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[dreiche2]

Fake-edit: Ok got beaten on the twin paradox, no wonder with my incredibly long post, but:

The twins paradox stops being a paradox when acceleration (the change in direction) is taken into account.  Acceleration speeds up the relatively traveling twin's time-flow (because of the direction of the acceleration), so they will be the same age when they get back together.

No. The one on earth will be older overall.

Anyway...

Anyway. I believe I asked this in one of the physics-related threads that popped up sometime in the past on this forums, but I don't recall getting a meaningfull answer.
So, is anyone able to explain to me the twins paradox? The guy who taught me physics relegated me to some book that I've never picked up, and I still can't wrap my head around it. Apparently it's somehow resolved during the actual acceleration, or maybe the reversing of movement direction that the ship would have to do at some point, to get back to the point of origin.
I can take some algebra, somewhat less of calculus, if you need to use it, but qualitative explanation would be enough.

Yeah, the twin paradox is a mean one because it seems to go against the whole spirit of relativity. To recapitulate, twin A stays on earth, twin B travels with his spaceship with nearly light speed to another planet, turns around, and comes back with nearly light speed. Now, apparently the theory says that the twin on earth will overall have aged more than the twin on the space ship.

From the viewpoint of earth this seems to make sense because from there it looked like time in the space ship was going slower throughout the journey because of time dilation. But, haven't we just just established that everything is relative? Because then, from the ship's point of view it was the earth that travelled away with almost c and then came back, so it should be the clocks on earth that should be going slower, i.e. people on earth should have stayed younger, not the other way around.

So that's the apparent paradox. Now, the short solution, and it definitely makes sense, is to say that the frame of references are not symmetrical in this case, because one was undergoing acceleration, namely the twin on the spaceship as he turned around at the distant planet. Movement is relative, but acceleration is not: When I pass an object in my spaceship, I can't say if it's me that is 'really' moving while the object is stationary, or the other way around; that's all relative. However, when I accelerate towards an object, yes, it will still look as if the object accelerates towards me from my point of view, however, only one of us, namely me, will experience an inertial force, pushing me back into my seat. So acceleration is not just a matter of definition.

Thus, in the case of the twin paradox, the short solution is to say that the frames are not symmetrical or equivalent, and because one experiences acceleration, special relativity does not necessarily tell you what is actually happening from his point of view.

It gets more complicated if you actually do want to describe what exactly happens. I didn't remember too much either, so I just had a quick look at wikipedia, and while wikepedia is not always reliable with science explanations, the article on the twin paradox seems reasonable, and I recognize one of the explanations from uni.

So here are two options of explaining what happens to the twin on the spaceship:

The first one is to think about what actually happened during the de- and acceleration at the distant planet. According to general relativity, experiencing acceleration and experiencing a gravitational pull are equivalent (or rather, the forces themselves might be equivalent). And we know that in a strong gravitational field, time will go slower (think of the astronaut approaching the event horizon of a black hole).

That means, as in terms of travelling at constant speed, the situation for both twins is equivalent, and both would observe time dilation in the respective other frame of reference. However, at the mid point of the journey, the spacefaring twin will experience an additional period of acceleration, which means that from his point of view the clocks in the outside world will go faster. And apparently the math gives you that this period of faster clocks on earth is more than enough to make up for the slower clocks on earth during the legs to and from the distant planet, yielding an overall net effect of more time having passed on earth once the twin arrives back home.

The other explanation just uses special relativity, and I remember that one from uni. I think I didn't find it very convincing back then, but having read about it again now on wikipedia, it kind of makes sense. I'm just going to comment on what is written there.

Basically, the argument goes that the space twin arrives at the planet in one inertial frame of references, then de- and accelerates, and then travels back in another frame of reference. So he switches inertial frames (so far no argument to be had here). Now, the argument is that whatever happens during that acceleration (again, which SR doesn't describe per se), you ignore that for now, but you only compare the initial condition, i.e. the first frame of reference, and the outcome of the acceleration, i.e. the second frame of reference (you could assume an instant switch of velocities). You apparently then find than the simultaneity planes have shifted..

uh haha that sounds like bollocks, but I think it actually makes sense (and my theoretical physics prof thought so as well). So: The point is that for any frame of reference, you can draw a Minkowski diagram and see which events at what locations are happening simultaneously in your frame of reference. For example, as the space twin is just about the reach the point of instant re-acceleration, from his point of view his twin on earth is about to lift a cup of coffee at his 25th birthday. Then the space twin changes velocities arbitrarily fast. If we draw again a Minkowski diagram to find out what events are simultaneous in the new frame of reference to the "now" of the space twin (which, for him, is almost the same now as before), we will find that the corresponding events on earth happen much much later on their timescale (e.g., years). So the earth twin might just celebrate his 30th birthday now.

