View Full Version : Faster than Light / Time travel

28th Jul 2001, 02:19
Further to the thread on "Some thoughts on sound", which I enjoyed a lot, I would like to ask my learned contemporaries out there:- Is it possible to travel faster than the speed of light? And, on the same subject, (some say), is time travel possible? The floor is yours!

Mr moto
28th Jul 2001, 03:48
Yes there is already something which travels faster than light. But it only happens at the quantum level. Its called 'action at a distance' and involves the uncertainty principle. The more acurately you measure the momentum or speed of a particle the less you can know of the other particle.

Current theory regarding time travel involves high density substances and super-gravity which bends space.

The numbers in space, as Slartiblartfast would tell you, are horrible.
For there to have been anything before the Big Bang, which of course there must have been, the theorists have to use 'imaginary time'.
Having now plotted the course if the start of the universe by the milli-second, and having measured the forces and masses required, and having now measured the mass of the universe as we know it(to be still expanding at its present rate)there is a slight problem. Nearly 90% of it is missing!

In my humble opinion it is highly unlikely that a species as primitive as Man will ever overcome these hurdles but I too would like to learn more about the subject.

If you want more practical faster-than-light travel you'll have to get into astral projection. As Jonathon Livingston Seagull's mentor says, "Perfect speed is being there!"

Now I need a lie down!

28th Jul 2001, 04:17
It would altogether be inconsequential whether one could travel at the speed of light, because the nearest galaxy is hundreds of light years distant.

Bally Heck
28th Jul 2001, 11:46
Apparently, warp factor 10 is so fast that if you reach it you would exist everywhere in the universe. Bet your quantum particle aint that fast :eek:

[ 28 July 2001: Message edited by: Bally Heck ]

28th Jul 2001, 13:40
Theory doesn't rule out time travel. But particles with mass can't be accelerated to (or past) the speed of light according to present theories (but there can exist particles that always stay faster than lightspeed).

Glueball...what's a few hundred years when you are not aging. :D

28th Jul 2001, 13:49
It can be shown that as the speed of a body approaches the speed of light the approximation of momentum as p = mv becomes innacurate and its actual relativistic momentum must be used which is given by p = mv / root(1-sq(v/c)) . We can see that this approximates to p = mv at values of v<<c.

The above equation for relativistic momentum is sometimes interpreted to mean that a rapidly moving body undergoes an increase in mass.
relativisticmass =rest mass / root(1-sq(v/c))

And we can see that as v increases so does the relativistic mass and thus a larger amount of energy is required to increase the velocity further, indeed if the velocity were to equal c then the relativistic mass would be infinite and an infinite amount of energy would be required to take the velocity past c. Therefore to all intents and purposes we cannot travel faster than light.

Tricky Woo
28th Jul 2001, 17:33

You've dashed my hopes, as I'd hoped to be able to travel faster than the speed of light sometime next week. Thursday, actually. I'd better call it off to save embarrassment.

Glad to see that time travel is still on the menu. Maybe the week after, huh?

Meanwhile, I'll leave you with a question, seeing as you're into this physics thingy: The chap who works at my dry-cleaners once told me that if you separate two 'entangled' particles, say putting 'em at either end of the universe, then change the quantum state of the one nearest you, then the quantum state of t'other will change instantaneously, i.e. without 'speed' at all.

How come? Why doesn't the C constant apply in this case? Are these particles being a bit naughty? Something should be done about it, that's what I say.


28th Jul 2001, 18:29
Time, Distance, Speed is all irrelevant if you wish to travel to the nearest galaxy or whatever. As humans we have to have a reference to something, just to be able to try and comprehend it. Mr Moto is on the right track when he refered to the latest ideas on super gravity etc, bend space!!

Bring that galaxy to you!!, thus no need for travel at the speed of light.

P.S Im not a scientist or proffesor of physics, i just fly a quadrapuff!!

