I think some one used the illustration of a rhebus loop to explain this.

The universe either respects causality or it doesn’t. If it does, and I think it must, then you cannot jump to a point in space faster than you could travel their via relativistic methods.

I think this is the rub here. The arguments are all well and good (in fact very well argued, you should be a science writer), but when minds as great as Einstiens refused to discard the possiblity of CTC’s, then I (and you) would be fairly arrogant to do the same, simply because ‘it must be so’. People discarded (and still do, despite endless practical proofs) relativity for the just the same reason: It seems so anti-intuitive to everyday existence that time warps and distorts so dramatically.

But for all the ‘obviousness’ of causality, cause precedes effect is still more a point of philosophical rather than scientific rigour.

I just breathed in Well what caused that? Because I breathed out a second ago Well what caused that? Well my brain sent an unconcious signal to my lungs And what caused that? Well, you see over the course of millions of years of evolution…

and on and on to the start of the universe where there was no identifiable root cause of any kind and, indeed, no concept of time at all. Law of causality broken, therefore not really a law at all. Next theory, please.

I’m not saying you can have an effect without a cause, that is certainly how the universe seems to work and science is, by definition, a useful way of describing how the universe works. But there are practical, identifiable clues that indicate FTL communication could exist, and frankly I don’t think the universe gives one jot or another if your grandfather is killed before he concieves your mother- only human arrogance would assume we’re so important that physics has to bend itself around what we consider to be the natural order of events.

I’m not saying I believe FTL communication to be possible or not. I’m saying that we’ve believed a lot of things that later turned out to be untrue, and if we can get our heads around a universe where you can spend thousands of subjective years getting torn to shreds by a black hole, we can probably also get our heads around a universe in which Battlestar Galactica can make lightspeed jumps and yet still not be able to send a message back to Caprica asking them to prepare for Cylon attack.

Speaking of trains and causes, can anyone explain to me why:

“We apologise for the late running of this service, this was caused by adverse signals”

is regarded by the train companies as an acceptable reason for their poor performance?

I have to ask, and I hope it’s not insulting, but did you think up the bomb-under-the-seat thing yourself or were you paraphrasing something you’d read? Because if you did I think that is particularly impressive.

I am pwned indeed.

Bravo. A piece of brilliance.

Traveling through a wormhole would do nothing in my opinion. Traveling through a wormhole is basically teleporting not traveling which i believe is two different topics. See my previous post on the last “the debate earlier on in the month”. Although a strange proposal maybe you would need time to travel back in time and the faster you go and the longer you would stay in a hypothetical craft that went faster than the speed of light the further back into time you would go. So the only way to travel back is an actuall craft not a wormhole and that is impossible. Maybe some1 could come up with a formula for this.

(this is the same thing as the last post mainly and doesnt have as much to do with the current topic but i didnt think i would get opinions on an older post)

[Comment from first post moved here - Adrian]

In my own perspective it seems that traveling through a wormhole would do nothing even if it were possible. You would not go faster than light because basically your teleporting yourself. If there were a distant star and i went through a wormhole to it the only way an observer would have seem me travel back in time was because the light hasnt reached him yet and he cant see me. However i am still at the same point of time as him when he cant see me. So it would appear to him that i was there maybe 6 years ago. So if i were to travel in a wormhole back home i could see my self. Its hard to explain and im not good w/words but it makes sence to me that the only possible way to time travel is by going very very fast which as we know is impossible to break the light barrier no matter how fast you go. So going faster than the speed of light and traveling back in time is impossible no matter how advanced technology gets.So for all those who want to curse my name I AM NOT ARGUING AGAINST EINSTEIN.

Please add comments since it is my idea and i want other opinions and to know if this has been proposed or is idiotic

I have a sore head (effect), ‘cos I read your bloody post (cause).

A thought on how to get from A to B in the universe faster than the speed of light without breaking causality. My appologies to Ian Banks (Excession).Just after the big bang the space newly created in the universe expands faster than the speed of light but everything in the universe still travels at light speed. So the laws are maintained but it explains why the universe is the same from horizon to horizon even though the infomation could not have travelled from one side to the other in the time the universe has been in existance. However if you could theoretically travel in the expanding edge you could go FTL-(which is how I understood Banks book).Once however you leave the universe there is no information of you to travel anywhere so even though you would arrive before light could get there you would not have broken causality because the information was no longer there to upset causality . If therefore a wormhole excluded the universe inside the hole you could pass from place to place via the wormhole FTL but not break causality.As one comment said this would be more akin to teleportation with all information of yourself being absent from the universe during the passage.

