# The Naked Scientists Forum

### Author Topic: What's the difference between general & special relativity?  (Read 2423 times)

#### thedoc

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##### What's the difference between general & special relativity?
« on: 11/10/2016 21:12:23 »
100 years ago, Einstein's theory of general relativity changed the world. Before that though, came special relativity.

Read the article then tell us what you think...
« Last Edit: 11/10/2016 21:12:23 by _system »

The following users thanked this post: chintan

#### chintan

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##### Re: What's the difference between general & special relativity?
« Reply #1 on: 03/12/2015 14:23:45 »
Relativity states that "the speed of light remains constant"..and special relativity for me simply states that "slow u go,fast u die!"..."fast u go the, slow u die!"
« Last Edit: 03/12/2015 14:28:06 by chintan »

#### ijaz

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##### Re: What's the difference between general and special relativity?
« Reply #2 on: 04/12/2015 09:32:54 »

#### Sean

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##### None
« Reply #3 on: 04/12/2015 19:00:23 »
As stated via Special Relativity(SR), there is no way in which you can detect whether or not you are at absolute rest in space, you therefore have no detectable absolute reference, and in turn, it also becomes impossible to measure absolute motion. Thus we are left with relativity instead. Physicist say that it is logical for these absolutes to be ignored, since these absolutes can not be detected. This then gives the false impression that the absolutes do not even exist.  In turn, for over 100 years, the simpletons of this world have been told to not be interested in the absolute cause.

How on Earth can SR be absolutely understood, if these absolutes are to be excluded. Obviously there is an absolute cause behind SR. SR does not just occur as the result of some kind of magic, meaning there is an absolute foundation of which SR resides within, the foundation which makes SR occur.

With the absolutes revealed, SR becomes so easy to understand that even the simpletons themselves can understand it. All of the bizarre phenomena of SR vanishes in a flash.

#### Bernd

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« Reply #4 on: 05/12/2015 19:23:46 »
... overthrowing Newtonian mechanics ...
I disagree. As long as the velocity is sufficient low Newton is my man. I would say 99,99 % of all calculations in mechanics are done by the way Newton showed us.

#### wolfekeeper

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##### Re: What
« Reply #5 on: 09/12/2015 00:09:40 »
This is a typical explanation of Relativity theory.

It's never done anything for me, and it's not actually how Relativity theory was developed.

Einstein didn't go "I'm guessing that the speed of light is constant-> Relativity!" no, he got the basic idea from Lorentz and worked it backwards. It turns out much easier to go backwards than forwards, like Lorentz did, but you still lose something, you lose the 'why'.

So, it's not well known, but you can actually *derive* Special Relativity from Newtonian Mechanics and Maxwell's equations. But you have to do it really, really, really carefully. And that's what Lorentz did.

If you do this, you find there's three things that happen to a system of charged particles (i.e. atoms and molecules) when they accelerate:

1) they get shorter ("Lorentz contraction")
2) they move more slowly ("Time dilation")
3) there's a fixed difference in time between the front and back ("Lack of simultaneity")

From these three systematic distortions that happen to material things, you can show that, due to what happens to your rulers and clocks, that a system of rulers and clocks will always measure the speed of light to be constant, and you can prove the principles of relative motion.

#### lightarrow

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##### Re: What's the difference between general and special relativity?
« Reply #6 on: 09/12/2015 13:26:52 »
"What's the difference between general and special relativity?"

The essential difference is that spacetime have not to be warped by masses or energy, to apply special relativity. If it is, general relativity have to be used.
So, for example, in absence of massive objects or others which can warp spacetime, we can use SR even to treat accelerating spaceships, as in the "twin paradox".

--
lightarrow

#### Space Flow

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##### Re: What's the difference between general and special relativity?
« Reply #7 on: 10/12/2015 11:35:51 »
Lightarrow,
So, for example, in absence of massive objects or others which can warp spacetime, we can use SR even to treat accelerating spaceships,
We can't use SR to teat accelerating spaceships.

