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starcanuck64
2011-Oct-08, 04:37 PM
Recalling a book I read years ago about a generation ship with a Bussard ramjet that was stuck at 1G acceleration, eventually the vessel achieved high relativistic speed and millions of light years away from Earth the crew witnessed the collision of two galaxies that happened at a high rate of speed from their perspective due to time dilation. What would happen if you actually could reach the speed of light in vacuum?

What would you see if you were able to achieve lightspeed?

macaw
2011-Oct-08, 05:04 PM
Recalling a book I read years ago about a generation ship with a Bussard ramjet that was stuck at 1G acceleration, eventually the vessel achieved high relativistic speed and millions of light years away from Earth the crew witnessed the collision of two galaxies that happened at a high rate of speed from their perspective due to time dilation. What would happen if you actually could reach the speed of light in vacuum?

What would you see if you were able to achieve lightspeed?

Massive objects (like the rocket) cannot reach the speed of light. At relativistic speeds one would see images like these, click on the links (http://www.google.com/search?q=relativistic+raytracing&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a)

starcanuck64
2011-Oct-08, 06:10 PM
Massive objects (like the rocket) cannot reach the speed of light. At relativistic speeds one would see images like these, click on the links (http://www.google.com/search?q=relativistic+raytracing&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a)

Maybe I'm not phrasing it right or the question has no meaning. I was trying to visualize what would happen if theoretically you could travel at light speed and time dilation became complete. Am I right in thinking that photons experience no time as they transit through a vacuum? What would that mean to a massive object?(ignoring the requirement for infinite energy to accelerate to light speed)

macaw
2011-Oct-08, 06:23 PM
Maybe I'm not phrasing it right or the question has no meaning. I was trying to visualize what would happen if theoretically you could travel at light speed

Theoretically (and practically) you cannot travel at the speed of light.


and time dilation became complete.

"Complete"? You mean "infinite"?




Am I right in thinking that photons experience no time as they transit through a vacuum? What would that mean to a massive object?(ignoring the requirement for infinite energy to accelerate to light speed)

You can't ignore reality.

pzkpfw
2011-Oct-08, 06:51 PM
macaw, let the person speculate/question a little. You don't have to worry that the foundations of science will crumble by answering the question as asked. You can simply ignore the thread now if you wish, you've given your answer twice.

starcanuck64
2011-Oct-08, 06:52 PM
Theoretically 9and practically) you cannot travel at the speed of light.

Wrong word, hypothetically then. Is it possible to predict the time dilation effects of travelling at light speed. I'm not claiming we can just trying to do a thought experiment.



"Complete"? You mean "infinite"?

As the relative velocity increases the relative time decreases so at light speed time effectively stops, or am I wrong on that point?





You can't ignore reality.

No but you can play around with concepts, which is what I'm attempting here.

jfribrg
2011-Oct-08, 10:37 PM
I you were travelling at lightspeed you would have infinite mass. Time would also stand still, which means that you could travel across the universe faster than an instant ( which explains the ability to see the galaxies colliding in real time when near lightspeed. However, any light you see would also be infinitely blue-shifted, which means that you probably wouldn't be able to see anything. The blue-shifted light would be in the range of infinitely high frequency gamma rays, so I suppose that you wouldn't survive the radiation very long anyway, but then again, at light speed, time has no meaning.

macaw
2011-Oct-08, 11:21 PM
macaw, let the person speculate/question a little. You don't have to worry that the foundations of science will crumble by answering the question as asked. You can simply ignore the thread now if you wish, you've given your answer twice.

So, we can turn Q&A into ATM? Is this what you are suggesting?

caveman1917
2011-Oct-08, 11:28 PM
As the relative velocity increases the relative time decreases so at light speed time effectively stops, or am I wrong on that point?

Pretty much. Add to that that the distances in your direction of travel will go to zero. So you'll be everywhere in your direction of travel in the same instant, not that you will have any time to notice that, and your demise will also occur instantaneously.


No but you can play around with concepts, which is what I'm attempting here.

You can certainly imagine the limiting case as you let the velocity approach c.