Damn you Sevitz, damn you to a world with no physics! I had been thinking about this yesterday morning while walking the dog and decided I wasn’t going to get into it again.

Something seems wrong about this whole thing and I can’t quite put my finger on it. As Destructor pointed out, some of it seems so anti-intuitive (and I think it’s the concept of spacetime that the theory of general relativity uses that bugs me. I’m sure there are some massive assumptions going on somewhere).

I’ll be back.

Stalker, the thing is that while it seems wrong (because our brains evolved in such a way that we didn’t need to take relativistic effects into account) the fact is that relativity has (so far) succeeded in every test we’ve thrown at it.

A real world example (outside a lab): The atmosphere is being constantly bombarded with high-energy cosmic rays (made of protons). In the upper atmosphere these protons hit molecules of gas, and occasionally these cause unstable sub-atomic particles (muons) to be released.

These particles shoot out at high speeds, significant fractions of the speed of light.

Now as these particles are unstable they decay within a few microseconds (half-life about 1.4 microseconds) - so that by the time they travel the few km from the upper atmosphere to the ground, even at the very high speeds involved, they should have decayed.

However, we do in fact detect these particles at ground level (there will be several going through you at any given time). The reason is that because the particles are at such a high speed, they appear to us as if their time clocks had slowed down to such an extent that they are able to reach the ground before they decay.

Unfortunately if we take all these kinds of examples together we have to accept that all our observations indicate Einstein was right. We aren’t making any assumptions, just inferring from what we observe.

…or perhaps I’ve spoken too soon:

E and mc2: Equality, It Seems, Is Relative

From that article “Some suggest, for example, the rate of the passage of time could depend on a clock’s orientation in space”.

This is the biggest issue I have - about time slowing down in the upper atmosphere. Perhaps it’s because I subscribe to Leibniz’s view of time, that it is an abstract concept with no “physical” properties. A second is defined as being some ridiculous number (9192631770 to be precise)of “oscillations of specified transition in Cs-133 atom”. Say we decided that it would be far better to make it a round number - like 10000000000 osciallations. Does that mean that time gets slower or simply that our measurements change?

Part of this whole issue is to do with observation. So much of it depends on what is observed and that depends on light.

Let’s look at this another way. I stand at point A holding a brick. There is a pane of glass at point B which just happens to be 1 light-hour away. Because I work out so much, I am able to hurl the brick really, really fast - say 1.5c (1.5 times the speed of light). The brick hits the pane of glass 40 minutes after it left my hand. But because of the speed of light, I see the brick only two thirds of the way there.

Is this breaking causality?

No, it’s not. Time is still going forward. I threw the brick at time t. At time t+0.66, the brick hits the window pane. At time t+1, the light hits the window pane (or whats left of it). However, the brick cannot break the window pane before it leaves my hand because time t is when the velocity 1.5c is applied to it and it starts moving. Even though it is travelling at more than the speed of light, it cannot reach a time t-1.

If we did a Xeno and stopped time at, say, t+0.8, what would we find? A broken window pane and a brick flying through the air towards it? An effect happening before it’s cause? Well, yes. But only because that’s what it looks like. The only way that this could be explained is because the brick that we see is only the light from the brick and not the actual brick itself. It’s an image of the brick, an illusion if you like. But the ‘solidity’ of the brick (which we obviously can’t see because it’s moving faster than the speed of light) is currently at point B, breaking the window pane.

Incidentally: I agree with Cornroller in that I don’t think wormholes are particularly pertinent in a discussion about going faster than light or causality.

Actually you did a pretty good job of explaining.

If we have to sets of cesium atom clocks, or a stop watch, a radioactive half life, or even your natural biology, these clocks will tick differently relative to each other if one of those clocks is travelling at relativistic significant speeds (i.e. fucking fast).

I get that this has been shown to happen and has been demonstrated repeatedly. However, to me it does not demonstrate that time is affected by travelling at fucking fast speeds - only that the clocks are.

