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## Excursions at the Speed of Light360

D4C5CE writes "S/F fans can finally find out what you really get to see at relativistic velocity, and tourists are one step closer to "doing Europe in a day" in these amazing Space Time Travel simulations of the Theoretical Astrophysics & Computational Physics department at the Institute for Astronomy and Astrophysics Tübingen. They put you in a driver's seat that both Armstrong the Astronaut and Armstrong the Cyclist would equally enjoy, in simulators built to ride a bike at the speed of light."
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## Excursions at the Speed of Light

• #### Re:G forces (Score:2, Informative)

<copeland.tj@ g m a il.com> on Sunday May 15, 2005 @09:01PM (#12539285)
Well, there aren't any G forces at the speed of light. Just getting to it and back down...
• #### Re:G forces (Score:5, Informative)

on Sunday May 15, 2005 @09:02PM (#12539288)
G-force is caused by acceleration. Assuming you accelerate slowly enough, you can get up to \$VERY_FAST without dying.
• #### Re:G forces (Score:4, Informative)

by Anonymous Coward on Sunday May 15, 2005 @09:06PM (#12539311)
G force is dependent on acceleration, not velocity. If one were to be accelerated too quickly to the speed of light, you would likely not survive. But if one were to accelerate to the speed of light under livable circumstances, it would not rip your skin off. Once traveling at the speed of light, you will feel just like you feel when traveling in an airplane
• #### The nerds have already seen (Score:5, Informative)

<slashdevslashtty ... m minus math_god> on Sunday May 15, 2005 @09:08PM (#12539326)
Lightspeed [sourceforge.net] is a simulator for velocities at c and below. Screenshots [sourceforge.net] are available.
• #### Re:Only one problem... (Score:1, Informative)

by Anonymous Coward on Sunday May 15, 2005 @09:09PM (#12539332)
Gravity is also a theory.
• #### This has been done before (Score:5, Informative)

on Sunday May 15, 2005 @09:12PM (#12539344) Homepage
I have seem something similar to this before. Check out:

http://www.anu.edu.au/Physics/Searle/ [anu.edu.au]

and

http://www.anu.edu.au/Physics/Savage/TEE/ [anu.edu.au]
• #### MOD PARENT IGNORANT (Score:2, Informative)

on Sunday May 15, 2005 @09:18PM (#12539379)
All of science is a "theory." Do you think that's air you're breathing now? Or are you a brain in a jar? My theory says the former, but it could be completely and utterly wrong.
• #### Re:Only one problem... (Score:3, Informative)

on Sunday May 15, 2005 @09:35PM (#12539442)
There is quite a bit of very convencing physical evidence for both special and general relativity. Here's the first google item returned, but there's lots more out there to read. http://math.ucr.edu/home/baez/physics/Relativity/S R/experiments.html [ucr.edu]
• #### Re:Anyone got an idea what's going on here? (Score:3, Informative)

on Sunday May 15, 2005 @09:46PM (#12539476) Journal
It's in the this explanation [spacetimetravel.org]. There's a diagram at the bottom which explains it much better than I can in words.
• #### Tübingen project got the colors wrong (Score:5, Informative)

on Sunday May 15, 2005 @09:49PM (#12539489) Homepage Journal
Very cool project - the screenshots [sourceforge.net] posted by the parent comment show nicely that the Tübingen Project forgot to adjust the colors - due to the Doppler effect, colors change dramatically.
• #### Mr. Tompkins in Wonderland (Score:4, Informative)

on Sunday May 15, 2005 @10:01PM (#12539537) Homepage
There should, I think, have been at least a nod given to George Gamow whose 1947 book, "Mr. Tompkins in Wonderland," attempted to explain relativity and quantum mechanics by putting Mr. Tompkins into situations like this. If I remember correctly, one of the episodes literally did involve his riding a bicycle in a Wonderland in which c was something like twenty miles an hour.
• #### Re:G forces (Score:5, Informative)

on Sunday May 15, 2005 @10:10PM (#12539576)
A human being can tolerate up to 5 G (fighter pilots can go to 9 G, but only for short periods of time). That is an acceleration of about 50 m/s^2. If you were able to sustain this acceleration all the way to light speed (which you wouldn't because near light speed the amount of energy needed to accelerate tends to infinity) you would have to keep accelerating for

300000000/50 = 6000000 seconds, or about 70 days.