Paul Wally
2011-Oct-08, 11:55 PM
In Carl Sagan's Cosmos documentary there is illustration of how the world would look as you travel closer and closer to the speed of light on a motor cycle or scooter, if I remember correctly. According to that episode, the world becomes more and more confined to a narrower field of view in front of the traveler, as he moves closer to c. So I suppose the limiting case would be a point.

R.A.F.
2011-Oct-09, 12:54 AM
So, we can turn Q&A into ATM? Is this what you are suggesting?

Not at all...he was suggesting that you have choices.

1) Go along with whatever is being speculated.

2) Leave the thread/don't post.


As you are now aware, there is no "3rd choice".



I am making no judgement/complaint regarding moderator actions...just stating the facts.

Jim
2011-Oct-09, 01:10 AM
Completely uneccessary meta-discussion, R.A.F. Let's all try to stay on topic.

Kuroneko
2011-Oct-09, 08:56 AM
macaw's first two posts are right if we're dealing with the question as asked in the OP: there is reasonable sense in which anything at all is or can be 'seen'; the question is like asking what happens if relativity doesn't apply and yet insisting on applying it anyway. There's no amount of speculation that can make that self-contradiction go away. At least one of the assumptions needs to be loosened: either you're just strongly relativistic but not outright luminal or something other than relativity applies.

But the reasons why that's so deserve more discussion than they got.

Wrong word, hypothetically then. Is it possible to predict the time dilation effects of travelling at light speed. I'm not claiming we can just trying to do a thought experiment.

As the relative velocity increases the relative time decreases so at light speed time effectively stops, or am I wrong on that point?
In the limit of v→c, a Lorentz boost turns both the timelike four-velocity of the observer (defining an observer's time axis) and the spacelike four-vector in the direction of propagation (on of the spatial axes) into a single four-vector of zero length. The "observer's" time direction is no longer timelike, but is instead null, and one of the spatial directions is lost.

Try it out: boosting some observer from rest in inertial frame and along the x-axis, the velocity 4-vector (time axis) goes from [t;x] = [1;0] to [γ;γv/c] and the corresponding spacelike vector [γv/c;γ]. The components get closer and closer together as v→c. If you don't rescale, you get [∞;∞], which makes no sense, but if you do (say, by keeping energy E constant and momentum p→E/c, as for a photon), you get something proportional to [1;1], which has length 0. Though it doesn't mean that you can't have a frame that travels at lightspeed, it does mean that there is no such inertial frame and that it's not found by accelerating an inertial frame in the limit of lightspeed (that's just a limit of Lorentz boosts, and it breaks). However, since a lightlike four-velocity has zero length, there is no reasonable in which such an 'observer' can 'experience' time, and so does not have any point of view.

Still not sure what you mean by "complete", though yes, a clock flung at lightspeed stops (in addition to turning massless), and so can't properly speaking be considered a clock anymore.

marsbug
2011-Oct-09, 10:40 AM
I dunno, asking nonsensical questions can be very educational. By asking 'what would I see at lightspeed' you get to learn why the question doesn't make physical sense and how everything breaks down if travelling at lightspeed. I think it' a valuable exercise.

At lightspeed, you'd have to be an entity with no mass, no time, etc else you couldn't be at lightspeed. At arbitrarily close to lightspeed you'd be an entity with arbitrarily huge 'mass', arbitrarily slow time as seen by an observer in a different reference frame, with the universe squashed into a narrow band and swarms of high energy photons coming at you from your direction of travel. You could probably see the cosmic microwave background ahead, blue shifted into the visual range! or not - if you were close enough to c then it would blue shift back out of you're visual range.

astromark
2011-Oct-09, 07:02 PM
Not that I can know the answer.. I do not. but will enter the discussion as a point of view if it could be...

In front of me I would perceive a very high intensity point of light.. while at any angle to the side or behind nothing...

If my velocity was c. no image can catch up from any angle other than directly ahead..

but that I have no time to see anything. As for me at velocity c. No time passes..

WayneFrancis
2011-Oct-09, 11:45 PM
Maybe I'm not phrasing it right or the question has no meaning. I was trying to visualize what would happen if theoretically you could travel at light speed and time dilation became complete. Am I right in thinking that photons experience no time as they transit through a vacuum? What would that mean to a massive object?(ignoring the requirement for infinite energy to accelerate to light speed)


You would see nothing but what ever you eventually ran into, as see by an observer in another frame, actually you won't really see that coming much either. It is a bit like asking how long a ship would take to get from one place to another at light speed. The answer is not even zero because to the ship they wouldn't know when to stop.

