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Relativity watch

1. Post here if you can say you genuinely understand it... I know I don't, but I'm getting there. How did you learn it? I'm finging that reading books is a pretty hard way, probably teaching or even just a conversation with someone who understands it will help.

I know what happens, but I'm not exactly sure why it happens! Like why, if two seperated events are simultaneous in one frame, in all other frames they are not simultaneous. And if two events happen at the same place and at different times in one frame, they occur at different places in all other frames...

It's basically really confusing. Sometimes I have flashes of understanding but can't keep them for long enough, then I just forget what made it all add up, or think of something that throws everything upin the air again.
2. The human brain wasn’t designed to comprehend relativity. Our brains are wired to understand Newtonian mechanics because this is what we experience everyday. I know precisely how you feel! I've managed the borrow this book off my friend. I've covered the first few chapters. I'm sure someone as capable as you would have no problems, however you have to really be prepared to grit your teeth and learn some new mathematical techniques if you want to understand relativity at its fundemental level.
3. Relativity isnt that hard to grasp really, are you sure you dont mean Quantum Mechanics?
4. (Original post by imasillynarb)
Relativity isnt that hard to grasp really, are you sure you dont mean Quantum Mechanics?
Yeah, general relativity, it's just a lark really innit.
5. you wont learn relativity from a popular science book, you really do have to be taught it (from a "proper" book or from lectures).
6. (Original post by elpaw)
you wont learn relativity from a popular science book, you really do have to be taught it (from a "proper" book or from lectures).
My teacher talked about how knowing E = mc^2 led to the concept of anti-matter.

Quite interesting.
7. (Original post by Momentum)
My teacher talked about how knowing E = mc^2 led to the concept of anti-matter.

Quite interesting.
i seriously doubt that. it was knowledge of symmetry and the conservation laws that led to the concept of antimatter.
8. (Original post by elpaw)
i seriously doubt that. it was knowledge of symmetry and the conservation laws that led to the concept of antimatter.
It was something to do with a squared term in the full equation, and it could have a -ve value as opposed to the +ve value. Apparently it went from there, with the concept of "opposite" matter - Anti-matter.

My teacher said so.
9. I used to understand it a few years ago but I havn't been any where near it since so I probaly don't any more - I'll have to have a re-read of my book.
10. (Original post by imasillynarb)
Relativity isnt that hard to grasp really, are you sure you dont mean Quantum Mechanics?
No, I find relativity much harder (and more interesting) than quantum mechanics. Light wave, particle, hedgehog, blah blah blah. Refraction, interference, photoelectric nonsense. It doesn't have a reason which annoys me. Relativity does have a reason, QM just describes things (I think).

I am reading the (very good book) Understanding Relativity for the second time. It's not 'popular science' like Briane Greene's books, I read them and basically it wasnt good enough. In this book it's a lot better in that it doesn't miss things out and it derives equations properly.

But there are some sentences that don't sink in no matter how many times I read them. I guess I'll have to wait until 1st year degree physics.
11. How deep are you looking to go into it? I understand the basics, which is pretty much the last 2 chapters of the CSA Cosmology book and various fragments of other books. You probably understand all this anyway, but this is how I've tried to realise it.