Deceleration would require the same amount of time. So the Tübingen experience would be a 140-day-not-very-pleasent-5-G bike ride :)

• #### Re:videos (Score:3, Informative)

on Sunday May 15, 2005 @10:15PM (#12539589) Homepage
I think what that video shows is what you see if you travelled at near the speed of light, and recorded the whole thing with a high speed camera, and then played the recording back.

Either that or the buildings and roads are so many thousands of times bigger than real life, in which case you would again see what the video shows.

Alternatively, you could set the speed of light very slow, and you would see the same effect even if you travelled at only 100mph and with normal sized buildings and roads.

I only wish they did the anim at 60fps instead of 30 frames per second. It'd look even nicer. "Oooh movies are at 30fps, so I must copy them".
• #### Re:G forces (Score:1, Informative)

by Anonymous Coward on Sunday May 15, 2005 @11:05PM (#12539813)
You can't actually accelerate to the speed of light, regardless of the acceleration rate (you have to start at the speed of light). Special relativity describes the behavior of particles of finite mass below the speed of light. As you approach the speed of light (from below), mass goes to infinity, and time and length go to zero.

l' = l0/gamma
t' = t0/gamma
m' = m0*gamma

gamma = 1/sqrt(1-Beta^2) (-> inf. as Beta^2 -> 1)

Beta = v/c (-> 1 as v -> c)

where v is the particle velocity, and Beta c is the speed of light.
• #### Re:Beyond cool! (Score:1, Informative)

by Anonymous Coward on Sunday May 15, 2005 @11:14PM (#12539848)
problem is, they ignore significant effects of relativistic speeds: length contraction and time dilation.

At the speed of light, your entire 180 degree front/back panorama becomes squished to a line that sits at the 90 degree mark.

And, no matter how far you travel at the speed of light (as long as it was a finite distance to the outside observer), the trip was instantaneous to you.
• #### Re:Sounds like a wonderful experience... (Score:4, Informative)

on Sunday May 15, 2005 @11:15PM (#12539853)
Yes, a person moving faster than another is affected by time differently. Time Dilation is one of the components of Einstein's theory of special relativity.
• #### Re:Sounds like a wonderful experience... (Score:3, Informative)

on Sunday May 15, 2005 @11:21PM (#12539885) Journal
t' = t / (sqrt(1-(v^2/c^2)))

Where v is your speed, c is the speed of light, t is the time that passes for someone at rest, and t' is the time that passes for you. If you plug a number in for the speed, say 30 kilometers meters per second (67k miles per hour) You would still be talking about a very small difference. Driving in your car at 80 Miles per hour would make the bottom of the fraction about equal to 1, meaning you wouldn't see any detectable difference.
• #### Re:G forces (Score:2, Informative)

on Sunday May 15, 2005 @11:31PM (#12539925)
it's a funny thing, what happens is that you can accelerate subjectively at 5G or whatever rate you want indefinitely, and you'll never reach lightspeed. An outside observer would see your rate of acceleration decrease as you approach the speed of light, such that you never reach the speed of light.
• #### Re:Welcome to happy vector land! (Score:4, Informative)

on Monday May 16, 2005 @12:02AM (#12540041)
No... Constant velocity = no acceleration. Constant high speed in a circle (such as in orbit)= lots of acceleration.
• #### Relativistic G forces (Score:5, Informative)

by Anonymous Coward on Monday May 16, 2005 @12:16AM (#12540095)
That's Newtonian. The relativistic acceleration equations are different. See this FAQ [ucr.edu] for the correct equations, which will tell you how long (in either proper or inertial time) it would take to reach a given speed, as measured by an inertial observer initially at rest with respect to the body -- with some calculations for 1 g acceleration.

