What textbooks did you read on that? I recommended one in my above post, I never moved beyond it (for some more advanced courses) as it's not my area, nonetheless I had a blast studying it.Flachmatuch said:Yep, that was what I meant when I said "a little bit more complicated". I just don't think I'll ever be able to understand general relativity
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Why nothing can travel faster than light: An easy-to-understand explanation
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Lostconfused
Member
Welp that explanation kind of breaks down towards the end. By pointing the arrow along the horizontal you complitely eliminate the vertical component thus you are never travelling through time so no matter what the speed you can be anywhere in the universe at once.
The explanation simplifies things too much by ignoring edge cases.
Of course considering that I know fuck all about physics or space time or whatever. I only took one course in general relativity and one in astrophysics in my 2nd year and failed both.
The explanation simplifies things too much by ignoring edge cases.
Of course considering that I know fuck all about physics or space time or whatever. I only took one course in general relativity and one in astrophysics in my 2nd year and failed both.
Not that they're always right, but people with decades of study on top of multiple university degrees tend to have better grasp of these kind of things compared to a GAF using science fiction shows as an argument.
I'm quite happy to avoid the headache and simply agree that dog birthday is adorable.
I'm quite happy to avoid the headache and simply agree that dog birthday is adorable.
IIRC (and I am probably wrong about this) the entanglement stuff doesn't count as FTL communication because the moment either particle is measured / collapses into a state, the other one collapses into the same state. Which apparently happens FTL (!) but since the person measuring it has no way of knowing whether the other person collapsed it already, you can't actually transfer information that way. The only way to do so would be to know when the other person was going to collapse it... and in that case you still aren't receiving information FTL, since you had to transmit that information to each other beforehand at FTL or sub-FTL speeds. Basically, it's a cool concept, but has no FTL transmission implications.
Caveat: the above could be totally made up or based on my grossly misunderstanding quantum mechanics / whatever explanation I was given.
Caveat: the above could be totally made up or based on my grossly misunderstanding quantum mechanics / whatever explanation I was given.
Nope. It actually makes perfect sense. There is no time flow for objects traveling with the speed of light. Keep in mind that (the basic postulate of special relativity) if the speed of light is the same in all frames of reference, there can't be a frame of reference for objects moving with the speed of light (ie. photons and gluons). If there's no frame of reference, there's no time flow in that frame of reference.Lostconfused said:
Lostconfused
Member
Ok then I should have phrased it better. The model it self is probably sound it's just the quoted explanation sucks.
I actually think there's still an interesting unanswered question here... if objects traveling at c (e.g. photons) don't have a time component, how do we see them moving around through the time that we experience? I suspect the answer has something to do with our perception of object motion being based on vector differences between our own 4D velocity vector and the objects', but since I don't really know I invite someone who actually does to elaborate. I do think the model is definitely hiding some complexity though.szaromir said:Nope. It actually makes perfect sense. There is no time flow for objects traveling with the speed of light. Keep in mind that (the basic postulate of special relativity) if the speed of light is the same in all frames of reference, there can't be a frame of reference for objects moving with the speed of light (ie. photons and gluons). If there's no frame of reference, there's no time flow in that frame of reference.
Lostconfused
Member
No I think I studied enough physics to be brainwashed into thinking that without the time component there is no motion at all. Thus you can either know the speed of the object which is the rate at which it travels through space time or its position in space time but never both. Or I could just be very confused.Sharp said:
And honestly I don't think about photons because I still haven't wraped my head around wave particle duality.
SciencePilot
Member
This explanation is incomplete because it only talks about what a "non-moving" observer thinks about your motion. From your own perspective, you can't even move through space; everything else just moves relative to you. As a consequence, from your own perspective, you can only move through time.
