Chasing light speed

[Carl]

This will sound stupid but I'm no Astro Phyisist (or Speller)
I woke up thinking this last week and no one has answered it yet and I'm looking for a simple explination.

I'm being chased by an enemy space ship. We are both travelling at the speed of light and for arguments sake we are 500m apart.The enemy turns their headlights on to see me better (it's dark out in space).

Question 1. Will the beam of light catch up to my ship, does the light travel at the speed of light plus the speed of the ship?

Question 2. When i look back at the enemy ship will I see the headlights?

Question 3. The enemy is carrying a standard issue rifle that shoots a bullet at approx 4500ft per sec. Will the bullet hit me. Is the bullet travelling at light speed plus the velocity of the bullit or only at the speed of the bullet.

Is the above the same principle as throwing a ball forward in a train or does it all change in space and at light speed?

Do the headlights and bullet theoretically travel faster than the speed of light?

I'm just a mug backyard astronomer but I cant get this off my mind an it's anoying the people i work with.
can anyone explain in plain English.

Regards
(not so bright)Carl

[Babalyon 5]

Wow! Cool dream! You musta done really pissed of those aliens!:lol:
Cant wait for the answers to this one!!

[Zuts]

Hi,

Question (1), the beam of light will be travelling at the speed of light so in your spaceship yes it will catch you up at the speed of light.
Question (2), particles with mass cannot travel at the speed of light. Regardless, adding the velocities of any two objects with a velocity less than the speed of light will always give an answer less than the speed of light, no matter how close to the speed of light the objects are.

The formula for adding relativistic velocities is quite straightforward, it is


w = (u+v)/(1+ uv/c2), where c is the speed of light, u, v are the velocities to be added.

(1) Plugging in the values for everyday speeds then it is roughly u + v as uv/c2 is very small. So this agrees with everyday observation.
(2) if u and V are both c, the speed of light as in your first question then plugging in the values you get c. So this agrees with the fact that the speed of light is a constant for all observers.
(3) your second question is not quite right. Bullets cannot travel at the speed of light. Anything with mass can only approach the speed of light. Plugging in values approaching the speed of light will always give a value less than the speed of light which agrees with the fact that particles with mass can never travel faster than the speed of light.

Cheers
Paul

[GTB_an_Owl]

Hi Paul

i have to question your answer to Q1
if A and B are both travelling at the speed of light and A is 500 metres in front of B, the light from B will never catch A.
to my mind it will always be 500 metres behind A ?

and i would love to be in ship A - cause i could shoot B - but B could not shoot me

geoff

[AdrianF]

I think it is all relative. The light from the headlights would catch up to your craft because the light would travel at the speed of light relative to your speed.
The rifle I have a little trouble with, because the projectile is travelling much slower than the speed of light the bullet would leave the end of the barrel (speed of light + 4500f/s, relative 4500f/s) but as the projectile slows (would it?) the aliens would be in danger of shooting themselves as they overtake the projectile.
Forgive the ramblings of a sick man (bloody flu)

Adrian

[sjastro]

From the frame of reference of the spaceships, the questions have no meaning if it was possible to travel at the speed of light (which it isn't). Both spaceships and the distances they travelled would be length contracted to zero. There would be simply be no spatial dimension in the direction of motion for both spaceships. The spaceships would behave as photons.

The speed of light from the cars headlamps is constant irrespective of whether the car is in motion or not. Additive velocities don't apply to light.

Regards

Steven

[Karls48]

Travelling at speed of light - Relative to what? The Earth, some distant galaxy receding from the Earth faster then speed of light or centre of the universe?
If Big Bang theory is correct the only meaningful measurement of time, motion and distance is referencing it to the point in the universe where the Big Bang occurred, (and the time and space come to existence).

[JohnH]

This is not right.

The theory of relativity is based on the abandonment of any absolute frame of reference for time and distance - this is the logical result of the (observed) fact that the speed of light is constant no matter where you are in the universe and no matter what your speed may be.