(For instance, to reach 0.77c requires 1 year of subjective time or 1.19 years of objective time; for 0.97c, it's 2 years subjective, 3.75 years objective; for 0.99999999996c it's 12 years subjective, 113,243 years objective.)
• #### Cosmos (Score:5, Informative)

on Monday May 16, 2005 @12:19AM (#12540106)
C'mon, surely someone else remembers the episode of Carl Sagan's series "Cosmos" where they did the relativistic motor scooter trick? In a small town in Italy, where the speed of light is only 40 km/hr (strictly enforced!) a young man leaves on a tour of the city at relativistic speeds, leaving his friend and younger brother behind. Sagan describes the effects of blue- and red-shifting, the contraction of the cyclist's length, and the dilation of time. It ends with the young man returning to the place he started, just a few minutes (in his frame of reference) after he left. Sadly, he finds all his friends gone, and only his once-younger brother, now an old man, still waiting for him.

I don't know why, but the bittersweet reunion of the two brothers, as well as the story of the late Wolf Vishniac in the "Blues for a Red Planet" episode, both make me cry.
• #### Re:Sounds like a wonderful experience... (Score:1, Informative)

by Anonymous Coward on Monday May 16, 2005 @12:25AM (#12540125)
Velocity does have a time correlated effect, even at atmospheric speeds below mach.

It's said that they set two atomic clocks to the same time and flew one around the world in a fighter plane while the other sat on the ground. Supposedly when it got back the time difference was a few millionths of a second or so.

What is being said is that people at different velocities experience time differently.
• #### Even earlier... (Score:3, Informative)

on Monday May 16, 2005 @12:26AM (#12540132) Homepage Journal
I recall seeing still shots of a speed-of-light visualization in a brochure from Carnegie-Mellon's supercomputing center, back in the early '90s.

I can't find the brochure online (this was pre-WWW), but I think the stills came from this paper [acm.org], from 1990.

Not that I think that this sort of thing is redundant. As technology advances, this is the type of visualization that's worth repeating on new hardware and new software.

k.
• #### Re:Welcome to happy vector land! (Score:5, Informative)

on Monday May 16, 2005 @12:41AM (#12540172)
Velocity is a vector quantity, basically speed times a direction vector. If you turn, it takes a force to push you in the new direction. Since F=MA, that means that you are being accelerated. If you were to drive a car in a clockwise circle at a speed of 100 MPH, it would be constantly accelerated to the right, but its speed would remain 100 MPH. However the net velocity would be zero, as the net spatial displacement would be zero (at least every time you come back to the start point.)

And orbiting bodies continually lost speed? What kind of troll weed are you putting in your pipe?
• #### Speed of light IS a constant. (Score:4, Informative)

<blinks@NOsPaM.acm.org> on Monday May 16, 2005 @01:21AM (#12540412) Homepage Journal
What you're talking about (the slowing down of light in glass, etc.) is the effect of light hitting a molecule of something, being absorbed by it, and then being reemitted out the other end.

Light's speed is a constant, c. It's the speed of absorbtion and reemission that changes it's apparent speed through substances.
• #### Re:Mr. Tompkins in Wonderland (Score:3, Informative)

<`ude.ogacihcu.inmula' `ta' `egnardts'> on Monday May 16, 2005 @01:22AM (#12540415) Journal
Check out the site. They did. It's in the main site on the "wheels" section. The wheels are freakin cool too.
• #### Re:Sounds like a wonderful experience... (Score:3, Informative)

on Monday May 16, 2005 @01:34AM (#12540470) Journal
No, G.P. is correct. As you approach the speed of light, distances along the line of travel contract. At the speed of light, you're trip is instantaneous, but to you, you've not gone anywhere, even though to the guy standing still, you've travelled 50 light years distant (or event 6 billion light years, but then the observer on earth wouldn't be around any more, whould he?).

The really interesting trips occur when you're travelling very near the speed of light, not at the speed of light.

