# Basic relativity

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What is relativity?

I'm sorry if that seems really basic it is just I am starting physics next year at school and I would like to have at least a little knowlege...

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In the shortest possible summary I can think of: The closer an object is approaching the speed of light --> the more its time slows down and its journey contracts.

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what does it mean by its journey contracts?

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If the object went on a journey to Centauri( 4 light years away), and it was traveling fast enough(say 3/4 the speed of light). The distance between its starting place and Centauri will have shrunken for the object.

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This pic is not Lorentz correct, but just a general illustration:

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oh ok

Thanks I get it now...

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More generally: There is no absolute reference frame. What you observe depends on your frame of reference.

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• 4 weeks later...

Einstein and Relativity.

St. Augustine thought that time did not exist before the beginning of the world and Einstein confirmed this one hundred years ago. Time was shown to be just another dimension like the three spatial ones we experience. Time and space are entwined so time too curves under gravity when the universe does.

Einstein thought about Galileo's relativity, that we did not feel the movement of the Earth because we were moving at the same speed as it. It is only a change in acceleration that can be felt and so Einstein realised that no scientific experiment could distinguish whether you were moving at a constant speed or if you were still without looking outside to see if anything else was moving. You are only moving in relation to what is around you. It is not just constant speed that can't be felt but a constant acceleration which is why it works for something which is not just travelling in a straight line. Because of this Einstein realised that the constant acceleration of gravity would be indistinguishable from any other type of acceleration.

Einstein saw that gravity wasn't something in the mass which pulled directly on the object. It pulled on space itself, space was soft like a 4 dimensional blanket and anything heavy placed on it bent the blanket. Anything too close rolled into the well it made. Light would roll into it too even though it was thought by most that light had no mass.

Einstein worked out that energy, invisible forces and the bright waves of light were interchangeable with mass, the particles that make up people. This meant that even light was effected by gravity and would grow colder the steeper the curve it had to fight against. Energy and mass were connected by a number that reoccurs in countless laws of the universe, the speed of light times by itself. It would take an infinite amount of energy to go over the edge and travel faster than light and so nothing ever could, and if they did they would find there was no universe to travel through. Because there is a limit to the speed that information can travel we can only ever know of the things which will have time to travel to us. We can not see past any light that will not have had time to reach us.

Light can be used to break down a wall and that matter can turn into pure white energy. Energy is like a ripple in universe as one thing changes into another.

Einstein realised that this meant that the shape of the universe depended on what it contained. There was perhaps no such thing as flatness and the shortest distance between two objects could often become a curve. Everything had to role around the bumpy currents of spacetime and nothing could accelerate faster than gravity allowed, no matter what their mass. Feathers fell on the moon at the same rate as kilos of lead. There were black holes as heavy as 250 million Suns and some as small as an atom. The shape of space could make time stand still in some places and go slower and faster in others. According to relativity the universe exists in this way with time laid out like the dimensions of space all inside a 4 dimensional sphere. The concept we have of presence is merely an illusion.

Einstein discovered that if the universe had too much mass close together then it wouldn't balance out like in Newton's universe and space could curve so much that it would bend back in on itself until it got so small that it disappeared. As space curves so does time and some heavy universes could curve in on themselves completely. When people looked at all the different possible universe's a man called Gödel found a possibility where time looped at every point, wherever you went time take you back to when you begun.

Knowing that the universe was pulled into shape by what lay inside it people could measure just how curved our universe was. A giant triangle was measured in the sky and the angles added up to a number greater than 180 degrees, the universe had curved in on itself.

As space curves and we accelerate around it space and time distort themselves so that the speed of light remains the same and so we measure lengths as being shorter as we accelerate past them as it takes us less time to get from one end to the other. Time is measured differently if you are in a space ship compared to someone on the Earth. Seconds take longer to tick by the stronger the gravitational force you are in, or the faster you go.

- a article on relativity from my website

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Einstein and Relativity.

St. Augustine thought that time did not exist before the beginning of the world and Einstein confirmed this one hundred years ago. Time was shown to be just another dimension like the three spatial ones we experience.

In order to experience something it has to be physical in some way. Are you saying that the first three spatial dimensions are physical?

Einstein and Relativity.

Einstein saw that gravity wasn't something in the mass which pulled directly on the object. It pulled on space itself, space was soft like a 4 dimensional blanket and anything heavy placed on it bent the blanket.

Space is soft like a blanket? Where did you get this information?

Anything heavy placed on it bent the blanket? Bent the blanket in what direction? What is this blanket existing in?

Who or what does this placing of heavy objects?

Please when answering science questions refer to established data on the subject. Otherwise arbitraries get introduced and the whole subject goes off the rails.