[Kal]

Carl - the simplest way to explain it is to say that no matter what speed you are travelling at, light will always appear to travel at the exact same speed to you. But some interesting changes occur. Lets say you jumped in a spaceship and started orbiting the earth at 90% the speed of light (relative to your stationary speed standing still on earth, or relative to an observer on earth). You orbit for exattly 24 hours going by your clock on your spaceship, but the people on earth observing you have actually had more time pass, for every 24 hours your clock ticks over, earth clocks tick over 55 hours! Also, to people on earth your ship would appear to be 2.29 times shorter, and 2.29 times heavier.

So as you can see, time, mass, and distance are all relative to the observer. You would look out and think your ship is 100 metres long, weighs 10 tonnes, and 1 day has passed, but people on earth would observe 2.29 days passed, a 43 metre long ship, and a ship weighing 29.2 tonnes. Which readings are true then? Well it is all relative to the observer!

[MrB]

I don't even pretend to understand this stuff, but..
How does one 'observe' a ships weight?
And how do you weigh something in weightlessness anyway?
Do you mean the mass would be increased to 29.2 tonnes?
I thought mass was mass and thats that, kinda like light speed is lightspeed...
Urrrgh.... think I'll leave this stuff to those that have bigger brains than I!

[Kal]

I won't pretent and say I completely understand it myself, I needed a relativity calculator (http://www.1728.com/reltivty.htm) to get those numbers, and I may have mis-interpreted them. If I'm wrong I'm sure someone will correct me though :)

[drmorbius]

Good question! I love this stuff... but it makes my brain hurt... :P

[bojan]

It is not weight but mass...
Mass increases with relative speed because the kinetic energy of moving ship has mass also (E=mc^2)
Mass can be measured.. for example, mass of the charged particles is measured in the magnetic field of known strength, where particle has curved trajectory instead of straight one. The radius of the trajectory is depending on charge, magnetic field strength and particle speed and mass.
To measure the mass of the space ship, travelling at nearly the speed of light is a bit more difficult obviously, but the same principle applies :-)

[Rick Petrie]

My thoughts or two bobs worth.
To any observer travelling at any speed, the speed of light would have to be absolute and relative to the observer.( I don't know of anyone who has seen anything travelling faster than the speed of light!)
If two spaceships could travel at the speed of light(improbable) in one direction they would therefore be able to see each other with light travelling between them at the same speed of light relative to each other. I think then in this case that ship A would see the beam of light from ship B at the speed of light between them.(Question 1 & 2)
Theoretically Question 3 would have to assume that the bullet (mass) can travel faster than the speed of light. If it could then the bullet would hit
ship A at 4500ft/sec.
Personally I don't think any matter can travel faster than the speed of light.
If that were the case then we are in deep s--t from any object in space on a collision course with Earth travelling faster than the speed of light. We definitely wouldn't have time to kiss our backsides goodbye.:whistle:

[Jeff]

I appreciate the analysis below, but I have another question...

Question 4: If a female astronaut was piloting one of the space ships at close to the speed of light, would she be able to put her lipstick on? I'm concerned that when she looks in the rear view mirror to check, the image reflected will be so significantly red-shifted that her eyes will not detect any visible image, irregardless of which shade of lipstick she chooses.

[PCH]

Agreed, - surely the light emitted from the headlight in Q1 wouldn't even move ahead of the headlight at all, as it is travelling at the speed of light already. Unless of course it's "speed of light + speed of light" if the velocities are added.

And the bullet being fired from the rear ship, - well if the velocities aren't added then it would disappear off backwards at a hell of a rate instantly and would therefore go straight thru the back of the gun and the person firing it as well. If the velocities are added, then what would happen when the bullet looses it's own velocity? Would it just 'stop' relatively speaking? Meaning in fact that it is now moving along at the speed of light since it doesn't have any 'extra' velocity left from being fired?

Just my 2c as a totally unscientific person :P

[sjastro]

Neither scenario would occur. Apart from the infinite relativistic mass, ships A and B would be dimensionless in the direction of motion let alone have a 500m gap between them.