In summary: Moving yardsticks shrink in the direction of motion. Moving clocks run slow. At the speed of light, Clock stops, Distance across the universe is 0 (All stars compress into a plane )

• #### Hint (Score:3, Informative)

on Monday May 16, 2005 @03:49AM (#12540957)
If you're going to misspell a word [google.co.uk], don't make it the one you emphasize in ALL CAPS...
• #### Not quite (Score:3, Informative)

on Monday May 16, 2005 @08:51AM (#12542169) Homepage

Normally we use the words absorbtion and re-emission to refer to electron energy-level transitions within the molecule: photons are absorbed and promote electrons to higher energy levels; then, at a somewhat random time and in a somewhat random direction (not uniformly), electrons drop to lower energy levels and re-emit photons. (Note that these transitions aren't instantaneous, nor entirely well defined in time, but we call them quantum events anyway).

A notable effect of complete absorbtion and re-emission events is the tendancy to randomise the direction and phase of the radiation.

When slight slows down in a substance, this is different. It's due to coupling between the light and the molecules of the substance. Photons aren't absorbed in the sense of electron energy-level quantum transitions, but rather the passing photon wave packets interact with the electron waves to modify the phase of the photons. You could think of it as fractional absorbtion and re-emission, each molecule affecting the path and phase of each photon only a very small amount.

There is a qualitative difference between the two effects: light slowing down in a substance usually only randomises the phase and direction very slightly.

Here's a daft analogy. Light slowing down is like running through a vast plain of spinning merry-go-rounds, occasionally touching one with your hand or foot so that it affects your motion. Absorbtion and re-emission is like occasionally jumping onto a merry-go-round, waiting for a little with your eyes closed, then jumping off again.

-- Jamie

• #### Re:The nerds have already seen (Score:3, Informative)

on Monday May 16, 2005 @09:04AM (#12542301)
A common misconception.

You cannot accelerate a mass > 0 beyond c.

Relativity does not prohibit travelling FTL, it just prohibits getting there from a speed < c. A subtle, but important distinction.
• #### Re:Sounds like a wonderful experience... (Score:3, Informative)

on Monday May 16, 2005 @09:09AM (#12542349) Journal
If you travel 50 light years distant at the speed of light, It takes 50 years with respect to a stationary observer. However, if you're the one doing the travelling, the trip is intantaneous, and also from your perspective, (while you're travelling), you've not travelled any distance. It would be like you've magically transferred from one place to another intantaneously. This is all because of the effects of relativity. However, practially speaking you cannot go AT the speed of light, you can only approach the speed of light. Even with this you could travel incredible distances in a lifetime, because the distances in your frame of reference would contract. There would be a long period of time (probably about 10-20 year) where you would be accelerating (at 1 or 2 G), and an equally long period of time while you were decellerating.

From the travellers perspective, they see that because the distance has shrunk, they're able to travel between the two very distant points in a lifetime. From an observers perspective, the traveller is able to do this because his 'clocks' all run super slow.

• #### No, there is no ether! (Score:2, Informative)

on Monday May 16, 2005 @09:12AM (#12542383)
I'm making one assumption in writing this. I assume that your cyclist never turns his head. This seems like a likely assumption, since, if he does turn his head, there would be no need for relativity to explain why he can see the lamppost after he's past it.

What you're saying is, that a cyclist going at high speed past a lamppost will at some point see a mirror image of the back of the lamppost. This is flat out wrong. Which parts of the lamppost that are seen by the cyclist, does not depend on his speed.

The mental image I get when I read your post, is that of a cyclist, 'seeing' a billiard ball photon being fired from a lamppost - just as he is passing it - curving in across his path so that he runs into it. This is the ether explanation for the constant speed of light, disproved by the Michelson-Morley (sp?) experiment.

In fact, in any inertial system light always behaves the same. The relative speed of the lamppost emitting the photon, does not affect the behavior of the photon in, say, an inertial system where the cyclist is at rest at origo - apart from deciding what frequency it has. He can see it if it is incident upon him within his field of vision, not otherwise.