- All things move relative to a frame of reference. When I'm driving on the motorway, to me I am stationary and the road is moving at 70mph towards me (while to someone standing on the side of the road, they are stationary and I'm moving towards them). We could also consider the view of a motorist travelling in the opposite direction, a slower/faster car or a passing bird. No one frame holds the true point of view, so there are an unlimited number of frames of reference. On the scale of the universe, it makes the concept of absolute space meaningless.
- The speed of light is unchanging from whichever frame of reference it is observed, as proven by the Michalson-Morsley (sp?) experiment, which showed no change in light speed shone in a direction along the Earth's path of motion and shone perpendicular to it. So if I shone a torch while driving, and took a measurement of c (ignore my disregard for road safety), both mine and the 'stationary' observer will measure c as the same value.
- If the observer attempted to measure my relative speed from the difference in our measurements of c, it would seem that I am relatively stationary. Obviously, I am not- my relative speed is 70mph. The implication of this is that, as speed = distance/time, and our measurements of each other's beam needs to be slightly different, and so our measurements of each other's distance and time has to be adjusted (needless to say, very slightly in this case).
- I'm sure you're aware of the thought experiments to explain time dilation and length contraction. When in doubt, try to refer back to these.
- Also, any object's mass increases to an observer in a frame of reference moving relative to that object being measured (I'm not sure why, if anyone knows I'd be interested) and any extra energy put in to accelerate the object will cause an apparent increase in mass, which tends to infinity as v tends to c. Therefore the speed of light cannot be exceeded (sorry, haven't explained this too well).
- The reason why General relativity works can also be explained in terms of thought experiments. This time though you need to refer to the principle of equivalence: an object inside a stationary spaceship on the Earth's surface will act in the exact same way as if the spaceship was moving though space with an acceleration of g. Hence acceleration is equivalent to gravity. Again, you probably don't need me to write down the thought experiments. It all implies that the path of light is distorted by mass- a horizontal light beam will seem to accelerate towards the mass (don't think of this as gravity pulling it in, but rather due to a distortion in the structure of space-time: think of the universe as a flat 2-D sheet of rubber, which is distorted by heavy masses).

Ok that's all for now as I'm pretty tired. Hopefully (apart from explaining mass increase) I've helped in some way to explain why it works, if not I'll be on again soon.
12. Ok that mostly made sense, except for this part:

"If the observer attempted to measure my relative speed from the difference in our measurements of c"

You wouldn't make different measurements of c, as it is a constant.

For general relativity I think of this:

Imagine a rotating disk, say 10 m radius. The acceleration of any point on this disk is towards the centre. The motion of the disk in the direction of this acceleration is always zero, and the motion perpendicular is the speed of the disk.

This means, from length contraction, if I jumped on this disk and recorded it's radius my recordings would agree with recordings made when the disk was stationary. But if I measured the circumference, becuase each part of the disk on the circumference is moving instantaneously in the direction of the tangent to the disk at that point, the length I measure of the circumference will be contracted.

Therefore 2pi.r will be larger than the circumference!

This can be related to making measurements considering the Earth is flat. If we measured the distance between 6 cities on the Earth, and try to plot them on a flat map they will not add up, because the Earth's curvature means some distances are larger than they should be if the Earth were flat.

The same applies here. The measurements, due to the circular motion, or acceleration, appear to show that there is another dimension. There is curvature, caused by the acceleration. Going back to the flat/curved example, if we try drawing a circle so that it's circumference is smaller than it should be, this is possible if we draw it on a sphere. (see diagram).

The only problem now is, last time I analysed this I recall coming to the conclusion that you would need to draw it on a "saddle" shape, a surface with the opposite (negative) curvature of the sphere. This makes me thing the circumference should have been larger, which doesn't make sense at all because it should definately be smaller due to length contraction. SHould it?
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13. Ok, that was pretty heavy reading, but here's what I've made of it.

Firstly, the fact that we would record the same measurements of c, despite travelling at different speeds is the whole idea behind relativity. If instead of a torch, both myself and the observer on the road fired a gun (again, ignore my disregard for road safety) and either tried to calculate the other's relative speed by calculating the difference in the speeds of the bullets there would be no problem in this method. For example, the observer may measure his bullet to be 500mph, and my bullet to be 570mph- conclusion: my relative speed is 70mph. If we used torches instead, he would measure both his and my beams to be c- conclusion: my relative speed is 0mph. This is obviously wrong, which implies that our measurements of our distance and time must differ from our measurements of their distance (or length) and time. Only light (and of course other EM radiation) has this property of having a contant speed (strictly speaking, in free space) from wherever it is being measured. That's the reason for the changes in observed time and length- for greater detail consult the thought experiments: the easiest are probably the two parallel trains covered in mirrors shooting perpendicular beams of light at one another for time; and the train entering a tunnel of the same length (when relatively stationary to it) and there are two lights- first at either end of the tunnel, then at either end of the train, each designed to flash when the train crosses the edge of its respective end of the tunnel. If you don't know which ones I am talking about I can explain them in greater detail.