Einstein and Relativity.

Anything too close rolled into the well it made.

This meant that even light was effected by gravity and would grow colder the steeper the curve it had to fight against.

As space curves and we accelerate around it space and time distort themselves so that the speed of light remains the same and so we measure lengths as being shorter as we accelerate past them as it takes us less time to get from one end to the other. Time is measured differently if you are in a space ship compared to someone on the Earth. Seconds take longer to tick by the stronger the gravitational force you are in, or the faster you go.

- a article on relativity from my website

Again, I am not trying to pick on you, but where are you getting all of this information that you wrote?

Anything too close rolled into the well. Why would it roll into the well, unless an outside force caused it to change course. Otherwise it would travel in a straight line and not vary off course.

Light growing colder? What?!!

In Einstein’s theory of special relativity it states that objects APPEAR to an observer to contract the closer they approach the speed of light, they do not actually contract. Clocks APPEAR to observers to slow down, they do not actually slow down.

That is why it is called relativity, things appear to change relative to other things. Nothing actually changes.

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Try checking out the wikipedia article: http://en.wikipedia.org/wiki/General_relativity

Just to outline the main concepts of relativity:

-Gravitational pull is due to the curving of spacetime by an object rather than a force, shown in this image http://en.wikipedia.org/wiki/Image:Spacetime_curvature.png

-"The general principle of relativity states that the laws of physics must be the same for all observers (accelerated or not)."

-"the curvature of spacetime and its energy-momentum content are related."

-due to the fact that spacetime can be curved by the presense of matter, and that spacetime is non-linear, non-euclidean geometry must be used.

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I'm sorry if that seems really basic it is just I am starting physics next year at school and I would like to have at least a little knowlege...

General Relativity is a mathematical nightmare. Don't be worried if you don't understand it, and unless you're doing something postgrad that deals with it I doubt it'll be presumed.

In Einstein’s theory of special relativity it states that objects APPEAR to an observer to contract the closer they approach the speed of light, they do not actually contract. Clocks APPEAR to observers to slow down, they do not actually slow down.

No, they do actually slow down, or contract. From our point of view, which is just as valid as theirs.

That is why it is called relativity, things appear to change relative to other things. Nothing actually changes.

Ever heard of the Twin paradox?

Space is soft like a blanket? Where did you get this information?

It's a metaphor that approximates the way gravity works in terms that can be understood by the layman. The rubber sheet analogy is one that I've seen time and time again, at every level of study.

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In Einstein’s theory of special relativity it states that objects APPEAR to an observer to contract the closer they approach the speed of light, they do not actually contract. Clocks APPEAR to observers to slow down, they do not actually slow down.

That is why it is called relativity, things appear to change relative to other things. Nothing actually changes.

As JaKiri has stated, they actually do contract/slow according to the other observers. Since there are no preferred frames of reference, you can't decide — based solely on physics — that one frame gives the correct view and all others are "illusions."

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No, they do actually slow down, or contract. From our point of view, which is just as valid as theirs.

It APPEARS from your point of view to slow down or contract. If things really did contract then Einstein would have said that it actually occurs and not use the term appears. Also, if space ships traveling near the speed of light actually would contract then you would notice your own ship contracting and not the other ship appearing to contract. It is all relative to your point of view, it does not actually happen. Go ahead and look at all the scientific papers written on this topic. It says appears, not actually happens.

Ever heard of the Twin paradox?

Yes I have heard of the twin paradox. It is a mental paradox since it is not a statement of an actual occurrence. It proves nothing, it is just something to think about.

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Yes I have heard of the twin paradox. It is a mental paradox since it is not a statement of an actual occurrence. It proves nothing, it is just something to think about.

Sorry. No. It is reinforced by these studies which were actually conducted:

Haefele and Keating, Nature 227 (1970), pg 270 (Proposal); Science Vol. 177 pg 166--170 (1972) (Experiment).

They flew atomic clocks on commercial airliners around the world in both directions, and compared the time elapsed on the airborne clocks with the time elapsed on an earthbound clock (USNO). Their eastbound clock lost 59 ns on the USNO clock; their westbound clock gained 273 ns; these agree with GR predictions to well within their experimental resolution and uncertainties (which total about 25 ns).

Vessot et al, "A Test of the Equivalence Principle Using a Space-borne Clock", Gel. Rel. Grav., 10, (1979) 181-204; "Test of Relativistic Gravitation with a Space borne Hydrogen Maser", Phys. Rev. Lett. 45 2081-2084.

They flew a hydrogen maser in a Scout rocket up into space and back (not recovered). Gravitational effects are important, as are the velocity effects of SR.

