d3f1

Ok, so this is where I started to puzzle myself last night. Light travels at... ( duh ) the speed of light, and it has been proven that the faster something is moving the slower time moves for it. So in the time that it takes for light to move from the sun to earth, approx 8 minutes in our time, exactly how old would that light be in its time ?!

saut

An interesting thought. I'm not much of a physics buff, so I can't answer your question. However, I have another one: Where does light go when it's dark? IE: You shut off the lights in your house, where does that light go? It just disappears?

FaderMonkey

I have no idea, but I'm curious.

I would try asking in Tilted Knowledge...you might find a quicker answer there.

iamtheone

If light is travelling at 'light speed' C within the Universe, and the Universe is moving in direction A, then light moving within the Universe along the line of the Universe' travel - A - would be travelling at C + A, therefore, faster than light speed......?

And any travelling in the opposite direction would be C - A...therefore less than light speed......?

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iamtheone

light is constant.

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tecoyah

As a photon (the quanta of light) carries no mass, it likely is uneffected by relativity as far as time goes. Light may not even exist in the fourth dimention for all we know.

And when you shut off the lights in your house, the photons are still moving away from the source. You simply cannot see them anymore.

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supersix2

No no no no no.
That is wrong.
The situation you describe is incorrect for a variety of reasons.
1. The speed of light is constant therefore it does not change no matter what inertial frame it is it. Example:
There is a high speed train with a laser emitter mounted on it. It fires a laser at a sensor which measures speed. Then a person standing still on the ground fires a laser at the same sensor. The speed of the laser is still the same even though the train is moving very fast towards the sensor. The situation is true if the train is moving away from the sensor as well.

2. The situation you describe is putting the universe in a inertial frame and comparing it to an inertial frame outside the universe which does not make sense because it cannot be done.

Now to answer d3f1's question:




The equation for time difference is this:

∆T = ¥T

T is the relative time and ¥ is 1/squareroot( 1- beta ) and beta is velocity/speed of light or v/c.

Light travelling at c would make beta equal 1 which would make the denominator of the equation equal to zero making the functions undefined. This means that the time for light cannot be defined. And as tecoyah said, light is a photon and carries no mass and it is unlike anything else in the universe.

zer0signal667

No, it becomes dark because there are no longer light waves flying around the room and then into your eyes. The ones that existed the moment before shutting the light off are absorbed very quickly by various things (walls, floor, you, random molecules, anything.)

NoSoup

Ah, the mystery of light. If photons had no mass whatsoever, they wouldn't be affected by gravity, as gravity only affects things with mass. However, light is bent into black holes, so we either need to rethink our theory on gravity and/or light particles...

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BrassSteve

speed moves at about 300,000 KM/second. and way i understand it, when going at light speed, the travel is basically instintanious. so if you were to go to the sun from the earth, it would be instintanious for you, but 8 minutes in time would have passed. this is how i think time travel could happen, but only going forward in time, not back

supersix2

Actually Einstien proved that if you could travel at the speed of light you could travel backwards in time. I'll look into this more.

d3f1

Ok, if something travels at the speed of light then it can go back in time...? So light is exactly where on the timeline of the universe !?

llama8

He actually didn't prove this. He just didn't disprove it! A VERY BIG difference! You would also need to travel faster then the speed of light (an impossibility?) to experience this "negative" time of arriving somewhere before you've left. A nice paradox to get to grips with I should imagine!

The big point is that everything is relative. There is no"absoloute" time line. The photons experience their own "personal" timeline as do you and everyone else. If your speed relative to someone else alters then also your individual concepts of time will. Luckily this effect only occurs at near to the speed of light or else it could lead to many problems in our daily lives

There are many interesting paradox associated with Special Relativity. Any good Undergraduate Physics text book can help or google a few. Try and get to grips with "the twin paradox" if you're interested.

BrassSteve

i dont think anything can go faster than speed of light.... i do know that there are galaxies billions of light years away that are moving away from us at speeds faster than the speed of light, but thats just because of the expansion of the universe... i think

Munku

You realize you contradicted your "nothing can go faster than speed of light" statement, with the last half of your post..

Nefir

I don't have very deep knowledge about space-time, time dilation, etc., but I'll take a stab at it:

Time is a very human concept. For light, traveling at the speed of light, time simply doesn't happen (at least not "our" time, that is, the time which we perceive) because displacement in time is being completely converted into displacement in space (as in space-time).

If a spaceship was built that could go at the speed of light without destroying itself, its occupants could arrive at their destination, then turn around and come back to Earth, and no time would have passed.

Am I close to the mark or did I just totally miss something important?

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llama8

Time dilation occurs in both frames.

I think time would appear to stand still on the spaceship (or atleast approach 0). Where as on Earth time would still continue to "flow" as you would expect for everyone else.

BrassSteve

well, the galaxies themselves are not moving away from us at speeds faster than speed of light. the universe itself is expanding, and the farther away a galaxy is from us, the faster it is moving away from us.

astronomers can tell how far away a galaxy is from the redshift. the greater the redshift, the faster it is moving away from us, the farther away it is. they could the use hubbles constant to guess how far away it is

nash

A person on the spaceship would experience time exactly the same way he did before he got on the spaceship. One second while traveling on the spaceship would be the same "one second" that he was used to back on earth. No one experiences time any differently whether on earth or on a near-light velocity spaceship.

Anyhow, it's really not all that hard to just google it and read about it:
http://en.wikipedia.org/wiki/Special_relativity
http://en.wikipedia.org/wiki/Twin_paradox

Have fun!

llama8

Sorry I meant to someone viewing the spaceship from Earth. The time alapsed on the spaceship would appear to have slowed down to something approaching stationary.

They are some good links.