# SR postulates

#### Stephanus

Dear PF Forum,
Before I go further, I'm stuck and need clarification.
Two postulates of SR

2. the velocity of light is the same for all inertial observers.
Okay..., this one I can understand, at least can grasp.
1. the laws of physics are the same in all inertial frames of reference;
What laws of physics that it means?
Time and distance, I already know that it differs.
And they say mass, too
When things move, will we see that waters in that space ships boils when it's cooled?
So, what kind of laws that are involved?
Thanks.

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#### Orodruin

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The things you mention are not physical laws, they are mostly physical properties. A physical law is something which tells you how to relate such properties to each other.

#### Stephanus

Wow, so fast. Thanks.
So, what physical laws that are affected by relativity?

#### Orodruin

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The entire point is that the laws are not affected by the choice of inertial frame.

An example of a physical law is the conservation of energy and momentum.

#### harrylin

Dear PF Forum,
Before I go further, I'm stuck and need clarification.
Two postulates of SR

2. the velocity of light is the same for all inertial observers.
Okay..., this one I can understand, at least can grasp.
1. the laws of physics are the same in all inertial frames of reference;
What laws of physics that it means?
Time and distance, I already know that it differs.
And they say mass, too
When things move, will we see that waters in that space ships boils when it's cooled?
So, what kind of laws that are involved?
Thanks.
First of all, the postulates of SR are:
1. the laws of physics are the same in all inertial frames of reference
2. the speed of light is a constant, independent of the motion of the source
(and this is a law of physics)
See the intro of http://fourmilab.ch/etexts/einstein/specrel/www/

Combining 1. and 2., you can derive the Lorentz transformations in a straightforward way.

#### Stephanus

First of all, the postulates of SR are:
1. the laws of physics are the same in all inertial frames of reference
2. the speed of light is a constant, independent of the motion of the source
(and this is a law of physics)
See the intro of http://fourmilab.ch/etexts/einstein/specrel/www/

Combining 1. and 2., you can derive the Lorentz transformations in a straightforward way.
A good article. But's rather hard for me.

#### harrylin

A good article. But's rather hard for me.
The intro should not be too hard, and IMHO most textbooks give a less good introduction.
If you combine the two postulates, then you get what you thought to be the second postulate:

The speed of light must be the same constant in any inertial reference frame.

The confusion is due to some textbooks whose authors "simplified" things. They merely economized 1 or 2 lines of derivation (corresponding to the logical deduction here above) at the cost of a less good understanding of the physics and the logical development.
http://fourmilab.ch/etexts/einstein/specrel/www/
What, by Einstein himself?? No surprise, it's hard for me.
Btw, is this "the" 1905 SR paper that he first published?
Yes it's that one (only, this is a translation as the original was in German).

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#### Stephanus

The intro should not be too hard
Yeah, until it hits Maxwell - Hertz.
Yes it's that one (only, this is a translation as the original was in German).
Of course. Only 50% is translated by the translator. They don't translate the 'math' portions in English

#### Dale

Mentor
What laws of physics that it means?
The laws of physics are the rules that govern the evolution and relationship between physical quantities. You can recognize them because they generally are expressed in the form of differential equations.

#### harrylin

Yeah, until it hits Maxwell - Hertz. [..]
With "intro" I meant the text until §1, which is exactly what you showed a picture of - no Maxwell-Hertz in that portion.

Poincare's relativity principle holds that all laws of physics should work the same in every inertial frame, so that no absolute inertial motion can be detected. Einstein gives in this paper a weak version of the relativity principle but it should be helpful for answering your question: not only the laws of mechanics but also the laws of electrodynamics and optics should work in every inertial frame. The second postulate is such a law.

#### Stephanus

Btw, how can we close our own thread?

Thanks.

#### Orodruin

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2018 Award
Btw, how can we close our own thread?
You cannot. This is a power reserved for the demi-gods known as "mentors".

If you feel a thread needs closing, or otherwise need to bring something to the attention of the demi-gods, you can always use the report link. Generally, threads do not require closing but tend to die a natural death once the issue is settled. We close threads that become too problematic by attracting off-topic posts or insults. Every now and then, very old threads are closed as well to avoid necro posting.

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