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question about flying close to c

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ThinLizzy

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Hi guys!

Please help me understand something about relativity.

Let's assume I am flying from the Earth to a star which is 8 ly away. As I understand it, if I fly very fast (like .9999 the speed of light), then I could get there in, let's say, 1 year. My question is how can I get there in 1 year, if light which is the fastest thing in the universe takes 8 years to travel from that star to the Earth?

Thank you!

PS: I am a philologist, don't laugh at my "innocence" :)
 
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ramparts

Guest
It would be a year (or some similar amount of time) from your perspective. The reason is that distance isn't absolute - the distance you measure between two objects depends on your speed relative to those objects. As you approach the speed of light, distances shrink, so an 8 ly separation as seen from Earth becomes a much smaller separation as seen by you.
 
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Mee_n_Mac

Guest
ThinLizzy":3briafsr said:
Hi guys!

Please help me understand something about relativity.

Let's assume I am flying from the Earth to a star which is 8 ly away. As I understand it, if I fly very fast (like .9999 the speed of light), then I could get there in, let's say, 1 year. My question is how can I get there in 1 year, if light which is the fastest thing in the universe takes 8 years to travel from that star to the Earth?

Thank you!

PS: I am a philologist, don't laugh at my "innocence" :)
The problem is you can't get there in a year, at least as far as someone watching from the Earth is concerned. Since you're going slower than light speed it'll take the ship longer that light would to get there, something in excess of 8 years. So that part makes sense.

Where it gets confusing is if you measure the time to get there with a clock onboard the ship. Special relativity says that time slows down for things travelling at appreciable fractions of the speed of light. The onboard clock actually runs slower than the one on Earth; for your 0.9999C example, about 71x slower (if I haven't screwed up the math). So for a person onboard the ship it will be less than a year of "subjective" time while over 8 years will have passed in the outside world. Which is the correct answer ? That's the really odd thing ... they both are correct. It just depends on where you are. The rate at time passes is not a constant ... which seems at odds with our everyday commonsense experience. Yet it's how the universe works.

http://en.wikipedia.org/wiki/Time_dilat ... e_velocity
 
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orionrider

Guest
Check this relativity calculator: http://www.1728.com/reltivty.htm

Very interesting if you plan a vacation on Gliese 581g ;)
If you leave you stepmother on Earth, notice how she will be long dead when you come back. But don't forget to buy a ticket for your wife. :? Or not. :lol:
 
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ThinLizzy

Guest
Great, thank you for your answers. So distance literally shrinks and time literally slows down if I am in the spaceship... wow! It's hard to wrap my mind around it.

But here's one more thing I can't picture. When you say that distance shrinks, does it mean that my ship shrinks, I mean do the atoms in my ship shrink? Is that even possible? Or does it just look like my ship shrinks to the person on Earth?
 
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orionrider

Guest
distance literally shrinks and time literally slows down
Some galaxies are receding at a good fraction of c, but the aliens don't feel a thing. In fact, I'm quite certain they say that WE are receding at incredible speeds while THEY are floating at rest. That is because the ship or the galaxy are not contracting/slowing time; it is SPACE itself that is changing.

Think of the effect that way: you fly close to 300,000 km/sec, which means you pass 300,000 km for each second. At this rate, the math (V=D/T) says it will take 4 years to reach Proxima Centauri. Yet you arrive after only one year on the ship's clock... It only computes if the kilometers became smaller or if the seconds became shorter, or both.

I think you will be interested by this link (thank you SpeedFreek). It's about the so-called 'big-bang', but you'll get the picture: http://www.mso.anu.edu.au/~charley/pape ... sSciAm.pdf
NB: the first page on this pdf is blank, you have to scroll down. ;)

All these things are related: the bigbang, the 'center' of the universe (or lack thereof), the speed of light, gravity, energy and mass, etc.
 