And I have to say again that while I understand your explanation about how travelling faster than the speed of light could potentially affect causality (and yes, I do get the model of the light cone), you can’t lump wormholes into this as they don’t affect the light cone at all and wouldn’t necessarily affect causality. A wormhole isn’t about travelling from A to B faster than the speed of light, it’s about taking a shortcut.

The first time I read about the Twin Paradox when I was about 7 years old, it messed with my head and it has done ever since. I even have “A Brief History of Time” next to a book called “What is time?” on my shelves. If it wasn’t the fact that I hated my physics teachers at school, maybe I would have gone down that path rather than the philosophy path. For example, the twin paradox is a thought experiment that has found substantive “proof” in an experiment with two atomic clocks, one on ground, one on a plane. After the experiment, the one on the plane was slower. Now this is fine but this is where I say “it doesn’t show that time has been affected, only clocks.” I’m afraid I can’t really come back on this.

I will come back on wormholes though. The whole concept of wormholes means that you take the the shortest distance between two points and make it shorter. Take a piece of A4 paper and draw a line down the length. Mark the ends A and B respectively. This is your origin and destination. The quickest distance between them is a straight line. But if you fold the paper, in half widthways, A and B suddenly become much closer. You can send information from A to B at normal speeds without breaking causality but far quicker than if you followed the straight line you originally drew.

That’s the theory behind wormholes. Granted, if you change the properties of a wormhole so that it became a timehole, you could theoretically travel back in time and affect causality.

That post is a tour de force. Congratulations.

If the clocks which measure time, tick at different rates, then how is this anything but showing that time has slowed down.

What might help to resolve the confusion (or alternatively make people more confused) is to remember that not only does time change with relativistic effects, but distance changes also - which helps to resolve a lot of the apparent paradoxes.

For example, suppose a spaceship is heading from our solar system to the nearest star, 4 light years away, and it is travellling at 0.8c so the journey as far as we on Earth are concerned takes 5 years.

Suppose the relativistic adjustment for the difference in these rest frames is 50% (the numbers probably need tweaking a bit but the principal is the same).

We on Earth look at the spaceship and from our point of view the spaceship looks to be half the length it was when it was stationary. We watch the spaceship and see it arrives at its destination 5 years later than it left. However if we look at a clock on board the spaceship (we have a very good telescope and it’s got big numbers on it) the clock has only ticked a time lapse for the journey of 2.5 years. Also the astronaut on board has aged 2 1/2 years.

This might look like a paradox but consider, what does the astronaut see?

From his point of view the distance between the Earth and the star is shortened, so it’s not 4 light years but 2. That’s why at 0.8c it takes him only 2 1/2 years of his own time to get there.

Note in this - when I say we see the spaceship arrive 5 years later, of course that’s after we allow for the time taken for the light to reach us - so in fact we don’t actually get the pictures of the spaceship reaching the star until year 9 - the spaceship takes 5 years to arrive, plus 4 years for the signal to get back to us.

(And the spaceship will look red to us during the journey due to the doppler shift)

Of course, this time dilation effect could be of use to science fiction authors/futurologists as it means that the finite speed of light isn’t necessarily that much of a limit to human exploration beyond the solar system.

For example if we could invent a way of propelling a spaceship near to the speed of light (while stopping the passengers from being squashed by the acceleration), then as far as the passengers are concerned the journey doesn’t feel as if it had taken nearly as long as it actually has taken. This would allow intergalactic exploration without wormholes, faster than light travel, hibernation or multi-generational colony ships.

The problem for science fiction authors is that it stops the intergalactic politics they like to imagine (difficult to run a galactic empire when in your subjective time it takes 100,000 years to even send a signal to the edge of the galaxy and back, let alone an army).

The problem for spaceship designers is that the closer you get to the speed of light, the more the mass of the spaceship increases and therefore the more energy you need to accelerate further (with infinite energy required to actually reach c).

As such I’ve not seen many examples of this in science fiction - the notable exception being the original Planet of the Apes (I haven’t seen the remake so don’t know if it also applies to that film too).

I just wanted to say that thanks for this post Adrian. Although I’m still not convinced about the whole time thing, I now think I understand why. What I never really got my head around before was about the whole “frame of reference” and how it’s all to do with, essentially, observers points of view. More than that, what I had not fully appreciated before was that that one of the postulates of relativity is that there is no absolute frame of reference.

