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Question: what is the theory of realativity?

Sharon Sneddon answered on 22 Jun 2010:
ok, here goes!!
Albert Einstein’s theory of relativity is actually two separate theories: his special theory of relativity , The Electrodynamics of Moving Bodies and his theory of general relativity , an expansion of the earlier theory, Einstein sought to explain situations in which Newtonian physics might fail to deal successfully with phenomena, and in so doing proposed revolutionary changes in human concepts of time, space, and gravity. Clear as mud!!so, in simpler terms ( I hope)
Special Relativity says that every person has their own time. One person’s clock says something different from another person’s clock. The reason a person’s time can be different from another’s is because of something called Time Dilation or the twin paradox.The Twin Paradox shows how time changes because of speed and mass. The faster someone or something moves or the more mass they have, then the slower time passes for them. This is shown by a pair of twins. Imagine that the first twin moves at a fast speed, near the speed of light for example, while the other twin stays in one general location. When the first twin returns from his trip, he will be younger than the second twin. Thus time and speed are related.
General Relativity adds gravity to Special Relativity. It says that space and time are joined together. Space is like a grid. Balls (planets) on the grid bend it because of their mass or speed. As one ball moves towards another it falls towards the other ball due to the dent in the grid. It can be shown by putting heavy balls on a trampoline. The dent in the grid is like gravity.
If space and time are one thing represented by a grid and heavy objects stretch out the grid, that means that time passes more slowly near heavy objects. This has been shown in experiments by comparing very accurate clocks on Earth (where Earth’s gravity is strong) with very accurate clocks on satellites or rockets in space (where Earth’s gravity is weak).I hope this makes some sense. I’m not a Physicist so these things are all a bit new to me too! I’m def learning a lot this week, but think I need a lie down to process all this relativity!

Jon Copley answered on 22 Jun 2010:
Relativity is the idea that the laws of physics should apply the same for everything and everyone, regardless of how a person or object is moving relative to another person or object. In other words, if you are moving and you make a measurement of something or do an experiment, you should get the same result as if you are standing still.
Galileo was one of the first people to think about relativity. He suggested that a person on a ship moving at a constant speed, looking out through a porthole at someone standing still on the shore, would not be able to tell just by looking whether they were moving and the other person standing still, or vice versa.
Now that sounds obvious, and in a way it is – but what Galileo did was to translate that into mathematics, so that if you wanted use maths to describe motion, then the sums for you moving forwards at a constant speed would be the same as if everything else was moving backwards at a constant speed (and that’s known as the “Galilean Transformation”). And that became a key part of the laws of motion worked out by Newton.
But much later, Einstein took it further. For the laws of physics to work the same for everyone, whether they are moving or not, Einstein deduced that time, length and mass could not be the same for people or objects that are moving very fast, compared with those that are standing still.
Now that sounds bizarre (and it is!), but we can measure the effect. For example, if you have two incredibly accurate clocks (called atomic clocks) and you synchronise them exactly, then send one to Australia and back on a jumbo jet, and then compare the clocks afterwards, you find the clock that was moving is now running slightly behind the clock that was left standing still.
In other words, moving clocks run slow (but only by a tiny, tiny fraction of a second at the speeds we usually travel). That was one of the amazing things about Einstein’s relativity – he realised that for the laws of physics to work the same for everyone, properties like time, length and mass would have to become relative.
The idea that time passes differently in different situations is perhaps not that odd – Einstein once joked: “Put your hand on a hot stove for a minute, and it seems like an hour. Sit with a pretty girl for an hour, and it seems like a minute. THAT’S relativity.” (But in his relativity, it really does pass at different rates for objects moving at different speeds, not just seem like it!)
All that is part of what is known as Einstein’s “Special Relativity”. He then took his ideas further to show that gravity is not really a force, but rather a consequence of the curvature of something called spacetime, in what became known as Einstein’s “General Relativity”.
There’s a lot more to it than that, so if you’re interested, I can recommend this book, called “Eyewitness: Time and Space” by John Gribbin:
http://www.amazon.com/EyewitnessTimeSpaceJohnGribbin/dp/0789455781

Zoe Duck answered on 22 Jun 2010:
The theory of relativity is pretty complex and I’m not sure I am the best person to explain it :S
This website might explain it a bit better: http://www.phy.syr.edu/courses/modules/LIGHTCONE/

Louise Dash answered on 23 Jun 2010:
Good question!
Actually, there are two theories of relativity – special relativity and general relativity, developed by Einstein in 1905 and 1915.
Special relativity tells us that the speed of light is the same for everyone everywhere, no matter how fast they’re moving. There are some strange consequences of this! As objects move faster and approach the speed of light (which is 299,792,458 metres per second – fast!) they get heavier, shorter, and they feel time slow down. The famous equation E = mc^2 also comes from special relativity – it means that mass (m) can in theory be converted to energy (E) – this is what actually happens in a nuclear bomb.
General relativity is more about the theory of gravitation – the force that keeps the solar system and the rest of the universe stuck together. It tells us about how spacetime can be curved by massive objects like stars, and how black holes can form.

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