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Author Topic: Headlights on my spaceship?  (Read 7532 times)

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Offline jgs001

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Re: Headlights on my spaceship?
« Reply #15 on: 05:03:33, 26 February, 2011 »
I never really got this stuff too well when I was studying applied physics, now... I just have a headache... ;)

Offline 661-Pete

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Re: Headlights on my spaceship?
« Reply #16 on: 23:16:36, 18 May, 2011 »
An old-ish thread, but I was just browsing and I love adding my 2p-worth to this sort of debate!  :embarrass:

I look at it this way - if an arrow leaves my archery bow at 80 kph when I'm standing still, then if I'm moving forward at 20 kph, then the arrow will leave the bow at 100 kph relative to the ground. If I'm travelling at the speed of light, then surely the light from my headlights would travel at twice the speed of light relative to a stationary object. Am I right?

Will we have to wait until we meet Spock before we get a definitive answer? Am I going daft? Please do not answer that last question (I already know the answer!)
So, to carry the analogy a bit further, and work in the explanations from Charles and others:

You are Robin Hood, galloping through the glen at the exact speed of 20 Km/h.  You see the Sherriff of Nottingham straight ahead, so you loose an arrow at a speed of 80 Km/h relative to you.  The velocity of light c = 3 x 105 x 3600 = 1.08 x 109 Km/h.

So the velocity with which the arrow plunges into the unfortunate Sherriff's chest, is 100 Km/h?  Not exactly.  The fomula is:
(20 + 80)/(1 + 20 x 80 /c2)

which works out at 99.999999999999862825788... Km/h approx.

Now suppose that Robin (ever a lousy shot) misses the Sherriff's chest, and hits him on the big toe instead.  How fast is the arrow closing in on his big toenail?

I'm assuming that toenails grow at about 3mm/month or 4.167 * 10-9 Km/h.  So the arrow closes in on the toenail at about 100.00000000416652949... Km/h - slightly over the 100 Km/h mark.

All goes to show, at terrestrial speeds, relativity's effect is rather small...
Pete

(Orion Europa 250, Vixen GP, Canon EOS350D) Telescope and mount in France, but not doing much at present..

Offline swashy

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Re: Headlights on my spaceship?
« Reply #17 on: 08:10:12, 19 May, 2011 »
Ouch! thats gonna throb a bit
Ade

Imagine...  above us only sky!

Offline mitchfry

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Re: Headlights on my spaceship?
« Reply #18 on: 20:31:27, 19 May, 2011 »
Of course if you were to drop into a black hole on your journey, all of the above would be irrelevant  ;). As a scientist myself, biological not physical, I can`t help thinking of the many theories that have been proposed, believed in, and followed without question, only ultimately (and painfully) to be proved wrong. It could be argued that these theories have held back research and understanding. Now I`m not about to knock Einstein, but what if he is wrong! We have tested his theories and they seem to hold up, but only because we are using techniques and approaches that are available to us. Why should the speed of light be limiting? And if something is travelling faster than the speed of light, then we can`t see it or measure it, but that does not mean it does not exist! There are physicists out there who are proposing theories contrary to Einstein`s doctrine, but they are very slow to be accepted, if at all. Are Einstein`s theories holding us back, are we all so blinkered that we are failing to see other things? We need a non-Einstein eureka moment, but in the meantime keep your headlights switched on, they may show you the way, most of the time.
Mitch
WO FLT132mm, CGE mount, SBIG ST-2000XCM. Captured and processed in MaximDL. Under the "sunny" skies of South Yorkshire :).

Offline Trow

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Re: Headlights on my spaceship?
« Reply #19 on: 08:40:54, 22 November, 2012 »
Hi, I can see this is a very old thread and so maybe nobody will see this reply - sorry don't really know the etiquette  :embarrass:

I'm sure you're right, we do need another eureka moment - there's always more to learn - but Einstein's theories are in no way holding us back. Most of Einstein's predictions have been proven experimentally, even the most counter intuitive. For example - The subatomic particles muons, usually only exist for 2.2 microseconds at rest. Muons have been accelerated to 99.94% of the speed of light, which Einstein's basic equation for time stretching predicts will increase the life of the muon by a factor 29. That's exactly what the scientists, who whizzed them up, observed - the muons lived 29 times longer - amazing?! There have been many other experimental results that match Einstein's predictions, so there's little doubt he was on to something, and, without his theories we would still be swimming about in the eather. It's great that physicists are challenging  current thinking, the trick is coming up with the experiment, the results of which match the predictions. His theories propelled us rapidly forward and opened loads of doors for theoretical physics - we just need to think as deeply and elegantly as he did and walk through one of these doors.
Bresser 8" f4.3 Newt, CGEM, unbranded Chinese digital eyepiece.

