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Noob question: Physicists say if you travel as fast as light, time is frozen/stops, if so, how come we can see light coming towards us, when in reality that light is supposedly "frozen" in time?
rcgldr said:I recall reading that the explanation for this is the rules for objects that only exist at light speed, such as photons, are different than the (special relativity related) rules for sub-light speed objects, which can never achieve [sic] light speed. This is why photons can have a frequency and why that frequency is affected the same manner as an ordinary (sub light speed) clock, being slowed down in a strong gravitational field (call gravitational red shift in the case of photons).
Yes, I should have made that more clear in my last paragraph. I was trying too hard to make the point.DaveC426913 said:There's a fair bit of misconception happening here.
1] No object with mass can ever reach the speed of light. No matter how long it accelerates, it will always be shy of c.
2] No matter how fast you travel, you will always measure light to move at c.
2] There is no such thing as the frame of reference of a photon. It is nonsensical to ask what the universe might look like to a photon.
That wasn't directed at you. Several earlier posters had some serious misconceptions about relativistic velocities.my_wan said:Yes, I should have made that more clear in my last paragraph. I was trying too hard to make the point.
http://en.wikipedia.org/wiki/General_relativity#Light_deflection_and_gravitational_time_delayAli Inam said:Gravity was mentioned earlier, I don't think that gravity will attract that substance which has no mass.
Gravity affects photons.Ali Inam said:Gravity was mentioned earlier, I don't think that gravity will attract that substance which has no mass.
hope4all said:According to Einstein's special theory of relativity, objects gain mass as they accelerate to greater and greater speeds. Now, to get an object to move faster, you need to give it some sort of push. An object that has more mass needs a bigger push than an object with less mass. If an object reached the speed of light, it would have an infinite amount of mass and need an infinite amount of push, or acceleration, to keep it moving. No rocket engine, no matter how powerful, could do this. In fact, as far as we know, nothing can exceed the speed of light.
Faradave said:I often hear this restriction to massive objects attaining lightspeed. I don't believe massive objects can attain lightspeed and remain part of this universe but the rocket example seems flawed and I would appreciate being corrected if wrong.
Suppose a rocket is accelerating by streaming propellant out its nozzel. As it accelerates, the rocket gains mass by virtue of its kinetic energy increasing. (Call it "radiant mass" as opposed to "rest mass". Effective mass would be the sum of the two.) Well, it would seem that the fuel (and thus the expelled propellant) has also gained mass, making it all the more effective at accelerating the rocket. That is, the capacity of the fuel to accelerate seems relativistically invariant.
Rockets aside, consider a particle on a path straight toward the center of a black hole. As it accelerates, it gains mass. That would only seem to add to the "force" of attraction (or steepening of spatial slope) experienced by the particle. Rather than a barrier to attaining lightspeed, increasing mass would be an advantage. It appears that lightspeed is indeed attained by massive particles at the event horizon where they effectively leave our universe.
I apologize for my stupidity. I forgot the apparent requirement at PF that the questioner must already know the answer prior to asking. Nevertheless, I thank you for condescending to reply to me, something others may have been too offended to do.DaveC426913 said:There are far too many flaws and misunderstandings in this to address them all. I'll just touch on a few of the most egregious.
I’m not trying to add to the dictionary here, just establishing a term for use in the current conversation. I would have thought that was obvious from my use of quotes and the preceding notice, “Call it”. Please remember that we outsiders have to endure an ever increasing dictionary of rather whimsical sounding terms emerging from physicists (e.g. “top” & “bottom” quarks, or is it “truth” & “beauty”, I keep forgetting.) Regardless, we need words to communicate, especially about real entities.DaveC426913 said:0] Please don't make up terms.
Are you sure? I just searched the term “effective mass” on PF I got back 371 threads as a result!DaveC426913 said:There is no such thing as "radiant mass" or "effective mass".
Then you see the source of my confusion. If we claim the rocket’s mass is invariant in its own frame, why are we then using infinite mass as a barrier to lightspeed? I could understand this concern if power were being applied from an external (at rest) source. In the rocket’s frame F=ma should give continuous acceleration as long as the fuel holds out, regardless of proximity to light speed. (Of course I could be wrong, even expectantly so, but that’s why I ask.)DaveC426913 said:1]Relativistic mass does not "count" in considering the effectiveness of a rocket exhaust. In the rocket's frame of reference, there is no increase in mass.
