# Faster than light

JesseM
all i want to know is if you travel a couple of thousand lyrs at close to the speed of light that you wouldnt die of old age before you got there
The question is ill-defined if you don't specify in which frame the distance between the starting point and the destination is 2000 light years. In the frame where the distance is 2000 light years, any ship will take more than 2000 years to get there, but in this frame the clocks on the ship will be running slow so they can tick a lot less than 2000 years over the course of the trip. In any frame where it takes less than 2000 years to get between starting point and destination, it's because the distance between starting point and destination is less than 2000 light years due to Lorentz contraction. No matter which frame you choose, no objects move faster than one light year per year when their speed is measured in that frame.

I was quite clear - as were others - to say that there are two different distances being measured from two different frames. I said explicitly in the last sentence how far he traveled and how long it took. Again: you cannot take a distance measured from one frame and a time measured from another and calculate a speed.

I've never heard of someoneone taking a cross-country plane trip across time zones and exclaim 'wow, looks like we just traveled at 900 mph!.
People understand different frames of reference in that context and never make such a calculation.
That's all the problem is here.
Non-sense,
I never suggested they calculate anything of the sort – only that traversing 10ly in only 8.9 years gives the appearance of traveling FTL.

If you "cannot take a distance measured from one frame and a time measured from another and calculate a speed"
Please explain what are scientist doing when the define “Proper Velocity” and when and where would they use it if not here?
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After working with several space-stations using light signals and radio communications to meticulously synchronize all Space-Station Clocks and Install Mile Markers at each space station. You’re suggesting that after travel like this you would ignore the mile markers you find at landing and instead rely only on some kind of in flight calculations to recalculate the distance you covered? (I don’t do that when I fly from Chicago to Columbus I look at the mile marks, just as I use my stop watch to measure elapsed time, your analogy of Time Zones has absolutely nothing to do with synchronization.)

What is your plan, everyone traveling with you is measuring the same elapsed time on their watches, 8.9 years does not justify traveling 10 ly. But the darn “10 ly from Earth” Mile Maker is right there at the station for all to see. For appearances sake do you ‘correct’ the mile marker with a 6.6 and get in a fight with the Station Master? (He already had one arrested for defacing his sign with 5 ly after getting off a ship traveling faster than you.)
Do plan to reset all their station clocks to your personal proper “time zone” as well?

Seeing the mile maker and measuring your proper elapsed time are directly observable facts. Denying them and not recognizing appearances is more irresponsible than failing to explain apparent FTL trip.

The appearance is real and is totally explainable with nothing moving FTL!
But you need understand SR, Simultaneity, and Proper Velocity (see Wiki) to explain it. And if you do not understand all three of those; then you have no business being the Near Light Speed Navigator – we’ll try to find you a job in the kitchen.

JesseM
If you "cannot take a distance measured from one frame and a time measured from another and calculate a speed"
Please explain what are scientist doing when the define “Proper Velocity” and when and where would they use it if not here?
Where have you seen scientists defining a "proper velocity"? Can you provide a link or a reference to a book by a physicist?

edit: never mind, found a wikipedia article with some references here. But I don't see anything in the definition that says the "dx" in dx/dtau must be measured in the object's current rest frame as you suggest--in other words, if I travel from Earth to Alpha Centauri and then come to rest there, there's nothing that says I should define my proper velocity in terms of the distance in the Earth/Alpha centauri rest frame as opposed to some other frame.

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Fredrik
Staff Emeritus
Gold Member
Non-sense,
I never suggested they calculate anything of the sort – only that traversing 10ly in only 8.9 years gives the appearance of traveling FTL.
But there's no frame in which the rocket moves 10 light-years in 8.9 years. There's a frame where it moves 10 light-years in 13.3 years, and a frame where it moves 6.7 light-years in 8.9 years. The speed is 0.75c in both of them.

...your analogy of Time Zones has absolutely nothing to do with synchronization.
The point of Russ's analogy was that if you calculate the elapsed time by taking the arrival time in one coordinate system minus the departure time in another coordinate system, you get the wrong result. It illustrates why it's a bad idea to use numbers from different coordinate systems in a calculation.

"10 light-years" is the distance in one coordinate system, and "8.9" years is the time in another. Dividing them to get a speed makes no more sense than calculating the time you spent on the plane the way I described above (for exactly the same reason).

edit: never mind, found a wikipedia article with some references here. But I don't see anything in the definition that says the "dx" in dx/dtau must be measured in the object's current rest frame as you suggest--in other words, if I travel from Earth to Alpha Centauri and then come to rest there, there's nothing that says I should define my proper velocity in terms of the distance in the Earth/Alpha centauri rest frame as opposed to some other frame.
In the context you are using here "Proper" values are defined in Earth/Alpha Centauri rest frame not the Traveler frame. All other frames would see the same "Proper Time" which would = the real time the traveler would see pass but very different than the time observed in that frame.
But each frame would define a different "Proper Distance" traversed in that frame; so of course the "Proper Velocity” of the traveler would be different in different frames.
Obviously it is important to keep track of these values based on which frame your considering as you work out collisions of high speed colliding from opposite directions. Not something we use in everyday life.

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shamphys
I assume you mean the statement in wiki:
" ... a constant 1 g acceleration would permit humans to travel as far as light has been able to travel since the big bang (some 13.7 billion light years) in one human lifetime. "
Would "appear" as a FTL speed or "change in location" - just remember that in SR the traveler still does not see a local FTL event at any time - and you will begin to understand what Einstein was saying with SR.

shamphys
I assume you mean the statement in wiki:
" ... a constant 1 g acceleration would permit humans to travel as far as light has been able to travel since the big bang (some 13.7 billion light years) in one human lifetime. "
Would "appear" as a FTL speed or "change in location" - just remember that in SR the traveler still does not see a local FTL event at any time - and you will begin to understand what Einstein was saying with SR.

i mean that if you could travel at very close to the speed of light(ignoring the acceleration problem for the moment another thread for another day)that you could travel very great distances within a heartbeat and this would give you the "appearance" that you traveled faster than light

can i say again i dont beleive you can travel at the speed light or faster

also any chance of answer for my question about how long it takes for a pulse of light to travel 10 lyrs ,from the pulse of lights point of view

JesseM