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Offline mcgillca

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Re: Headlights on my spaceship?
« Reply #20 on: 13:57:36, 22 November, 2012 »
Hi, Trow. I completely agree.

Although there are other ideas out there (especially modified gravity theories), most do not match the observed data as well as Einstein's theories. Although they may solve something specific (e.g. constant rotational velocities seen in spiral galaxies), they (to date) fall down somewhere else. Moving from Einstein's theories to something else will almost certainly involve quantum gravity, for which string theory is the best candidate, although without any supporting experimental proof.

As for why something can't move faster than the speed of light, its much deeper than just the properties of a particle. One of Einstein's key insights was to realise that this is a property of space and time itself. Its impossible for any particle to move faster than that because space and time adjust to make that impossible.

For example, in the example of moving towards M31, space contracts (as seen by the rocket), so that the journey seems to take only 50 years.

From the point of view of someone in M31, time on the rocket slows down so that the journey seems to take 1 million years, but the person on the rocket only ages by 50 years.

Colin

Main scope: Ikharos 8" RC
Camera: Atik 460ex with Atik EFW and Baader LRGB Ha, OIII and SII filters
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Offline Trow

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Re: Headlights on my spaceship?
« Reply #21 on: 12:18:25, 21 March, 2013 »
Slightly different take on it but as space is a (near) vacuum there wouldn't be anything for the light to reflect off so you wouldn't see the light, however an observer in front of you would be able to.

The speed side of it is covered by Special Theory of which I understand very little except it has something to do with time dilation and space contraction. :confused:


I know this a really old thread but I'm dead interested in this stuff at the moment.

I thought it would be the other way round. As you stand at the bridge of you space ship travelling near the speed of light, you switch you lights on and the lights shine out in front at the speed of light relative to you. A stationary observer (relative to the space ship) wouldn't see the ship approaching until it was almost there, and surely wouldn't see the lights approaching?
Bresser 8" f4.3 Newt, CGEM, unbranded Chinese digital eyepiece.

Expanding Universe denier.

Offline mcgillca

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Re: Headlights on my spaceship?
« Reply #22 on: 23:24:40, 21 March, 2013 »
Almost. The light would still travel faster than the space ship.

The stationary observer would see the light beam, travelling towards them at speed of light. Suppose the space ship was travelling at 0.8c, and turned on its lights when the stationary observer measured it to be 1 light year away.

The light would arrive after 1 year, and the spaceship after 1.25 years. The observer would then see the headlights for 3 months before the ship arrived.

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 Trow

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Re: Headlights on my spaceship?
« Reply #23 on: 13:46:14, 15 May, 2013 »
Wow! You seem to really understand this stuff?

I understood that, for the passengers on the ship, the lights would shine out ahead, but I was imagining the ship travelling a lot faster when they flicked on the light switch - say 0.999...C. Surely then, the observer on the planet, towards which the ship was travelling, would never see the light until the ship either slowed down or crashed into them?

How about this scenario then (if you, or anyone, else are bored enough to indulge me)...

2 planets, 1 ly apart. A ship on each planet is launched toward the other planet. The pilots turn their headlights' on at the moment of launch. Assume instant acceleration to almost light speed, and ignore the need to decelerate when they arrive.

a) What do the pilots observe of their counterparts' ship and headlights?

b) What do "stationary" observers on planets see?




Bresser 8" f4.3 Newt, CGEM, unbranded Chinese digital eyepiece.

Expanding Universe denier.

Offline Nomis Elfactem

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Re: Headlights on my spaceship?
« Reply #24 on: 15:47:19, 15 May, 2013 »
a) What do the pilots observe of their counterparts' ship and headlights?

Well from my understanding (which ranges from grasping the concept to no clue whatsoever) the ships will take just over a year to reach their respective planets and they would pass (assuming they plotted the correct course) each other halfway after 6 months.  Because they aren't traveling at the speed of light the each pilot would see the others headlights before they saw the ship.

Quote
b) What do "stationary" observers on planets see?

They will see a launch then a year later they will see the lights of the oncoming spaceship before it arrives just over year after it set off / the other ship was launched...