Alright quit with the fancy thought experiments that could never actually take place and just answer me a simple question:

The Battlestar Galactica makes an FTL jump away from Caprica and leaps 30LY away from it. A few weeks later, Starbuck steals a Cylon raider and makes an FTL back to Caprica.

What, according to relativity, would Starbuck have found apon her return?

d

So basically, you don’t know.

Hmm… I’ve been away for too long.

CornRoller, Adrian has stopped talking to you, and I suspect it’s because you state ‘fact’ that contradicts the argued points without any justification, despite the huge efforts that he and various other people are making to justify everything they say.

Moving on, I’m not going to say anything new and ground-breaking here, but I am going to repeat some points in different terms. It helps me work them out, and it may help some others who aren’t yet convinced.

There is no universal frame of reference. (Going to use that term a lot, so I’ll shorten it to FoR). Even without getting close to c, you can appreciate this effect with sound-based Doppler effects, if you consider that the ambulance sounds different to you once it has passed you than it does to the guy 100m up the road; same time, but different observers get different measurements of something that we might otherwise consider constant (in this case the pitch of a sound).

Time as humans know it is defined solely by their experience of it, such as by clocks and certain chemical/atomic decay events. After everything I said last time we were debating this, I’d like to take that as a given. To claim that humans have a better grasp of time than by measurement and experience would be a little odd, to say the least.

The claim that FTL (specifically the ability to leave your light cone) circumvents causality is true for some frames of reference (and I’m with Adrian in saying that causality void in one FoR enough to consider it void in general). Let’s return to the Stalker’s example: in throwing a brick at 1.5c he throws it out of his light cone. He throws it, sees it travel, sees it hit the target; no problem there. An observer placed at the point of impact sees it hit, then sees the Stalker throw it. Causality violated. (I think I’m right in saying he doesn’t see it approach).

I don’t want to speculate on the effects of violating causality. Perhaps you might get away with it. Perhaps you might disappear in a puff of logic. Hopefully there’s some built-in mechanism that will stop it ever happening; certainly the current combination of theories seems to do just that. I don’t take ‘FTL ~ Time Travel’ as proof that FTL isn’t possible, and I will still moan at Adrian whenever he asserts fact about things that actually are based on perception and hence cannot be logically proved, but it’s been a very enlightening debate, and I thank everyone involved.

See you all in the next thread, for more trying to catch up, and maybe more necroposting ;-)

If there’s no universal frame of reference for time, why do we assume that there’s a universal frame of reference for reality? Why CAN’T the brick break the window for one person and not break the window for the other person? Other than the fact that we think it shouldn’t, I mean.

You’re nearly there, QE, although not quite.

Your example of the brick being thrown assumes there is a contradiction in the fact that you see the brick hit the target before you see it being thrown. This is not, however, a contradiction in itself. To understand that this is not a contradiction consider a supersonic missile.

If you are standing near the target of such a missile (and far away from the launch site) you will hear the explosion first, then subsequently (assuming you survive!) you will hear the missile being launched. In between you will hear the sound of the missile’s flight (and the sonic boom), but in effect you will hear it backwards. This will seem odd as we are not used to hearing noises backwards but isn’t a contradiction to causality.

In the same way, if we postulate a universe with a finite speed of light but following Newtonian rules (which is not in itself contradictory - it is just that we don’t happen to live in such a universe) then a faster than light missile (which would be permitted in such a universe) would indeed appear to fly backwards if you looked at it.

Your understanding appears to be that the perception of events differs according to your point of view, not the actual order of the events.

The point Adrian and I are making is that in consequence of (1) the fact that in our universe the speed of light is the same for all observers (a measured fact), and (2) the principle of relativity, i.e. the laws of physics are the same for all observers (also a measure fact) then we must conclude that (3) there is no possibility of transmitting information faster than light.

It’s difficult to prove this without using mathematics but I’ll try (Adrian, if you’re up to doing some diagrams for this and not bored of this thread that would be great):

Consider two ‘events’ A and B. An ‘event’ is a point in spacetime - something which happens in a particular place at a particular time.

Suppose firstly that A is not in B’s lightcone (and vice-versa, which is true by definition).