On to the rotating disk problem, it took me a good 10-15mins to try to get my head around what was written down. For a start, I hate trying to think in circles- believe it or not there are easier ways to understand relativity! The main mistake you've made is this:

"But if I measured the circumference... the length I measure of the circumference will be contracted."

Your measurement of the circumference will be the same to what you measured it as when you were stationary. However, if you introduced an observer relatively stationary to the centre of the disc (placed anywhere), his measurements of the circumference will be less, and his measurements of your time will be slower (including your heart-rate - lol). Perpendicular from the face of disc, from the centre it will seems to the observer that the circle will uniformly shrink, looking at the edge of the disc it will seem to be distorted in weird and wonderful ways- it's probably best not to even go down that road. This is actually special relativity. The only way this can provide an explanation of general relativity is if the observer was at a constant distance between you and the centre (so you each have different radii), rotating with you. Because you'd be under different accelerations, person 1's measurements of 2's time/length would differ from 2's measurements of their own. However, because the one further from the centre would experience greater acceleration, it is an unsatisfactory analogy, because acceleration (or gravity, same thing) increases the nearer you get to a surface of a planet.

If the last paragraph was too confusing, don't worry- it took me a while to write down exactly what relativity tells us would happen (if I've understood it correctly, that is). My advice would be to drop the whole disk thing and instead look at simpler explanations of relativity. The highlighted sentence is probably the main thing I would look at: it's a common mistake people make when understanding relativity. There are two popular thought experiments for general relativity, each involving spaceships accelerating through space (with motion nice and linear, that's how I like it!). The best advice I could give would be is to read the last 2 chapters of the CSA Cosmology book, if you can get your hands on one, as it gives a simple-ish explanation of why relativity occurs, from the basics. If you look at something a bit heavier, it will probably assume the reader has a basic knowledge of the topic and set it at such a level that leaves the reader knowing what happens but not understanding why (try reading the first chapter of 'The Universe in a Nutshell' ). It will take a dedicated effort, and a load of re-reading, but the reward of picking so many holes in science-fiction films means it'll be well worth it.
14. In the words of my physics teacher: there's only been 4 people to fully understand relativity... and Einstein was three of them.
15. (Original post by el GaZZa)
In the words of my physics teacher: there's only been 4 people to fully understand relativity... and Einstein was three of them.
so who was the fourth?
16. I don't know, but as you could imagine it gave us all great confidence for the exam...
17. (Original post by el GaZZa)
I don't know, but as you could imagine it gave us all great confidence for the exam...
There is quite a difference between understanding relativity at A level and understanding the deep mathematics of general relativity
18. Well obviously I don't think any of us thought that this was as far relativity went, though if it's notorious at this level, imagine what it's like for physics undergarduates!

Mik1a: Did you mange to get a copy of the CSA Cosmology book? If not, there's a thought experiment on page 21 of A Brief History of Time that explains your question in a way that's probably easier to understand.

In the diagram a beam of light is shone from X to Y. Using Newtonian mechanics, absolute space does not exist (my first point in my first post disproves absolute space without the need for relativity) but absolute time does. A measures the distance between X and Y to be one value, B measures this to be slightly smaller, C smaller still. Because of absolute time, the speed (distance/time) of the beam is different for each of them. However since the speed of light is constant, and the distance measured differs, this implies that the time measured for each observer is different (hence, there is no such thing as absolute time). This is why time dilation / length contraction occurs.
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19. (Original post by el GaZZa)
Well obviously I don't think any of us thought that this was as far relativity went, though if it's notorious at this level, imagine what it's like for physics undergarduates!
Hard, but interesting (for me anyway). The maths in special relativity isn't that hard though, there are a few equations to learn (Lorentz transforms, length contraction, time dilation, etc). Haven't done any general relativity, so I can't say what that's like.
20. Lorentz transforms??

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