Right... I suppose the real issue I have is with the vector itself. He describes it as a velocity vector plus time. In that case, it's no longer really a velocity vector, just a position vector in 4 dimensions. What this picture fails to illustrate is (1) how the vector can change, and (2) how on earth can it change? It's either implicitly assuming a current "window" of time that passes along the time axis that represents some consensus "present," in which case a (much) better explanation is required, or there's supposed to be some intuitive understanding of "time" that doesn't fit into the 4D vector model. Maybe what we plug into the equations is really just the derivative of some 4D structure frozen in spacetime, I dunno. Either way, I would love for someone who knows this stuff really well to help clear this up.SciencePilot said:
Oh, duh, I totally didn't see that part.szaromir said:
Here's what never seems to be touched upon in these explanations. If someone could clarify this, then great:
1. You pick a starting point (say, the moon) for a race in your extremely fast spaceships. You go in one direction away from the moon at 3/4ths the speed of light. Your friend travels in the opposite direction at 3/4ths the speed of light.
2. Since motion is relative, if you consider the moon stationary, and the two ships moving, no one is exceeding the speed of light. However, if you consider your spaceship stationary, and your friend's ship moving, then your friend is travelling away from you at 1.5 times the speed of light!
What gives? How does the time dimension factor in to this to make it not possible?
1. You pick a starting point (say, the moon) for a race in your extremely fast spaceships. You go in one direction away from the moon at 3/4ths the speed of light. Your friend travels in the opposite direction at 3/4ths the speed of light.
2. Since motion is relative, if you consider the moon stationary, and the two ships moving, no one is exceeding the speed of light. However, if you consider your spaceship stationary, and your friend's ship moving, then your friend is travelling away from you at 1.5 times the speed of light!
What gives? How does the time dimension factor in to this to make it not possible?
There's no absolute position, velocity and acceleration. You are always standing still and oonly moving forward in time in your frame of reference (and by you, I mean a single paritcle not entire human body), it's about how you're perceived by other objects (or how you perceive them). Of course those changes in perception happen via interactions but that's a broader subject. I think you guys try to overthink it.Sharp said:
I was going to give a long relativistic explanation, but there's actually a much simpler one to be had--since you're moving away from each other at a speed of > c, the light from his ship will never reach you; the rule is that you can't receive information faster than light. You will only see his ship once you slow down enough that you are no longer moving away from each other at a speed > c.Okin said:
So essentially, we could choose a particle (that wasn't going at light speed), arbitrarily decide that its velocity vector was the "true" one and it was experiencing time at the "proper" rate, and then deduce all the other velocity / time vectors from there... except things moving at c which are postulated to always move at c for everyone. That makes sense.szaromir said:
This is true but I don't think it applies in this case... if we shoot two beams of electorns out of guns 180 degrees from each other at highly relativistic speeds I am pretty sure the space between them would grow at a rate greater than c, regardless of the velocity shift.szaromir said:The consequence of Einstein's postulates (particularly the second one) is that speed does not add up linearly, hence solving your paradox in no time.
Lostconfused
Member
By the same argument you made it would be impossible to observe one beam from them perspective of another beam. You would have to observe them from a perspective in which they are both moving at some factor of c.Sharp said:
Edit: I am sorry, I probably shouldn't have made this post. I don't entirely understand the implications of what you are trying to say.
Ignoring the space expansion (which is outside of the scope of special relativity), you cannot have two objects moving FTL relatively to each other. His example is extremely simply to solve.Sharp said:
c=1
v1=v2=0.75=tanh(u)
u=atanh(v1)
V=tanh(u+u)=tanh(2atanh(0.75))=0.96
So they move with the relative speed of 0.96c to each other (ignoring space expansion and stuff)
EDIT: putting v1=v2=1 gives obviously no result.
From the perspective of the observer on the moon, or from each other's perspective? If from the perspective of the observer on the moon, could you point me at the relative articles so I can understand what you're doing with the equation? If it's from the perspective of the particles, it makes a bit more sense to me, but it's still unclear to me that you've demonstrated that they aren't moving away from each other at speeds > c... only that they can't be observed to do so.szaromir said:Ignoring the space expansion (which is outside of the scope of special relativity), you cannot have two objects moving FTL relatively to each other. His example is extremely simply to solve.
c=1
v1=v2=0.75=tanh(u)
u=atanh(v1)
V=tanh(u+u)=tanh(2atanh(0.75))=0.96
So they move with the relative speed of 0.96c to each other (ignoring space expansion and stuff)
EDIT: putting v1=v2=1 gives obviously no result.
