Been reading up on a lot of FTL stuff lately. It seems like one of the biggest problems with traveling faster than light is violating causality (relative to some reference frame).
I was wondering: couldn't it be enough that a sequence of events is causal in one reference frame even if it isn't in another? My thinking is that simultaneity is relative so why shouldn't causality be? Is there a major error in this line of reasoning? Has this been discussed before? I'd be very interested.
Also I'm a mechanical engineering student so treat me like a brainlet.
I don't think something can violate causality in reference to one reference frame but not with respect to another, because the speed of light is constant.
Eli Bailey
The biggest problem with FTL isn't causality breaking. That is just a quirk of how special relativity works. I don't think any one could even answer if FTL would actually violate causality, because its the same theory that says its impossible to go faster than light. The theory clearly just doesn't work at that point.
Relativity just makes it very clear that FTL is not an option.
Ryan Morris
Well I meant specifically something along the lines of the Alcubierre Drive or a wormhole that allows you to arrive somewhere faster than light could without actually "moving" faster than light. As I understand, both of those things are valid in special and general relativity but still violate causality.
Traveling faster than light can violate causality depending on the reference frame in which an event is observed.
Leo Harris
There's no causality violation, it's a misunderstanding of the theory. The theory assumes that faster than light speed is impossible, so if enter something that travelled faster than light you get the only possible consistent answer - that it 'actually' didn't travel faster, it just started in the past.
In other words, ftl is fundamentally incompatible with special relativity.
>As I understand, both of those things are valid in special and general relativity but still violate causality.
They are not valid in special relativity. General relativity simplifies to special relativity at infinitesimal units of spacetime. Alcubierre drivers and wormholes create a 'highway' in space that you can travel with
Brandon Cox
How do we get around the heavy particle buildup issue and destroying your destination on arrival? Smaller 'jumps'?
Kayden Mitchell
Okay so let me see if I understand what you are saying.
Analyzing the causality of any kind of spacetime distortion to travel with "apparent" FTL speed simply doesn't work in special relativity? Like, you need general relativity to even think about it?
Also, you're saying in general relativity, neither of the two methods inherently violate causality?
I don't know about either of those issues actually, could you explain to me?
James Cooper
Forget causality it just confuses the issue. Special relativity really says there is no known way to travel faster than the speed of light.
Ryder Davis
> no known way to travel faster than the speed of light. And even that's not exactly true due the behaviour of quantum entangled particles.
Angel Perez
Okay. I get that. I guess I kind of have two questions now.
1. Are there any objections to wormholes and alcubierre drives in general relativity? Aside from needing exotic matter to construct and the possibility that a theory of quantum gravity could make the theory behind them invalid.
2. My original question, sort of: Could two events have a causal relationship in one reference frame and be noncausal in another frame in real life? Is there any argument against that other than our general experience with the universe? I'm just interested in this idea now.
Sorry if my questions are dumb, I'm just trying to learn.
Your understanding is confused because you don't understand how special relativity is constructed. I recommend reading a book about relativity. There are a lot of textbooks written that introduce the math with the subject material, it's not inaccessible if you've already had diffy Q's, calc 1-3 and physics 1-2, which I assume you had since you're MechE.
William Scott
Yeah I've taken a modern physics course before but it was mostly quantum. And I read Einstein's own treatise on relativity but that was for fun mostly, I wasn't working out the math along side it. Evidently a lot if it was over my head haha. Thanks for being patient with me. Any texts in particular you'd recommend?
This is weird as heck, I've never heard of it before
Camden Wright
cool hey :)
Jackson Morgan
>Could two events have a causal relationship in one reference frame and be noncausal in another frame in real life? Not really. Maybe the illusion of such, weird things happen at relativistic speeds with multiple observers. In general causality remains intact.
Josiah Cooper
>Are there any objections to wormholes and alcubierre drives in general relativity? Aside from needing exotic matter to construct and the possibility that a theory of quantum gravity could make the theory behind them invalid. > You're in an area that scientists in the field often argue about. But you need a lot of background knowledge to discuss the matter.
Aiden Brown
>heavy particle buildup >sydney.edu.au/news/physics/1737.html?newsstoryid=8801 If the particles are not in the warp bubble they can't travel FTL so will be left behaind. If they get into the warp bubble then you might need some leakage system to get rid of them.
Austin Cook
...
Oliver Morris
If anything travels faster than light then causality is broken. Then you've got time-travel and other bullshit. Unless we're only talking about tachyon telephones in which case we're probably OK.
Nolan Brown
>Alcubierre drivers and wormholes create a 'highway' in space that you can travel with
Wyatt Ramirez
If we discover that faster than light speeds are possible, then its clear that we have to rethink special relativity. If you put causality up against the theory of SR, I would definitely put my money on causality holding true before SR. But its all irrelevant since its impossible to go faster than light anyway.
I just think its stupid to use the same theory that says its impossible, to say that it breaks down.
Kevin Sanders
>I heard black science man reference particles building up in front of the ship during warp speed travel on discovery channel and so it must be true
Its all hypothesizing at this point. They dont know anything about what happens when you travel FTL or if its even possible. Its just as likely that the entire universe is torn apart by vacuum decay the instant you turn on your warp drive. Its not an "issue" that needs to be worked out when we dont even know what technology and/or laws of physics need to be exploited to make it happen.
Jackson Richardson
(Checked) To be fair the vacuum decay would only be happening around the path you'd travelled. Everything else would be fine for a while.
Michael Green
Im just saying, there is no specific problem to work out with warp drives when we have no idea how to build one in the first place, or if its even physically possible.