Length contraction might be a difficult concept to grasp but there is a practical application in the construction of particle accelerators. For particles with small half lives accelerated at 0.99c, the distance between the particle beam source and the target is calculated taking into account the relativistic effects.

Regards

Steven

[GTB_an_Owl]

and i didn't understand one word you just said Steven

you sure your not into politics rather than astronomy ?

geoff

[Rick Petrie]

I'm utterly confused at warp factor five Mr Spock.:)

[sjastro]

Well I suppose politics is the science of BS but not in this case.:)

Let me give an example.

Particle physicists frequently bombard atoms with unstable nuclei that have very short half lives. If the distance beween the atoms and the source of the bombarding nuclei is too great then most if not all of the nuclei would have decayed before hitting the target.

In the laboratory frame of reference the distance travelled when 50% of the nuclei decay is simply the (speed of the nuclei) X (half life measured in the laboratory).

If however the speed of the nuclei is near the speed of light (say 99%), the half life measured in the laboratory is much greater than the half life of a stationary nuclei due to time dilation. The percentage change for time dilation is the same as percentage shrinkage in distance.

Suppose the distance in the the laboratory frame between the source and the atoms is calculated to be 100 metres. (Without considering the effects of length contraction).

The corresponding distance in the nuclei's frame of reference at 99% speed of light is
100X(1-(0.99)^2)^0.5 = 14 metres.

The distance between the source and the atoms needs to be set at 14 metres for 50% of nuclei to reach their target.

As one gets closer to the speed of light the degree of contraction increases. At the speed of light contraction to zero length would occur (which of course is not possible as matter can't travel at this speed).

Hope this makes sense.:)

Steven

[Rick Petrie]

I'm glad you sorted me out on that one Steven.:atom:

[gmbfilter]

The more follow this thread the less I follow this thread

Since both spaceships are in the same frame of reference, everything is normal, lights work backwards and forward, and bullets do what they do.

For anyone in a different frame of reference eg IIS things would appear quiet different to what the crew on spaceships perceive.

A good physicist should be able to explain it, a couple of belts of wild turkey makes it all seem OK

[sjastro]

You would be correct only if the spaceships were travelling at less than the speed of light.

Since this thread was based on spaceships travelling at the speed of light (which of course is impossible) the scenario is very different. The spaceships will behave as photons. There is no front, back, gap or any specific spatial characteristic that can be assigned to photons.

Regards

Steven

[Karls48]

