Essay about General Relativity

[iloveknowledge]

Hey folks, i am an S1 student in Scotland ( it should be 7th grade for the usa ). 
I've been asked to write a 6 pages essay about General Relativity and it's implications. 

First, i looked some documentaries on Youtube to understand better how space is connected to time and how both are not absolute as newton described. I think i understood what Albert is trying to say pretty good now. 

What i don't get is how is it possible that gravity bends space-time, is it the matter that bends it ? or the relation of the mass and the space-time, like in the classic view of gravity ? or does spacetime alone "do the job"?. 

One last thing i don't really understand in General Relativity is that gravity "seems not to exist", like it is not something defined as a force or a photon. So does gravity exist in Einstein ideas? 

(Also, but this is for my curiosity: why is it called "the fabric" of space-time ?)

 

Excuse me for the many questions, but i find this theory really really hard to understand even if physics is my favourite subject right now. 

See you,

Ryan.

[StringJunky]

3 hours ago, iloveknowledge said:

Hey folks, i am an S1 student in Scotland ( it should be 7th grade for the usa ). 
I've been asked to write a 6 pages essay about General Relativity and it's implications. 

First, i looked some documentaries on Youtube to understand better how space is connected to time and how both are not absolute as newton described. I think i understood what Albert is trying to say pretty good now. 

What i don't get is how is it possible that gravity bends space-time, is it the matter that bends it ? or the relation of the mass and the space-time, like in the classic view of gravity ? or does spacetime alone "do the job"?. 

One last thing i don't really understand in General Relativity is that gravity "seems not to exist", like it is not something defined as a force or a photon. So does gravity exist in Einstein ideas? 

(Also, but this is for my curiosity: why is it called "the fabric" of space-time ?)

 

Excuse me for the many questions, but i find this theory really really hard to understand even if physics is my favourite subject right now. 

See you,

Ryan.

Gravity doesn't bend spacetime; gravity is bent spacetime.  Matter tells spacetime how much too bend. Conversely, spacetime tells matter how to move i.e. its trajectory. 'Fabric' is just a metaphor and not to be taken literally. I think the term originates from the  'heavy ball on a trampoline' idea of bent spacetime, which the fabric represents. It only shows two dimensions; when there's actually 4. It's a very crude idea, not really representing the facts but just gives a glimpse of the concept.  'Spacetime' is actually a 4 dimensional map of the gravitational distribution in a space...it's a mathematical construct. It allows people to plot mass- energy values in time and space on co-ordinates in the spacetime map and see how they change under different scenarios... amongst other things. As  you should see, spacetime is not a 'thing' but a way of describing how things  behave and interact in space. That behaviour is what we call 'gravity'.

Edited March 10, 2019 by StringJunky

[studiot]

I know this is for an essay,

But I think it is legitimate to ask your questions in the main relativity section.

Homework help requires you to show your working here, which is not appropriate at your level.

So ask (send them a private message) a moderator to move this for you.

There are some really good  folks here with great knowledge of relativity.

I don't know how what your timescale is but this book could have been written for you.

 

My local public library has a copy.

 

Note that

Special Relativity is founded on two principles

General Relaticvity introduces a third principle and also the Einstein Field Equations.

Note the plural - there are 10 of them, usually rolled into one big one that you will not yet have the maths to work.

 

Go well with the writing, and tell us the timescale.

[Strange]

7 minutes ago, studiot said:

I know this is for an essay,

But I think it is legitimate to ask your questions in the main relativity section.

!

Moderator Note

I agree. We are not being asked to write the essay!

 

3 hours ago, iloveknowledge said:

What i don't get is how is it possible that gravity bends space-time, is it the matter that bends it ? or the relation of the mass and the space-time, like in the classic view of gravity ? or does spacetime alone "do the job"?. 

It is energy that bends space-time. And (as we know from E=mc²) mass and energy are equivalent.

4 hours ago, iloveknowledge said:

One last thing i don't really understand in General Relativity is that gravity "seems not to exist", like it is not something defined as a force or a photon. So does gravity exist in Einstein ideas? 

Gravity is just how we perceive this curvature of spacetime. It causes things to move together, even in the absence of a force.

