High School Light Behaviour at High Speed: Explained

[Daveopg]

First, I am way out of my field of understanding here so please keep it simple. I watched some videos on E=MC2 which led to how light reacts differently than matter at high speeds causing time to slow down when moving fast.
My question is, if I was to shine a flashlight perpendicular (90 degrees) to the direction traveled am I correct to say if I was moving at half the speed of light the beam would actually be at a 45 degree angle and when traveling at the speed of light the beam would be horizontal (0 degrees). This would also be true whether the beam was inside or outside of the spacecraft , correct?

 

[Ibix]

The most important thing to realize is that there's no such thing as "moving fast". You can always regard yourself as stationary, and there is no experiment that will say differently. You can only be moving fast or slow relative to something else, so all of the relativistic effects you read about are things I determine happen to you and you determine happen to me, but we never directly experience them ourselves. So "light reacts differently than matter at high speeds" doesn't make any sense. Nor does it make sense to talk of whether the light beam is "actually" moving at one angle or another - there's only your description and mine. Both are equally valid.

Secondly, it's contradictory to talk of an object with mass traveling at the speed of light. It cannot happen.

However, you have your question more or less right. If you shine a laser perpendicular to what I would call your direction of motion then I will say that the beam keeps up with you (you won't see anything unusual). But if you are doing 0.5c I must measure the beam at 60° (not 45°) to your direction of travel, so that its velocity is c and the component parallel to your direction of travel ($c\cos 60$) is 0.5c.

 

[PeroK]

My question is, if I was to shine a flashlight perpendicular (90 degrees) to the direction traveled am I correct to say if I was moving at half the speed of light the beam would actually be at a 45 degree angle and ...

Unwittingly you've assumed an absolute frame of reference here. There's no such thing as "actually" moving at a 45 degree angle. Your trigonometry might be not quite right either.

For example, at what speed and in which direction are you moving now? Relative to your computer, the centre of the Earth, the Sun, the centre of the galaxy, the Andromeda galaxy ...?