How can we sure that physical laws are always the same in all the universe?
We can't be!
Its just an assumption which has shown to be a good assumption till now and we'll continue to use it till its revealed that its in fact not a good assumption.
We can never be sure of anything in science. The scientific principle is based on falsification and finding a good description of what we can observe. As Shyan said, it is an assumption that has been working very well so far and it could have been falsified several times over if it was not correct.
Any information cannot travel faster than light ,so our universe is what we can observe.There is always scope for a possibility of a physical law to change. Newton's laws,Laws of electromagnetism and other laws are holding in this SCENERIO of the observable Universe.
There are other types of laws like Thermodynamics laws which are there holding and there so far is nothing contrary to these.
It is expected that newton's laws should be derived out of subtle quantum laws(not sure about exactness of this statement).
General relativity is more General theory of gravitation.
Special relativity reduces to Newtonian concepts at low speeds.
One very strong indication is that we can look at the light from very distant sources and the spectra (apart from the red shift) are identical to the spectra of local light sources. That implies that the same 'laws' about what goes on inside atoms, must apply out there. So if those laws apply, why not all the laws?
To generalize on what Sophiecentaur said, we can say that we have not seen any evidence of different laws. We can make theories about what lies far out there, but we can't test those theories against evidence.
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