# What is relativy ?

#### heaven eye

what is relativy ???

actually I heard about theory Einstein made it was "relativy" , well am so confused , I mean each time I ask about it I receive different answers but what I could to understand that there is relativy in time , mass , velocity , volume ....... and so much which made me want to understand it will.

Related Special and General Relativity News on Phys.org

#### HallsofIvy

First of all, the word is "relativity" not "relativy" and that concept was not new with Einstein! Classical Physicists talked about "Gallilean relativity" that was developed by Gallileo who maintained that if you were inside a completely sealed carriage moving at a constant speed in a constant direction, there is no experiment you could do to determine your speed or even if you were moving! Basically that is because F= ma- force is proportional to acceleration, not speed, and what we really measure are forces. Speed can only be measured "relative" to something else, not in any absolute terms.

The development of electro-magnetic theory put that in question. The force due to a moving charge depends on its speed, not acceleration. That implies that there exist an absolute speed, not "relative" to anything else. The Michaelson-Morley experiment was designed to measure our "absolute" speed and gave a null result- there was no such thing. Einstein's theory of relativity explained why, even with electro-magnetic forces, speed is still "relative".

If, as I suspect, you want a general explanation of Einsteinian relativity, I can only suggest that you read other threads in this forum. That's what they are all about!

#### εllipse

The reason you may feel like you get different answers each time you ask is because there is a lot to explain in relativity and a few different ways to explain each thing. The best thing you can do is get a book on it: http://math.ucr.edu/home/baez/physics/Administrivia/rel_booklist.html

There are two theories of relativity, the special theory and the general theory. The special theory was completed in 1905 as a remedy for the conflict between Maxwell's electromagnetism and Galilean mechanics. This conflict was really only something Einstein cared about, but his special theory of relativity also happened to explain something else physicists were confused about at the same time, the results of the Michelson-Morely experiment which seemed to show that the speed of light is the same to everyone, no matter how fast they're moving. Einstein assumed the results of this experiment (that the speed of light is a constant) on the basis of the principle of relativity (an expanded version of a Galilean postulate) and Maxwell's theory of electromagnetism, and ended up with a new system of mechanics which reduces to Galilean mechanics when velocities are low relative to the speed of light.

The general theory was completed in 1916 as a remedy for the conflict between the special theory of relativity and Newton's theory of gravity. The general theory also solved an experimental problem physicists were confused about at the time, although not quite as accidentally, the perihelion shift in the orbit of Mercury. Newton's theory of gravity doesn't predict the orbit of Mercury around the Sun accurately, but Einstein's general theory of relativity (the replacement of Newtonian gravity) does give accurate predictions for the orbit of Mercury.

Some unfamiliar/weird insights from relativity:
(1) Something moving fast will be shorter than it would be if it is standing still.
(2) A clock moving fast will tick slower than it would if it was standing still.
(3) Two events which happen at the same time from one person's point of view can happen at different times from someone else's point of view (provided the two people are moving fast relative to each other).
(3) Matter and energy are united as two different aspects of the same thing.
(4) We live in a four dimensional universe; space and time are united as the spacetime continuum.
(5) In a strong gravitational field, lengths contract and time slows down.
(6) Gravity isn't a "force" in general relativity; it is a curvature in spacetime, which violates Euclid's parallel postulate (allows two inertial bodies moving parallel to each other to fall toward each other).
(7) Nothing can ever accelerate to a faster speed than the speed of light.

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#### pmb_phy

heaven eye said:
actually I heard about theory Einstein made it was "relativy" , well am so confused , I mean each time I ask about it I receive different answers but what I could to understand that there is relativy in time , mass , velocity , volume ....... and so much which made me want to understand it will.

Albert Einstein published the article On the electrodynamics of moving bodies, Albert Einstein, Annalen der Physik, 17, 1905. The theory which Einstein created and published in this paper has come to be known as special relativity. The "special" because it applies only to inertial frames (local or not, for you SR/GR perfectionists). The "relativity" term means that observations and measurements make no sense and have no meaning until you specify an observer which may or may not be in motion relative to you.

Now my turn. I helped you now you help me. For more info on this and how to learn it see - http://www.geocities.com/physics_world/sr/sr.htm

Start at the top and work your way down. Let me know if you find something in error or something confusing. Thanks.

Peter

#### heaven eye

well i back to see the answers and i like to thank each obe helped me lets go to the first answer :-

mr.HallsOfITY :-well you are right its relativity not relativy
then is thats means that Einstein didn't make the relativy or anything new in
relativy? is that true?????

mr.ellipse:- thank you for the website
and for these ideas is that true that if i moved faster time run slower?? but when i move the watch still the same ? maybe i missunderstood the idea
but what is the theory of electromagnatism? i just wish if i get a simple answer because Iam 16 years old !!!

mr.bmb phy:- I added the website in favourite because it has really useful information , I promise you that when I find any difficult I will ask you

well I asked a profissor in a university by email about this law E=mc^2 he said in atomic reaction the Uranium he said if the Uranium mass before the reaction was 4g then the mass of the Uranium after the reaction would be 3.6g what about the .4g to did it go? does it disappeard because it moved in speed of light????? this contrast the relativity in mass??? I think I well not ever be like Einstein or even understand Einstein or maxwell ???

"What is relativy ?"

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