# how long would it take to travel a light-year?

Discussion in 'Astronomy, Exobiology, & Cosmology' started by spankyface, Nov 29, 2001.

1. ### Janus58Valued Senior Member

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A photon doesn't have a "perspective".

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3. ### paddoboyValued Senior Member

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Agreed...Just a thought experiment.

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5. ### Write4UValued Senior Member

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This could be true, but if there is infinite time dilation for a photon traveling at "c", what happens at superluminal speed?

Comes to mind two possible answers:
a) does anything traveling at "c" or faster, still experience infinite time dilation?
b) if time stands still at "c", might time go backward at faster than "c"?

This would be a logical result if the passage of time slows downs in relation to the rate of speed and stands still at "c". But then, what happens to time if the speed exceeds "c"?

Last edited: Jan 9, 2015

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7. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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I'm not sure the speed of the planet is most important. Janus58, if still reading may discuss, I hope. After all it the integrated force on the rocket ship, a vector, during the encounter that changes the speed of the rocket. Also interesting and possible important, is how close to the surface you get. Jupiter is not as dense as Venus but much more massive. My first intuitive guess, is you would have a greater "skim by" altitude (and thus safety) for the same speed increase with Jupiter . Janus58, surely knows, and I have never found him to be wrong.

8. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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You can use a planet to change the direction of flight regardless of its movement but if you want to increase (or decrease) the speed of the ship it is only the orbital speed of the planet that is used. Imagine a planet that is 'not moving' (yeah I know), as you coast towards the planet you would increase in speed due to the gravity, you would then swing around the planet and leave the orbit. The gravity experienced as you moved towards the planet would be exactly the same gravity that you experienced as you moved away from the planet, so there would not be any net gain in speed.

In reality when you use a gravity assist you are 'stealing' the momentum of the planets orbit and transfering it to your ship.

Here is a nice little explanation of Gravity Assist.

9. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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This is akin to asking what would happen if pigs could fly.

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10. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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Thanks. That is an excellent link. Makes it clear why planet's orbital speed, is so important, contrary to my guess. Obviously a far-a-way from surface pass, gets less speed boost than a close "near surface skimming" one does and link has little to say about that. So I am still wondering how the speed boost goes if safety (and navigation correction /controls) sets a minimum of say 10km at point of nearest approach. I am almost sure the speed gain at that closest instant would be greater for Jupiter, than for Venus, but completely undone in the "planet frame" as the space craft recedes from the planet, and also agree that when switching back to the "sun frame" you do just add orbital speed; But in the link's example, only `~60% of it is gained by the space craft.

As this 60% does depend upon how close you came, I still have wonders if a closer approach can not compensate for a slower planetary speed? - I don't see how if it is say 10Km for both cases, but still have this doubt.

11. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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=The OP asks: "with current technology, how long would a light-year-long trip take?" Obviouly that depends on the magnitude and duration of your acceleration and also on wheather or not you want to return to Earth. I'll assume you do, and that at the 0.5 Light year from earth (in the earth's frame) you turn the space craft 180 degrees (in neglect able time) and accelerated by with the constant magnitude acceleration you used on the out-bound leg. Furthermore I assume that acceleration is Earth's gravity, g = 980 cm s−2 = ~ 1.03c yr−1 = ~ c/0.97 yr for all the trip.

These values were used here: http://www.astro.cornell.edu/pdfs/relrocketderiv.pdf where a detailed analysis of a one way trip coming to rest at Alpha Centaurus with total travel distance of D = 4.5 light years. As they also start to decelerate at the half way point, I just added the 180 degree turn at the half way point to use their result but ending up back on earth with a soft landing in my posts here: http://www.sciforums.com/threads/time-travel-is-science-fiction.140847/page-71#post-3252458
During this 4.5 light year trip, the astronauts age 3.62 years, whereas their counterparts on Earth age 6.14 years. I. e. the traveling twin extends his death date 2.52 years later into the future. Never smoking, exercise, regular medical check-ups, and healthy diet, can do at least twice that, on average, compared to typical beer drinking "couch potato" watching TV, at millions of times less cost.

The maximum speed attained in this journey at half-way point is vmax = 0.954C, and the mean speed is <v> = 4.5/6.14 = 0.733 C.

The link gives equation (11) for adjusting these numbers to other distances, but I must stop now and return later to apply it. I just want to note that with hydrogen fusion power source the mass ratio of lift off mass to final stop mass is m(0)/m = ~ 2 × 10^13. I. e. You need for reactor's fuel more than half the mass of Jupiter, the largest planet, inside your fuel tank, at lift off from Earth! (even if it is only hydrogen, not a 50/50 mix of D/T.)

Thus, I don't think one could actually make a 1 LY instead of this 4.5LY trip. Obviously in a trip of only 1 LY you would never get an average speed as great so would not extend your life into the future even a year but might only need buy H2 fuel equal to say 5 % of Jupiter's mass, I think without grinding thru the numbers.
Also who would want to spend 4.5 years of their life cooped up alone in a space craft and billions of dollars just to post pone the day they die on by 2.52 years, when just taking care of your body as best as is now known can add 10 years to your life?
Also space has very low density Hydrogen atoms in it. Run into just one with >0.9 C and there will be at best a hole in your space craft the air is leaking out of. I'm to lazy to see how that energy compares with a high speed bullet, but bet it is more.

Last edited by a moderator: Jan 9, 2015
12. ### paddoboyValued Senior Member

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We are never going to achieve light speed or FTL directly.
But if we could ever master technology to enable us to warp and shape spacetime at our whim, certainly a perception of "c"or FTL travel could be achieved.
In actual fact then, it is spacetime doing the travelling, taking us along for the ride...just as the current expansion of spacetime is taking the galaxies along for a ride.

13. ### Write4UValued Senior Member

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Does a pig have the potential for flight? No potential, no action.

However, a hurricane has great potential, enough even to make a pig fly.

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14. ### Write4UValued Senior Member

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If you look up all the definitions of the word and definition of Potential, you will find that fundamentally "potential" is described as a "latent excellence or ability which may become expressed in reality".

When we exceed "c" things are no longer physical but meta-physical, where potential is no longer associated with properties of physical mass/energy, but as a probability factor. This is why we describe a photon traveling as a "probability wave function", which becomes expressed in reality as a particle of energy (light).

IMO, the fabric of the universe functions as a "probability wave" with (near) infinite potential for expression in reality.

15. ### SwiglordRegistered Member

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Correction, if you are referring to Gene Roddenberries idea of "warp drive" then warp 10 isn't possible. The highest possible warp achievable is warp 9.9 repeating.

16. ### Write4UValued Senior Member

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No, I was not referring to anything physical. That speed limit is @ *c*

Also, I am using the word *potential* in context of inherent but latent probabilistic abilities. No Sci-Fi.