Hi Geoff, is not modern cosmology and theoretical physics wonderful? It can explain everything in whole universe.
So our two ships are receding from the Earth at speed of light or even faster. It seems to be confirmed observable fact (based on red shift) that more distant the galaxy is, faster it will recede from us. At some distance it will recede from as at speed of light and faster. If we take some distant galaxy that is receding from us at 70% of light speed and launch our spaceships at 35% of light speed from there, those ships would be travelling faster then the light relative to the earth and therefore backwards in time. Impossible, according to theory of Relativity. Relative to the galaxy the ships started from, nothing special will be taking place. This scenario (and some other things like massive monopoles that supposed to exist just after Big Bang) would render theory of relativity invalid. To salvage the theory our wonderful scientists invented assumption that the space itself is expanding. Not proven by any experiment, except other assumption that speed of light is constant in vacuum and ultimate speed that anything can move. So although some object in the universe appears to travel faster then light, according to current scientific dogma those object are travelling slower then speed of light but space between it and the Earth is expanding and therefore it appears that it is travelling faster then light. Well. Because of obvious problem with this assumption (the Earth and the rest of the solar system planets should be getting farther apart from each other) space expands only between objects that are not gravitationally bound. As no one come up yet with explanation of mechanisms that cause such a phenomena scientist have to invent other mysterious things as Dark Matter and Dark Energy. So we have Space that can be bend (curved), stretched, compressed (as it did not existed before Big Bang so it must be either compressed inside of object that exploded as Big Bang or created by some sort of conversion of matter/energy to space). It is then logical to assume that it can be also converted back to what it was before Big Bang. Where the Theory of Relativity stands on this? What is the speed of light in Dark matter?
At any given point in time in our history the intellectual elite of the society (shamans, priests, scientists) was absolutely convinced that they have all the answers to everything that is surrounding us, that what we consider as Universe today. And almost without exception their observations on which they based their conclusions were based on assumption that the earth is the centre of the Universe. Nothing really changed even today. We assume that basic law of nature that we observe here are universal and work everywhere. Well what about the Black holes? According to the scientists the laws of nature will break down inside of black hole and cannot work as they do elsewhere in the Universe. So logically – either Black holes don’t exist or universality of laws of nature is wrong.
Back to our two spaceships. The Enterprise is chasing Kiligons spaceship. Relative to the Earth the ships are receding at 105% of light speed but because of space expansion assumption they move only at 35% of light speed and therefore they exist as real object. From ships frame of reference ships are 500m apart and nothing unusual is taking place. Booth ships main weapons systems and engines are damaged so Capt. Kirk finds somewhere old M16 and order Scotty to mount the rifle in ships nose. Kiligons do same mounting similar rifle in rear section of their ship. Ships commence firing on each other. With every shot the Enterprise fires, rifle recoil will slow it down. Every time the Enterprise is struck by Kiligons bullet it will slow down even more. Exactly opposite will happen to Kiligons ship as they fire on Enterprise and are struck by the bullets from Enterprise. They will accelerate away from the Enterprise. Eventually the Enterprise will not be able to hit Kiligons ship as it will be moving faster then the bullets fired from the Enterprise. Also both ships will not be able to hit each other as the distance between them increase. No rifle will shoot consistently in same spot, the spread of shots over the large distance will cause bullets to miss. That’s applied science and it works. Theoretical science, especially in cosmology requires one important element. You have to believe in what it is telling you. Well, you may just as well believe in creation.
And one last thought about the high mathematics. Mathematicians can calculate temperature and size of the Universe milliseconds after the Big Bang (among other amazing things). Just ask them to calculate flip of the coin (head or tails). They will fail miserably. Although such event is taking place in same universe and is influenced by same law of nature with enormously smaller number of variables and possibilities. Make your conclusion about such mathematical proofs.

[Rick Petrie]

I like your assumptions Karl. There are just too many questions about our Universe that are unanswered and who is right and who is wrong?.:thumbsup:

[sjastro]

That's quite a tirade Karl.

A ship travelling at 35% light speed towards a galaxy, will be travelling at that speed irrespective of the recession velocity of the galaxy.

The recession velocity of the galaxy is based on the expansion of space-time coordinates. The velocity of the ship is based on local coordinates which are independant of space-time expansion. You can't add both velocities.

I'll leave it at that.

Regards

Steven

[gmbfilter]

Ladies and Gentleman,

Google up "The relativity express "
There is some great stuff
especially on u tube.

Don't miss "Do-it-Your-Self-Relativity " Parts 1 and 2

Pay attention, there will be a short quiz

Simultaneity is also cool

Paradoxes only seem that way!

[Jeff]

Is there a known point in the universe where the Big Bang occurred?

My current understanding is that we have a viewable horizon which is fairly homogeneous in every direction such that we are close to the centre of the observable universe. However, the observable horizon from an outlying galaxy would be completely different ... and it would appear to be at the centre of its observable universe.

Can a Big Bang Origin point be deduced (eg. from the CMB dipole and the modelled inflation/expansion history of the universe), or it a meaningless concept becasue the origin has expanded and is therefore now all around us? I understand that the CMB dipole for the solar system is estimated to be 368 km/sec ... can we draw any implications from this?

[sjastro]

Jeff,

It's meaningless because the implication is that the universe was created in existing space. Space-time was created at the BB. From a topological perspective any point in the universe can be considered the "centre".

Regards

Steven

[MrB]

Blah, thats like saying a falling tree doesn't make sound because no one is around to hear it.

Forget what was, or wasn't, there before.
Once there were no landmarks on earth, and no possitioning relative to them. Relative possitioning is done now, but using you're logic that shouldn't be possible, because those landmarks once weren't there and neither was our concept of space and distance etc.
There is matter now, and we should be able to find the point of origin of the expanding bubble.
We now have a reference, all the observable galaxies, so where is the origin(referenced to observable/mappable galaxies) that the red-shifted galaxies are moving away from?

