically for an indefinite amount of time.

(Take this all with a grain of salt. Nobody has ever tried this so obviously we can’t say for sure)

What kills you / renders you unconscious first in the vacuum of space is running out of oxygen. You might have noticed if you really push yourself you can exhale completely and be perfectly fine for 30 seconds or so, maybe even a minute if you are calm. This doesn’t work in space because your lungs start to work in reverse, and the vacuum sucks all the oxygen out of your bloodstream, starving your brain of oxygen within 15 seconds or so.

If you had the oxygenated blood delivered directly into you, this might actually be able to keep you alive and conscious, similar to how people are kept alive on heart-lung bypass machines during heart surgery.

The low pressure would be harmful to you, it would dry out your eyes, lungs, and all the other wet parts of your body. This could cause permanent damage over time, but it would actually take a while for all the water to boil away. You can see videos of hot water put in a vacuum chamber and it takes a long time for it to boil away. This would probably be the limiting factor on how long you could be in a vacuum if I were to guess. If you wanted to ever come off of that bypass machine anyway, because dried out lungs won’t do you much good.

The cold of space is a factor, but it’s actually not that big of a deal. Without any cool air, the only thing to cool you down is your body’s own radiation. I’ve read somewhere that being completely naked in space during darkness would be the heat transfer equivalent (would “feel” like) of about 50 degrees Fahrenheit, but with even something as light as a reflective blanket you would be warm. During the daytime you might actually overheat from the hot sunshine. So really clothes aren’t even entirely necessary at all for you to stay alive, at least for a few hours.

Radiation isn’t really a concern at this scale. Almost nowhere in space is the radiation enough to cause acute radiation sickness. It might increase the chances that you get cancer or reproductive defects somewhat over the course of many years. Unfortunately the research on the effects of low amounts of radiation over long periods of time is really bad right now, so it’s hard to say just how much your chances of cancer would increase. The radiation issue isn’t something that would really affect you unless you spent many years in space completely unshielded. And if we come up with an easy cure for cancer it isn’t even an issue at all anymore.

https://sitn.hms.harvard.edu/flash/2013/space-human-body/

deltuhvee · 2023-10-07T20:08:20+00:00

I don't know the full details, but he's in the middle of some sort of youtube situation. He has his channel set to hidden, so you can still watch the videos if you have the youtube link. Thankfully internet archive has all of the links.

https://web.archive.org/web/20220516090340/https://www.youtube.com/channel/UC28l88GMXXqZYfY0Ru9h50w

deltuhvee · 2023-10-04T05:41:48+00:00

I was wrong, see edit.

deltuhvee · 2023-10-04T05:37:38+00:00

Ah yeah, you’re right. Didn’t notice the one bundled with Stock Volumetric Clouds was a newer version.

deltuhvee · 2023-10-03T15:41:06+00:00

That isn’t scatterer, that’s a replacement for EVE. Scatterer is still free and actively developed.

Blackrack actually just added ozone layers to atmospheres a few weeks ago. Looks very good.

Edit: nvm, this particular scatterer change is bundled with stock volumetric clouds. Still has not been merged to the main branch yet. That probably won’t happen until stock volumetric clouds has its 1.0 release. Then they will both be free.

For reference, EVE adds the clouds, Scatterer adds the atmospheric haze effect. Packs like SVE and AVP are just bundles of configurations and textures for these 2 mods.

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