Lightspeed and motion

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j_rankin

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Weird thought just came into my head.<br />This was probably calculated by Einstein if not before, but if an object which emitted light (not reflect) was moving away from the Earth at the speed of light, how would we observe it?<br /><br />I am confused as to whether we would see no motion at all in the object, or whether the object would appear to be moving at half the speed of light.<br /><br />If the former - Would the object just appear to get less and less bright over time?<br /><br />Someone please help me out.

B

bbrock

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I'm not the best one to try and answer this, but I'll give it a guess. I think if an object reached the speed of light, it would no longer be an object that we would understand, but rather energy. So assuming it is just shy of acquiring "c", it would appear greatly red shifted. <br /><br />Bill

J

j_rankin

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That's what confuses me. I understand that because of an object's motion, its particle density increases with velocity. Does that mean that if it were travelling the speed of light it would have infinite density and therefore infinite mass? I should read more about einstein<br /><br /><br />

K

killium

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Going with the same logic, what about that:<br /><br />We send a probe at 1/2c in one direction. We send another probe at 1/2c in the other direction. The second probe is equiped with a lot of detection instrument.<br /><br />What would the second probe "see" if it looks at the first probe ?<br /><br />Now, same question but, both probe are sent at 3/4c ?<br /><br /> <div class="Discussion_UserSignature"> </div>

J

j_rankin

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I think the Doppler effect can answer that one, there will just be a huge red shift.<br /><br />You have to think about the position of the original light being emitted. The original light from the first probe is always going to be further along in the direction of the second probe than the second probe itself. So the second probe will see the first, just with a very large red shift.

S

Saiph

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1) Infinite density does not mean infinite mass. It can also mean zero volume and a finite mass.<br /><br />2) We'd see it moving. however time would not appear to pass for the object.<br /><br />3) The light emitted, or reflected, from the object would be redshifted to infinity, so we wouldn't actually see it. If we could, 2 still holds.<br /><br />4) As the distance increases, the object would appear to dim, assuming the light isn't emitted at you, and only you (like a "perfect" laser beam). <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

F

farse

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First of all, : <br />nothing except light can ever reach the speed of light, it would take an infinite amount of energy. however, if an object did manage to reach 99.999% of the speed of light, you would still observe the light to be traveling at the speed of light, this is one of einstein's postulates for the theory of relativity : the speed of light is the same regardless of the speed that one is traveling. so being at the speed of light remains the same, one would notice a doppler shift in frequency of the light (the redshift mentioned above). this means that if it was moving away from you the wavelengths would stretch out and you would observe the light to have a longer wavelength and thus a lower frequency, but the speed ALWAYS remains the same. pretty crazy, i hope this helps. <br /><br />i forgot to mention as well : due to the hubble expansion of the universe, the deeper we obseve into the universe, the faster galaxies and the like are moving away from us. there is a point at which galaxies are receding from us faster than the speed of light...this isn't any sort of particle or thing moving faster than the speed of light, it is the expansion of space itself... so in this case we are not able to observe these galaxies which are moving RELATIVE TO US at faster than the speed of light

S

Saiph

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Good explaination Farse. Thanks for reminding us all that you can't actually go C (which I omitted for some reason...) <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

K

killium

Guest
The light comming from the probe towards us at light speed independently from its speed is one thing, but the signal transmitted by this light is another.<br /><br />Suppose the probe is carrying a clock which we look at. The faster the probe goes, the slower we would see the time ticking because each "frame" would have a greater distance to go before reaching us, thus arriving late (and later, and later ....).<br /><br />If that probe reach C (suppose we are on the second probe), we would see it stand still because we would be "moving" at the same speed as the information carried by the light, so we always see the same information.<br /><br />If the relative speed of the 2 probes is /> C, then we would start to see the clock ticking in reverse time, this is because, at each second, we would be capting a "frame" that was sent earlier (we're going faster than the informtation carried by light. This doesn't change the fact that if we measure the speed of the light (not the speed of the information), it would be c.<br /><br />If we use that information to measure the relative speed of the probe, we would never see it going faster than c, not even reach it. This is what we would see, it is not the reality though. As far as we are concerned, we would see the probe going slower in time, and slower, and slower.....as it accelerates. and it would never reach c, for us.<br /> <div class="Discussion_UserSignature"> </div>

S

Saiph

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and it never reaches C, and it's never measured at c, and it never sees us gonig at C. And as such the relative velocities cannot be greater than C. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