[sjastro]

Your line of reasoning contradicts the Cosmological principle. If you use red shift to calculate the origin you will find the BB to have occurred in your own backyard.:eyepop:

Red shifted galaxies are moving away from you, or from an observer from any point in the Universe. Doppler red shift is not an absolute parameter but is relative to the position of the observer.

At the scale of the universe, galaxies are essentially evenly distributed. This is a direct consequence of space time expansion which makes the universe homogenic and isotropic.

If you want to use a bubble analogy think of the universe as the surface of the bubble rather than its volume. You will find that every point on the surface is moving away from every other point as the bubble expands. There is no specific origin.

Regards

Steven

[Rick Petrie]

Yes there is - The centre of the bubble, easy.;)

[sjastro]

I'll assume the wink face is sarcasm.:)

[Rick Petrie]

Sorry Steve, Just my warped sense of humour. Couldn't help myself.:)

[MrB]

Never heard of it, so it's somebody else's problem... hehe

True. You can probably tell I've never read/learnt anything about this and I'm kinda making it up as I go along.
The furthest galaxies are moving at the same speed aren't they?
Does the level of redshift vary between these galaxies or is it the same for all?
I suppose if the observer(us) is too close to the 'origin', or the galaxies are at such distance that we appear to be close to the 'origin', then the delta-redshift (redshift's-shift(!)) between each galaxy would be minimal.
But assuming the observer is sufficiently removed from the origin, then the amount of redshift each galaxy has, their apparent speed, would be an indication of angle of movement relative to the observer?

Not from the 3d plots/maps I've seen, there are clusters and strings and swirls etc etc. The position of the galaxies relative to each other is of no consequence anyway, only their true velocity/apparent velocity and the vector calculated(if possible?) from them.

Yep true, but again, observed from any point within the bubble, (or outside for that matter) other than the 'origin', the velocity of the surface at any point other than those perpendicular to the observer, will have different apparent velocities due to the angle of observation.
I failed physics at high school, it bored me shickless, but do remember vector diagrams... these should apply here right?

Ofcourse there was, we just havent found it yet.
Hope my dribble above makes sense?

[sjastro]

There is a linear relationship between redshift and velocity (and distance) between the observer and galaxy provided the galaxy is not gravitationally influenced. If the furthest galaxies are at the same distance from the observer they will have the same velocity and redshift.

If galaxies are not gravitationally influenced they will not have an apparent speed nor travel at an angle different to the line of sight of the observer.
The point is that galaxies don't travel through space, they are in a fixed position and are swept along by the expansion of space time. Since space time expands in all directions galaxies are always in the line of sight of the observer.

If galaxies are gravitationally influenced such as being part of a cluster things are more complicated. The red shift/distance/velocity relationship is not as straightforward. The motion may no longer be in the line of sight of the observer and may even be approaching the observer. That's were your vector diagrams will come into play.:)



The 3D slices only represent a small percentage of the Universe. Due to the isotropic nature of the Universe an astronomer ten billion light years away will find the same sort of configurations in their own maps.



The surface of the bubble was meant to be an analogy for the expansion of the Universe nothing more. I'm sorry I raised the issue.:)



You queries are perfectly sensible.

Regards

Steven

[MrB]

Heh, I'm enjoying this discussion, however, the more I read your understanding of the universe, the less I understand it.:D
And I suspect the same for you and mine?! :lol:

BTW, I was aware of the 'bubble' analogy, in my view it makes no difference, if it suits you, replace the 'surface' with the most distant observable galaxies.

[sjastro]

What I understand is the common misconception that the Universe was created in existing space. Then you can pinpoint the origin by retracing the steps.

Unfortunately there are some dire consequences on the way, galaxies can travel faster than light, but then again they probably not, as matter would have collapsed onto itself long beforehand.