SR works with non-accelerating reference frames. Constant velocities. As such it is a good way to demonstrate and explain relativistic effects.

Real life situations are a bit more complicated than that and that's where General Relativity is used.

#### puppypower

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##### Re: What's the difference between general and special relativity?
« Reply #8 on: 10/12/2015 12:45:20 »
One thing General relativity and Special relativity have in common, are both can alter space-time and lead to the exact same changes in space-time. These two are equivalent, relative to space-time, at some level. They are two paths leading to the same place.

General relativity changes space-time via mass and distance but not by time. It does not matter if gravity cause the mass to collapse quickly or slowly, the final space-time well is only dependent on the final mass-geometry. With special relativity changes in space-time occur via mass, distance and time, with the starting mass not critical to the result. The  seems to imply an equivalence of mass and distance (GR) and time and distance (SR). This reduces to mass equivalent to time.

The equivalence of mass and time.

Consider two space-time references that exist side-by-side. The one on the left has time moving faster and the one on the right has time moving slower. In this hypothetical example, I am in the left reference (faster) and I place my hand in the right reference (slower) to dribble a basketball. Because time is moving slower in the right reference, where my hand and ball is, if I try to dribble the ball at the correct speed for my reference, I notice the ball moves slower.

What I will need to do is push harder, with greater force, to make the ball move at the correct speed for my reference. After the ball hits the ground and rebounds, as I go to push down on the ball, I notice the ball, although moving at the correct speed, now appears to have more inertia. The difference in time, between these two reference, has the impact of altering the inertia of mass. While the higher force at the same speed seems to add to more mass, since velocity is normalized.
« Last Edit: 10/12/2015 12:48:02 by puppypower »

#### lightarrow

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##### Re: What's the difference between general and special relativity?
« Reply #9 on: 11/12/2015 13:07:17 »
Lightarrow,
So, for example, in absence of massive objects or others which can warp spacetime, we can use SR even to treat accelerating spaceships,
We can't use SR to teat accelerating spaceships.
Guess why I have specified that? Because your is a common mistake.
Quote
SR works with non-accelerating reference frames. Constant velocities. As such it is a good way to demonstrate and explain relativistic effects.
And were I wrote that you have to work in the accelerated reference frame? You have a spaceship accelerating with respect to an inertial reference frame and you make your computations in that inertial frame.
Quote
Real life situations are a bit more complicated than that and that's where General Relativity is used.
But you can't invoke the equivalence principle in the sense that spacetime does not become warped only because you are in an accelerating (instead that inertial) frame: the spacetime stays the same (if that is what you intended to say).

--
lightarrow
« Last Edit: 11/12/2015 13:10:34 by lightarrow »

#### puppypower

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##### Re: What's the difference between general and special relativity?
« Reply #10 on: 11/12/2015 13:49:48 »
One main difference between SR and GR is special relativity SR defines one space-time reference, while GR usually defines a range of space-time references; space-time well.

With SR we can go anywhere in the rocket ship and be in the same space-time reference, since the entire rocket is in motion with the same velocity=V. With GR the surface of the planet is in a different space-time reference than the core of the earth. The core of the earth is like the twin that ages slower, since it is deeper in the space-time well.

With GR, if I was on the surface and I could stretch my arm to the center of the earth and dribble a basketball , I would notice changes in inertia relative to the surface. This is due to the equivalence of time and mass. This equivalence allows  the space ship, via SR, to simulate any reference of the earth by altering speed. The time bridge used to alter inertia is relativistic mass.