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ArcCentral

Guest
ThinLizzy":2mjn9d8s said:
Great, thank you for your answers. So distance literally shrinks and time literally slows down if I am in the spaceship... wow! It's hard to wrap my mind around it.
I'm very doubtful that the distance shinks, you will still travel 8 light years distance, and I don't think that time slows down, but the markers for time do, rather the rate the individual fundamental entities that make up all matter go about their cycles.

But here's one more thing I can't picture. When you say that distance shrinks, does it mean that my ship shrinks, I mean do the atoms in my ship shrink? Is that even possible? Or does it just look like my ship shrinks to the person on Earth?
There is a length contraction that takes place, such that if you were 6 feet tall and headed toward a star at close to light speed, wherein your head was facing toward the star, and feet away, you might contract down to and inch tall for instance.
 
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orionrider

Guest
you might contract down to and inch tall for instance.
YOU are not contracting, SPACE is. Inside this ship, you are still 180cm tall. Only the 'centimeter' has changed relative to the same unit on Earth. As far as you're concerned, it's the world outside the windows that is expanding.
 
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csmyth3025

Guest
orionrider":2tpe8zgf said:
you might contract down to and inch tall for instance.
YOU are not contracting, SPACE is. Inside this ship, you are still 180cm tall. Only the 'centimeter' has changed relative to the same unit on Earth. As far as you're concerned, it's the world outside the windows that is expanding.
...It's the world outside the windows that is shrinking (along your axis of travel), I think.

Chris
 
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origin

Guest
csmyth3025":jfarqwau said:
...It's the world outside the windows that is shrinking (along your axis of travel), I think.

Chris
Yes, to the observer on the space ship the universe has flattened along the axis of travel. PLanets look like pancakes and your distance of travel would seem impossibly short.

Of course to someone watching you fly by, your ship would appear to be a flying flap jack, with the flat part facing the axis of travel.

edit to add - of course this not really see planets as pancakes because the light would be red and blue shifted and you would begin to only have a 'tunnel' in front of you that you would be able to see.
 
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emperor_of_localgroup

Guest
ThinLizzy":29cgqtur said:
Hi guys!

Please help me understand something about relativity.

Let's assume I am flying from the Earth to a star which is 8 ly away. As I understand it, if I fly very fast (like .9999 the speed of light), then I could get there in, let's say, 1 year. My question is how can I get there in 1 year, if light which is the fastest thing in the universe takes 8 years to travel from that star to the Earth?

Thank you!

PS: I am a philologist, don't laugh at my "innocence" :)

If an earthling keeps the time, he would find it takes 8.0005 years to reach the star.
If a spaceman on the flying ship keeps time, he would find it takes .113141 year to reach the star,
because to him the star is only .11313 LY away.

Anyone can correct my numbers, I'm not 100% sure.
 
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csmyth3025

Guest
emperor_of_localgroup":6qr2fp2e said:
ThinLizzy":6qr2fp2e said:
Hi guys!

Please help me understand something about relativity.

Let's assume I am flying from the Earth to a star which is 8 ly away. As I understand it, if I fly very fast (like .9999 the speed of light), then I could get there in, let's say, 1 year. My question is how can I get there in 1 year, if light which is the fastest thing in the universe takes 8 years to travel from that star to the Earth...

If an earthling keeps the time, he would find it takes 8.0005 years to reach the star.
If a spaceman on the flying ship keeps time, he would find it takes .113141 year to reach the star,
because to him the star is only .11313 LY away.

Anyone can correct my numbers, I'm not 100% sure.
Those numbers look about right. From the viewpoint of the earthling, the travel time would be (8 LY/0.9999 LY/year)=8.0008 years. The relativity calculator link provided by orionrider is here:http://www.1728.com/reltivty.htm
Putting 0.9999c in the input box yields a relativistic ("shrinkage") factor of 70.71. (8 LY/70.71)=0.113138 LY (the distance the space traveler would "see" the star as being away from the Earth). This apparent distance divided by 0.9999 LY/year gives the space traveler a travel time of 0.113149 years.

Chris
 
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