Is the speed of light really constant ?

Gordon Gekko · Feb 7, 2004

Quote from jem:

aphexcoil. In school I always questioned my physics professors about the concept that two spaceships travelling at 99C shine a light at each other and the light still moves at C. No one ever answered my questions satisfactorily. Is that because as you state an observer only sees one C but for light it gets there instantly. Perhaps none of my professors knew enough to explain the observation vs getting there instantly thing.

Are you sure about light can travel a distance of 10 light years instantly?

Two, if light is not a constant does that effect time. Would the earth be older or younger if light is slowing down?

I just saw the other posts perhaps my professors were correct. Nevertheless I would like to find out the question about if light is slowing down do we have to recalculate things like the age of the earth, and the size of the universe.

http://www.elitetrader.com/vb/showthread.php?s=&postid=327003#post327003

maxpi · Feb 7, 2004

Those measurements of the speed of light done in the late 19th and early 20th century had a funny characteristic in that if you average them over time to avarage out the "noise" of measurement errors, the speed of light was slowing down. It did not stabilize until "science" started using the atomic clock as the measure of time around 1965. If light is slowing then probably the atomic clock is slowing at the same rate and therefore "Einstein was relatively right" for a few decades!!

Max

marketsurfer · Feb 7, 2004

Quote from inandlong:

Aphie I don't understand your logic here. It seems that you are saying observed speed and actual speed are two different things.

A light-year is an actual measure of distance that assumes a constant speed. The distance is not covered instantly. It is covered in a year. If I am cruising at the speed of light, it will take me a year to get to something one light year away. And it will feel like a year, not an instant.

if one travels at the "speed of light" time stands still for the traveler making any distance traveled feel like an instant. however, those observing the traveler will still be in the constraints of time-- a year in this case. isn' this the basis of einsteins theory ? i think aphie is correct in this case.

best,

cold surfer

ARogueTrader · Feb 7, 2004

So you know how it "feels" to travel at the speed of light?

Quote from marketsurfer:

if one travels at the "speed of light" time stands still for the traveler making any distance traveled feel like an instant. however, those observing the traveler will still be in the constraints of time-- a year in this case. isn' this the basis of einsteins theory ? i think aphie is correct in this case.

best,

cold surfer

marketsurfer · Feb 7, 2004

Quote from ARogueTrader:

So you know how it "feels" to travel at the speed of light?

i wish.

surfer

Gordon Gekko · Feb 7, 2004

Quote from ARogueTrader:

So you know how it "feels" to travel at the speed of light?

so you have a personal relationship with the creator of the universe?

LOL.....i find that a little hard to believe.

omcate · Feb 7, 2004

Quote from marketsurfer:

if one travels at the "speed of light" time stands still for the traveler making any distance traveled feel like an instant. however, those observing the traveler will still be in the constraints of time-- a year in this case. isn' this the basis of Einstein's theory ? i think aphie is correct in this case.

best,

cold surfer

Time dilation and length contraction are always fascinating.

Both relativity and quantum mechanics give many predictions that are counter-intuitive and yet supported by experiments. I guess it is becoming more and more difficult to understand nature.

TGregg · Feb 8, 2004

Quote from aphexcoil:

If you are in a spaceship that is going the speed of light, you would reach another star that was 10 light years away instantly, but people on Earth observing your trip would notice it took you 10 years to make the trip.

Since there is no universal time, it depends on the reference point of the observer.

One thing I always wondered about that. To the space traveller, it appears as if Earth is blasting away at the speed of light, while his destination flies forward also at the speed of light. Shouldn't that mean that the "observer" in this case sees the Earth spend 10 years in travel?

Another way to explain this question is the typical science thing we all got in school. One twin jumps on a rocket and flies around for a while at a significant percentage of light speed. Then he comes home to find his Earth-bond twin to be much older than he is. But from the Rocketship twin's point of view, the Earth was doing all the travelling, so shouldn't he expect to see his twin younger than him?

bdixon619 · Feb 8, 2004

Quote from TGregg:

One thing I always wondered about that. To the space traveller, it appears as if Earth is blasting away at the speed of light, while his destination flies forward also at the speed of light. Shouldn't that mean that the "observer" in this case sees the Earth spend 10 years in travel?

Another way to explain this question is the typical science thing we all got in school. One twin jumps on a rocket and flies around for a while at a significant percentage of light speed. Then he comes home to find his Earth-bond twin to be much older than he is. But from the Rocketship twin's point of view, the Earth was doing all the travelling, so shouldn't he expect to see his twin younger than him?

The rocketship twin is returning to the frame of reference of the earth twin. The faster one travels the less time it takes to travel. Someone said the same thing on the other thread using jet travel and time zones.

http://www.phy.ntnu.edu.tw/java/relativeVelocity/relativeVelocity.html

aphexcoil · Feb 8, 2004

Quote from TGregg:

One thing I always wondered about that. To the space traveller, it appears as if Earth is blasting away at the speed of light, while his destination flies forward also at the speed of light. Shouldn't that mean that the "observer" in this case sees the Earth spend 10 years in travel?

Another way to explain this question is the typical science thing we all got in school. One twin jumps on a rocket and flies around for a while at a significant percentage of light speed. Then he comes home to find his Earth-bond twin to be much older than he is. But from the Rocketship twin's point of view, the Earth was doing all the travelling, so shouldn't he expect to see his twin younger than him?

This is one of the harder questions that I've researched and it took some time for it to sink it. All of this is anti-intuitive. I will give you a very quick answer and then point you to an excellent site that does a much more complete job of explaining this "paradox."

I was told that the simplest way to explain this problem is the two frames of reference are not equal. The person who experiences "acceleration" is the one actually doing the traveling. Although the spaceship twin could look at Earth and say that it is moving away, in reality the people on Earth will not feel that acceleration and therefore there is a difference between the two frames.

That isn't the best explanation, but here is a website to get you started.

The best way for anyone to begin to understand this subject is to realize two very important things.

a) There is no universal time
b) Space / Time are deeply connected and not completely seperate

http://mentock.home.mindspring.com/twins.htm