K

killium

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Explain me how the relative speed of the 2 probe couldn't reach c, if we throw them in opposite directions at 3/4c ? <div class="Discussion_UserSignature"> </div>

K

kmarinas86

Guest
Suppose the year is 2004 on Earth and we want to send two ships to a distance of .75 LY away from Earth. If these ships are traveling at .75 C, by 2005, time will have passed 0.661437 years on the space ships<br /><br />http://www.nasaexplores.com/show_912_student_st.php?id=030107161805<br /><br />dialated time = stationary time * sqrt(1-VV/CC)<br />0.661437 = 1 * sqrt(1-(.75*.75)/(1*1))<br /><br />When the two spaceships reach their destination, after they completed their 1 "on-board" year, 1.511857 years would have already passed on Earth.<br /><br />If both ships are equally "in the future", then how come they are 1.5 LY away from each other now? Well, travelling 1.5 LY in 1.511857 years... that's almost a light year per year. But whose light years? Earths? Let try different numbers.<br /><br />Suppose the two spaceships travel .9 LY in 1 "on-board" year in opposite directions, starting from Earth in the year 2005.<br /><br />1*(1-(.9*.9)/(1*1))<br /><br />By the time Earth reaches 2006, it will have been 0.435889 "on-board years" for the occupants of the space ships. By the time the occupants have traveled for one "on-board" year, 2.294157 years will have already passed on Earth. The distance between the two spaceships is now 1.8 LY, but 2.294157 years have already passed on Earth.<br /><br />So from the perspective of Earth, the space ships are not travelling away from each other faster than light speed.

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Saiph

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because velocities don't add normally at high fractions of the speed of light.<br /><br />http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel2.html<br /><br />Vobs=v1+v2 / (1+v1*v2/c^2)<br /><br />In your case: 3/4 +3/4 = 1.5c<br /><br />Divided by<br /><br />1+3/4*3/4= 1.5625<br /><br />giving .96c total. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

K

kmarinas86

Guest
something is weird about the formula I've been using...<br /><br />Column 1: rocketship speed relative to earth<br />Column 2: "onboard" years per earthyear<br />Column 3: earthyears per "on-board" year<br />Column 4: the velocity (C) of the craft-seperation from Earth's perspective<br /><br />The last column is weird... it says the velocity of an object approaching very close to light speed appears motionless when viewed from earth. That's doesn't make sense to me. Any light coming from the spaceship towards Earth is extremely redshifted I think.<br /><br /><pre>0.100000 0.994987 1.005038 0.198997<br />0.150000 0.988686 1.011443 0.296606<br />0.200000 0.979796 1.020621 0.391918<br />0.250000 0.968246 1.032796 0.484123<br />0.300000 0.953939 1.048285 0.572364<br />0.350000 0.936750 1.067521 0.655725<br />0.400000 0.916515 1.091089 0.733212<br />0.450000 0.893029 1.119785 0.803726<br />0.500000 0.866025 1.154701 0.866025<br />0.550000 0.835165 1.197369 0.918681<br />0.600000 0.800000 1.250000 0.960000<br />0.650000 0.759934 1.315903 0.987914<br />0.700000 0.714143 1.400280 0.999800<br />0.750000 0.661438 1.511858 0.992157<br />0.800000 0.600000 1.666667 0.960000<br />0.850000 0.526783 1.898316 0.895531<br />0.900000 0.435890 2.294157 0.784602<br />0.950000 0.312250 3.202563 0.593275<br />0.960000 0.280000 3.571429 0.537600<br />0.970000 0.243105 4.113450 0.471624<br />0.980000 0.198997 5.025189 0.390035<br />0.990000 0.141067 7.088812 0.279313<br />0.991000 0.133862 7.470387 0.265314<br />0.992000 0.126238 7.921553 0.250456<br />0.993000 0.118114 8.466372 0.234575<br />0.994000 0.109380 9.142433 0.217448<br />0.995000 0.099875 10.01252 0.198751<br />0.996000 0.089353 11.19154 0.177992<br />0.997000 0.077402 12.91964 0.154339<br />0.998000 0.063214 15.81930 0.126175<br />0.999000 0.044710 22.36627 0.089331<br />0.999100 0.042417 23.57553 0.084757<br />0.999200 0.039992 25.00500 0.079920<br />0.999300 0.037410 26.73080 0.074768<br />0.999400 0.034636 28.87184 0.069230<br />0.999500 0.031619 31.62673 0.063206<</safety_wrapper></pre>

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