Thus, with these considerations, we could conclude that whatever happened during acceleration (which we cannot describe with SR), apparently the outcome was a shift in time on earth. And if we were to accelerate more smoothly instead of almost instantly, one would expect that during this time the clocks on earth would go faster from our point of view. Which is the same conclusion we draw in the other solution, but now without appealing to gravity and general relativity.

So. For me the second explanation kind of makes sense, but my feeling would be that a full explanation always would involve general relativity.

Hope that made some sense.

[alway]

The best way to understand the Twins Paradox is imprecise math (for simplicity's sake). First, understand that due to relativity, there is no maximum speed when one compares time onboard a spaceship and distance outside. There is no point at which acceleration from that frame of reference (time onboard vs distance) will decrease.
Let's say you have accelerated to the point where from the perspective of the crew onboard, this time onboard/outside distance is 300c. What does this mean? That you are traveling 300 lightyears for every year which passes onboard.
Since the ship at this point would be traveling very close to c, we can approximate the time passing outside to be aproximately 300 years outside per year inside, since the ship travels 300 lightyears at a speed of aproximately c every 1 year onboard.

[Sean Mirrsen]

So, in essence, accelerating in space accelerates you in time? If yes, then the plot of Gene Rodenberry's Andromeda makes sense. If your ship is sucked into a black hole (and doesn't fall apart), orbiting it at a significant fraction of c, you will effectively enter stasis. I guess I can't hope to explain time compression with just the ambient energy theory (which isn't too different from aether, but still different). I'll go study them particles. We still have to see if it actually works that way though. As mythbusters would say, we have the small-scale results, time to move up to full-scale! (I'd love to see 40th century equivalent of Mythbusters, btw. "Today on Mythbusters, we test the myth that a hypernova can destroy a black hole!")

Btw. Black holes. If it wasn't for all the random atomic crap orbiting them, they'd be perfect for catapulting off into space with the gravity slingshot. Or not. I suspect the gravity gradient would become too steep near the thing's surface, and the ship would be quite effectively torn apart when its bow starts to weigh twenty million times more than its aft.

Also, something strikes me as odd. What exactly prevents you from continuously accelerating if the lightspeed is a limit only for an outside observer? Sure, for the observer your acceleration quickly approaches zero as you approach lightspeed, but if you keep accelerating, to you it should seem that you keep going faster and faster.

[Zironic]

Antimatter does not have negative mass. It as opposite electric charges. Antimatter electrons have positive charges, antimatter protons have negative charges. The same holds true at the quark level.

I have no idea where people got the idea that it has negative mass.

You're thinking of Exotic Matter, which actually does have negative mass, but has yet to be discovered anywhere.

I was waiting for someone to say this. Yeah, this thread seems to be 90% made up using the fantasy of many theorist who themselves are becoming discredited over time mainly because their assumptions are never completely explanatory. Someone was able to disprove part of Hawking's theory on Hawking Radiation (that it emits 100% of the particles absorbed, which would explain black hole dissapation and conservation of mass) and showed some mass/energy is permanently lost.. Hawking protested this by saying in other timeline for other universes, it exists... Basically a made up theory to cover a new hole that can't be truly explained. Science about particles becomes dangerously siding on erreonuous qualities when we live off assumptions. That's why if you do not work at a particle accelerator, I don't find you 100% credible.


Edit: Just because we assume the speed of light is the limit, it doesn't mean it is. Think: We call it the limit because light is the fastest thing we have seen. It is the only thing we can see. So, technically there could be ways of faster travel, an unlimited amount, but we cannot see it or detect it unless it abides by the effects of light, otherwise, it'd move so fast, it would literally be invisible.

[andrea]

sean, I think the point is that the person inside the spaceship would also see distance compressed, not only time. I don't know much about this ( haven't studied it yet and probably won't do it at high school, sadly) but shouldn't this prevent you from observing an higher than light speed?
of course, using time measured in the ship and distance measured outside, it would seem you went faster than light, which is quite convenient. In fact, for all you, pilot, care you traveled faster than light, without actually breaking the annoying limit of c. Everyone else would, unluckily, disagree and age accordingly.

[Sean Mirrsen]

Yeah, this is kinda odd. I'd imagine that for a pilot travelling at C, length contraction would kick in to compensate for the limit. So you'd see sprite planets. (correction: you would if you could) Fun.

I think an Alcubierre drive (or stutter warp) is the best way to travel. Hack the universe to allow you to instantly jump a short distance, then repeat as fast as possible. That way you get the ludicrous speed, but still get to enjoy the scenery.