28th Jul 2001, 19:16
I think you need to bring your dry cleaning friend up-to-date; Bell's Inequality was put into serious doubt by the results of Alain Aspects' experiment in 1982. I believe our only hope(s) of time travel will come from Everetts Many Worlds Theory.

Vapour Trail
28th Jul 2001, 20:28
I posted this one Turbofans thread on sound, but it applies here more as it was to do with faster than light travel etc. Bear in mind this is just my unserstanding of it so if I am wrong just let me know :rolleyes:

"......that is where E=mc2 comes in. And also the rocket will reach a constant velocity when its forward speed is equal to the speed of the gases being emitted from the rocket, as it is newtons reaction (equal and opposite) that makes the rocket accelerate.
.....with reference to the speed of light, scientists have already found particles that travel faster than the speed of light, called quarks, but they only exist for microseconds, but I was theorising that maybe we can only detect them for microseconds because due to traveling faster than the speed of light, they are traveling through time, or maybe they are flicking in and out of sub either, or even existing in more than one place at a time due to traveling in the 4 th dimension....that being time........Just some food for thought :D :eek: :confused: "

Gravity powered ships would be a good start, considering that balck holes are so strong that even light can't escape them!!!! :eek:

28th Jul 2001, 22:06
HARRY999...Of course according to the textbooks all of this is absolutely true...However in forming his theory on relativity, Is it not true to say that EINSTEIN took the speed of light (c) in your expression as an absolute i.e. nothing can travel faster than light ...not even a lightwave notionally launched from the crest of another lightwave....so until someone comes up with some alternative that works better than the current theory...we shall continue to live in a bounded universe... :confused: :confused: :confused:

29th Jul 2001, 00:20
I once took off from Manchester in 1999, and landed 45 minutes later in the Isle of Man for a night stop, and it was suddenly 1972.

So I guess time travel does exist...

29th Jul 2001, 01:04
Blimey. Absolutely brilliant everybody, keep it up. I'll have to re-read A Brief History of Time again just to keep up. I really don't understand how time travel can be possible because if one travelled back in time and then changed the past the whole issue could be completely different. We would possibly be living different lives, presuming we were born at all. But we must have been, in order to travel back through time in the first place. See what I mean?

29th Jul 2001, 02:03
Did you know that pilots are aging slower than non flying individual, simply because of an effect called "time dilation"...????? ;) ;) ;)

29th Jul 2001, 03:48
Interesting thought TR3.
Being a rather bored and sad individual I have done a few back of the envelope calculations to see how much younger a pilot might be at the end of his career compared to somebody with a stationary desk job.

I reckon that a pilot doesn't even save himself a second. Perhaps you would like to check the maths.

Based on a pilot of a 747 with a cruise speed of 600mph = 270m/s who spends perhaps 15 hrs per week in the air and does so for perhaps 30 years, i.e. 84 x 10^6 seconds. Now I'm not a pilot (soon to be rectified fingers crossed) so don't quote me on the above stats.

The pilots time is given by lorentz transormation:

t'= t/root(1-sq(v/c))

where t' is time experienced by pilot and t is that experienced by our office clerk if you will.

t'=84 x 10^6 / root(1-sq(270/3x10^8))

And by my reckoning the difference in time is a mere 0.000034 seconds. So nothing to get too excited about eh? :(

29th Jul 2001, 14:04
Bloody 'Eck......am I on the right site!?

Interesting stuff chaps......

Dont forget the paradox of time travel.....if you travelled back into the past from the now, then the past will include your arrival from the future, and ips facto you should still exist in the future, because you came back.......

Hope that helps, and before anyone quotes maths at me, Remember that I am only an O Level boy made good!


29th Jul 2001, 14:12
Oh, and just another quick question for you thorists out there.....

If I could notionally drive my car at the speed of light in a dark tunnel, and turned the headlights ON, would the light still illuminate in front of the car. Would the speed of light emitted from the lamp be added to the speed of the vehicle?


