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Author Topic: Headlights on my spaceship?  (Read 6878 times)

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Offline swashy

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Re: Headlights on my spaceship?
« Reply #30 on: 13:54:53, 31 May, 2013 »
Just to come back to the origin of the thread, as I'm indeed getting further confused, will an object at velocity which emits photons, add its velocity to the photon, so that the photon will be traveling at C+v

I figure it should, but if I read correctly, spacetime itself will limit this so the answer is no.   :gum:
Ade

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Offline mcgillca

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Re: Headlights on my spaceship?
« Reply #31 on: 14:02:26, 31 May, 2013 »
You are again correct. Photons always travel at the speed of light, so even if emitted from a spaceship at 0.9999c, an observer on the ship would see it going away at c, and an observer on the ground  would see the same thing.

All observers agree that a photon travels at the speed of light - and as you say, space/time alters in such a way to make sure that this is the case.

Colin
Main scope: Ikharos 8" RC
Camera: Atik 460ex with Atik EFW and Baader LRGB Ha, OIII and SII filters
Guide scope: On Axis Guider with Atik 314L+
Mount: Paramount MX located in Astrocamp, Nerpio, Spain

Offline AJG

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Re: Headlights on my spaceship?
« Reply #32 on: 12:27:17, 03 June, 2013 »
Thanks for your reply Colin.

I am not sure I fully understand:


...Simultaneity is only agreed by every observer if it occurs at the same point (e.g. two photons arrive at the middle of the ship at the same time).


I thought in this example crew members and external observers would have a different view of when and where the signals pass (or photons meet). Even if the signals where sent simultaneously both would agree only on the time of signals meeting each other (0.5s) but for the crew members it would be in the middle of the ship and for the external observer 0.1 light second from the stern. With a 0.2s delay between the signals, crew and external observer would agree on neither time nor place.

Is this correct?

Cheers,

Adrian.

(Also realised that the calculation was awry in my first post as I forgot that the 0.2s delay between the signal was shipboard time and the external observer would see it as 0.333s).


Offline mcgillca

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Re: Headlights on my spaceship?
« Reply #33 on: 13:55:11, 03 June, 2013 »
Almost.

Again, the key is that simultaneity is only agreed if occurs at the same point in space time. If the two events are simultaneous in the rocket ship, but are some distance apart (stern and prow), then an observer on the earth would see them occuring at different times.

However, the crossing point (lets assume that both beams are sent at the same time as observed on the ship) is the same point in space time - so all observers agree.

What they won't agree on is that both sets of beams were sent at the same time.

To be more precise:

a) Firstly, because of Lorentz contraction, the space ship will appear smaller on the earth. The contraction will be sqrt(1-0.8^2) = 0.6, so the spaceship will appear 0.6 light seconds long.
b) Working backwards from the point at which the beams cross, the stern will be at location 0.3 light seconds plus 0.8*t, where t is time before crossing.
c) So when distance traveled by light = t = 0.3+0.8t, the beam would be at the stern. This occurs at t = 0.3/(1-0.8) = 1.5 seconds before the crossing.
d) Going the opposite way, the the prow is at location 0.3 - 0.8t (the prow is closer earlier in time).
e) Again setting t = 0.3 - 0.8t gives 1.8t = 0.3, or t= 1/6 second.

So, the observer on earth would see a light beam emitted from the stern 1.5 seconds before the beams cross, and a light beam emitted 1/6 second before the beams cross from the prow.

Any more of this and I'll have to get out my text books...

Colin
Main scope: Ikharos 8" RC
Camera: Atik 460ex with Atik EFW and Baader LRGB Ha, OIII and SII filters
Guide scope: On Axis Guider with Atik 314L+
Mount: Paramount MX located in Astrocamp, Nerpio, Spain

Offline AJG

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Re: Headlights on my spaceship?
« Reply #34 on: 21:57:29, 03 June, 2013 »
Thanks, I think I will need to mull that one over to fully appreciate it. The Lorenz contraction was the element I was missing - rings a dim and distant bell but no more.


Any more of this and I'll have to get out my text books...


...and this is still Special Relativity - we haven't got onto General Relativity yet!

Adrian.

Offline mcgillca

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Re: Headlights on my spaceship?
« Reply #35 on: 09:46:43, 04 June, 2013 »
Time dilation and Lorentz contraction are straight forward to derive. For example, see http://www.drphysics.com/syllabus/time/time.html.

The maths is no more complicated than Pythaogras' theorem, plus relative motion. Its only because we have years of low velocity "common sense" that causes the issues!

Colin




Main scope: Ikharos 8" RC
Camera: Atik 460ex with Atik EFW and Baader LRGB Ha, OIII and SII filters
Guide scope: On Axis Guider with Atik 314L+
Mount: Paramount MX located in Astrocamp, Nerpio, Spain

 

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