[alway]

Here is the effect which would make it still appear to be going sub-lightspeed to the ship: http://en.wikipedia.org/wiki/Length_contraction
This is why I specified distance from outside's perspective.

[dreiche2]

So, in essence, accelerating in space accelerates you in time?

Well, it makes everyone else's time speed up, so to say, but yes.

Also, something strikes me as odd. What exactly prevents you from continuously accelerating if the lightspeed is a limit only for an outside observer? Sure, for the observer your acceleration quickly approaches zero as you approach lightspeed, but if you keep accelerating, to you it should seem that you keep going faster and faster.

Hm, I have to concede that I haven't fully thought that specific case to the end. I mean, by virtue of how things work according to the theory, if you stop accelerating at any moment in time and just travel with constant speed, then you will still see your surroundings travelling at almost c. But this is with regards to your current measures of length and time. And your measures of length will find distances having contracted from when it was the last time you took a break from accelerating.

During the acceleration however, you will find lengths contracting more and more, clocks going slower and slower, but also some weird effects of acceleration itself... yeah I don't know what exactly happens. One probably would have to look at the maths.

Edit: Just because we assume the speed of light is the limit, it doesn't mean it is. Think: We call it the limit because light is the fastest thing we have seen. It is the only thing we can see. So, technically there could be ways of faster travel, an unlimited amount, but we cannot see it or detect it unless it abides by the effects of light, otherwise, it'd move so fast, it would literally be invisible.

Well, it's not just a matter of not having seen something. According to the theory, which is currently our best description of the world, travelling faster than light is impossible. Or at least without some major complications with causality. Something that is travelling faster than light could not only not be seen, but wouldn't be allowed to have any measurable effect on the world either.

That doesn't mean it is impossible. The theory could simply be wrong. But it's not just a matter of finding something additional. It would involve a major overhaul of the current theory.

Well, having said that, there are weird non-local effects in quantum mechanics such as entanglement, but even they have the peculiar property that they cannot be used to actually transmit some signal or causal effect in the macroscopic world, so again, they do not invalidate that nothing can travel faster than light (more precisely: no signal, nothing that could have a causal effect).

The thing with entanglement is properly more a sign that we don't really understand (microscopic) reality in the first place... but hey, better not bring up another discussion... I'm exhausted 

[Sean Mirrsen]

One more interesting question I-haven't-thought-about-earlier-but-that-is-probably-covered-already: If you fire a laser, then accelerate to lightspeed, what do you see of the laser? Presuming you can somehow see the laser, of course.

[alway]

Since it is impossible to accelerate to light speed, that question is irrelevant. It is like asking what tan(90 degrees) is.
However, accelerating to very close to the speed of light, time dilation would mean the light still moves away from you at what appears to be the speed of light.

[Sean Mirrsen]

But that would mean, essentially, that l light accelerates along with you, no? Or it would mean that light has an effectively infinite (not necessarily actually infinite) speed, and we only see it going as fast as it does due to relativity?

[andrea]

actually, you can't see a laser unless it hits your eyes, so you should be able to travel faster than light and position yourself in front of the laser to see it. which invalidates the whole thing...

of course, that is just a practical limitation. theory can go on.

and no, light doesn't accelerate... it is something about time and space. Remember that we can only say how fast we see light traveling, and that is based on concepts of time and space. At the end, time dilation and space contraction would probably make it look like it is going at light speed for any observer. which means that light speed is, for all practical purposes, c. Because that is what we will always see.

I really shouldn't speak much here... not until I reread that little book about relativity, quantum physics and string theory
( hateful quantum physics... the cat is either dead or alive, and a day I'll prove it! what is worst is that since at the moment that is the best way to describe the world, I have to accept it and I will have to use it, until a best one is discovered.)

disclaimer: I know that the cat example is not good, sice it is macroscopic

[Sean Mirrsen]

And so, lightspeed is always c,
Because that's what we'll always see.

I would never expect physics to do a double-roundhouse and pack a rhyming pun.

[Neruz]

If i remember correctly, light speed in a vaccuum is a constant in relation to observers. It doesn't matter how fast you are going; light will always travel at light speed relative to you. The reason it can get away with this is cause of that time and space dilation that goes on.

I could be wrong though, the math starts making horrible tortured sounds when you get to this bit.



andrea; not only is the cat a bad example, it was originally thought up by Einstein and Schrodinger to show why Quantum Physics could not possibly be right and was clearly rediculous.

[andrea]

yes, I know about that, it was already told in this forum. It is just popular enough that people will understand what you are speaking about no matter how wrong it is.

If the cat was an electron, however, that is when problems start.