# A question

#### kant

In regard to einstein s second postulate on special relativity:

The speed of light is the same in all frame of reference.

____________

Can physics explain why this is true?

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#### Hootenanny

Staff Emeritus
Gold Member
If you are looking for proof, then I am afraid you will be disappointed. Postulates can not be proven, they are assumptions or statements made.

#### kant

Can you reduce this postulate to a more fundemental postulate in nature? How do we know this postulate is true? Can more modern physic explain this postulate?

#### Hootenanny

Staff Emeritus
Gold Member
kant said:
Can you reduce this postulate to a more fundemental postulate in nature? How do we know this postulate is true? Can more modern physic explain this postulate?
The definition of a postulate is that it cannot be described using more basic statement since there are non. We believe the invariance of c to be true because experimental evidence agrees with this postulate (The Doppler effect for light would be a good example).

#### pizza1512

What I don't understand about Einstein's equation is that can you calculate Energy of humans using E=mc^2

#### Hootenanny

Staff Emeritus
Gold Member
pizza1512 said:
What I don't understand about Einstein's equation is that can you calculate Energy of humans using E=mc^2
Dare I ask what you mean by "the Energy of humans"?

#### Pete81t

No matter how hard I try, I still can't grasp Einstein's second postulate of special relativity. It seems to defy common sense, yet we accept it as true. Another example of "doublethink"...

#### pervect

Staff Emeritus
What most people get confused by in relativity is the fact that simultaneity is relative. There is no "doublethink" involved, it's just a case of a non-intuitive result.

Mathematically, SR can be understood by studing the Lorentz transforms

$$t' = \frac{t - {\color{red}v\,x/c^2}}{\sqrt{1-v^2/c^2}}$$
$$x' = \frac{x - v\,t }{\sqrt{1-v^2/c^2}}$$

The relativity of simultaneity is due to the term in red. This is the key and non-intuitive point in relativity. Note that the Lorentz transforms have the mathematical property that

$$c^2 t'^2 - x'^2 = c^2 t^2 - x^2$$

this can be interpreted as the constancy of the speed of light, because x=ct implies x' = c t', both of which imply that the value of the Lorentz interval (the equation above) is zero.

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#### robphy

Homework Helper
Gold Member
Pete81t said:
No matter how hard I try, I still can't grasp Einstein's second postulate of special relativity. It seems to defy common sense, yet we accept it as true. Another example of "doublethink"...
It does defy common sense... that's why it's not intuitive.
However, the theory makes predictions (which differ from the "common sense" physics of Galileo and Newton) that can and have been measured experimentally with precision instruments. So, we know that "common sense" isn't correct.

However, with modern technology, our "common sense" is changing and may one day make relativity more "common sense" to us. If our daily lives involved traveling with relative-speeds close to the speed of light or, more practically, we rely on precision wristwatches, we'd begin to appreciate relativity. (The GPS is an often cited example where relativity is needed.)

#### russ_watters

Mentor
Pete81t said:
No matter how hard I try, I still can't grasp Einstein's second postulate of special relativity. It seems to defy common sense, yet we accept it as true. Another example of "doublethink"...
"Doublethink" is holding two mutually exclusive ideas in your head at the same time and accepting both as true. In this case, we know from evidence that this postulate is true. So you just plain need to stop believing anything else because those other things go against experimental data we have.
robphy said:
It does defy common sense...
It only defies common sense when that common sense is based on certain preconceptions about how the universe should operate. Drop those preconceptions and look at the data available and it becomes logical.

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#### Pete81t

russ_watters said:
It only defies common sense when that common sense is based on certain preconceptions about how the universe should operate. Drop those preconceptions and look at the data available and it becomes logical.
Easier said than done...I remember learning for the first time that there's really no real force called centrifugal force and that gravity isn't an actual force. We should remember that our modern view of the universe didn't develop overnight, but rather, was a gradual process spanning over centuries, and took such minds as Galileo and Newton to come up with them. Something to think about...

#### pess5

kant said:
In regard to einstein s second postulate on special relativity:

The speed of light is the same in all frame of reference.
____________
Can physics explain why this is true?
kant,

You ask a most challenging question indeed. If you can answer this with mathematical proof, you will hold a nobel prize if you live long enough to receive it. They only award it to living folks, and they generally are not quick to give it out.

One typical answer is this ...

All mediums support energy at a constant speed. The fabric of space should be no different. Hence, spacetime supports light at only speed c.​

The big question, which is yours here, is WHY is it c in the first place?

No one knows yet! If I were to make an educated "guess", I would venture that it has something to do with the expansion of spacetime. That the speed at which light goes by is the speed of spacetime.​

But then, what do I know :-)

pess

#### GautamAishwarya

kant said:
In regard to einstein s second postulate on special relativity:

The speed of light is the same in all frame of reference.

____________

Can physics explain why this is true?
the light postulate has been exhaustively verified a number of times. a good example is comparison of speed of light emitted by high speed pions in an accelerator and that by a stationary source. interestingly, both were found to be equal.

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