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What's wrong with this Hypothesis?


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Lets say you had a star 5 Light Years (LY) away. Right next to the star you have a space ship that is just as bright as the star itself. Both visible to the naked eye.

If the spaceship were to travel at the speed of light toward Earth then would you see It's entire journay at once? Here's my thinking.

When the ship is at 5LY away the photons that leave the ship will take 5 years to get here as will the ship if it's travelling at the speed of light.

One year later it is 4LY away. The photons that left ship at the start will have 4 years until they reach earth, as will the photons that are just leaving the ship at that point.

This applies for each year/month/day/hour/minute/second that the ship is journeying so surely all those photons would reach earth at once?

Edited by Ganymede12
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http://what-if.xkcd.com/1/

That makes sense. I love being wrong, it gets me one step closer to being right!

Good. So it's safe to try this out for myself then?

I think the light emitted in the forward direction would be blue shifted to infinite energy, so you would vaporize anyone trying to observe the answer to your question.

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I think the light emitted in the forward direction would be blue shifted to infinite energy, so you would vaporize anyone trying to observe the answer to your question.

I don't get what you mean? Doesn't the speed of light remain constant no matter what, therefore It can't be any more blue shifted than the light from a stationary source?

Edited by Ganymede12
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I don't get what you mean? Doesn't the speed of light remain constant no matter what, therefore It can't be any more blue shifted than the light from a stationary source?

It stays at the same speed, but it's frequency can change. The same way sound goes up when an ambulance approaches, but the sound waves still travel at the same speed.

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Receding objects redshift light, that's how we know the universe is expanding.... Approaching objects blue-shift the light - so can see for example we are on a collision course with M31!

It is the same with sound - the ambulance siren is higher pitch when the ambulance is approaching you, but the sound is still going 330m/s.

EDIT.... typed too slowly.

Edited by Ags
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Ant, if an object in space is travelling towards you it will be blue shifted, and if its travelling away from you it will be red shifted. therefore the light from the ship will be blue shifted but no one would be vapourised because photons carry zero mass, so there will be zero momentum because of Momentum = Mass x Velocity, so if you multiply the speed of light by zero you get zero momentum and no one dies:D

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Ant, if an object in space is travelling towards you it will be blue shifted, and if its travelling away from you it will be red shifted. therefore the light from the ship will be blue shifted but no one would be vapourised because photons carry zero mass, so there will be zero momentum because of Momentum = Mass x Velocity, so if you multiply the speed of light by zero you get zero momentum and no one dies:D

Well not quite

  • photons have momentum, proved by the Compton experiment as well as theoretically.
  • if you blue shift something enough it turns into x-rays or gamma rays, and they are pretty harmful!
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About 2.5 million light years away.

I think I see what your getting at. Try the experiment using Andromeda. Unfortunately it isn't moving fast enough. It's just like JulianO said:

You can't travel at the speed of light, unless you're a photon or similar - so you can't do the thought experiment.

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Well not quite

  • photons have momentum, proved by the Compton experiment as well as theoretically.
  • if you blue shift something enough it turns into x-rays or gamma rays, and they are pretty harmful!

So maybe I should get a hazmat suit for the experiment?

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Well not quite

  • photons have momentum, proved by the Compton experiment as well as theoretically.
  • if you blue shift something enough it turns into x-rays or gamma rays, and they are pretty harmful!

ok if we are going that deep into things photons have no rest mass, I still dont think the gamma rays from the light emitted by a space ship will vapourise us? oh unless? we assumed the same luminosity as a star didn't we, not sure in that case? :D

I should know how harmful X-rays are I'm a Radiographer:D

Edited by starfox
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It stays at the same speed, but it's frequency can change. The same way sound goes up when an ambulance approaches, but the sound waves still travel at the same speed.

think thats the doppler effect see as explained on the big bang theory Edited by dtr42
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