One analogy that might help, is to consider two people separated by a few miles and both travelling due north. As they get closer to the North Pole, they will get closer together (following the lines longitude). But nothing is pushing them together, it is just the curve geometry of the Earth's surface.

 

4 hours ago, iloveknowledge said:

Also, but this is for my curiosity: why is it called "the fabric" of space-time ?

It is just an analogy to visualise something that is stretched and curved (and even twisted) by the presence of mass.

23 minutes ago, studiot said:

General Relaticvity introduces a third principle and also the Einstein Field Equations.

Note the plural - there are 10 of them, usually rolled into one big one that you will not yet have the maths to work.

Even without understanding the math, you might get something out of this description of how the equations work: http://math.ucr.edu/home/baez/einstein/

(If you find it confusing, don't worry. It is!)

[iloveknowledge]

Thank you so much to everyone, i am understanding much more now! I have some update becouse, yesterday many other students in my class complained about the difficulty of the topic and so the teacher changed it to "Newton's law of gravity"for those who wanted. I was the only one to stay with dad Albert; so i want to do this very good and i would like to have a complete view of the theory as far as i can understand. 

13 hours ago, Strange said:

It is energy that bends space-time. And (as we know from E=mc²) mass and energy are equivalent.

How does e/m do that ? A couple of years ago when i was 10 i started studing physics in my class and since then it seemed to me that every effect, even the thiniest one, had a cause: like the gluon causing the strong force that helds the quarks togheter ( at least that's what i know ). So i still can't figure out how in details is space-time bent. 

 

14 hours ago, StringJunky said:

As  you should see, spacetime is not a 'thing' but a way of describing how things  behave and interact in space.

Oh ok, this is pretty mind blowing to me. I always thought space ( in particular becouse time seems a more abstract and not fundamental concept to me ) as something existing, as something we can't see but "it's there" like the higg's field or the electromagnetic field. I guess i would need a much higher level of mathematics to understand the equations ( i am currently learning polynomial equations) from which things should get more clear i guess.

Describing in the essay the implications of general relativity i thought of writing about : 1) Gravitational Lensing 2) Black Holes and time dilatation effects 3) Problems in space research travel missions ( like in interstellar ). Is there anything else which is quite cool and simple to describe that i am missing? 

 

Also thank you, everyone, for your support in making me understand this concepts which are much harder to approach without the help of my dad (who was a physicist), which sadly passed away last year. I joined this forum yesterday and i looked up some discussion about topics of my interest and i immediately realized this a forum of giants, people that knows a lot and which i want to become in the future. I don't know if i can understand even 1/3 of what discussed in most of topics but i'll try to. 

 

See you,

Ryan

13 hours ago, studiot said:

Go well with the writing, and tell us the timescale.

I need to deliver the essay this thursday and the best of them gets published in the monthly journal of our school in the science section.

Edited March 11, 2019 by iloveknowledge

[Strange]

38 minutes ago, iloveknowledge said:

How does e/m do that ?

That is a tough one. How do electric charges attract one another?

It seems to be just in the nature of spacetime to be affected by the presence of energy. (One person, whose opinion I respect, said that the reason is because that is one definition of energy: the thing that curves spacetime!)

There might one day be a deeper theory that provides an explanation, but for the moment, all we can say is: that is the way nature behaves...

[iloveknowledge]

9 minutes ago, Strange said:

That is a tough one. How do electric charges attract one another?

It seems to be just in the nature of spacetime to be affected by the presence of energy. (One person, whose opinion I respect, said that the reason is because that is one definition of energy: the thing that curves spacetime!)

There might one day be a deeper theory that provides an explanation, but for the moment, all we can say is: that is the way nature behaves...

Oh i see, thanks. I think however, one day we'll figure it out . Personally i don't think events happen without a precise cause which determines them.

9 minutes ago, Strange said:

How do electric charges attract one another?

Isn't this becouse of photons which gets emitted or absorbed by the involved particles ?

