Speed of Light vs Light Years

[jacksnap]

Hi,

This is probably a beginners question, but ill ask it anyway as it has me confused.I have read quite a few posts from clever people here (so I assume its correct) that the closer to light speed you get, the slower time gets, and also distances get shorter.
Effectively meaning that, say for a photon, the universe is very small in its direction of travel and can be traversed in, practically, no time at all.

Then, why do we always measure distances in light years? we say the Andromeda Galaxy is roughly 2.5 million light years away, so traveling at the speed of light it would take us 2.5 million years.
Dont these 2 statements conflict, surely if we went at the speed of light (which I know is impossible) we would get there in a VERY short time.

Also if the first statement is true, then why does it take time for light from the sun to get to us etc.I suspect its to do with the reference frames, and perhaps the zero time is what the photon 'feels', but from our reference frame it takes the longer time?
I personally would love to measure everything from the frame of the object moving, but I guess humans want to measure everything from their own reference frame and that's why we say its 2.5 million years.

Any extra info would be great.

Jason

 

[Doc Al]

I have read quite a few posts from clever people here (so I assume its correct) that the closer to light speed you get, the slower time gets, and also distances get shorter.

That's not quite right. A better way to say it might be: Moving clocks will be measured to run slowly, moving objects/distances will be measured to have contracted along the direction of motion.

So, if you are traveling very fast with respect to Earth, in a spaceship say, then to you time (and your clocks) run just like they always do. But if you were to make measurements (using your onboard clocks) of the Earth clocks, then you'd say that Earth clocks were running slow. And if you measured the distance between Earth and Mars, say, you'd say that the distance was shorter.

Effectively meaning that, say for a photon, the universe is very small in its direction of travel and can be traversed in no time at all.

Let's leave the photon out of this, since it doesn't have a reference frame. But your point is correct: If your spaceship could reach a speed of 0.999c, then you could traverse ordinarily great distances in a very short amount of time as measured by your clocks.

Then why do we always measure distances in light years, we say the Andromeda Galaxy is roughly 2.5 million light years away, so traveling at the speed of light it would take us 2.5 million years.

According to Earth clocks, not according to your spaceship clocks. According to your clocks, it wouldn't take much time to get there.

I personally would love to measure everything from the frame of the object moving, but I guess humans want to measure everything from their own reference frame and that's why we say its 2.5 million years.

To avoid insanity, we naturally measure things with respect to ourselves. (Just like folks on the spaceship would naturally measure time and distance using their moving spaceship clocks and metersticks.) Of course, we know that things will appear quite different if we were moving with respect to the Earth.

 

[Entropia]

,

Thank you for your question. It is a common misconception that the speed of light and light years are interchangeable terms. In reality, they represent two different concepts.

The speed of light, denoted by the letter "c", is a fundamental constant in physics and is the fastest speed at which energy, including light, can travel through space. It is approximately 299,792,458 meters per second. This means that if you were to travel at the speed of light, it would take you about 8 minutes to reach the sun, which is about 93 million miles away from Earth.

On the other hand, a light year is a unit of measurement used to describe distances in space. It is defined as the distance that light travels in one year, which is approximately 9.46 trillion kilometers. So, when we say that the Andromeda Galaxy is 2.5 million light years away, it means that it would take light 2.5 million years to travel from the galaxy to Earth.

Now, coming to your question about time dilation and the apparent contradiction with light years. Time dilation is a phenomenon predicted by Einstein's theory of relativity, where time appears to slow down for an observer moving at high speeds. This means that from the perspective of a photon traveling at the speed of light, the universe would appear to be very small and distances would seem to be traversed in no time. However, from our perspective as stationary observers, light still takes 2.5 million years to reach us from the Andromeda Galaxy.

This is because we are measuring the distance and time from our own reference frame, which is different from the reference frame of the photon. This is why we use light years as a unit of measurement, as it takes into account the time it takes for light to travel a certain distance.

In conclusion, the speed of light and light years represent different concepts and cannot be used interchangeably. The apparent contradiction with time dilation is due to the different reference frames from which we are measuring distance and time. I hope this helps clarify your confusion.