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LostInParadise's avatar

Anybody here able to help me with this question regarding relativity?

Asked by LostInParadise (30436points) 1 week ago

I have been doing some reading trying to at least make some sense of the special theory of relativity, which is said to be much easier to understand than the general theory.

Here is my question. Suppose I have two flashlights, one pointing left and the other pointing right. I flash a signal from the two of them at the same time. After a second goes by, one light signal will be 300,000 km to my left and the other will be 300,000 km to my right, so the distance between them is 600,000 km, meaning that they are traveling twice the speed of light relative to one another. What have I got wrong?

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16 Answers

kritiper's avatar

The speed of light, no matter what your perspective is, remains the same.
What you think about the speed of light being twice as fast IN RELATION TO ONE ANOTHER is correct. But as far as you yourself are concerned, both beams of light are travelling 300,000 KPS.

flutherother's avatar

The distance between the light signals is 600.000 km relative to you but not to the light signals as for them the distance is compressed and time is dilated due to their motion.

RayaHope's avatar

Each light is still going the same speed. You have a flaw in thinking it is twice the speed because you are measuring the entire distance traveled by both individual light sources, that is wrong.

LostInParadise's avatar

@flutherother , So you are saying that it is okay for me to speak of their relative velocity as being twice the speed of light, but not from the point of view of the two light beams.

@RayaHope , If the distance between the two light beams starts as 0 and goes to 600,000 km in a second then their relative speed is twice the speed of light. It is the same idea as two cars traveling in opposite directions, each traveling 60 mph relative to me. They would be separating from each other other at 120 mph from my point of view.

seawulf575's avatar

You have 2 separate light sources. The photons are not from the same source. They are going in opposite directions but still have nothing to do with each other. You are the divider. You are the starting point for both. Take either, see how far they go in one second and you have the speed of light. But both are starting from different points (the separate flashlight bulbs).

RayaHope's avatar

@LostInParadise Yeah but the light source is from two different places. It is NOT from the SAME source.
Both cars are still only going 60 mph each. The cars would cover twice the distance (as one car) but still at 60 mph.

Caravanfan's avatar

The speed of light through a vacuum is constant no matter what your frame of reference is. So if you are moving at 99% of the speed of light and you point a beam of light forward and backwards, they will travel away from you at the speed of light. An observer at rest relative to you will also see the beams of light moving at the speed of light.

And here is a freaky thing. A photon experiences no time. So let’s say you see the light from a star from a galaxy 15 million light years away. From your point of view the photon took 15 million years to hit your eye. From the photon’s point of view no time has passed whatsoever.

filmfann's avatar

Let me change the example, to simplify your understanding.
Instead of beams of light, let’s use cars the can travel light speed.
A passenger in either car will look backwards, and see the other car still standing next to the point where they were next to each other.
The light from that car is traveling along with the other car.

JLoon's avatar

You’re not crazy, but you’re not exactly right either.

The fact is no one has actually measured the speed of light in a standard laboratory experiment – because it’s physically impossible :

“Specifically, relativity forbids you from measuring the time it takes light to travel from point A to point B. To measure the speed of light in one direction, you’d need a synchronized stopwatch at each end, but relative motion affects the rate of your clocks relative to the speed of light. You can’t synchronize them without knowing the speed of light, which you can’t know without measuring.”

So implying that there could be variations in the theoretically constant 186,282 mps speed of light may not be out of line – up to a point. But your in your example light speed doesn’t double just because you’re firing 2 beams of light in 2 different directions at once. That result seems logical if you use regular Newtonian algebra – but it’s wrong in the physics of Einstein’s special relativity. That’s because length and the rate at which time passes both change appreciably as the object’s speed approaches the speed of light, meaning that more variables and more complicated mathematics are required.

Recognizing the mistake is the easy part though. Learning from it is something else. And everyone is still working on that part :

Caravanfan's avatar

@JLoon The guy who publishes Universe Today is a friend of mine.

And just to clarify, it’s length in the direction of travel. And time passes normally for the individual who is travelling near the speed of light. It’s just that the observer at rest sees the clock moving slower in the craft that is travelling towards lightspeed.

JLoon's avatar

@Caravanfan – Glad to be corrected by someone who knows someone who knows ;-)

Caravanfan's avatar

@JLoon I edited to say “clarify” because nothing you wrote was wrong.

JLoon's avatar

@Caravanfan – I’ve gotta be wrong about something. My reputation is at stake ;D

Caravanfan's avatar

@JLoon Tragically, it’s not this.

flutherother's avatar

@LostInParadise Their velocity relative to you is the speed of light, their velocity relative to each other is also the speed of light. You have to be consistent with your frames of reference. The 600,000 km isn’t fixed and it exists only in the observer’s frame of reference you can’t use it to work out the speed of the light signals relative to each other.

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