.... I think  ;) :laugh:

Simon

Scopes: Astro-Tech AT-111EDT Triplet, TS65ED Quad, Orion ST80, Modded PST-90 Solar Scope, PST Cak (on loan)
Cameras: SXVF H694, Atik 16ic, Canon EOS 600d, DMK41, DMK21, QHY 5L-II (mono & colour)
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Offline mcgillca

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Re: Headlights on my spaceship?
« Reply #25 on: 11:22:01, 16 May, 2013 »
Wow! You seem to really understand this stuff?
Courtesy of a misspent doctorate

Quote
I understood that, for the passengers on the ship, the lights would shine out ahead, but I was imagining the ship travelling a lot faster when they flicked on the light switch - say 0.999...C. Surely then, the observer on the planet, towards which the ship was travelling, would never see the light until the ship either slowed down or crashed into them?

How about this scenario then (if you, or anyone, else are bored enough to indulge me)...

2 planets, 1 ly apart. A ship on each planet is launched toward the other planet. The pilots turn their headlights' on at the moment of launch. Assume instant acceleration to almost light speed, and ignore the need to decelerate when they arrive.

a) What do the pilots observe of their counterparts' ship and headlights?

b) What do "stationary" observers on planets see?


b) is easier - as Simon says, they will see the lights of the oncoming spaceship just before the spaceship arrives. The maths is identical to my original example, but with 0.999c, they will arrive 1/1000 of a year, or about 8 hours after the light arrives.

a) Again, Simon is right. The light beams would cross at the mid-way point, and the rockets would be just behind (about 4 hours as observed by an observer on one of the planets). On the ship, time would slow down by a factor of sqrt(1-v^2/c^2), or about 1/20th of time as observed by the stationary observer. So, on either the ship, they would see the light beam, and 12 minutes later the other rocket would pass them. Incidentally, the combined relative speed of the two rockets would be 0.9999995c, and they would observe time to pass at 1/1000 of their speed on the passing rocket. Pretty fast!

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 Trow

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Re: Headlights on my spaceship?
« Reply #26 on: 14:46:30, 29 May, 2013 »
Thanks Simon/Colin - you guys must be as bored as I am?

So the pilots would experience a journey time of 18days?

An observer on one of the planets, one year after takeoff, would look through their theoretically good telescope (capable of resolving down to a spaceship width at 1 ly!) and see the whole journey of the inbound ship, takeoff to landing, last little more than 8 hours?!?!
Bresser 8" f4.3 Newt, CGEM, unbranded Chinese digital eyepiece.

Expanding Universe denier.

Offline AJG

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Re: Headlights on my spaceship?
« Reply #27 on: 22:24:18, 30 May, 2013 »
So much for the easy bits, I have been mulling this over and here is the part that is baking my noodle:

Lets suppose that our spaceship, travelling at 0.8C relative to an external observer, is 1 light second long. At each end are two crewmembers, John in the prow and Jane in the stern. They send light signals to each other; John Prow sending his signal 0.2s after Jane Stern sends hers (as previously agreed and having synchronised atomic clocks)

To the crew of the ship, John Prow receives his signal (Event A) 0.2s before Jane Stern receives hers (Event B).

However, to our external observer, Event B occurs 4.44…s before Event A.

The order of the events is switched depending on the observer’s position.

What have I misunderstood or should I surrender my sanity now?

Adrian.

Offline Nomis Elfactem

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Re: Headlights on my spaceship?
« Reply #28 on: 09:29:04, 31 May, 2013 »
I'd surrender your sanity personally  :laugh:

 :thumbup:
Simon

Scopes: Astro-Tech AT-111EDT Triplet, TS65ED Quad, Orion ST80, Modded PST-90 Solar Scope, PST Cak (on loan)
Cameras: SXVF H694, Atik 16ic, Canon EOS 600d, DMK41, DMK21, QHY 5L-II (mono & colour)
Accessories: SX USB Filter Wheel, SX OAG, Baader LRGB Ha SI OII Filters, SharpSky Focuser
Mount: EQ6 (EQMOD), SW Star Adventurer, plus a lot (and I mean a lot) of other bits and pieces

Offline mcgillca

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Re: Headlights on my spaceship?
« Reply #29 on: 10:56:48, 31 May, 2013 »
Dear Adrian,

You are quite right. 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).

Otherwise, you can observe event 1 occurring before event 2, but another observer will see it the other way round.

On the other hand, causality is agreed by every observer. So if Jane Stern's light beam arrived at John Prow's position and that caused him to trigger his light beam, then all observers would agree that Jane's light beam was sent before John's (and in fact is a corollary of the fact that all observers agree on simultaneity at the same point in space time - every one agrees that Jane's light beam arrives at John's location exactly the same time that John's is released).

Special relativity always sounds so simple - just two postulates (physics only depends on relative motion, and the speed of light is constant for every observer), and the maths is straightforward (just Pythagoras' theorem), but the consequences are profound.

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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