Mr moto
29th Jul 2001, 15:38
At the quantum level time doesn't exist or rather that the flow of time is irrelevent, one way or the other.
Also, due to certain particles being the anti-particle of others you have particles that only exist for a moment.

Thus, we have the basis for many worlds.

Anyone for Schrodinger's cat?

29th Jul 2001, 15:41
Yes and No.
Yes your headlamps would be on and anyone stood infront of your car would see the light from the headlamp (slightly before being splattered into a million pieces!)and no you would not see the effect of your headlamps being on (assuming you were the driver) as the speed of your car will not add to the speed of the light being emitted; which means if it was dark you would not be able to see where you were going and would mostlikely end up killing yourself fairly quickly (at the speed of light perhaps!) ;)

29th Jul 2001, 16:41
HARRY999...I'm impressed with your calculation...Imagine what the world would look like if one travelled at~c for say 4 years ...Icarus...what you have said is true however, to avoid an absurd mathematical expression c or the speed of light is accepted as absolute...HARRY999...could maybe you tell us why...??? ~ :confused: :confused: :confused:

Nick Lappos
29th Jul 2001, 16:44
For Harry999,

I crunched the numbers for a Space Shuttle at 17,000 mph, and it works out to about .7 seconds for a week in orbit. After only about 1.4 million shuttle missions, you can retire a year early! (Unless the bean counters hear of this and factor it into the next contract).

29th Jul 2001, 17:21
Something to think about:

I've often wondered if thought ( the thing that rushes around inside your brain ) is faster than light. Discussed it once with an eminent scientist and while he said the idea had not occurred to him he did think it was just possible. The question is though how to measure it ?


tony draper
29th Jul 2001, 18:05
I thought the sum total of mass/energy in the universe was constant, if you travel back in time you would be adding to this total.
Each molecule in your body would already be in use, ie the water molecules could be spread out in rockpools or combined into the bark of a tree somewhere, the trace iron in your blood could be thousands of feet beneath the earth.
The only way to do it would be to transfer a equal amout of mass /energy forward to your time at the same instant you left.
Think time travel is a no no.

29th Jul 2001, 18:12
I must disagree with your answer to Sky Yachts question. Yes, the person standing in front of the car will see the light being emmitted from the headlights but only at the same time as he is hit by the car because the car is travelling at the same speed as the light.
You say that the driver does not see the light emmitted from his own headlights. This is a false statement. It is a cosequence of Einstiens special theory that the speed of light measured by any observer whether moving or stationary will be the same (c is absolute as TR3 kindly points out). You infer however that in relation to himself, the driver measures the speed of the light emitted from his headlights as zero.
This result contradicts our elementary notion of relative velocities, and it may not appear to agree with common sense. But common sense is intuition based on everyday experience, and this does not usually include measurements of the speed of light.

Incidentally this answers the question posed by the original post. Nothing can travel faster than the speed of light. Suppose a spacecraft is travelling at the speed of light relative to an observer on the earth. If the spacecraft now turns on a headlight the principle of invariance of c asserts that the earth observer measures the headlight beam to be also moving at c. Thus this observer measures the beam and spaceship to be always at the same point in space. Our invarience principle also asserts that the beam moves at c relative to the spaceship, so they cannot be at the same point in space. This contradictory result can only be avoided if it is impossible for the spaceship to move at c.

[ 29 July 2001: Message edited by: Harry999 ]

29th Jul 2001, 18:17
The speed of thought is actually rather slow. Action potentials propagate along the fastest neurons in the human body at only about 120 metres per second. That's only 432km/h.

[ 29 July 2001: Message edited by: stagger ]

29th Jul 2001, 18:39
I don't pretend to be a professor on the topic. I am but a lowly 2nd year physics student and can't answer your question "why is c absolute?" with any great confidence. But as far as I know it is merely a theory which agrees with experiment. The experiment being the Michelson Morley expt. If you would like further details you'll have to ask and I'd be happy to describe it unless anyones got any objections.