The Pisa myth is so good, it ought to be true, so I concede it. But to your point, increasing relativistic mass as a particle descends into a black hole is no advantage. But neither is it a barrier to lightspeed. Acceleration should continue regardless of proximity to lightspeed. For neutrinos, initial velocity is already pretty close. No?DaveC426913 said:2] A heavy object and a light object fall in a gravitational field at the same speed, the heavier object does not fall faster. Galileo demonstrated this 400 years ago from the top of the Tower of Pisa.
(my bold).In the rocket’s frame F=ma should give continuous acceleration as long as the fuel holds out, regardless of proximity to light speed. (Of course I could be wrong, even expectantly so, but that’s why I ask.)
That statement makes no sense. You're not the asker, you're the answerer. Wouldn't it behoove you to have the answer? All I expect is that you stick to known physics, and if you don't know it, at least put a proviso that you don't.Faradave said::uhh:I apologize for my stupidity. I forgot the apparent requirement at PF that the questioner must already know the answer prior to asking.
It's probably not, but the lesson is still valid.Faradave said:The Pisa myth is so good, it ought to be true, so I concede it.
Faradave said:Acceleration should continue regardless of proximity to lightspeed.
DaveC426913 said:Gravity affects photons.
In Einsteinian general relativity, gravity is not a "force that pulls", gravity is the warpage of spacetime. Photons, like everything else, follow this warpage.
DaveC426913 said:1] Relativistic mass does not "count" in considering the effectiveness of a rocket exhaust. In the rocket's frame of reference, there is no increase in mass.
Why do you think this is counter-intuitive? Why would you expect the Moon, which is moving at ~1km/s to move on the same path as light, which is moving at 300,000km/s?Oldfart said:I keep hearing about objects following paths in spacetime, rather than being pulled by gravity. Yet the moon and photons, for instance, will obviously not follow the same path no matter how we arrange the experiment.
stone1 said:Here is a way to travel faster than light -- fall into a black hole. To an outside observer you will pass the light speed limit and your light will not be seen again. You can also rent a spaceship and go far enough so that the expansion of the universe carries you away faster than light. So traveling at the speed of light (or even faster) is possible but it comes at a price, you can never return to the original space-time referenc frame to tell the story.
Inasmuch as distant galaxies are currently moving away from us at faster than c, so we can say that any spaceships in those galaxies are also moving away from us at faster than c - which is what you're saying.Dmitry67 said:No, he is right, it works: spaceship flies faster than c... in some coordinate systems. But in GR for distant objects there is no surprise that it works.
Remember that rockets work by conservation of momentum. As v->c the momentum of the rocket is unbounded in the rest frame, which means that the momentum of the exhaust is also unbounded in the rest frame. Transforming to the rocket's momentarily co-moving inertial frame we find that the momentum of the exhaust is even larger (greater than unbounded is also unbounded). So, by conservation of momentum, you would have to have exhaust with infinite momentum in the rocket's frame. So the "infinite mass" argument is valid although I don't like it since I don't like relativistic mass in general.Faradave said:Suppose a rocket is accelerating by streaming propellant out its nozzel. As it accelerates, the rocket gains mass by virtue of its kinetic energy increasing. (Call it "radiant mass" as opposed to "rest mass". Effective mass would be the sum of the two.) Well, it would seem that the fuel (and thus the expelled propellant) has also gained mass, making it all the more effective at accelerating the rocket. That is, the capacity of the fuel to accelerate seems relativistically invariant.
I am not sure if any of these thoughts are connected, or if they follow from my comment.Dmitry67 said:But we can get to another galaxies. When Universe will be expanding faster and faster, you will cross the event horizon in finite time. In Big Rip scenario, it happens to ALL observers.
DaveC426913 said:Why do you think this is counter-intuitive? Why would you expect the Moon, which is moving at ~1km/s to move on the same path as light, which is moving at 300,000km/s?
Dave, thanks for your reply, the putting green analogy was very clear. I wasn't thinking of anything being counter-intuitive, it was more of a semantics problem, I guess, which you straightened out. Thanks!
Dmitry67 said:Because as expansion accelerates because of the Dark Energy, distant galaxy which is not beyond the cosmological horizon now can be behind it in the future.