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Space-Time, Relitivity, and Phone Calls





dgrowe
We all know that one could not travel at speed c (186,000 miles per second), but the closer you travel to c, the slower your time goes around you. Example: if you traveled in a spaceship at a super high speed for 10 years, earth would have aged 100 years (you age 10 years, earth ages 100). Therefore, people on earth view your time as "slowing down". If it were possible, and you travel at a great rate of speed, and made a phone call to earth; would the phone call seem "normal" to you, but sound slower to people on earth (the speed of your voice seem slower and longer to speak)?
ocalhoun
dgrowe wrote:
If it were possible, and you travel at a great rate of speed, and made a phone call to earth; would the phone call seem "normal" to you, but sound slower to people on earth (the speed of your voice seem slower and longer to speak)?

Any relativistic effect (though present) would be drowned out by the effects of Doppler shift.


In this pictured example, we'll need to change a few things to make it more applicable.
The car is your spaceship.
The sound from the car is the radio (or light or whatever) waves from your 'phone'.
The observers are the Earth, depending on where your ship is relative to it.

So, you can see, that if you're going towards the Earth, it'll be much faster, and if you're headed away, it'll be much slower.
dgrowe
But the Doppler effect doesn't include the variable of time, which in this case is the problem. Since you're aging much slower due to your speed in the spacecraft, would you be speaking slower to someone on earth.

If you're using a cell phone, the sound in front and behind the observer is not changed due to there is no "compression" from to air molecules. This works great for objects traveling on earth; but doesn't work for space situations that are near vacuum. The variable in this case, is time. Would you sound slower because of your high rate of speed (and thus, time travel)?
Bikerman
dgrowe wrote:
But the Doppler effect doesn't include the variable of time
Of course it does.
f0=[(v+vr)/(v+vs)]*f1
(where v is the velocity of waves in the medium, vs is the velocity of the source relative to the medium
and vr is the velocity of the receiver relative to the medium).
Since velocity is the rate of change of position, time is implicit.
dgrowe
But the Doppler shift isn't the question here; time is. When speaking to astronauts in the space station (via phone), the Doppler effect isn't noticed here on earth to people on that call. As the astronauts orbit the earth, the phone call doesn't go slower (as they pass the earth person), than faster (as they orbit earth and come back around the other side) due to their speed relative to those on earth, right?

The original question was if time effects a phone call. Therefore, if the International Space Station station where to speed up 1,000 times faster in orbit than it is now (and it were to stay in orbit by propulsion), would the time-relativity theory make the conversation for someone on the space station to someone on earth go much faster?
Bikerman
Quote:
As the astronauts orbit the earth, the phone call doesn't go slower (as they pass the earth person), than faster (as they orbit earth and come back around the other side) due to their speed relative to those on earth, right?
Well, the doppler effect isn't really relevant there since they are not shouting through air, but using em radiation to carry the call, and, as we know, the speed of light is constant for both the astronaut and the 'earth observer'.
The thing is, that time is not constant - each experiences time differently according to their relative motion.
Indi
Bikerman wrote:
Quote:
As the astronauts orbit the earth, the phone call doesn't go slower (as they pass the earth person), than faster (as they orbit earth and come back around the other side) due to their speed relative to those on earth, right?
Well, the doppler effect isn't really relevant there since they are not shouting through air, but using em radiation to carry the call, and, as we know, the speed of light is constant for both the astronaut and the 'earth observer'.
The thing is, that time is not constant - each experiences time differently according to their relative motion.

i don't think you need air for the Doppler effect. >_< i think that in the case of EM radiation propagating without a medium (self-propagating), the Doppler effect is due only to relative velocity of the source and receiver.

So, in this case, the Dopper effect will mean observer 1 will hear the message coming in very slowly (redshifted, like the batteries running down on an old-fashioned tape player), while observer 2 will hear the message coming in very quickly (blueshifted, like chipmunks talking).

You also have to consider the time dilation to consider how much slower the people on the ship making the phone call are moving relative to the receiver on either end.

