Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: amalia on 26/11/2019 15:59:14
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Stuart got in contact with us to ask a question:
I saw on space.com they reported an object travelling faster than light, apparently it was a star that had been ejected, how is this possible? Doesn't the mass of the object become massive according to general relativity?
Do you know the answer?
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I saw on space.com they reported an object travelling faster than light, apparently it was a star that had been ejected, how is this possible? Doesn't the mass of the object become massive according to general relativity?
Never heard of anything getting shot out at super speed and still managing to remain intact enough to be classified as a star.
Perhaps https://www.space.com/41724-neutron-star-merger-superfast-jet.html
Headline:
"Faster Than Light? Neutron-Star Merger Shot Out a Jet with Seemingly Impossible Speed"
But it goes on to correct itself: "In the 155 days between two observations, the jet appeared to move 2 light-years, a distance that would require it to travel four times faster than light. This "superluminal motion" is an illusion created as the jet is pointed nearly toward the Earth; it is actually moving at about 97 percent light speed."
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You do not give us enough detail to identify precisely the report you talk about.
However there are several cases of observations of astronomical "objects" apparently moving faster than light. The reason is that the moving source of light is not in fact moving but is a shell or layer that is illuminated by a pulse of light or a moving beam of light or particles that cause light emission.
Consider pointing a powerful continuous and steerable laser up at a low angle into the clear sky illuminating clouds in the distance and changing its angle by one radian (around 60 degrees) in one millisecond. Say there was a cloud 30 kilometres away This is an experiment that can be done.
The spot of illumination by the laser on the cloud would move 30 kilometres in one millisecond that is at an apparent speed of 30,000 kilometres per sec. Now if this cloud was 300 kilometres away the spot would be moving at the speed of light. Any further away and the spot will be moving faster than the speed of light. Nothing is actually moving faster than light but it is possible to see a bright spot moving faster than the speed of light.
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A key is understanding the difference between "apparently" moving faster than light and actually moving faster than light.
Simple example: And object starts 8 light mins away traveling towards you at 0.8c.
The light from the moment it starts at the distance of 8 light min, arrives 8 min after it starts. The object itself arrives in 10 min, or only 2 min after the light of it being 8 light min away does. Thus it would appear to you that the object crossed the 8 light min distance in just 2 min and thus traveled at 4c. But this is just an visual effect caused by the fact that the object is following closely behind its own light. The object was always traveling at 0.8c You merely saw its 10 min trip compressed into 2 min.
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I would like to thank everyone for there quick responses and i apologise for seeming a bit foolish but was worried that einstien was wrong but glamorised titles need to be eliminated from stuff like this because as a layperson i am very susceptible in making mistakes, but err is to be human. Plus i can say with ease even our top scientist are doing a best guess scenario, and that is no disrespect because im pretty sure astrophysics or any science for that fact isnt easy and i will generally trust scientist best guess. hopefully my next question to you fine people might be more challenging and maybe invoke a good debate. thanks again everyone
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If A and B were simultaneously ejected from a point in opposite directions at 0.6 relative to the start point, each would be travelling at 1.2c relative to the other, but neither would know it because any signal from A could only travel towards B at c.
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If A and B were simultaneously ejected from a point in opposite directions at 0.6 relative to the start point, each would be travelling at 1.2c relative to the other, but neither would know it because any signal from A could only travel towards B at c.
Think of it this way, If You were at rest with respect to the start point, after 1 sec, A and B would be 1.2 light sec apart. ( assume A move to the left and B to the right)
A sends a signal. That signal travels at c relative to you. After other second the front of that signal is 1 light sec from where it
was emitted. A has moved 0.6 ls to the left and B another 0.6 ls to the right. Distance between signal and B is 1.8-1 = .8 light sec. In one more sec the signal has moved another 1 ls to to right from and B 0.6 ls to the right. New distance = 1.4-1 = 0.4 ls In one more sec, the light travels 1 ls, B .06 ls, and the distance is 1-1 =0. the light has caught up with B. 3 sec after emission, the light reached B.
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I've read about this but I have never found a definite explanation.
I see 3 possibilities:
1- Soul Surfer is correct in his explanation.
2- They have used Newtonian mechanics. (I've seen this explanation but I was skeptical)
3- Cherenkov radiation. (I've seen this explanation but it couldn't have been 3 or 4 times the speed of light)
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Think of it this way, If You were at rest with respect to the start point, after 1 sec, A and B would be 1.2 light sec apart. ( assume A move to the left and B to the right)
A sends a signal. That signal travels at c relative to you. After other second the front of that signal is 1 light sec from where it
was emitted. A has moved 0.6 ls to the left and B another 0.6 ls to the right. Distance between signal and B is 1.8-1 = .8 light sec. In one more sec the signal has moved another 1 ls to to right from and B 0.6 ls to the right. New distance = 1.4-1 = 0.4 ls In one more sec, the light travels 1 ls, B .06 ls, and the distance is 1-1 =0. the light has caught up with B. 3 sec after emission, the light reached B.
Perfectly true, but irrelevant. The light reaching B will be Doppler shifted so that
fB = fA√(1-β)/(1+β) where β = v/c = 1.2
So if A transmits, say, a 1 GHz pulse of radio waves, B will be able to calculate their relative speed as the received pulse will be at 0.4 GHz.
And indeed we have observed astronomical objects with extreme redshifts.
The conundrum is resolved by remembering that you only require infinite energy to accelerate a massive object from rest to c with respect to its starting point. There is no reason why two objects shouldn't travel in opposite directions at > 0.5c relative to their origin.
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Think of it this way, If You were at rest with respect to the start point, after 1 sec, A and B would be 1.2 light sec apart. ( assume A move to the left and B to the right)
A sends a signal. That signal travels at c relative to you. After other second the front of that signal is 1 light sec from where it
was emitted. A has moved 0.6 ls to the left and B another 0.6 ls to the right. Distance between signal and B is 1.8-1 = .8 light sec. In one more sec the signal has moved another 1 ls to to right from and B 0.6 ls to the right. New distance = 1.4-1 = 0.4 ls In one more sec, the light travels 1 ls, B .06 ls, and the distance is 1-1 =0. the light has caught up with B. 3 sec after emission, the light reached B.
Perfectly true, but irrelevant. The light reaching B will be Doppler shifted so that
fB = fA√(1-β)/(1+β) where β = v/c = 1.2
So if A transmits, say, a 1 GHz pulse of radio waves, B will be able to calculate their relative speed as the received pulse will be at 0.4 GHz.
And indeed we have observed astronomical objects with extreme redshifts.
The conundrum is resolved by remembering that you only require infinite energy to accelerate a massive object from rest to c with respect to its starting point. There is no reason why two objects shouldn't travel in opposite directions at > 0.5c relative to their origin.
The formula you give is for Doppler shift do to ordinary motion ( rather than Cosmological Redshift) In this case, then v/c has to be determined using the Relativistic addition of velocities as noted by Halc. This gives a value of 0.882... for v/c and ~0.7288 for the Doppler shift. B would see a frequency of 0.7288 hz. If you try to use 1.2 for v/c, you get (1-1.2)/(1+1.2) = -0.909 under the radical, which leaves you with trying to get the square root of a negative number.