C. Alley, "Proper Time Experiments in Gravitational Fields with Atomic Clocks, Aircraft, and Laser Light Pulses," in Quantum Optics, Experimental Gravity, and Measurement Theory, eds. Pierre Meystre and Marlan O. Scully, Proceedings Conf. Bad Windsheim 1981, 1983 Plenum Press New York, ISBN 0-306-41354-X, p363-427.

They flew atomic clocks in airplanes which remained localized over Chesapeake Bay, and also which flew to Greenland and back.

Bailey et al., "Measurements of relativistic time dilatation for positive and negative muons in a circular orbit," Nature 268 (July 28, 1977) pg 301; Nuclear Physics B 150 pg 1-79 (1979).

They stored muons in a storage ring and measured their lifetime. When combined with measurements of the muon lifetime at rest this becomes a highly-relativistic twin scenario (v ~ 0.9994 c), for which the stored muons are the traveling twin and return to a given point in the lab every few microseconds.

Muon lifetime at rest:Meyer et al., Physical Review 132, pg 2693; Balandin et al. JETP 40, pg 811 (1974); Bardin et al. Physics Letters 137B, pg 135 (1984). Also a test of the clock hypotheses (below).

The Clock Hypothesis

The clock hypothesis states that the tick rate of a clock when measured in an inertial frame depends only upon its velocity relative to that frame, and is independent of its acceleration or higher derivatives. The experiment of Bailey et al referenced above stored muons in a magnetic storage ring and measured their lifetime. While being stored in the ring they were subject to a proper acceleration of approximately 1018 g (1 g = 9.8 m/s2). The observed agreement between the lifetime of the stored muons with that of muons with the same energy moving inertially confirms the clock hypothesis for accelerations of that magnitude.

Sherwin, "Some Recent Experimental Tests of the 'Clock Paradox'", Phys. Rev. 129 no. 1 (1960), p17.

He discusses some Moessbauer experiments that show that the rate of a clock is independent of acceleration (~1016 g) and depends only upon velocity.

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As JaKiri has stated, they actually do contract/slow according to the other observers. Since there are no preferred frames of reference, you can't decide — based solely on physics — that one frame gives the correct view and all others are "illusions."

ACCORDING to OTHER observers. If things actually contracted and slowed down, then if you were on a ship traveling close to the speed of light don’t you think that all this contracting would effect the propulsion system in some adverse way. The appearance of these effects is based on viewpoints of the observer, and not based on what is fact. Also if things contracted and time slowed down how do these work together ? Contracting is a motion which involves time, a motion that can be timed, yet if time slows down for the object then so does the contracting of that object, the closer to the speed of light the more it is supposed to contract yet the slower the time for that object would be. So the slower it would contract. Seems like it would all balance out and nothing would change.

To iNow and Lockheed.

So are you saying that time is a physical thing that is effected by motion?

In all of those experiments regarding time, only two things could have been acted on by motion, Time or The measuring device. Who is to say which was effected? Believe what you want, but for time to be effected by an outside influence it would have to be a physical thing, with some sort of atomic structure. Do you really think that time is a thing that is at the effect of other forces? Besides this time change that occurred in a specific location around the test object, was there some kind of bubble around it so that only the time in that small area changed and no change for the measuring device, or the airplane or the area outside the plane, or the Earth, or the universe. If there is no protective time bubble around this test object then what is to stop this time change from happening to everything connected to this test object? Why the time change in only what was being tested? If objects moving around can influence the rate of time…. Well you can see what a mess that would be. Planets, photons, comets etc.

So an airplane flying around can effect the rate of time, but only in what is being tested, how convenient. Man has a tendency to find in his experiments what he is looking for to prove that his beliefs are true.

Again, if an airplane flying around can effect time why is time only effected in what is being tested? What are the mechanics behind this pin point time change in things that man is testing? It is either that, (pin point time change) or time changes all over the place and that would include the measuring device and the plane flying slower and my clocks going slower all from one airplane, now include all the other things that are moving as fast or faster then these airplanes. Wow! Look at all of the things that effect this thing called time.

I will not get into the idea that time speeds up or slows down depending on which direction you are traveling in. Can anyone describe the mechanics of how directions effect the rate of time? How is time connected to direction? I wonder which way time is going for all those satellites circling the Earth?

I am sure that those experiments that you posted gave the desired results that were being hoped for.

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Eric5, you do know that GPS is dependent on time actually being at different rates for different observers, right?

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It APPEARS from your point of view to slow down or contract.

And does, from your point of view, and the point of view of any rest frame that has it non-stationary (which when acceleration is involved is all of them by definition.

If things really did contract then Einstein would have said that it actually occurs and not use the term appears.