Objects going past you at relativistic speeds will indeed appear to be rotated. This is because the perspective you get of the closer part of the object becomes mixed with the perspective of the further off part, which is from an earlier time.
Imagine that a rod has two synchronized watches, one in each end. When the rod is some way off, you have a head-on perspective of it; as you go past it, you will see more of its side. Imagine that your eyes are so fast, that you can tell that the further off watch appears to be behind (whether the rod is moving or not), due to the fact, that the image of that watch has farther to travel. At relativistic speeds, you would then see the closer part of the rod curve away from you, since the side perspective, of the closer part of the rod, becomes mixed with the head-on perspective, of the further off part. (Drawing pictures would help at this point.)
However, the constituent perspectives in all this, are still the same that you would see, if you went past at a non-relativistic speed.
• #### Re:G forces (Score:1, Informative)

by Anonymous Coward on Monday May 16, 2005 @09:15AM (#12542413)
> But you are correct - it's the accelleration that kills you (or decelleration, in the case of defenestration...)

It's actually differential acceleration that's the killer isn't it?

You can survive any level of acceleration as long as it applies uniformly to every part of you, (and in fact you won't even feel it) - like gravity does.

The problem when you smack into something is that only part of you is being directly deccelerated, and your body is left to provide the internal forces needed to get the rest of you up/down to speed.
Which it can do to a point. Beyond that you turn into a paste.

• #### Re:mass increases (Score:1, Informative)

by Anonymous Coward on Monday May 16, 2005 @09:30AM (#12542584)
The point of relativity is that speed is relative. Therefore, traveling near the speed of light is no different to you; in fact, you yourself are, right now, traveling at near the speed of light with respect to some observer. In your rest frame, nothing special happens. To some other observer, they see your relativistic mass increase, but forces (e.g. the pumping of blood by your heart) transform likewise, to make everything consistent.
• #### Re:Sounds like a wonderful experience... (Score:2, Informative)

by Anonymous Coward on Monday May 16, 2005 @09:46AM (#12542739)
No, photons are massless particles. The modern usage of the term "mass" is that of "invariant mass" (of which "rest mass" is a special case, when applied to massive particles which can be at rest). A particle can travel at the speed of light if and only if its (invariant) mass is zero.

It is possible to define an "effective mass" for a photon of E/c^2, but that's not the sort of mass that is important in deciding whether something can travel at the speed of light (which is one of many reasons why the use of that kind of mass is deprecated).
• #### You are not correct. (Score:2, Informative)

<jgardn@alumni.washington.edu> on Monday May 16, 2005 @09:54AM (#12542831) Homepage Journal

First, it's not a perception only that objects contract in length in the direction of motion (remember, the frame of reference you are observing is always at rest! It's the universe that is moving, not you.) It's an actual contraction. Time dilation is likewise. The reason this must occur is because of the simple fact that the speed of light is the same in ALL frames of reference. This means the particle of light you see is travelling the exact same speed relative to you as the particle of light someone in one of the buildings sees as you zip past them.

There has to be some "give" in the universe to allow this to hold true. That "give" is the actual contraction of size and expansion of time.

The relativity effects are not simple perception distortions; the actual distance shrinks and time dilates. Objects get distorted in reality.

Finally, to you, those particles of light weren't "bending" to get to your eye. They travelled straight from the lamppost (or wherever the lamppost was when the light was bounced off of or emitted from it) to you. You can't see the back of the lamppost.
• #### Re:Could someone explain the relativistic time thi (Score:1, Informative)

by Anonymous Coward on Monday May 16, 2005 @10:25AM (#12543189)
But with everything being relative, couldn't you see it that the Earth was flying around at near light speeds in relation to you, while you were standing still?

Yes.

So then why does the time pass slower for you than the other way around?

The Earth sees time pass slower for you, and you see time pass slower for the Earth. However, if you go out and come back, everyone will agree that you're younger than people who stayed behind on Earth. Reconciling this is the basis of the twin paradox [ucr.edu].
• #### Re:If by travelling at the speed of light.... (Score:1, Informative)

by Anonymous Coward on Monday May 16, 2005 @11:56AM (#12544105)
I've read that the light from a star which is 10 light years far away from us will take 10 years to reach us.

Yes, according to us. No time elapses for the photon; the 10 "outside" years pass by instantaneously from the photon's perspective.

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