Rhaedas
2011-Oct-10, 02:50 AM
The book mentioned in the OP is likely Poul Anderson's Tau Zero. The title is taken from the time contraction variable that approaches 0 as the ship approaches c.

astromark
2011-Oct-10, 06:42 AM
...And it must be noted that the bright intensity light that seems to be in your way, is,.
and you are seconds from total destruction... as your molecular structure is torn asunder...
Which is not something you will enjoy or can avoid...

Paul Wally
2011-Oct-10, 05:15 PM
I wonder what is the difference/similarity between the bright intensity light point "observed" when moving at light speed and a white hole, theoretically speaking of course? Time stands still in a black hole and it also stands still in a ship moving at light speed. There seem to be some parallels between black holes opening up as white holes in a different universe, and a ship moving at light speed, but this is perhaps just a superficial similarity. Those who know the advanced mathematics involved here might know whether the two cases are mathematically related.

Jeff Root
2011-Oct-10, 07:05 PM
'Tau Zero' Is almost at the top of my list of classic stories
that I need to read.

If you travel from one place to another at the speed of light,
you simply will do so instantaneously, from your point of view.
You leave Earth at noon and immediately find yourself at your
destination on Pluto, give your kid his bag lunch which he left
behind, and immediately return. It will be 10 PM when you
get back, but the trip will have taken no time at all as far as
you are concerned, and you can start thinking about fixing
lunch for yourself.

If you have to accelerate to near light speed, then your trip
will take a lot longer. You probably won't want to endure an
acceleration of much more than 1 g for more than a few hours,
so it could take months to get close enough to light speed
that time dilation would make a big difference.

If you aren't concerned about the acceleration, you could
launch yourself with a super cannon at millions of g's for a
few seconds, and get close to lightspeed before you leave
the end of the cannon. So the portion of your trip that you
are inside the cannon would be the longest part of the trip.

-- Jeff, in Minneapolis

Kuroneko
2011-Oct-10, 11:55 PM
I wonder what is the difference/similarity between the bright intensity light point "observed" when moving at light speed and a white hole, theoretically speaking of course? Time stands still in a black hole and it also stands still in a ship moving at light speed. There seem to be some parallels between black holes opening up as white holes in a different universe, and a ship moving at light speed, but this is perhaps just a superficial similarity.
Time does not stand still in a black hole, but a stationary external observer watching an infalling object will see it slow down and hover above the horizon, growing ever-fainter, and someone stationary near the horizon (with powerful rockets or whatnot) will be gravitationally redshifted (time-dilated) relative to a stationary far observer. At the horizon of a Schwarzschild black hole, there is only one stationary trajectory, and it's no longer timelike, but null, meaning only light can 'hover' directly at the horizon. The gravitational redshift there is infinite relative to a stationary far observer. So yes, there is some similarity, but that's about the extent of it.

speedfreek
2011-Oct-12, 01:23 PM
If you were able to impossibly travel at c, the universe would cease to exist for you. The whole universe would have zero length, and you would travel that length in zero time.

How are you ever going to slow down again, seeing as there is no time passing for you in order for you to do anything?

:)

Cougar
2011-Oct-12, 02:09 PM
I dunno, asking nonsensical questions can be very educational.... I think it' a valuable exercise.

If you were able to impossibly travel at c, the universe would cease to exist for you.

Therefore, as v --> c, the educational value of this exercise --> 0 ?

Kuroneko
2011-Oct-12, 03:00 PM
Therefore, as v --> c, the educational value of this exercise --> 0 ?
Other way around: the educational value is almost entirely in the v→c limit, not in what happens at v = c. Frames that travel at the speed of light are not found by simply taking a limit of inertial frames v→c. Things break in the limit, and understanding why that's so requires looking at strongly relativistic but subluminal speeds.

starcanuck64
2011-Oct-13, 04:09 PM
Some very interesting posts that help clear up some of my confusion with Special Relativity.