OT : Relativity question.

I agree with all four, especially 4.

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Albert van der Horst, UTRECHT,THE NETHERLANDS 
Economic growth -- being exponential -- ultimately falters. 
albert@spe&ar&c.xs4all.nl &=n http://home.hccnet.nl/a.w.m.van.der.horst

It it is if you want to have the laws of physics be the same for all observers in an inertial reference frame.

Because whilst that works fine at the sorts of speeds we are used to dealing with it doesn't work at all well at relativistic speeds. People would have a much better intuition if the speed of light were closer to the speed of sound so that relativistic effects were more obvious.

You have to be very careful about observing an event and seeing it since the latter includes the light travel time from the event to your eye. What you see with a relativistic jet beaming looks like a superluminal velocity but is an illusion due to the geometry.

That isn't a helpful way to look at it.

Photons in essence don't see time or age at all in their rest frame.

You can derive special relativity from the invariance of the speed of light c in a vacuum and careful consideration of the mutual events of two metre rules passing each other at a relativistic velocity v.

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Regards, 
Martin Brown

It seems to me the two questions are:

1) Will the OP will see on his speedo, operating as described, a speed greater than c?

2) Can he use that displayed speed, along with his printed distance charts and simple division, to work out how long it will take him, by his watch, to reach his destination?

I'd say yes to both, what do you say?

Cheers

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Clive

Yes, both are true. But when he reaches his destination he will find his watch is some time slower than the local time. So did he in fact travel faster than the speed of light or did he just think he did because his time frame slowed down?

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Rick C

Tricky. The outcome is the same, so in one sense, both views are valid. However, the laser beam he switched on when he left (didn't I mention that?) has far outpaced him, and that *is* light so must be travelling at the speed of light, so he must be slower.

This is where Albert's point that light speed is in some sense infinite seems valid. But I'll stop now before I prove that black is white and get run over on a pedestrian crossing.

Cheers

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Clive

Lol, I thought you were talking about the *other* Albert... No, I don't agree that light is in any sense traveling at infinite speed.

As pointed out in the twin paradox, there *is* an unaccelerated frame of reference which is easily distinguished from the accelerated frame. Once you consider that then it all makes better sense.

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Rick C

Oops, sorry, I meant Albert van der Horst of this parish.

But, from Mr Photon's POV, he's just crossed the universe in what his (very small) wristwatch says is no time at all, so in some sense...

Cheers

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Clive

I already explained that once he comes to rest with respect to his original reference frame he will see that his universe was distorted by the motion. He will then understand that he has not been traveling faster than light, but rather that his reference frame while at motion was different from the other. Once you take that into account, it is easy to see that he was never traveling faster than light nor was light itself.

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Rick C

I see this thread is still going.

To the OPs original question.

If you could travel along with a light beam. (this was Einsteins original though process by the way)

You would get to your destination in no time. (by your own clock) as I said way back a few month ago.

Thereby, using your ordinary "speedometer" it would read infinity. Your velovity would seem to you to be infinity.

This is not controversial or untrue.

We observe light to travel at the speed of light

light observes itself to travel at infinite speed.

From the lights frame, it is emitted and absorbed simultaneously, no matter what we think the distance or time it covered.

So for a more physical view, if you burn more and more more fuel, you measure YOURSELF going faster and faster WITHOUT LIMIT all the way to infinity.

It is only an outside observer that sees your velocity reach a limit of c.

But your question is about your own speedometer in your own frame. In your own frame you can reckon you are going infinity fast, as does a light beam by it's own reckoning.

Mark

Correct. But that is because your clock is running slower due to SR.

Only if you insist on incorrectly dividing the distance measured in a stationary reference frame by time measured in a moving reference frame.

This is about as logically consistent as dividing apples by cheese.

In a vacuum. Otherwise its speed can be altered by refraction.

True enough but it isn't a helpful way to look at it. For all inertial reference frames travelling at a speed v So for a more physical view, if you burn more and more more fuel, you

Only if you insist on dividing two quantities measured in different reference frames to make a "speed". If you make a round trip then you will discover the true time that it took you to reach your destination as measured by the people you left behind on the Earth.

That isn't a helpful interpretation though. It is more like for a light beam there is no passage of time. Dividing by zero is not a good idea.