29th Jul 2001, 20:44
HARRY999...The most significant "negative-result"experiment ever performed,This baffling result was put to bed by Einstein after publication of "Special Theory" in 1905.Again this notion that light propagated through the "Ether"until the lack of "fringe-shift" proved otherwise ...Some readers will be unfamiliar with terms such as "Inertial Frames of Reference" if we continue with this thread,then perhaps, we should start with what relativity is all about, for readers who are interested but have not been exposed to the concept before...Any takers???

Vapour Trail
29th Jul 2001, 20:50
If you were in a capsual traveling away from the earth at the speed of light, then if you could watch the earth, it would apear to stand still because you are traveling awayfrom it at the same speed as the light being emitted from it. If you then went faster than the speed of light, the earth would appear to rotate backwards, and it would appear that you are travelling back in time, where in actual fact you are just overtaking the light emitted from the earth.

Another interesting thought is imagine if you were staionary at a point in space, and a space ship that was traveling faster than the speed of light, slowed down and stoped infront of you......It would instantly appear and then a while after, you would see the ship approaching as the light emitted from the ship catches up :eek:

29th Jul 2001, 22:32
I believe the two foundation stones of the special theory of relativity are the principle of constancy (Michelson-Morley) and the principle of relativity (Galileo).
1. The velocity of light 'in a vacuum' is the same in all frames of reference (for al observers) moving uniformly, relative to each other,
I am not certain that the post mentioned that the car was being driven in a vacuum?
2. All laws of nature are the same in all frames of refernce moving uniformly, relative to each other.

The speed of light only 'appears' to be constant doesn't it? it is the measuring instruments that change from one frame of reference to the other in just such a way that the speed of light appears to be the same. :confused:

29th Jul 2001, 23:50
Vapour trail,for every point along a given line that this ship would pass,then,light would reflect from the ship from each individual point it passed.So, the net effect is perhaps a large tail appearing in the objects wake...but a much smaller object...

30th Jul 2001, 00:27
Why would the measuring instruments "change"? wouldn't that defy your second postulate?
If the speed of light only "appeared" to be constant then there would be no problem with the absolutness of time. The fact that the speed of light is the same with respect to two observers in different inertial frames leads us to the conclusion that the two observers do not share the same time scale hence time travel. Also isn't the speed of light in a vacuum near as damn it the same in air?

Howzat? Maybe I'm wrong - I look forward to your reply :confused:

Mr moto
30th Jul 2001, 01:54
It may sound like it comes from a Richard Bach story but I like the idea that thought is not only faster than light but more all-encompassing. Thus, an idea or piece of inspiration is a finished model and already exists, you've just got to make it exist for you! The Bach book 'ONE' is a quite understandable explanation of the many worlds theory.

Tony Draper
The mass/energy in the universe are constant in theory and relativity theory is based upon it being a closed system, which it is not; its expanding and we're missing nearly 90% of what was present at the Big Bang. Yes everbody stood well back!

30th Jul 2001, 02:27

"The velocity of light 'in a vacuum' is the same in all frames of reference (for al observers) moving uniformly, relative to each other,
I am not certain that the post mentioned that the car was being driven in a vacuum?"

Just a quick question; If the speed of light is considered as constant then why does it matter if this is in a vacume or not? surely this would therefore make no difference to c?


