I've never understood ____

[vox nihili]

You know that one thing that you always hear about, but have no idea what it really is?
What about something that you've always tried to read up on, but never quite got?

This thread will [hopefully] have your answers!

The purpose of this thread is to pose any questions that you've never been able to answer or always want the answer to, so somebody can swoop in, be your Superman and help explain the concept in a judgment free environment. You can ask absolutely anything you like and about any topic you like as well

So I'll ask the first question!

Can somebody give a general description of general relativity?

[ShortBlackChick]

I dont understand why posts like this are in the Victorian Education Discussion board???

[The Brightest Witch]

I've never understood what reversing the polarity of the neutron flow means exactly, and whether it's actually possible or what.

[vox nihili]

I dont understand why posts like this are in the Victorian Education Discussion board???

It's been moved. I put it there though because the posts here are what would fit under "genuinely helpful" and thus should encourage upvoting, hence not putting it in other general discussion in the first place Don't particularly mind though, but that's my rational.

[vox nihili]

I've never understood what reversing the polarity of the neutron flow means exactly, and whether it's actually possible or what.

That sounds frightening! Anybody know?

[BigAl]

General relativity is the advanced theory of gravity. In Newtonian mechanics, big mass attracts the small mass and vice versa. In this theory, mass and also energy can affect gravity. Gravity is not only an attractive force but also bending of space-time. I have no formal education on general relativity yet..but I'll be taking associated units in third year level maths. If you want some further reading, look at differential geometry because Einstein turned a mathematical concept into a physical one.

[vox nihili]

General relativity is the advanced theory of gravity. In Newtonian mechanics, big mass attracts the small mass and vice versa. In this theory, mass and also energy can affect gravity. Gravity is not only an attractive force but also bending of space-time. I have no formal education on general relativity yet..but I'll be taking associated units in third year level maths. If you want some further reading, look at differential geometry because Einstein turned a mathematical concept into a physical one.

That puts a bit of it in perspective! It's such a tricky area. I've been reading The Fabric of the Cosmos but even that's not sticking properly. Thank you

[T-Infinite]

I've never understood the concept on time travelling. I've seen countless numbers of science videos on time travel and they say something like if you travel fast enough you could.. go back in time. BUT HOWWW???! I just don't get it, "go back in time" as in YOU going back to the past and will you physically BE in the past? For example, would I be able to see the stuff that happened years ago again, be there, and literally change the past (i.e do my bio exam again... hahaha).  GAHH! I don't know how to explain it without sounding so confusing.

[BigAl]

I've never understood the concept on time travelling. I've seen countless numbers of science videos on time travel and they say something like if you travel fast enough you could.. go back in time. BUT HOWWW???! I just don't get it, "go back in time" as in YOU going back to the past and will you physically BE in the past? For example, would I be able to see the stuff that happened years ago again, be there, and literally change the past (i.e do my bio exam again... hahaha).  GAHH! I don't know how to explain it without sounding so confusing.

Those videos lead to some speculations. What astronomers mean by time travel is that they are able to observe the universe as it was 13 billion years ago and make prediction whether or not those observations match the current state of the universe. I don't think we'll ever see those machines which launch you back in time. These speculations come with their paradoxes whose solutions are physically possible but then the thing itself is impossible.

[BigAl]

I hope I'm making sense. English is my second language...so sorry for sloppy expressions/misunderstandings. I'm sure someone will correct me

[vox nihili]

I've never understood the concept on time travelling. I've seen countless numbers of science videos on time travel and they say something like if you travel fast enough you could.. go back in time. BUT HOWWW???! I just don't get it, "go back in time" as in YOU going back to the past and will you physically BE in the past? For example, would I be able to see the stuff that happened years ago again, be there, and literally change the past (i.e do my bio exam again... hahaha).  GAHH! I don't know how to explain it without sounding so confusing.

All right, this is a tricky one, and I may not get this 1000000% correct, but hopefully I'll have the right kind of idea. It's all about special relativity.

First and foremost you need to accept that the speed of light never changes. So we'll let the speed of light be "c".

If you're stationary and you measure a beam of light, the speed of that light will (obviously) be c.
Now let's say that you hop in your super fast car and go 1000kph. Whilst you're driving that fast, you measure the speed of light. So if you're approaching from the opposite direction, what would you expect? You'd expect the speed of light to be c-1000  To your surprise though, this is not the case. It's still c. That's because it NEVER changes no matter how you look at it. That's a big point.

What is the importance of that?

Ok, so let's do another thought experiment to explain. You're standing on a bus travelling at 100kph and you bounce a ball. What's its motion? It goes straight down and straight up. So it's a really, really tight parabola. Basically up and down in straight lines. To the person in the bus, this is exactly how the motion appears, and exactly how it should appear. Pretend the bus is see through, and somebody on the side of the road sees the ball bounce. What do they see? The ball doesn't just go down and up, remember, the bus is moving along. So if you tracked its trajectory, it would go down in a really deep arc. The ball appears to travel further to the person on the outside of the bus.
Let's say both did measurements. The guy on the bus says that the ball travelled 2 metres. 1 down, 1 up. The guy outside the bus calculates that the ball travelled 12m. That's 6x difference! So who got it right? The answer is both. It depends on which way you look at it. It's relative.

