Relativity question

Post by **matthew1** » 04 Aug 2020, 11:22

You're in a ship traveling away from Earth at 50% the speed of light, relative to Earth.

You turn on a bright light pointing at Earth.

How long does the light take to reach Earth? Say you're one light minute away from Earth, traveling at a constant velocity, for the sake of argument.

I don't know the answer, I'm asking.

I know the light will appear red-shifted when viewed from Earth, but will it take twice as long because the ship is moving .5 speed of light? Or is C an absolute (IIRC, Einstein said this is the case)?

Post by **pgill** » 04 Aug 2020, 15:58

The short answer is the speed of light in a Vacuum is constant.

The speed limit isn't affected by velocity of the source.

If you really want to brake your brain then you need to ask what happens if a particle exceeds the speed of light (think particle accelerator)

For this second example you get Braking Radiation (Bremsstrahlung to be exact)

https://en.wikipedia.org/wiki/Bremsstrahlung

https://en.wikipedia.org/wiki/Synchrotron_light_source

Post by **matthew1** » 04 Aug 2020, 21:42

Thanks Paul. Very interesting. It's hard to wrap my mind around this constant. i.e.: that you could be traveling so fast away from something yet that speed won't affect the speed of light it's emitting. I enjoy thinking about how this stuff happens whether I wrap my mind around it or not

.

I've been reading a lot of Alastair Reynolds, who writes (mostly) hard science fiction, and these concepts come up frequently in his books.

Post by **j-dawg** » 11 Aug 2020, 20:16

Everything in special relativity is tied together by all parties observing light traveling at precisely c, no matter the source and target. All the weird space travel effects you read about in sci-fi - such as the time dilation effect that plays such an important role in the Ender's Game books - follow from this.

It is hard to wrap one's mind around. At the moment one understands it one asks why this seemed so hard before (at least for special relativity, which you can understand conceptually without doing too much math). If you can find a copy of TM Helliwell's Special Relativity, which seems to be out of print now, it's a great intro that's easy to follow and really helps you build an intuition for it. I've completely forgotten SR, but I remember that book being the only reason I was able to pass the class. If only I could find my copy.

Post by **manovlov** » 07 Sep 2020, 13:14

What brand, the ship ?

Post by **abscate** » 07 Sep 2020, 14:09

If it’s a p80, light is stopped p0455 code

Post by **manovlov** » 08 Sep 2020, 02:49

Post by **gnalan** » 25 Nov 2020, 17:39

Mind if I tweak your question a bit?

You're in a Volvo driving on the freeway at 75mph. You have 150 miles left to drive, but only 15 minutes of daylight left before it's pitch black out.

If you reach over and flip your headlight switch on do your headlights come on?

Post by **volvolugnut** » 25 Nov 2020, 17:48

When you drive 75 mph in lesser cars, you may not see the light from your headlights because you have over driven the light beam. However, if your Volvo headlights do provide light when you turn them on, your headlight wiring may have crumbled.
volvolugnuty

Post by **RickHaleParker** » 26 Nov 2020, 00:41

matthew1 wrote: ↑04 Aug 2020, 21:42 yet that speed won't affect the speed of light it's emitting.

That is why a blue or red shift does happens. The ship's velocity adds or subtracts space not velocity to the wave. That is why the word length is in the term Wave Length.

IF the ship velocity did effected the speed of light there would be no shift because the wave would occupy the same length of space it did in the ship, therefore no change in wave length.