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Regards, 
Martin Brown

No it's the same frame, the frame of the traveler.

I think the entire fundamental point of relativity is that there is no one "TRUE TIME"

For the traveler on the spaceship traveling with the light beam, he gets to the destination in near 0 time, his speedometer correctly reads near infinity and for the gist of the the OPs question, that is HIS true time and true speed.

For others left on Earth there is a different true time and speed.

Both are correct and equally valid.

A scientific example of alternate facts?

:-)

Mark

I don't think so; the "pre measured distance" between the posts was measured *at rest*.

And when moving in fact those distances will shrink - the view ahead becomes compressed.

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John Devereux

one "TRUE TIME"

this is very interesting

book title Can Star Systems be Explored?: The Physics of Star Probes

if you could make a rocket that could accelerate at 1G indefinitely,

you could reach the edge of our observable universe in about 25 years of " ship time". Time on Earth of course will have passed 1000's of years.

mark

"If wishes were horses, beggars would ride."

A baseball (5.5 oz) moving at a mere 0.9c would have a KE of several megato ns (see xkcd).

"For thousands more years the mighty ships tore across the empty wastes of space and finally dived screaming on to the first planet they came across - which happened to be the Earth - where due to a terrible miscalculation of scale the entire battle fleet was accidentally swallowed by a small dog."

"Those who study the complex interplay of cause and effect in the history of the Universe say that this sort of thing is going on all the time, but that we are powerless to prevent it. 'It's just life' they say."

(Douglas Adams was a genius. RIP.)

Cheers

Phil Hobbs

"Tau Zero" by Poul Andersen.

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John Devereux

Exactly. The mile posts were put in by somebody who moved at a walking pace (or > And when moving in fact those distances will shrink - the view

The distances become compressed when you measure them from a moving frame of reference. And you can only measure intervals between point events defined in both frames of reference. It is at the fundamental core of special relativity that the laws of physics are the same for all observers in an inertial frame of reference.

What you *see* when moving at light speed is that things coming towards you look squished and rotated and for favourable geometries you can see

5 sides of an approaching cube simultaneously. Its a Physics FAQ

Therein lies the problem. This is standard relativistic dynamics as taught at undergraduate level and somewhat misquoted. At 1g you can get to the centre of our galaxy in about 20y and Andromeda in 28 if you want to stop and actually take a look at what is there. See

The engineering difficulties would be beyond impossible.

You are hopelessly optimistic. ie Wrong!

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Regards, 
Martin Brown

I believe the trip by earth time would be in excess of 15 billion years since that is the time it would take light to travel that distance, hence the term "observable universe". Remarkably quick actually.

If the universe started in a very small space and could not expand faster than the speed of light, how could any part of the universe be bigger than 15 billion light years?

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Rick C

You got that backwards. Some bloke noticed that objects are red-shifted more or less proportionally to their distance. Attributing this to the Doppler effect, that would imply that objects recede with velocities proportional to their distance. The distance at which a linear extrapolation of this velocity reaches c got declared to be the edge of the universe, never mind relativity. The big bang is what you get by extrapolating this the other way. Then a bunch of non-physical patches got tacked on to 'explain' some discrepancies.

Jeroen Belleman

As I pointed out earlier this is wrong. You would be able to get almost to the Andromeda galaxy in 25 years at 1g but that is about it.

And in that 15 billion years travel the universe will have had another

15 billion years of what now seems to be an accelerating expansion. There are parts of our universe forever inaccessible to us since they are already receding away from us with a Hubble velocity >c.

We can only see back to the surface of last scattering where the universe first became transparent to electromagnetic radiation.

That restriction doesn't apply to spacetime itself. A full GR treatment allows (indeed needs exponential expansion early on to get the universe to match the boundary conditions we see today in the standard model).

This summary by Alan Guth summarises it far better than I can:

The strict restriction is that you cannot ever send information faster than the speed of light. You can expand spacetime in GR so that some parts of it are moving away from each other at speeds >c.

Cosomologists are very careful to distinguish an event horizon, a particle horizon and an absolute horizon for a given observer, trajectory and spacetime. Essentially mapping out the region of space that can be seen or reached starting from a given position.

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Regards, 
Martin Brown