So it's a really complicated situation that i can't just figure out off the top of my head without crunching numbers. But... i suspect you'd get a situation like this:

Ship calling observer 1: Observer 1 will hear people on the ship talking very slowly, and vice-versa.
Ship calling observer 2: Observer 2 will hear people on the ship talking very quickly, and vice-versa.

And in either case, the time delay between sending and receiving will be huge.
Bikerman
Yes, OK, mea culpa. Of course the Doppler effect occurs regardless of the medium.
It is indeed complex and I'm going to have to think about it before saying much more (if I get the time and can be bothered Smile )
TomS
ocalhoun wrote:
Any relativistic effect (though present) would be drowned out by the effects of Doppler shift.


Doppler effect appears if the ship moving towards or away from the observer.
What if the ship circles the observer. Then the distance is constant and there's no doppler effect, right?
Indi
TomS wrote:
ocalhoun wrote:
Any relativistic effect (though present) would be drowned out by the effects of Doppler shift.


Doppler effect appears if the ship moving towards or away from the observer.
What if the ship circles the observer. Then the distance is constant and there's no doppler effect, right?

Correct, but there's still time dilation.

It's too bad i can't do equations here, or it would be easy to show what happens, and you could experiment by playing with the values.
TomS
Indi wrote:
Correct, but there's still time dilation.


And that was the original question of this thread. Until the doppler effect was brought into play.

Indi wrote:
It's too bad i can't do equations here, or it would be easy to show what happens, and you could experiment by playing with the values.


Try http://math.at.gg/
It's only available in german, but eventually you figure out how to use it. Mathematics is a universal language.

For example:
Bikerman wrote:
Of course it does.
f0=[(v+vr)/(v+vs)]*f1

Indi
TomS wrote:
Indi wrote:
Correct, but there's still time dilation.


And that was the original question of this thread. Until the doppler effect was brought into play.

The Doppler effect does matter, and - depending on a lot of factors - may even dominate.

TomS wrote:
Indi wrote:
It's too bad i can't do equations here, or it would be easy to show what happens, and you could experiment by playing with the values.


Try http://math.at.gg/
It's only available in german, but eventually you figure out how to use it. Mathematics is a universal language.

ich werde hineindenken. ^_^

Ok, here's the relationship:


f(s): the frequency of the source signal.
f(o): the frequency you observe.
v: speed of the ship.
θ: the angle the ship is travelling at with respect to the observer (0 is flying away, 180 is towards).
c: speed of light.

The first term is the time dilation factor, and it is always (0, 1] so it always slows down the frequency (makes f(o) less than f(s)) unless you're standing still. It is dominant at higher speeds.

The second term is the Doppler shift factor, and it ranges from (0.5, ∞), so it can slow down the frequency a bit (but not as much as time dilation), but it can speed it up infinitely. It is dominant at low speed, or when the ship is coming right at you at high speed (but is somewhat cancelled out by the time dilation).

When both time dilation and Doppler are combined, the frequency shift is (0, ∞)... basically the final frequency can be anything - higher or lower than the original signal - so you could hear ultra fast messages or ultra slow ones. If the ship is moving away or perpendicular to you, then you will always hear slow messages. If the ship is moving towards you... that's where it gets complicated. Depending on the exact angle and speed, you could hear the message as faster, slower, or... even normal speed. (In fact, if you have time on your hands, you could use the quadratic formula to calculate what angle you would have to fly at (θ) to get normal message speed for any v.)
Chinmoy
i did not get a ting you all are talking about! What is this phone call doing with the topic?
Bikerman
Chinmoy wrote:
i did not get a ting you all are talking about! What is this phone call doing with the topic?
If you have a specific question then by all means ask it, but I suspect that you don't understand the basics, in which case it would be better to remain silent.
lovescience
Does that mean if we get a phone from a spaceship traveling in light speed, in order to listen its original speaking voice, we will need to know its traveling speed and angel to the earth, then to change the phone call frequency?