Lostconfused
Member
That's the point of relativity. You can't observe any single object moving at speed greater or equal to the speed of light. Other than light. At least as far as I understand it.Sharp said:
Sure, but if in one year ship A ends up in a spot .75 lightyears away from the moon and ship B arrives in a spot .75 lightyears away from the moon in the opposite direction, I would argue that they have moved apart from each other at a speed faster than light, even if no information was transferred at FTL speeds. That's why I would like to know from whose perspective the 0.96c was derived... one of the ships', or the observer on the moon? Would what I'm saying here (they are 1.5 lightyears apart after 1 year) be true regardless of whose perspective the calculation was from?Lostconfused said:
Lostconfused
Member
But ship A wont see ship B arrive at it's destination at same time because the light from ship B has more distance to travel and didn't reach ship A. So if some one on Ship A would assume that ship B left at the same time and calculated the speed of ship B it would not be greater than c.Sharp said:
They both have velocity of 0.75 from the perspective of the observer on the moon. The 0.96 result is for their relative velocity. My calculation here is an equivalent of Lorentz's transformation (which you probably had in high school?) for the two dimensional example (ie. one spacial dimension and time), it's just much simpler to use tanh than powers and square roots.Sharp said:
Probably all your curiosity will be satisfied if you study the first two chapters of A First Introduction to General Relativity by Bernard Schutz, very intuitive and clear mathematically. I studied the proper and lengthy explanation of Lorentz groups in chapter 3 of Relativistic Quantum Mechanics by Walter Greiner.
You look at time as something that's fixed, whereas it's different for both ships and the observer on the moon. I'll paint a picture in paint if you want to.
I see what you're saying as far as "1 moon-year != 1 ship-year" but, again, is this strictly relevant? All I'm asking is from whose perspective the relative velocity of the two ships is being calculated. If it's one of the ships, I pretty much understand (not really, but vaguely). If it's from the moon, I have a hard time understanding it. I find it hard to believe that their relative velocity is the same from all vantage points so this is sort of a critical piece of information.szaromir said:They both have velocity of 0.75 from the perspective of the observer on the moon. The 0.96 result is for their relative velocity. My calculation here is an equivalent of Lorentz's transformation (which you probably had in high school?) for the two dimensional example (ie. one spacial dimension and time), it's just much simpler to use tanh than powers and square roots.
Probably all your curiosity will be satisfied if you study the first two chapters of A First Introduction to General Relativity by Bernard Schutz, very intuitive and clear mathematically. I studied the proper and lengthy explanation of Lorentz groups in chapter 3 of Relativistic Quantum Mechanics by Walter Greiner.
You look at time as something that's fixed, whereas it's different for both ships and the observer on the moon. I'll paint a picture in paint if you want to.
Ace Harding
Member
Lostconfused said:
This reminds me of a situation I keep thinking will get me close to explaining this all, then my brain starts to hurt.
You and a buddy get some really powerful microscopes - powerful enough to see another person on a planet 1 million light years away.
Imagine (somehow) your buddy instantly travels to this other planet, 1 million light years away. He's got his telescope with him, and he takes a peek at Earth. He's looking at Earth as it appeared 1 million years ago, because that's how long it takes light to reach his telescope. And you look for him in your microscope but can't find him, because the light reflecting off of his body won't reach Earth until 1 million years from now.
But you guys are still looking through your microscopes at the same time, both looking at things as they appeared a million years ago. You'd both have to sit there and wait for a million years to see each other. Never more than a million years, because he traveled at infinite speed.
Think about what happens in this same scenario (you guys observing each other with very powerful telescopes) if your buddy does not move instantly but travels in a ship at light speed. Then 99% light speed.
What happens?
This is where my head explodes.
Lostconfused
Member
If its possible to observe his ship while its travelling at that speed you would just see him reach that planet. Of course that would take almost a million years.Fong Ghoul said:
He on the other hand would probably be seeing you about a million years ago observing him a bit after start of the journey?
