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proberecall
United States104 Posts
My question is: what happens if they both carry a cellphone and they keep in constant communication? My guess is that since the waves travel at the speed of light, the signal would be discrete and would frequently break, until it gets to a point where the Earth twin dies and no longer sends a signal. Am I right or too far with this? | ||
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Jombozeus
China1014 Posts
In other words, its probably just not going to reach him within his lifetime after 2 days if hes traveling at 99% speed of light. Just do the math. | ||
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AmericanUmlaut
Germany2594 Posts
I guess I don't understand the point of the thought experiment, though. What about the situation is changed in an interesting way by having the two individuals in contact? | ||
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Nawyria
Netherlands140 Posts
Name the twin that stays on earth twin A and the travelling one twin B; assume further that they communicate by way of a continuous stream of light that varies mildly in wavelength around 550 nanometres (yellow). 1B) As twin B moves away from the earth at high speed, the relativistic doppler-effect redshifts the light signal, the signals he receives are in the red or infrared. He will also receive the signals at a lower pace due to the fact the light signals have to catch up to him, and thus the intensity will be much lower and the transmitted message plays very slowly. 2B) Just after twin B reverses his direction of travel (which we will assume he does instantly), all the signals he has been travelling ahead of will catch up to him in a very short amount of time. Since twin B is now travelling towards earth, the signals will be blueshifted and the signals he receives are in the blue, ultraviolet or beyond. He will now receive the signals at a much much higher pace, so the intensity will be much higher and the transmitted message plays extremely fast. 3B) After this short burst has passed and twin B is travelling back towards the earth, we are in the inverse of situation 1B. The light signals are blueshifted (as in 2B), the intensity is higher (though not as high in 2B) and the message players faster (though not as fast as in 2B). 1A) After twin A says his farewell to twin B, he will receive messages at a very slow pace, with the same redshift, intensity and message speed as in 1B. 2A) As twin B nears earth, all the messages that he has sent since turning around will reach earth in a very short fashion, much like what happens in 2B, only the intensity will be even higher. Edit: Assuming the distances are vast enough for twin A to die as twin B is travelling, it depends on the speed of twin B whether he receives words of twin A's death as he is travelling or soon after he turns around. The faster twin B travels, the less messages he will receive before turning around, thus increasing the chance he will hear of it after he turns around. | ||
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iNSiPiD1
United States140 Posts
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nam nam
Sweden4672 Posts
On July 22 2011 01:03 iNSiPiD1 wrote: If you assumed that they were actually able to keep in touch in real-time, which as others have pointed out is not possible with a cell phone, then yes, the twin on the spaceship would simply listen to the twin on earth die way before he dies. What if the twin on earth get hit by a car? | ||
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skunk_works
United States109 Posts
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japro
172 Posts
Aside from that I don't see why it wouldn't be possible to communicate with any kind of radio transmission (like a cell phone). Relativistic corrections have to be taken into account even at way lower velocities/acceleration differences. GPS satellites for example are essentially clocks and their signal has to be corrected for relativistic effects to obtain correct coordinates. So there is no "physical" reason why you shouldn't be able to communicate. A cellphone just wouldn't work for technical reasons. | ||
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oxidized
United States324 Posts
On July 22 2011 01:07 skunk_works wrote: This is from Einstein's theory of relativity. You can read about it.how does traveling at the speed of light keep you from getting old? | ||
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iNSiPiD1
United States140 Posts
If the twin on earth gets hit by a car then the twin on earth gets hit by a car. I think that answer the question? | ||
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AmericanUmlaut
Germany2594 Posts
Serves him right if he's talking on his cell phone instead of paying attention... | ||
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Stropheum
United States1124 Posts
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KimJongChill
United States6429 Posts
On July 22 2011 01:02 Nawyria wrote: The Lorentz transformation in flat space-time is a continuous transformation, so the signal will not be discrete; however, the signal will be distorted due to the relativistic doppler-effect. Name the twin that stays on earth twin A and the travelling one twin B; assume further that they communicate by way of a continuous stream of light that varies mildly in wavelength around 550 nanometres (yellow). 1B) As twin B moves away from the earth at high speed, the relativistic doppler-effect redshifts the light signal, the signals he receives are in the red or infrared. He will also receive the signals at a lower pace due to the fact the light signals have to catch up to him, and thus the intensity will be much lower and the transmitted message plays very slowly. 2B) Just after twin B reverses his direction of travel (which we will assume he does instantly), all the signals he has been travelling ahead of will catch up to him in a very short amount of time. Since twin B is now travelling towards earth, the signals will be blueshifted and the signals he receives are in the blue, ultraviolet or beyond. He will now receive the signals at a much much higher pace, so the intensity will be much higher and the transmitted message plays extremely fast. 3B) After this short burst has passed and twin B is travelling back towards the earth, we are in the inverse of situation 1B. The light signals are blueshifted (as in 2B), the intensity is higher (though not as high in 2B) and the message players faster (though not as fast as in 2B). 1A) After twin A says his farewell to twin B, he will receive messages at a very slow pace, with the same redshift, intensity and message speed as in 1B. 2A) As twin B nears earth, all the messages that he has sent since turning around will reach earth in a very short fashion, much like what happens in 2B, only the intensity will be even higher. Edit: Assuming the distances are vast enough for twin A to die as twin B is travelling, it depends on the speed of twin B whether he receives words of twin A's death as he is travelling or soon after he turns around. The faster twin B travels, the less messages he will receive before turning around, thus increasing the chance he will hear of it after he turns around. Wow, are you an astrophysicist? | ||
