[thedoc (OP)]

Luke Pullar asked the Naked Scientists:
   If I move backwards at the speed of light (bear with me here) and turned on a torch, what would happen to the start of the beam of light? Would it be a glow frozen in one place? Would stationary bystanders be able to see a terminating light source, dangling in the air/space/whatever?
What do you think?

[Zer0]

A----------B----------C

Considering B is the point you stand still with your torch.
You move from B towards A, backwards holding the torch.
Light beam from the torch moves from B to C forwards.

If I am Observing you from a distant point, I would see you moving from B to A at the speed of light, and a light beam moving from B to C at the speed of light.

Although I might be totally wrong, and if you would observe any redshift/blueshift from your point of observation is still unclear to me.

I've read about a Theory/Law that irrespective of the Observers point of Observation or Velocity, the observed speed of light would remain the same.

But Honestly, I haven't understood this completely as yet.
Although nothing to worry about, cause we are at the right place where all questions regardless of how stupid they sound or how meaningless they look, they will be answered with due importance in the right manner.

P.S. - (Apologies to Luke Pullar for the incredibly delayed response, how on earth did this question go unanswered is out of my perceptions.)

[Kryptid]

If such a thing was possible, I'd say that the light beam would be redshifted out of existence. However, since no physical object can reach the speed of light, such a thing won't actually happen.

[geordief]

The source of light cannot move at the speed of light in a vacuum.

If it (the source) is moving at any other speed its frequency and wavelength  will be altered

So if the flashlight is retreating from the observer at a speed close
to the speed of light it will appear to that observer as very faint (red shifted) but with the same speed in a vacuum.

[Colin2B]

@

P.S. - (Apologies to Luke Pullar for the incredibly delayed response, how on earth did this question go unanswered is out of my perceptions.)

Very easy. If there are lots of posts at a particular time it doesn’t take long for a question to disappear off the page and get missed.

Yes, extreme Redshift will occur close to light speed.

Why does speed of light remain constant for all observers? The roots of this go back to electromagnetic theories of Faraday, Amper, Gauss and Coulomb but was never recognised until Maxwell did his famous equations and even he didn’t really understand the full implications. I’m afraid you will need to dig into these equations in order to understand.

[elidaesilva]

The source of light cannot move at the speed of light in a vacuum.

If it (the source) is moving at any other speed its frequency and wavelength  will be altered

So if the flashlight is retreating from the observer at a speed close
to the speed of light it will appear to that observer as very faint (red shifted) but with the same speed in a vacuum.

That makes sense. All object will fail to gain light speed. Light does not have resistance but other object have.

[Zer0]

@

P.S. - (Apologies to Luke Pullar for the incredibly delayed response, how on earth did this question go unanswered is out of my perceptions.)

(1) Very easy. If there are lots of posts at a particular time it doesn’t take long for a question to disappear off the page and get missed.

(2) Yes, extreme Redshift will occur close to light speed.

(3) Why does speed of light remain constant for all observers? The roots of this go back to electromagnetic theories of Faraday, Amper, Gauss and Coulomb but was never recognised until Maxwell did his famous equations and even he didn’t really understand the full implications. I’m afraid you will need to dig into these equations in order to understand.

(1) Yes I understand out of sight is out of mind.
I'm a Newbee n learning how this forum works, but I've realized that before posting a question/op it helps to use the Search Option n look for keywords/tags n check if a same/similar question was asked before n then take it forward from there. 👍
In fact I would also recommend newbies/newcomers to just browse through Page 1 2 3 4 etc and explore multiple pages as there are a lot of hidden treasures to be found. 👌

(2) Thanks for the feedback. ✌

(3) Looks like I'm gonna be doing a lot of reading now, surely its going to take me a whole day to come to speed.
But I'm ready as always with my shovel of curiosity not afraid to dig deep into the ground of knowledge going where a lot of great men n women have gone before. 👊

[Zer0]

Twisting/Modifying the OP.

A--------------------B

Total distance from A to B - 1 light year.
I race with a beam of light, start point A n finish point B.
Both light beam n my velocity is same = speed of light.
Considering we start at the same time, both will require a year to reach finish point B.

Now...
(1) Who would win, me or the light beam or none/draw ?
(2) While I'm travelling parallel to the beam of light travelling in a straight line, what would I observe red/blue/white/black ?
(3) Irrespective of my velocity which is the speed of light, will I still calculate the light beam travelling at the speed of light ?