30th Jul 2001, 02:46
Hey guys... Lets not get hung up on semantics,SMUDGER has raised a super topic and if anyone out there can explain how this universe works. Then I, for one will be an avid reader...Just think of those long flights when all the papers are read,Tell us what you think...who knows perhaps we can contribute to the search for the "Theory of Everything"...

tony draper
30th Jul 2001, 02:48
Yes but if the missing mass has been converted to energy ie, hydrogen to helium,the excess mass converted to energy this has been spread around, entropy, but it will still be there surely.
It always seemed to be a better reason for time travel being impossible than the grandfather paradox. :)

30th Jul 2001, 03:14
This is a facinating topic isn't it guys?
TR3 has already mentioned time dilation, well try to get your heads round this one... length contraction - If L0 is the distance between two points that are at rest in a particular frame of reference L0 is called the proper length. If this frame moves with constant speed u relative to a second frame and the distances are measured parrallel to the motion, the distance L between the points as measured in the second frame is


So if our aeroplane could travel at c its length would become zero. And I have stated in a previous post that its mass would become infinite. So that means that our aeroplane would become infinitely dense- a singularity if you will. And you know what that means don't you? A black hole. :eek:

AA I guess that the actual speed of the beam within a medium remains c. I account for the apparent slowing of the speed of the light in that the distance travelled by the light in a medium is increased as a result of reflections off molecules of the medium. The general direction of the beam remains the same within the medium. Note that the spaces between the molecules are vacuum.

[ 29 July 2001: Message edited by: Harry999 ]

30th Jul 2001, 04:05
A tangent here butÖ

Mr moto - can I assume that you're a dualist; i.e. you don't believe that mind and brain are one and the same thing. Or perhaps you didnít see my little post on the previous page. I just pointed out that action potentials propagate along the fastest neurons in the human body at only about 120 metres per second. That's only 432km/h. So if thoughts are neuronal activity then they are rather slow and many contributors to this forum routinely travel faster than the speed of thought. :D

30th Jul 2001, 04:16
747 pilot airborne for 15 hours a week? thats 60 hours a month! who doeshe work for? who do i send the cv to? :D

30th Jul 2001, 04:32

30th Jul 2001, 07:54
There's one major problem for devotees of Einstein's theories. Einstein, being a scientist, defined his frames of reference; those things to be considered as given in order for the theory to work. A constant value for 'c' to all observers was one of them. So, 'c' is a constant for all observers simply because the theory requires it to be so. Subsequent experimental testing confirms that the theory explains the physical world better than Newton's Laws, while conveniently confirming Newtonian physics for the everyday velocities experienced by human beings. Relativity moves on to new technology, in applications such as satellite navigation, with corrections for relativity effects routinely factored into GPS navigational computations.

The problem now is in the observing, since all human observers live within the framework of space and time. For any development to the next stage, (as when Einstein stood on Newton's shoulders so that he could see further) a new frame of reference must be developed. Effects that seem to be beyond Relativity are already being observed. Further advances in physics will come from explaining these events. Personally, I'm not convinced that the velocity of light cannot be exceeded, though I don't see how super-velocities could be observed. But then I'm not a Particle Physicist, just a humble avionics technician.

Nevertheless, its interesting that even humble instrument bashers like me need to understand at least a bit about relativity as part of my job. Everyday acceptance and use is the driving reason for scientific research. No matter how esoteric it may be at the outset, good science eventually becomes practical. The mind boggles at what might come from chaos theory. Oh! I forgot! The weather forecasters already use that to help predict the movement of storms.

Mark my words. 'c' is NOT a constant in the REAL universe... :cool:

Through difficulties to the cinema

30th Jul 2001, 10:58
While these are interesting technical questions, I don't think that they are involved with aviation, in the spirit of PPRuNe. The internet abounds with discussions on particle physics and the theory of relativity. Have a look at:

A basic tutorial on faster than light travel (http://www2.abc.net.au/science/k2/stn-archive1/posts/topic42526.shtm), and
Other frequently asked questions (http://www2b.abc.net.au/science/k2/stn/faq.htm#light)

From the (Australian) ABC's Self Service Science (http://www2b.abc.net.au/science/k2/stn/) website.

Given that Techlog is for matters aviation, I am going to close this discussion.

That's what moderators do! :D