Not sure about the last part. Actually I am not sure about any of this.
UrbanRats said:week-old bump, but worthwhile topic
So this means that, if i had a space ship that could travel at that speed, it would take 4 years to get to Alpha Centaury, but it would be like mere minutes for me on the ship?
No, it would be like thousands of lifetimes on earth, still years for you.
Edit: I think, someone who actually studied in science class probably can explain it. I only read novels.
Well, if i travel at the speed of light, it takes 4 "earth" years to get to Alpha C, right? But since i'm traveling at C, the time goes slower for me, on the ship.. so while on earth 4 years have passed, only minutes (or whatever) have passed for me.
This was my reasoning, at least.
While instead if i travel for 4 years, counting them from the ship (that travel at c) i'd go far beyond Alpha Centaury and millions of years would pass on Earth.. i guess.
This was my reasoning, at least.
While instead if i travel for 4 years, counting them from the ship (that travel at c) i'd go far beyond Alpha Centaury and millions of years would pass on Earth.. i guess.
Still depressing, to see this picture:mike23 said:
http://upload.wikimedia.org/wikipedia/commons/b/b6/Earth's_Location_in_the_Universe_(JPEG).jpg
And to think that we can't even go to Mars yet.
ThoseDeafMutes
Member
UrbanRats said:week-old bump, but worthwhile topic
So this means that, if i had a space ship that could travel at that speed, it would take 4 years to get to Alpha Centaury, but it would be like mere minutes for me on the ship?
Yes, that is correct. Minutes from your perspective, years for an observer on Earth or Alpha Centauri.
To me, it was very clear instead.Coldsnap said:
I got another question (more than one, actually, but i'll go with this one for now):
When you guys says that gravity travels at the speed of light, what exactly do you mean?
It may sound dumb, but i can't visualize it.. gravity isn't a particle, right? so what "travel"? Is it an invisible force? Sometimes they show it like a wave in a grid-like structure (that represent the space between objects) but that also isn't very clear, because ok, that wave travels at c, but what is that wave made of, in the physical world?
AceBandage
Banned
DarthWoo said:
Wouldn't the center, technically, be stationary?
You mean is there a point in space and time which can be considered the "center" of the universe and that all other points are just expanding away from it?
I have no idea. I assume there might've been in the moment the universe came to being, when it was still a singularity.
I have no idea. I assume there might've been in the moment the universe came to being, when it was still a singularity.
AceBandage
Banned
Astro-Physics was never my strong point.
I still can't even understand how a single point in space could create all matter for the entire universe.
I still can't even understand how a single point in space could create all matter for the entire universe.
ThoseDeafMutes
Member
AceBandage said:
There is no centre of the universe, but this is going to be one of those things where you just have to say "the math makes sense, don't ask" because IMO the analogies explaining this are very poor.
Here is a link that will attempt to explain it, but the tl;dr is that there is no centre or every point is the centre, but none can lay special claim to it.
The Technomancer
card-carrying scientician
Gravity actually is a particle. Well...theoretically. We've never observed a graviton, but it pretty much has to exist because all forces are mediated by particles. Photons, gluons, bosons and gravitons.UrbanRats said:
AceBandage
Banned
Screw it. I'm going with the Star Trek theory. Space, Time and Thought are all the same.
We think it, therefore, it exists.
We think it, therefore, it exists.
ThoseDeafMutes
Member
AceBandage said:
If you're imagining that there was a point in space which was a little matter and energy factory, then it's pretty understandable that you would be confused. To give you a little bit of clarity, consider this; the big bang is not a creation event, it is an expansion event. Energy already existed at t=0, and thus by definition, was actually "eternal" (exists at all points in time) and just expanding and changing form with the big bang.
Ah, now i remembered a video i saw, where it said that [is thought that] gravity was fused with other foundamental forces, in a period called "Planck Epoch":The_Technomancer said:
http://www.youtube.com/watch?v=b2U_ZYqlry8&hd=1&t=6m27s
The whole video is very beautiful though. (took me a while to find it, lol)
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