# Boring, probably stupid questions about light

**URL:** <https://boards.straightdope.com/t/boring-probably-stupid-questions-about-light/169373>\
**Category:** Factual Questions\
**Created:** [April 17, 2003, 5:57pm UTC](https://boards.straightdope.com/t/boring-probably-stupid-questions-about-light/169373 "2003-04-17T17:57:10Z")\
**Posts on this page:** 3\
**Page:** 2

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**Author:** ![MegaDave](https://avatars.discourse-cdn.com/v4/letter/m/db5fbb/32.png) [@MegaDave](https://boards.straightdope.com/u/MegaDave)\
**Post date:** [April 18, 2003, 4:39pm UTC](https://boards.straightdope.com/t/boring-probably-stupid-questions-about-light/169373/21 "2003-04-18T16:39:24Z")

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if you suppose that the above is correct, and you would have infinite mass, then it stands to reason that you should also have infinite gravity, since there is a direct link between the size of the object and the power of that objects gravity

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**Author:** ![SlowMindThinking](https://avatars.discourse-cdn.com/v4/letter/s/dc4da7/32.png) [@SlowMindThinking](https://boards.straightdope.com/u/SlowMindThinking)\
**Post date:** [April 18, 2003, 7:35pm UTC](https://boards.straightdope.com/t/boring-probably-stupid-questions-about-light/169373/22 "2003-04-18T19:35:03Z")

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> [@](#):
>
> As to how a perfectly rigid scissors (if such could exist, which they can’t) could transmit information FTL:

The pair of scissors I was referring to is Ashbless’ in which the scissors are pushed by rockets which are synchronized. I.e., the moment of closing is already known along the whole length of the scissors, as is the rate of closure. Therefore anyone at any point along the scissors can calculate the rate at which the “sciss” intersection moves. I don’t see how any information is transmitted by that pair of scissors.

> [@](#):
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> If you can transmit information FTL, then you can also transmit it back in time.

So the statment that relativity “really says that information can not travel faster in light” is, to really nitpick, not quite true? The information statement supposes that the principle of causality (macro and micro) hold, which special relativity, stricitly speaking, does not. (It seems to me that at one time there were some quantum field theories that allowed the principle of causality to be violated on some quantum scale.)

MegaDave, relativity contains two conceptually different types of mass. Inertial mass and gravitational mass. Inertial mass is the mass that relates to how hard it is to accelerate an object. The principle of equivalence relates the two types of mass.

Relativistic momentum differs from classical momentum by the 1/sqrt(1-v^2/c^2) term. You can look at an object’s inertial mass as the product of that term by the rest, inertial mass. That does not imply the gravitational force of the object increases.

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**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\
**Post date:** [April 18, 2003, 8:06pm UTC](https://boards.straightdope.com/t/boring-probably-stupid-questions-about-light/169373/23 "2003-04-18T20:06:53Z")

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> [@](#):
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> \*Originally posted by MegaDave \*  
> \*\*
> 
> i am sure if i looked, i could find dozens more cites, but i didn’t think it would be neccassary \*\*

It’s better to stick with quality over quantity. From the Physics FAQ:

[http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html](http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html)

> [@](#):
>
> Of the two, the definition of invariant mass is much preferred over the definition of relativistic mass. These days, when physicists talk about mass in their research, they always mean invariant mass. The symbol m for invariant mass is used without the subscript 0. Although the idea of relativistic mass is not wrong, it often leads to confusion, and is less useful in advanced applications such as quantum field theory and general relativity. Using the word “mass” unqualified to mean relativistic mass is wrong because the word on its own will usually be taken to mean invariant mass. For example, when physicists quote a value for “the mass of the electron” they mean its invariant mass.  
> A common example is the mistaken belief that a fast moving particle must form a black hole because of its increase in mass (see relativity FAQ article If you go too fast do you become a black hole?).

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