Edited March 11, 2019 by iloveknowledge

[Strange]

40 minutes ago, iloveknowledge said:

Oh ok, this is pretty mind blowing to me. I always thought space ( in particular becouse time seems a more abstract and not fundamental concept to me ) as something existing, as something we can't see but "it's there" like the higg's field or the electromagnetic field. I guess i would need a much higher level of mathematics to understand the equations ( i am currently learning polynomial equations) from which things should get more clear i guess.

I think it is better to think of relativity describing the (relative) relations between times and distances.

It turns out that this can be described in terms of geometry. For special relativity, where there is just relative motion, this geometry is "Euclidean" (it is the same geometry we use everyday).

For General Relativity, this geometry is more complicated, and is the geometry we have to use on curved surfaces (where the angles of a triangle do not always add up to 180° for example). And it is the effect of those changed times and distances which causes the effect that we call gravity.

Note that we normally think of gravity as a force. This is similar to the thing you have probably been told: "there is no such thing as centrifugal force". What that really means is that when we look at something spinning from our perspective, the force is inwards not outwards. But if you are in a centrifuge, or fairground ride, being spun around you will feel the force outwards. So the repsence of the force depends on who is measuring it. Gravity is like that: we "think" we feel a force because the curvature of spacetime is making us accelerate towards the ground.

(There are some subtle questions here (that you probably don't want to get into in your essay!) such as: is spacetime really curved, os is this just a mathematical description of distances and times that happens to work!?)

 

1 hour ago, iloveknowledge said:

Isn't this becouse of photons which gets emitted or absorbed by the involved particles ?

Yes (roughly).

But ... why do those (virtual) photons get emitted and absorbed! (See what I mean, you can always ask another question!)

[studiot]

3 hours ago, iloveknowledge said:

I need to deliver the essay this thursday and the best of them gets published in the monthly journal of our school in the science section.

Given your background and the timescale involved,  I would recommend concentrating on the Physics of Relativity, rather than the Maths.

The Maths for Special Relativity is accessible to those with high school Maths, Gerneral Relativity is not.

 

So in your essay go into the (Physics) principles of importance in Relativity.

1) The Principle of Relativity, both the pre Einstein version and the updates Einstein introduced.

2) The Principle of invariance of the speed of light to all observers.

3) The principle of equivalence.

 

The first two form the basis of SR adding the third, ugrades to GR.

 

(1) Expresses the desire in Physics to have isotropy and homogenity in space. (ask if you don't know what that means).
       Another way to put this is the desire to have the laws of Physics look the same to all observers. Well call this ' Form Invariance'.
      this can be made to work for the Laws of Mechanics (Newton's Laws) but not the Laws of electromagnetism (Maxwell's equations)
 

(2) Is introduced to make Maxwell's equations compatible with Principle (1)

(3) The first two are local principles. That is they apply to a small region ( a point) in space. (3) is used to create the Field equations which extends relativity over all space and time.

 

I will post a couple of sketches showing the implications of (3) and the relationship to curvature.

Please avoid the 'trampoline analogy'. It is just plain misleading.

 

Is your maths up to knowing that a sphere can be represented by the equation radius = a constant so the position on the surface can be represented by the coordinates

[math]\left( {r,\theta ,\varphi } \right)[/math]

Where r is the radius, theta and phi are angles?

[iloveknowledge]

50 minutes ago, studiot said:

(3) The first two are local principles

Why are those local principles (expecially the invariance of the speed of light) ? 

 

50 minutes ago, studiot said:

The Principle of invariance of the speed of light to all observers.

This was formulated to escape the apparent paradox which suggested that if one could catch up to a photon, that one would see a stationary electromagnetic wave right ?

 

50 minutes ago, studiot said:

Is your maths up to knowing that a sphere can be represented by the equation radius = a constant so the position on the surface can be represented by the coordinates

(r,θ,φ)

Where r is the radius, theta and phi are angles?

 I know r,θ,φ  but honestly i've never "applied" them in something concrete so i don't know if i would understand any use of them. (I'll add what i know about them )


Also what is LAMBDA g v in EFE ? In my textbook there is nothing written about it. It just shows the equation and describes roughly what everything means except LAMBDA g v.

Thank you and sorry for my ignorance!