He didn't use the term "appears", though (not least because he was writing in German). He was also far from the first to describe the Lorentz transformations.

Also, if space ships traveling near the speed of light actually would contract then you would notice your own ship contracting and not the other ship appearing to contract.

It appears to contract, and does. It's not some optical illusion, it's a mathematical consequence of the speed of light being constant for all observers.

It is all relative to your point of view, it does not actually happen.

The first statement does not exclude the possibility that it does "actually happen", which it does.

If it does not, there is no way to explain, for example, the huge discrepencies between the decay rates particles moving at different velocities with an otherwise predictable decay rate, like muons. As it is, muon decay agrees with SR - both from the point of view of the muon, and from the point of view of the lab (in non-mathematical analysis, the muon "gets further" than it should travelling in the lab either because it's time dilated (from the lab's point of view) or because the lab has spatially contracted (from the muon's point of view)).

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ACCORDING to OTHER observers. If things actually contracted and slowed down, then if you were on a ship traveling close to the speed of light don’t you think that all this contracting would effect the propulsion system in some adverse way. The appearance of these effects is based on viewpoints of the observer, and not based on what is fact. Also if things contracted and time slowed down how do these work together ? Contracting is a motion which involves time, a motion that can be timed, yet if time slows down for the object then so does the contracting of that object, the closer to the speed of light the more it is supposed to contract yet the slower the time for that object would be. So the slower it would contract. Seems like it would all balance out and nothing would change.

This is, in essence, introducing a preferred frame — yours. A consequence (or observation) of relativity is that there is no preferred (inertial) frame of reference, and when we couple that with the notion that whatever I experimentally measure is real, then one has to say that time has slowed and moving things contract. Neither the shipboard nor outside obsever can claim — from a physics standpoint — that the other is wrong, only that their measurements disagree.

If you reject the notion that measurements reflect reality, you will have difficulty doing science. You will have no way to determine what is "real" or "illusory" in a non-arbitrary way.

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ACCORDING to OTHER observers. If things actually contracted and slowed down, then if you were on a ship traveling close to the speed of light don’t you think that all this contracting would effect the propulsion system in some adverse way. The appearance of these effects is based on viewpoints of the observer, and not based on what is fact. Also if things contracted and time slowed down how do these work together ? Contracting is a motion which involves time, a motion that can be timed, yet if time slows down for the object then so does the contracting of that object, the closer to the speed of light the more it is supposed to contract yet the slower the time for that object would be. So the slower it would contract. Seems like it would all balance out and nothing would change.

To iNow and Lockheed.

So are you saying that time is a physical thing that is effected by motion?

No. Relativity says that "Time" is a relative measurement. Or put another way, time is "what clocks measure" and clocks in relative motion to each other measure time differently. They will not even agree whether given events are simultaneous or not.

In all of those experiments regarding time, only two things could have been acted on by motion, Time or The measuring device. Who is to say which was effected? Believe what you want, but for time to be effected by an outside influence it would have to be a physical thing, with some sort of atomic structure. Do you really think that time is a thing that is at the effect of other forces? Besides this time change that occurred in a specific location around the test object, was there some kind of bubble around it so that only the time in that small area changed and no change for the measuring device, or the airplane or the area outside the plane, or the Earth, or the universe. If there is no protective time bubble around this test object then what is to stop this time change from happening to everything connected to this test object? Why the time change in only what was being tested? If objects moving around can influence the rate of time…. Well you can see what a mess that would be. Planets, photons, comets etc.

So an airplane flying around can effect the rate of time, but only in what is being tested, how convenient. Man has a tendency to find in his experiments what he is looking for to prove that his beliefs are true.

Again, if an airplane flying around can effect time why is time only effected in what is being tested? What are the mechanics behind this pin point time change in things that man is testing? It is either that, (pin point time change) or time changes all over the place and that would include the measuring device and the plane flying slower and my clocks going slower all from one airplane, now include all the other things that are moving as fast or faster then these airplanes. Wow! Look at all of the things that effect this thing called time.

I will not get into the idea that time speeds up or slows down depending on which direction you are traveling in. Can anyone describe the mechanics of how directions effect the rate of time? How is time connected to direction? I wonder which way time is going for all those satellites circling the Earth?

I am sure that those experiments that you posted gave the desired results that were being hoped for.

The problem you are having is that you are still stuck on the idea of time as an universal absolute concept rather than the relative one that it is.

Absolute time behaves something like the directions of North and South. Put two people in a room and no matter how they are facing, they both agree which direction North is.

Relative time behaves like the directions left and right. Two people standing in the same room can disagree on which direction "right" is, depending on how they are facing relative to each other. One's left may be the other's right.

It is this second example that is closer to how real time behaves.

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