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tfoster
2006-Nov-06, 03:08 PM
OK, here's a question I've been pondering.

Your friend is on a spaceship 50 lightyears away and takes off towards you at the speed of light. That speed is attained instantly, i.e., no gradual acceleration (I know that that is not possible but this is just a thought experiment).

In 50 years time our friend will reach Earth. You, armed with your unimaginably large telescope, can see the image of your friend 50 lightyears away (the light from that event just now reaching Earth). So you can see your friend standing next to you and simultaneously 50 lightyears away.

Halfway though his journey, his ship occupied a point 25 lightyears away from earth. The light from that event took 25 years to reach Earth, as does your friend's ship. Both will reach Earth 50 years from when he first took off (25 years for the first half of the journey and 25 years for the remainder). Similarly, the light from every "event" along the path of the journey will also arrive on Earth 50 years from the start of the journey.

So the instant your friend arrives on earth would you see a brief "light trail" that stretches for 50 lightyears, in effect a solid (but extremely brief) rod connecting you and the original position of the ship? Is this "accumulation of light" been observed in objects that are extremely far away of us (and moving extremely fast?).

To take the thought experiment one step further, imagine your friend is traveling in a faster-than-light ship. He would arrive before the light source of him leaving his original position, so that a few years down the road you could conceivably see your friend standing next to you and blasting off in his ship 50 lightyears away. As you continue to observe the ship it comes closer and closer until it lands, what happens then? Does his "image" land, walk over and merge into one with your "real" friend? Is this perhaps a paradoxical argument for the impossibility of faster-than-light travel? (kind of like "if time travel were possible then where is everybody"?).

I know time passes differently for observers moving at different speeds, but these are all observations made by one observer. A bit confusing I know, but the answer to the thought experiment can surely help one to have a better understanding of relativity.

Thanks all!

antoniseb
2006-Nov-06, 03:22 PM
If a spacecraft were somehow able to instantly accelerate up to the speed of light, and your friend was coming toward you, all light emitted by his/her craft would come to you in a flash of very high energy gamma rays (blue shifted considerably) all at once.

If the spacecraft were able to go faster than light, there would be some physics going on we don't understand, especially concerning the fate of photons it emits during its journey. However, there would be no such event such as the ghostly image of your friend joining the real friend walking up to you. The observed backward journey only covers the faster than light part, not the part after he/she lands and walks over to you.

tfoster
2006-Nov-06, 06:07 PM
Then I guess the question is what happens when the ship lands? Is the person (or the image of the person) in two places simultaneously?

As for the first part of your answer, any thoughts on what it would actually look like? I'm thinking a Han Solo-Millenium Falcon light streak thing, or perhaps the entire event cannot be seen, since you would see every point of the journey at the exact same time. And perhaps by "exact same time" we mean an interval of time that equates to a point of singularity (existing but unmeasurable). Meaning your friend would just "appear".

I guess another way of phrasing the question is this:
There are paradoxical arguments that would seem to imply that time travel is impossible (the Grandfather paradox for example), unless you resort to elaborate theories of alternate universes and timelines.
Are there also paradoxical arguments that would imply that faster-than-light travel is impossible as well?

antoniseb
2006-Nov-06, 06:14 PM
If faster than light travel were possible, there is no paradox in simultaneously seeing your friend here, and seeing the light he emitted earlier in his journey.

In a real world analogy, if a high powered rifle shot a bullet in your general direction and hit a rock near you, you would hear the impact to the rock, and then some time later hear the bullet being fired. That isn't a paradox is it? It doesn't make you think there had to be two bullets in parallel universes... right?

All that being said, I don't think we have any evidence supporting the idea that faster than light travel is possible.

astromark
2006-Nov-06, 06:35 PM
You have answered this question well. I would only add that the universe does not record light images. You would need to be looking in the right direction and at the right time in order to see your friends light image arrive. Just as hearing that riffle shot after the bullet has arrived requires ears.
If we could see these things I would sagest a blinding flash of light as the whole of that transmitted light would arrive in the same instant. Best not look. That might be more light than your retina could process.

Corgon
2006-Nov-06, 06:56 PM
If we could see these things I would sagest a blinding flash of light as the whole of that transmitted light would arrive in the same instant. Best not look. That might be more light than your retina could process.
As antoniseb pointed out, that light would be shifted well into the gamma range. Depending on how much light the ship reflected on the course of its trip, it might be a good idea to replace 'don't look' with 'get behind a nice piece of radiation shielding'.

Kristophe
2006-Nov-07, 02:30 AM
...
To take the thought experiment one step further, imagine your friend is traveling in a faster-than-light ship. He would arrive before the light source of him leaving his original position, so that a few years down the road you could conceivably see your friend standing next to you and blasting off in his ship 50 lightyears away. As you continue to observe the ship it comes closer and closer until it lands, what happens then? Does his "image" land, walk over and merge into one with your "real" friend? Is this perhaps a paradoxical argument for the impossibility of faster-than-light travel? (kind of like "if time travel were possible then where is everybody"?).

I don't think this has been stated explicitly yet, though I saw it suggested. If your friend was traveling faster than the speed of light, what you'd see is his ship appear at the point where it slowed to below the speed of light. From there, there would be two images: One of your friend's ship landing, and the other of it traveling backwards towards the position of your friends departure.

There wouldn't be any Star Trek style "warp lines" or "light tubes" or anything trailing behind the ship. It's just be *pop*-there's-the-ship, and then it'd fission, with one ship landing, and the other whipping off backwards, as if it were designed to fly that way.

ozark1
2006-Nov-07, 12:54 PM
Antoniseb's point about blueshifting would have another impact. On accelerating to light speed (or thereabouts), all the stars ahead of the ship would become intense gamma radiation sources. This would immediately vaporise the ship and the astronaut (by heating the ship up to above 6000 K)

Kristophe
2006-Nov-07, 01:34 PM
Antoniseb's point about blueshifting would have another impact. On accelerating to light speed (or thereabouts), all the stars ahead of the ship would become intense gamma radiation sources. This would immediately vaporise the ship and the astronaut (by heating the ship up to above 6000 K)

It would also give every grain of space dust (near) infinite momentum. It wouldn't be a comfortable trip in any regard.