Ryan

P.S. I don't need/have to put math in my essay i just wanted to understand the equation myself.

 

Edited March 11, 2019 by iloveknowledge

[studiot]

37 minutes ago, iloveknowledge said:

Why are those local principles (expecially the invariance of the speed of light) ? 

At that time light was regarded as a wave phenomenon.

For all waves, including light, the characteristic speed is independent of the speed of the emitter.
The emitter is considered as a point source with its own 'speed'.
That is once the wave has been lauched it is entirely controlled by the medium of transmission.

An observer, travelling relative to the medium will observe a different velocity.
So supersonic aircraft can 'catch up' as you put it with a sound wave.

Einstein's innovation was to say that for light, the observed speed is independent of the observer's speed or the same for all observers.
That is quite different form every other wave.

This discussion is not about a transmission medium for light so don't get diverted into that.

Now both of these are local in that the emitter and observer are points and they can only measure the speed near to themselves and in relation to themselves.
They cannot measure a 'global' speed that is 'for the galaxy' or wherever.

So they must rely on a principle such as Einstein's.

57 minutes ago, iloveknowledge said:

This was formulated to escape the apparent paradox which suggested that if one could catch up to a photon, that one would see a stationary electromagnetic wave right ?

No.

You can see stationary em waves using lecher lines for instance.

https://www.google.co.uk/search?source=hp&ei=Y22GXOLOF4GclwTz4oTQBQ&q=lecher+line+experiment&oq=lecher+lines&gs_l=psy-ab.1.1.0j0i10l2j0i22i30l2.826.2804..4822...0.0..0.214.1498.5j6j1......0....1..gws-wiz.....0..0i131j0i22i10i30.JYtj1z6og1w

It was, as I said, formulated to recast Maxwell's equations in a form invariant way.

I can write these out if you wish but this is a lot of Maths.

Remember that our whole basis of Physics and Astrophysics/Astronomy is that the Laws of Physics are the same in Alpha Centauri as on Earth.

That is a global statement that forms the basis of Cosmology, which is about the development and evolution of possible/credible universes.

If the laws were not the same then we could not rely on spectroscopy to tell us what the stars are made of and so on.

 

Thank you for your sketch.

That's good I can do something today.

 

 

[studiot]

Ok to make a start on curvature.

It's late so I am sorry that the sketch is rather rough.

 

The point is that a sphere is a surface (mathematicians call the solid round object a ball) and is two dimensional.

That is we need two numbers to specify any point on it.
Conventionally we use latitude and longitude. Both of these are angles. Note that the radius is not needed to specify any point on the 2D surface.
The radius takes us into 3D.

But a different pair of numbers, representing x and y would give us a plane surface
So what is the difference?

Geodesics.

These are the lines which have the shortest distance between any two points A and B on the surface.
In a plane any straight line is a geodesic.

Usually the gridlines are geodesics but this is not true of latitude, although it is true of longitude.
Geodesics on a sphere are 'great circles'  like the equator and lines of longitude.

Geodesics also occur in higher dimensions than 2D surfaces, but they are still lines.

Why so much emphasis on geodesics?

Because they are the path taken by light rays through space and spacetime.

Now look at the sketch, showing triangles on the surface of the earth.
The 0^o and the 90^oW lines of longitude, AN and CN  intersect at the pole, N,  at 90^o.
The also intersect the equator at 90^o, as do all lines of longitude.

So the angles of triangle ACN on the surface of the Earth add up to a total of 90^o +90^o +90^o = 270^o, rather larger than the 180^o for a plane triangle.
This is called spherical excess (over 180^o) and is dramatically large as the excess depends upon the area of the triangle and ACN is a large triangle.
The excess for a smaller triangle, say ABN, can be seen to smaller at 60^o.

Furthermore if we follow AN to D and BN to E we have a yet smaller triangle, but the apex angle at N is still 60^o.
So the angles of intersection at D and E must be less than 90^o to achieve a smaller spherical excess.

 

 

So I have set up a real world example, sailing across the North Sea from Brora in Scotland to Kristiansand in Norway, which conveniently happen to be on the same 58th parallel of latitude.