« Last Edit: 11/12/2015 13:51:36 by puppypower »

#### dhjdhj

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##### Re: What's the difference between general and special relativity?
« Reply #11 on: 11/12/2015 14:12:39 »
I have always wondered about space time and why the speed of light should be a posit and not an effect of something more fundamental. I respectfully suggest the answer may be within the atom. If the fundamental particles with mass that make up the atom were to spin with a tangential velocity of C. The effects of SR are the same. A sphere spinning about its axis has an interesting property. If you move the axis in any direction the mean velocity of the spinning object does not change provided the spin velocity is greater than velocity of the axis movement. As the axis moves the forward motion is deducted from the backward velocity as the edge spins away and adds to the forward motion as the opposite edge spins towards the motion. The effect is the mean velocity remains constant. If you do the vector diagram you will find that the resultant combined velocity is √c²-v². as the distance S is equal in each case t1=s/c and t2=s/√c²-v². so λ=t2/t1=c/√v²-c²=1/√v²-c².This of course is SR. In addition if these particles have a total mass m. Then the kinetic energy of the particles spinning is ½mc². If this is the total energy trying to get out then the same energy must be available to stabilise the atom. So we get energy in the atom to be E=mc². All the results of SG are identical in each case, but we now have fundamental time relating to the fundamental particle and space being just space. This obviously causes problems with GR. If you follow the argument further then C must be the speed of energy transfer, gravitational lensing becomes automatic and there are other explanations for orbital distortion in mercury.

#### dhjdhj

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##### Re: What's the difference between general and special relativity?
« Reply #12 on: 11/12/2015 15:56:45 »
Did anyone spot my deliberate mistake? the calculation is right but Time dilation is given by λ=1/1-v²/c². Sorry not used to the formula producing system on the web site

#### syhprum

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##### Re: What's the difference between general and special relativity?
« Reply #13 on: 11/12/2015 22:18:00 »
Simple I can understand SR with my schoolboy maths but GR defeats me with tensors and whatnot

#### puppypower

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##### Re: What's the difference between general and special relativity?
« Reply #14 on: 13/12/2015 13:03:59 »
In my last post I pointed out the observation that with SR the entire object in motion with velocity V has the same space-time reference. While in GR, an object, like a planet or star will exist in range of space-time references; space-time well.

Based on this difference we can simulate GR with SR if we accelerate the space-ship. This allows the space ship to move along the space-time well via incremental increases in velocity. Since velocity is d/t and acceleration is d/t/t, we added time to help us get closer to the impact of GR mass.

GR brings up an interesting consideration. Since GR can define a space-time well, that will set up a range of references, then the core of the earth or star exists in a different space-time reference than the surface. The core is like the twin in the twin paradox that is aging slower. The surface twin ages faster. In this example, GR sort of simulates two SR references; stationary and moving twin. How does the time difference between core and surface, impact the interaction of the core with the surface? GR is creating a potential in time that is not only perpetuated by the well but is expanding over time. Distance is contracted but does not compound like time. In the twin paradox, one twin is younger but he is not permanently thinner in space.

One way to answer this is to start with two identical factories that make widgets, side by side, but in two different space-time references. Both factories are making 1 defect per hour, and both using X energy per hour.  The one moving slower in time, will appear to use less energy and generate less entropy on a normalized time scale. The 2nd law has less impact on the slower core reference in a normalized scale. Entropy will still increase but at a slower rare; normalized.

The result will be a dual potential appearing between surface and core connected to energy/entropy, due to the time difference. This is reflected, in part, in the pressure of the core generating heat; more energy, as well as limiting the degrees of freedom in the matter of the core for less entropy. One might also expect a flux of energy/entropy from core to surface, which in turn, would reflect the time potential difference. However, the well is also causing the references of the core and surface to linger, expanding the time gap.

One interesting observation is the core of the earth rotates faster than the surface. One way to explain this is connected to the release of energy and entropy in the core, due to the lingering time potential with the surface. The expanding universe shows a net flow in the direction of expanding reference.

Another way to look at this is the core is expressing the past, when the earth rotated faster.
« Last Edit: 13/12/2015 13:13:23 by puppypower »

#### lorentzviolator

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« Reply #15 on: 06/01/2016 13:20:55 »
This is a great article, very clear and well explained, certainly has improved my understanding of relativity and I love the diagrams!

#### The Naked Scientists Forum

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« Reply #15 on: 06/01/2016 13:20:55 »