(Hopefully in a day or two I shall be in a position to comeback n answer theses questions, until then any/all opinions/views are Welcome).  🙏

21st Nov 2017.
I'm back, a bit wiser than before I would say...
" You cannot travel at the Speed of Light. " Thanks for the Info geordief 👍

(1) Irrelevant. 👎
(2) Irrelevant. 👎
(3) Irrelevant. 👎

If anyone ever reached 50% speed of light, and tried to measure/calculate speed of light beam it would still be 'c' constant considering their clock/stopwatch would be ticking 50% slower. ⌚⏰

Light which is made of Photons have zero rest mass hence 'c'. If a particle or body with mass would try to match 'c' speed then it would require infinite energy/force cause as the mass body accelerates its mass would increase infinitely. 🚀

Light is often simply considered as the one visible to human eyes, but in fact it has a lot of unseen variants.
A bit of reading on ElectroMagnetic Radiation would help one to understand different variants such as xrays, gamma rays, infrared, micro n radio waves n so on and also provide explanations as to why they are different due to frequencies & wavelengths. 🍭💎

P.S. - (Yeah I travelled back in time n answered my own questions. Why? In order to Help a layman/newbie who one day might stumble upon this OP.) ✌
Thanks TNS for being such a time traveller friendly forum! 🙏

[geordief]

I race with a beam of light, start point A n finish point B.
Both light beam n my velocity is same = speed of light.

You cannot travel at the speed of light.

No matter how close you come to the speed of light (as measured from your point of departure,as well as that of the beam of light) the beam of light ,if you were able to measure its speed would still be c .

The only difference would be its frequency .

[Bill S]

You cannot travel at the speed of light.

There are a few who argue against that, but I doubt they are taken seriously.  However, is it only "experts" who are permitted to speculate about possible effects of doing so, without being shouted at?

Einstein did it with outstanding success, and J Richard Gott has a go.

   In "Time Travel in Einstein’s Universe" he formulates an interesting argument under the heading “Why Rockets Can’t Go Faster than Light”.  He says: 

If an astronaut’s rocket were to travel by us at faster than the speed of light, a light beam he sent forward could never catch up with the front of his rocket.  The light beam could never catch up because the front of the rocket would be moving faster and have a head start.  Any athlete should know that catching another runner who is running faster and has a head start is impossible.  The astronaut’s observations would be most peculiar: he would take out a flashlight and shine it towards the front of his rocket, but he would never see the beam of light arrive.  That’s not what an observer at rest would see: rather than perceiving he was at rest, this astronaut would know he was moving, and that’s not allowed by the first postulate.

I find this interesting because he seems to be saying that the astronaut (in his/her RF) would not observe light as travelling at c.

[geordief]

Well I was actually wondering if I might be contradicted since I am far from expert(although I did feel confident on that point having   so often heard it said.

Yes ,though I do think it is fair and prudent to leave  speculations in the main to those who have mastered the essentials of the subject.

Which counts me out.

[Bill S]

Yes ,though I do think it is fair and prudent to leave  speculations in the main to those who have mastered the essentials of the subject.

Which counts me out.

If you stop speculating, how do you know you are still alive?

[geordief]

If you stop speculating, how do you know you are still alive?

We all speculate but only based on what we think we already think we know.
My own speculations would be of far less interest to another person than the speculations of someone with  learned knowledge and experience.
If I speculate I can easily discover the truth by inquiring of someone who knows the subject ahead of me .

[Bill S]

My own speculations would be of far less interest to another person than the speculations of someone with  learned knowledge and experience.

Not necessarily. Just because someone has a lot of knowledge in a particular area does not mean that he/she can always use that knowledge to the best advantage, or draw the "best" conclusions from it.  Don't undervalue your own efforts.

I spent much of my early childhood deep in the Cornish countryside.  I still retain "pearls" of wisdom that came from people who could hardly read or write.  One of these was:  "Never pretend to understand something you don't really understand: and never believe something unless you understand it".  OK, one can pick holes in that, but it contributed to my being known as "What if", because I questioned everything.  (Not aloud, if they happened to be my mother's edicts )

[jeffreyH]

The gamma factor for an object travelling at c is infinite. Since this determines both time dilation and length contraction the object will experience no time or motion. If you could reach the speed of light you would be trapped at that speed. That is until you collided with another object.

[Bill S]

The gamma factor for an object travelling at c is infinite.

Please could you simplify that a bit (a lot )for me.  It could be something I've been looking for.

[jeffreyH]

A quick read of this should clarify a little.
https://en.m.wikipedia.org/wiki/Lorentz_factor

[jeffreyH]

Our factor is . When v = c the result of the function is infinite.

[Bill S]

Thanks.  Are you saying that because the definition of γ doesn’t include mass, the equations of SR can (should?) be extrapolated to, or even beyond, “c”?

[jeffreyH]

No. I am saying the opposite. You cannot match or exceed c.