From the figures it can be seen that sailing along the parallel is about 1km longer than sailing a true great circle geodesic.

 

Now translating this to the first experimental verification of general relativity (by Sir Arthur Eddington) which is conveniently shown here

Light from a star hidden behind the Sun was deflected by the Sun's gravity a minute but measurable amount exactly as calculated by Einstein.

 

There is more discussion of this and possible implications of GR

https://www.scienceforums.net/topic/106314-what-warps-space/?tab=comments#comment-993433

https://www.scienceforums.net/topic/114528-gr-and-the-principle-of-reversibility-of-light/

 

Edited March 12, 2019 by studiot

[iloveknowledge]

Thanks to everyone ! My essay ended up being pulished in the monthly journal of  my school!

[Strange]

1 hour ago, iloveknowledge said:

Thanks to everyone ! My essay ended up being pulished in the monthly journal of  my school!

Congratulations!

[studiot]

Thank you for coming back to tell us what happened.
I'm so glad how well it turned out for you. +1

Please give us more notice next time.

[Phi for All]

2 hours ago, iloveknowledge said:

Thanks to everyone ! My essay ended up being pulished in the monthly journal of  my school!

Outstanding!

[studiot]

49 minutes ago, Williamgibson said:

I couldn't believe such smart people hang out on forums, you made me register here.
A bit of prehistory, I have now got to engineering college and am passing the homework date, just in this thread I found the answer to my question, fascinating.
Besides this I will be an active member!

Whilst I can see this is not the best place to put an introductory post, I really can't see how it merits a negative rep point so I have balanced that out.

William, welcome and start with a clean sheet. Well done for starting engineering college , I look forward to productive post from you, but please think about whare you place them.

[Phi for All]

3 hours ago, studiot said:

Whilst I can see this is not the best place to put an introductory post, I really can't see how it merits a negative rep point so I have balanced that out.

William, welcome and start with a clean sheet. Well done for starting engineering college , I look forward to productive post from you, but please think about whare you place them.

!

Moderator Note

The latest spam tactic is to later add the link to their essay writing services in the quote they use, rather than in their own writing. The delay must fool the system, so it needs to be dealt with manually.

 

[studiot]

20 hours ago, Phi for All said:

!

Moderator Note

The latest spam tactic is to later add the link to their essay writing services in the quote they use, rather than in their own writing. The delay must fool the system, so it needs to be dealt with manually.

 

Thanks for the heads up.

Glad we have some alert mods here.

Pity though, as real new members should always be welcomed.

[Phi for All]

3 hours ago, studiot said:

Pity though, as real new members should always be welcomed.

Indeed, though it seems lately that we have to sift through 5 spammers, 4 religious extremists, 3 crackpots, and 2 science deniers before we get a sincere science enthusiast.

[AlexTinny]

According to Einstein, there is no "gravitational field" in the sense of an electric field, and we will never find a "graviton." However, modern physicists stubbornly continue to search for it.

Edited December 11, 2022 by AlexTinny

[Mordred]

7 minutes ago, AlexTinny said:

According to Einstein, there is no "gravitational field" in the sense of an electric field, and we will never find a "graviton." However, modern physicists stubbornly continue to search for it

I wouldn't say stubbornly we hope to find it but I don't believe we achieved the necessary energy levels yet. The reason isn't because we need the graviton to explain spacetime curvature aka gravity. The graviton if ever found would be a tremendous help to renormalize gravity.

[Markus Hanke]

15 hours ago, AlexTinny said:

However, modern physicists stubbornly continue to search for it.

There are no searches for the graviton taking place, for two fundamental reasons:

1. There is simply no physically reasonable detector that would be able to unambiguously detect individual gravitons, since their interaction cross section is so extremely small

2. The concept of ‘graviton’ comes from applying the tools of quantum field theory to General Relativity, in the same way as we do for the other forces. We already know that the resulting model cannot be renormalised - it contains infinities that cannot be removed, and thus it is impossible to extract physically meaningful predictions from such a theory. Gravity just doesn’t work the same way as the other interactions. It is therefore highly doubtful that the naive concept of ‘gravitons’ has any physical meaning at all.