Then according to relativity there is SOME frame of reference (i.e. a constant velocity which someone could travel which is slower than light) (call it FoR1) in which A occurs before B (i.e. the observer (Ob1) in FoR1 would say that A occurs at a time before B AFTER THAT OBSERVER TAKES INTO ACCOUNT THE TIME IT TOOK FOR THE LIGHT TO TRAVEL FROM A AND B TO HIM (This last capitalised part is important).

There is also a different FoR (FoR2) in which an observer (Ob2) would say that B occurs before A, again taking into account the light travel time.

This isn’t the observer’s personal perception - in Ob1’s “world” A really did “happen” before B. In Ob2’s “world” B really did “happen” before A. Both are right.

That means that A can’t have caused B to occur (because Ob2 knows that B happened first) and B can’t have caused A to occur (because Ob1 knows that A happened first). A and B are independent events and cannot influence one another in any way - i.e. no information whatsoever can go between A and B.

(There is also a third FoR in which A and B happen at the same time).

In fact for events outside each other’s light cone it’s not really sensible to say anything about the time they occured with respect to one another, as it depends on the observer.

Now secondly, consider an event C that lies inside A’s future light cone.

Clearly there is a FoR at which C happens after A (for example the FoR of an observer travelling from A to C).

Now if you do the maths you find that there is no FoR* at which C happens before A. This makes sense as C could easily be influenced by A (by sending a light signal from A to C) and A could therfore be a cause of C. If C happened before A for any observer then that observer would say that C can’t have been caused by A.

*This assumes that FoRs (i.e. observers) can only travel at speeds less than light. If you found a way of travelling faster than light (whether moving through space or taking shortcuts via wormholes) then you could find a FoR at which C does happen before A - which is why FTL travel causes a violation of causality.

Destructor, to answer your question, we don’t assume it, we observe it.

If we ever found a window which was broken for one person but fixed for another then we would throw the theory of special relativity out of it (although one of those people might have to open it first).

Sorry Fer, I can’t see where it is you and I are disagreeing.

You’ve proved (assuming your assumptions, which I’m happy to do at this stage) that events lying outside one anothers’ light cones cannot be causally related, and also that for a cause-effect pair there exists no FoR for which effect precedes cause.

The Stalker is proposing an effect (brick landing) that is outside the light cone of its cause (brick being thrown), by means of the brick travelling FTL. This gives a situation that violates both relativistic causality and a more intutive layman’s causality, since there exists an FoR for which effect precedes cause.

I didn’t refer to it as a contradiction (a term I would normally reserve for logical impossibilities rather than concrete unlikelihoods), but I do consider it a violation (as stated) of both kinds of causality, and proof of the case that Adrian is arguing.

No disagreement from me ;-)

Fer,

can you please answer my question about Battlestar Galactica in a direct manner, since Adrian refuses to.

Let’s return to the Stalker’s example: in throwing a brick at 1.5c he throws it out of his light cone. He throws it, sees it travel, sees it hit the target; no problem there. An observer placed at the point of impact sees it hit, then sees the Stalker throw it. Causality violated.

I don’t think I said any of this. In fact, you’ve totally misinterpreted what I said. Leaving relativity out of it (because I don’t want to make Adrian sigh any more) my sequence of events does not involve any observer A, B or C being able to see the brick hit the window or the window breaking BEFORE the brick has left the throwers hand. Certainly they may see the window breaking before they see the brick hit it but definitely not before it leaves the throwers hand.

As for the BSG discussion, that’s easy. She found the survivor bloke in the museum but he was only 56 days older than when the cylons invaded. She, however, was 57 days older because of a warp in the Script Continuity bubble which I believe was first identified as a possibility by Einstein but turned into a viable theory by Alan J. Smithee.

I’ve not misinterpreted what you said, I’ve just applied some of the assumptions (many of which are well observed, but I will call them assumptions nonetheless) of relativity to it.

With time as a universal constant, then it is true that nobody sees the brick hit the window before the brick was thrown. However, since time as it’s accessible to humans doesn’t work like that, the pertinent point is that there is a frame of reference in which it is seen to hit before it is seen to be thrown, and hence according to a particular observer cause followed effect.

Make of it what you will. I’m afraid I can’t prove anything from there without assuming the accuracy of Fer’s last big post, which I can’t actually justify and you may not accept.