Now let's expand our thought experiment. Rather than the ball, let's pretend we do the same with a beam of light. Obviously, the light moves a lot faster, so the difference in distances would be minute, but let's just pretend for a moment that the light is really slow and moves as quickly as the ball.
The guy on the bus measures the distance travelled as 2m, whereas the guy outside the bus measures the distance travelled as 12m. Both are right, remember? Because it's relative. Light is a bit different though. The speed remains the same. In both cases, the speed of the light beam is c. Is the speed is the same, but the distance is different, what does that mean? Well given that speed is distance over time, it means that the time taken has to be different for both people.
On the bus, time is slower than outside the bus. That seems weird right? But if you follow our experiment, and accept that light is always travelling at c, it has to be right, doesn't it?


So how does this all relate to time travel?

Buses aren't going to help you travel time. But, on the larger scale, the differences can be huge. If you got on a spaceship and crank it up to 99% of c and start flying around the earth, what do you observe? For the people on the space ship, the distance the light travels is 1m. If people on earth had a super good telescope and could measure the beam of light, it would travel 500 times that distance! (this isn't necessarily the right number, but thereabouts). So what does that mean, if the speed is measured to be the same for both observers (it has to be remember?). It means that time on earth is travelling 500x quicker!. So after a year, when the astronauts make their return, 500 years have passed on earth!

So this explains how to travel into the future, theoretically.

Also theoretically, to travel into the past, one would have to exceed the speed of light, thus going backwards. It's thought to be physically impossible though, so sadly no Back to the Future!

[vox nihili]

I've never understood the concept on time travelling. I've seen countless numbers of science videos on time travel and they say something like if you travel fast enough you could.. go back in time. BUT HOWWW???! I just don't get it, "go back in time" as in YOU going back to the past and will you physically BE in the past? For example, would I be able to see the stuff that happened years ago again, be there, and literally change the past (i.e do my bio exam again... hahaha).  GAHH! I don't know how to explain it without sounding so confusing.

As BigAl has made me realise is that those videos may also be referring to galaxies far, far away (cue Star Wars music). If a galaxy is 1 billion light years away, it means that what we observe is actually what happened 1 billion years ago. Because obviously it takes 1 billion years for the light to travel to us so we can observe it

[Robert243]

I've never understood why these thread hasn't been sticked yet .

[T-Infinite]

All right, this is a tricky one, and I may not get this 1000000% correct, but hopefully I'll have the right kind of idea. It's all about special relativity.

First and foremost you need to accept that the speed of light never changes. So we'll let the speed of light be "c".

If you're stationary and you measure a beam of light, the speed of that light will (obviously) be c.
Now let's say that you hop in your super fast car and go 1000kph. Whilst you're driving that fast, you measure the speed of light. So if you're approaching from the opposite direction, what would you expect? You'd expect the speed of light to be c-1000  To your surprise though, this is not the case. It's still c. That's because it NEVER changes no matter how you look at it. That's a big point.

What is the importance of that?

Ok, so let's do another thought experiment to explain. You're standing on a bus travelling at 100kph and you bounce a ball. What's its motion? It goes straight down and straight up. So it's a really, really tight parabola. Basically up and down in straight lines. To the person in the bus, this is exactly how the motion appears, and exactly how it should appear. Pretend the bus is see through, and somebody on the side of the road sees the ball bounce. What do they see? The ball doesn't just go down and up, remember, the bus is moving along. So if you tracked its trajectory, it would go down in a really deep arc. The ball appears to travel further to the person on the outside of the bus.
Let's say both did measurements. The guy on the bus says that the ball travelled 2 metres. 1 down, 1 up. The guy outside the bus calculates that the ball travelled 12m. That's 6x difference! So who got it right? The answer is both. It depends on which way you look at it. It's relative.

Now let's expand our thought experiment. Rather than the ball, let's pretend we do the same with a beam of light. Obviously, the light moves a lot faster, so the difference in distances would be minute, but let's just pretend for a moment that the light is really slow and moves as quickly as the ball.
The guy on the bus measures the distance travelled as 2m, whereas the guy outside the bus measures the distance travelled as 12m. Both are right, remember? Because it's relative. Light is a bit different though. The speed remains the same. In both cases, the speed of the light beam is c. Is the speed is the same, but the distance is different, what does that mean? Well given that speed is distance over time, it means that the time taken has to be different for both people.
On the bus, time is slower than outside the bus. That seems weird right? But if you follow our experiment, and accept that light is always travelling at c, it has to be right, doesn't it?


So how does this all relate to time travel?

Buses aren't going to help you travel time. But, on the larger scale, the differences can be huge. If you got on a spaceship and crank it up to 99% of c and start flying around the earth, what do you observe? For the people on the space ship, the distance the light travels is 1m. If people on earth had a super good telescope and could measure the beam of light, it would travel 500 times that distance! (this isn't necessarily the right number, but thereabouts). So what does that mean, if the speed is measured to be the same for both observers (it has to be remember?). It means that time on earth is travelling 500x quicker!. So after a year, when the astronauts make their return, 500 years have passed on earth!

So this explains how to travel into the future, theoretically.

Also theoretically, to travel into the past, one would have to exceed the speed of light, thus going backwards. It's thought to be physically impossible though, so sadly no Back to the Future!

Your experimental examples are so much easier for me to comprehend!

[BigAl]

Also you talked about going into the past and change it. For example, you could go back in time and shoot your grand, grand, grandpa so you won't born in the future. That's physically impossible because you're actually existing now (you might break the fundamental conservation laws) So how do we solve this problem? Well, you could travel back in time but can't change anything. We observe the universe as it was 13 billions years ago but can't do anything. That baby universe will/was evolved over time and took its current shape. See the similarity?