Regards

Steven

[MrB]

Sorry, haven't time to reply in length, but I get the feeling we're both doing a little of this:http://www.clipartof.com/images/emoticons/xsmall2/1920_.gif

I'll add this to my growing list of other POV's(religion, politics, war, nuclear power etc etc etc) I don't understand.

edit: I have no idea why the forum is converting an icon and image BBCode tags into a link?!.... icon shows fine in post-preview.... and in edit.

[skwinty]

What I find extremely difficult to understand is the concept of space not existing prior to the big bang. If space was created at the time of the big bang, what was space expanding into, if not more space.

[sjastro]

I agree but cosmology is built around a mathematical model, where space is defined mathematically instead of our perceived notion of space.

Apart form assigning coordinates to space, mathematicans define space by it's metric tensor which is a function that describes the shortest pathway between 2 points in space. For Euclidean space this is simply a straight line, for spherical space it's a length of an arc etc.

Cosmology uses a special metric http://en.wikipedia.org/wiki/Friedmann-Lema%C3%AEtre-Robertson-Walker_metric.

The cosmological model requires galaxies to be fixed in space and expansion to occur of the coordinate scale. Galaxies are therefore fully constrained in the coordinate system. It's therefore not possible to describe the Universe as expanding in space.

Another way of looking at it is to assume that if the Universe does expand in existing space what are the consequences.
Existing space would be an absolute frame of reference for the Universe. Galaxies would be able to exceed the speed of light in this frame of reference.
Another consequence is that since galaxies are now travelling through space the inflation event would not have occurred shortly after the BB. The Universe would not have time to become large enough to prevent matter from collapsing onto itself due to gravity.

Regards

Steven

[Karls48]

"Existing space would be an absolute frame of reference for the Universe. Galaxies would be able to exceed the speed of light in this frame of reference."

That’s what I have been saying in this and other treads.

"Another consequence is that since galaxies are now travelling through space the inflation event would not have occurred shortly after the BB. The Universe would not have time to become large enough to prevent matter from collapsing onto itself due to gravity."

No one knows how big was the original object that caused Big Bang, or what was the velocity of the matter expelled by it. If the velocity has high enough the inertia should overcome gravitational collapse.
This scenario makes Big Bang theory more acceptable to me. Consequences of this would make General Relativity wrong.

[sjastro]

The speed of light has been measured since the 17th century, is found not to exceed c, and be independant of the motion of the observer. All of which means there is no absolute frame of reference or absolute space.

There was no "object" at the BB. The universe was simply too hot. The first hydrogen atoms began to form around 400000 years after the event, the first galaxies around 500 million years later. Any inertial effect from the BB would come from the expansion of space time. You can't refer to inertial effects of expanding matter at the BB because there was no ejection matter in the first place.

I suggest you have a look at this.
http://en.wikipedia.org/wiki/Timeline_of_the_Big_Bang

Regards

Steven

[AGarvin]

Fom the viewpoint of General Relativity and BB theory, this statement is correct.

GR is a theory of geometry, and within that geometry you choose a set of coordinates (generally comoving coordinates (http://en.wikipedia.org/wiki/Comoving_coordinates#Comoving_coord inates)). As the universe expands with the Hubble flow (http://en.wikipedia.org/wiki/Hubble_flow), these coodinates, including "Cosmic" time (http://en.wikipedia.org/wiki/Cosmic_time), move with it. So even though the distance between two points increases with cosmic expansion, a point in space remains the same with respect to another, providing an absolute frame of reference. Of course ALL objects also have local peculiar motion (http://www.daviddarling.info/encyclopedia/P/peculiar_motion.html) through space, and Special Relativity comes into play.



Steven has commented on this but just to clarify. What he is saying is that matter didn't expand at all in the BB, the universe did. Matter is inside the expanding universe. Matter only has local peculiar motion. This is why objects can receed from us greater than c....they are moving with the Hubble flow which has no speed of light restriction. It's the local peculiar motion through space, governed by Special Relativity, that has the c restriction.

Anrdew.

[Chippy]

Great thread. Thanks to the posters who have explained some of the concepts in detail (and with references). I thought I had a pretty good handle on it - but there's always someone who knows more! ;-)