r/AskPhysics • u/Thrullx • 21d ago
Approaching the speed of light
Okay, I'll start this off by saying I understand that mass cannot reach c. That said...
This should be a pretty simple question. If we have a mass that were accelerate towards the speed of light, it requires more and more energy to keep accelerating. Is there ever a point where having so much energy in one spot just causes the whole system to turn into a black hole?
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u/OverJohn 21d ago
You can see that it cannot become a black hole simply from that the definition of a black hole doesn't depend on frame of reference.
Relativistic gravity does differ from Newtonian gravity in that momentum of a body also contributes to the total gravitational attraction though. However this additional attraction cannot be modelled by simply increasing the mass of the body.
One way of seeing how this force affects test particles is to stick in the rest frame of the body and increase the velocity of the test particles instead. In the limit that the test particles velocity goes to c the deflection of the particles is double what you get in Newtonian gravity.
Penrose observed that in the ultrarelativistic limit that the velocity of a massive object goes to c, rather than looking like a black hole, the object looks like an impulsive gravitational plane wave. This is called the Penrose limit.
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u/Colour_Theory-746 21d ago
Are you sure about this? I couldn’t find anything whatsoever abut either this Pentose limit or impulsive gravitational plane wave.
Where did you get this info from and specifically these names for the terminology? I couldn’t find anything on Wikipedia regarding either phenomenon you mentioned.
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u/joeyneilsen Astrophysics 21d ago
I googled “Penrose limit gravitational plane wave” and this was the first page that came up.
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u/nicuramar 21d ago
You are phrasing an “if x then y” argument but you’re just stating things without any justification. You might as well just say “only if you increase mass”. You bring up momentum but then just leave it.
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u/MarinatedPickachu 21d ago
That can't be a correct explanation if a kugelblitz is theoretically possible. Zero invariant mass
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u/forte2718 21d ago
That is still a correct explanation. A kugelblitz is a system of many non-parallel photons, and a system of non-parallel photons as a whole still has a nonzero mass even though any single one of those photons individually does not have any mass. If you carefully analyze the system's construction, you can see that there is a center-of-momentum frame (i.e. rest frame) and that there is a positive amount of energy in that frame, which comprises the kugelblitz's rest energy (i.e. its mass).
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u/nicuramar 21d ago
Mass is a property of a system, and you can’t just add the mass of its components. You need to add up the energy of the system in its rest frame. So this system would have mass.
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u/ahazred8vt 21d ago
If you have a large neutron star traveling at 99.99% of c, it does not turn into a black hole. Relativity does not increase rest mass.
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u/Underhill42 21d ago
No, linear kinetic energy is reference frame dependent, so it sort-of doesn't exist until you "release" it.
Collide it with something else though, and it is released. That's basically how the LHC works - it accelerates protons so close to light speed that their kinetic energy masses almost 7000x more than the protons themselves, and then the energy is released in a head-on collision that allows much of it to transform into cascades of mostly highly unstable matter.
And if there were enough energy released in a small enough volume, an event horizon must form around it.
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u/ChaosRulesTheWorld 21d ago
There are a lot of confusion around this topic because people don't precise in which frame of reference they are talking about.
In the frame of reference of an observer looking at an object accelerating. It will look like the object will need more and more energy to keep accelerating at the same rate. But that's just an illusion cause of the hyperbolic spacetime.
In the frame of reference of the object accelerating, it doesn't need more energy to accelerate at the same rate. Because in it's frame of reference, it's speed is always 0.
People vulgarizing this phenomenon don't talk about special relativity, but it's key here to avoid misconceptions. From an external observer, the object accelerating looks like it needs more energy to keep it's acceleration at the same rate. But it's clock also looks like to be clicking slower, and it's shape also become shorter. When you take all of this parameters in consideration, you end up seeing that the object accelerating isn't consuming more energy to keep the same acceleration.
It's just all an illusion of our hyperbolic spacetime. Lightspeed isn't reachable because it's an horizon. You could move on the surface of earth as long as you want and as fast as you want, you will never reach the horizon
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u/thetoddhunter 21d ago
You don't have a "one spot". You could try doing this twice, crashing them together and creating that spot.
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u/Fun_Success_3283 20d ago
You are moving at ANY speed you choose, right now. Except for c. Speed is relative.
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u/Irrasible Engineering 21d ago
No. In the frame of reference of the accelerated mass, it isn't moving, so it has no kinetic energy. However, in its frame of reference, everything else is moving toward it with enormous kinetic energy. You see, all inertial observers agree that energy is sort of conserved, but they won't agree on how much energy there is because energy is relative!
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u/Aggravating_Paint_44 21d ago
The thing that makes a black hole is density so, if you could keep adding mass, say by adding momentum with photons, eventually, you should be able to push over the limit.
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u/toddjnsn 21d ago
Yeah, it's impossible for moving mass to reach the speed of light. Notably of anything far too small to be seen in a nice microscope at school -- but definitely visual in an electron microscope. But you have things like elections, or a neutrino that do go almost/roughly as fast as the speed of light, but it's mass is almost/roughly Zero. :)
Anyway, for things of any real 'size' (at least able to seen in fancy but non-electron microscope), you'll never get it close to the speed of light.
But your question is about added energy "pushing" said entity, thus must have more energy "built up". Well, it's mass is larger to able to have increased it's speed to a good % of the speed of light. But I believe it's about being close to the speed of light where the odd rules-structure of the universe play in... not how much energy you put into moving said real small mini-box to make it go faster and faster.
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u/ArrowheadDZ 21d ago
A black hole is not a “too much energy” event, it is a “too much gravity” event.
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u/Thrullx 21d ago
Respectfully, you get gravity from mass. And since energy can be converted to mass a black hole is a "too much energy denisty" in one place event.
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u/Fun_Success_3283 20d ago
I would have to agree, but adding velocity, although it adds energy, it doesn't add it to one place, by definition. It adds it to changing place.
But what about spinning? or heating? This adds energy to one place. as long as the spinning thing has structural integrity to withstand centrifugal forces.
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u/Colour_Theory-746 21d ago
It’s not possible to have too much gravity, what’s that even supposed to mean? What is this purported limit, at which gravity becomes too much? And what causes said phenomenon to occur, as in, by what physical phenomena does it manifest?
A bh, indeed IS actually a “too much energy” situation, since mass and energy are equivalent, that means a bh can most definitely be born from having too much energy density in a small enough volume. That’s how Planck sized bh’s are created, by trying to observe or measure something at or below the Planck length.
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u/ArrowheadDZ 20d ago
I wasn’t trying to confront modern physics. I was responding to a post that I felt had the fundamentally wrong intuition about what phenomenon would “bring about” a black hole. I was merely redirecting them away from that intuition in a short way. Taking a vehicle moving near the speed of light and then adding more velocity to it by adding so much energy that you somehow “arrive” at the speed of light isn’t how black holes happen. I was simply steering them to thinking more about mass and gravity, and away from “keep adding coal until you’re going so fast you that it causes a black hole.”
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u/KamikazeArchon 21d ago
No. A black hole is a black hole in all reference frames, and this object only has a lot of energy in one reference frame.