I wonder does NASA have machine traveling in space sending sound recording signal back to the earth. Does the machine send back the sound in a file(pre-recorded) or in real time?
Bikerman
It isn't necessary. Current spacecraft have a maximum velocity of around 20,000 mph. That is only 0.003% of the speed of light, which is insignificant when considering dilation effects.
Indi
lovescience wrote:
Does that mean if we get a phone from a spaceship traveling in light speed, in order to listen its original speaking voice, we will need to know its traveling speed and angel to the earth, then to change the phone call frequency?

Correct!

Suppose you are receiving a signal from a spaceship travelling near light speed relative to you. If you don't know how fast the ship is travelling relative to you, and you don't know what frequency it is broadcasting at, you're out of luck trying to reconstruct the original signal.

Take Voyager 1 for example. Suppose you received a 4 GHz signal from Voyager 1. What was the original signal? What is the speed it's travelling at? You have no way to find out. This is the equation you need to use:


You know c (it's a constant), and you know f (it's 4 GHz), but you don't know f₀ or v. So, one equation, two unknowns, you're screwed.

If you know v then you can figure out the original frequency. But what actually happens in real life more often is that you don't know v, but you know the original frequency, because either:
  • you have a physical law that predicts the frequency - for example, you know that helium emits at 510.29 THz, so if you get a helium spectra at 400 THz, you can calculate the speed of the object relative to you (this is how Hubble discovered the universe is expanding); or
  • you built the transmitter - for example, in the case of Voyager 1, the transmitter is an 8.4 GHz transmitter, so if you receive a 4 GHz signal from it, you can figure out its speed.

So, in reality, what usually happens is you get a signal from your spaceship at some frequency, and you know the original frequency, so you just speed it up or slow it down. You don't really need to know the relative velocity.

lovescience wrote:
I wonder does NASA have machine traveling in space sending sound recording signal back to the earth. Does the machine send back the sound in a file(pre-recorded) or in real time?

Oh, NASA has launched thousands of machines into space, but i think only a few hundred are currently operating. That includes everything from deep space probes like Voyager 1, to GPS satellites to television broadcasting satellites. Actually, frequency shifting and relativistic effects are a real problem for them. Famously, GPS satellites have to perform anti-relativistic corrections to keep their on-board atomic clocks accurate, and GPS receivers have to take Doppler into account because even though the satellites are transmitting at 1.57542 GHz the frequency you receive will probably be shifted.

Generally speaking, most satellites do not continuously transmit, because that's a waste of power. Instead, they store up a bunch of stuff, compress it, then burst transmit.
lovescience
thanks your posting! I am inspired to think of a spacecraft capable of traveling in light speed, the substance detection using frequency, and the signal decoding.

Voyager is a great project. It took Voyager years to go from earth to out of our solar system. Does human already have a technology to build a machine that can travel in space faster than Voyager to go from one place to another?
Indi
lovescience wrote:
Does human already have a technology to build a machine that can travel in space faster than Voyager to go from one place to another?

In space, the magic word is acceleration, not speed. What you would need is something that can accelerate hard enough, for long enough, to match Voyager 1's speed. We have many things that can accelerate far, far faster than Voyager 1 ever could.

Voyager 1 also got boosts from the gravitational wells of Jupiter and Saturn, which is why it is currently the fastest travelling space probe we have.

But we could make something that travels far, far faster, if we really wanted to. There's not really much reason to want to, though. Anything travelling that fast would shoot through all the interesting stuff to see in our solar system too fast, and then there would be a whole lot of nothing for decades, or even hundreds of years, until it encounters another star.
paolord
Are cellphone and radio signals just as fast as lightspeed? I mean would it be possible for a spaceship travelling at lightspeed recieve signals from earth?
Bikerman
Yes, essentially. Radio waves are 'carried' by photons - as is the whole electromagnetic spectrum.


The actual speed will depend on the medium which the waves move through. The photons collide with particles and this slows them down, so only in a vacuum does light travel at 'full speed'.
Indi
Actually, it depends. If a spaceship were travelling directly away from Earth at light speed, a signal sent from Earth would never reach it.

(But that doesn't require relativity to understand. If a car is driving away at 100 km/h and i shoot a rocket after it that travels at 100 km/h, the rocket will never hit it (unless the car slows down or stops before the rocket).)
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