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Wonderballs
Canada253 Posts
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Nawyria
Netherlands140 Posts
Not an astrophysicist, but just a physicist. Einstein's Theory of Relativity has interested my for the larger part of my life; I wrote a paper on visualising relativity some time ago. | ||
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proberecall
United States104 Posts
So I understand that cell phone wouldn't work. Does your explanation also apply for any other radio signal or any other way of keeping in communication ? Also. Now that I see the signals are continued, if they keep talking at all times... at what point does the time difference start to notice ? meaning when does travelling twin start noticing Earth twin is growing older? | ||
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Sbrubbles
Brazil5776 Posts
On July 22 2011 01:15 AmericanUmlaut wrote: Serves him right if he's talking on his cell phone instead of paying attention... Which leads me to the conclusion that, if you have a twin in outer space traveling at close to the speed of light, you shouldn't buy a cell phone. | ||
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Nawyria
Netherlands140 Posts
On July 22 2011 01:47 proberecall wrote: Nawria, thank you So I understand that cell phone wouldn't work. Does your explanation also apply for any other radio signal or any other way of keeping in communication ? The reason cell phones wouldn't work is because the transmitters inside the phones or the large antennae we use to communicate on earth wouldn't do us any good over the vast distances involved in the twin paradox. Still, cell phones, radio's, skype calls etc. all work by means of sending electromagnatic waves, which travel at the speed of light. As long as the signal is strong enough to travel the distance without getting drowned out by background noise, the same reasoning holds as I gave for the yellow light signals. However most messages we send through our cell phones, for example, are encoded and a doppler-shift in the wavelength of the signal may not correspond 1-to-1 with a voice message being played at a higher or lower frequency or speed. The reason I used the example of yellow light signals to communicate is so that the similarity between the doppler-shifted light and the doppler-shift we frequently hear when an ambulance speeds past us is maintained. | ||
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Blazinghand
United States25559 Posts
On July 22 2011 01:47 proberecall wrote: Nawria, thank you So I understand that cell phone wouldn't work. Does your explanation also apply for any other radio signal or any other way of keeping in communication ? Also. Now that I see the signals are continued, if they keep talking at all times... at what point does the time difference start to notice ? meaning when does travelling twin start noticing Earth twin is growing older? They won't be able to keep talking due to the relatavistic "bend/stretch." Imagine, if you would, a train or car going past. It goes "wweeeeeeeee-ooooooooh", with the sound being high pitched as it approaches and low pitched as it passes away. This is because of the frequency of the sound being emitted sounding different to you due to the motion of the car. A similar, but somewhat different effect happens with "time dilation" where if I communicate, say, through a series of pulses, each pulse and the gap between each pulse becomes shorter and shorter as my reference frame moves faster and faster. This would make the establishment of some sort of communications standard very difficult, since anything using frequency would be out the window, or extremely hard to calculate. The traveling twin would immediately start noticing the earth twin is growing older, the instant his journey had been underway, because the earth twin will be a second or two older, etc, very quickly. This sort of thing has been done with atomic clocks and it doesn't take long for them to become desynced. | ||
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Nawyria
Netherlands140 Posts
On July 22 2011 01:58 Blazinghand wrote: They won't be able to keep talking due to the relatavistic "bend/stretch." Imagine, if you would, a train or car going past. It goes "wweeeeeeeee-ooooooooh", with the sound being high pitched as it approaches and low pitched as it passes away. This is because of the frequency of the sound being emitted sounding different to you due to the motion of the car. A similar, but somewhat different effect happens with "time dilation" where if I communicate, say, through a series of pulses, each pulse and the gap between each pulse becomes shorter and shorter as my reference frame moves faster and faster. This would make the establishment of some sort of communications standard very difficult, since anything using frequency would be out the window, or extremely hard to calculate. When considering though experiments like these, I usually idealise things and assume that communication is possible. However, practical issues do present a new variety of interesting problems as it all depends on the velocity of the travelling twin. At, say, 0.95c or 0.99c the associated γ-factor is 3.2 and 7.1 respectively and should be well within our capacity to handle. But you are more and more right when we continue adding 9's, seeing as 0.999c corresponds to γ=22.3, 0.9999c to 70.7 and it only gets much worse from there. | ||
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