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black-hole

Can a Really, Really Fast Spacecraft Turn Into A Black Hole?

15 Feb , 2010

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This question was posed in an Astronomy Cast episode a while back. It offers an interesting thought experiment, although a reasonably definitive answer to the question can be arrived at. 

Imagine a scenario where a spacecraft gains relativistic mass as it approaches the speed of light, while at the same time its volume is reduced via relativistic length contraction. If these changes can continue towards infinite values (which they can) – it seems you have the perfect recipe for a black hole

Of course, the key word here is relativistic. Back on Earth, it can appear that a spacecraft which is approaching the speed of light, is indeed both gaining mass and shrinking in volume. Also, light from the spacecraft will become increasingly red-shifted – potentially into almost-blackness. This can be partly Doppler effect for a receding spacecraft, but is also partly a time dilation effect where the sub-atomic particles of the spacecraft seem to oscillate slower and hence emit light at lower frequencies. 

So, back on Earth, ongoing measurements may indicate the spacecraft is becoming more massive, more dense and much darker as its velocity increases. 

But of course, that’s just back on Earth. If we sent out two such spacecraft flying in formation – they could look across at each other and see that everything was quite normal. The captain might call a red alert when they look back towards Earth and see that it is starting to turn into a black hole – but hopefully the future captains of our starships will have enough knowledge of relativistic physics not to be too concerned. 

So, one answer to the Astronomy Cast question is that yes, a very fast spacecraft can appear to be almost indistinguishable from a black hole – from a particular frame (or frames) of reference. 

But it’s never really a black hole. 

Centaurus A with jets powered by a supermassive black hole within - the orange jets are as seen in submillimetre by the Atacama Pathfinder and the blue lobes are as seen by the Chandra X-ray space telescope.

Special relativity allows you to calculate transformations from your proper mass (as well as proper length, proper volume, proper density etc) as your relative velocity changes. So, it is certainly possible to find a point of reference from which your relativistic mass (length, volume, density etc) will seem to mimic the parameters of a black hole. 

But a real black hole is a different story. Its proper mass and other parameters are already those of a black hole – indeed you won’t be able to find a point of reference where they aren’t. 

A real black hole is a real black hole – from any frame of reference. 

(I must acknowledge my Dad – Professor Graham Nerlich, Emeritus Professor of Philosophy, University of Adelaide and author of The Shape of Space, for assistance in putting this together).



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dimtick
Member
dimtick
February 15, 2010 8:42 AM

this question may have been answered at length.
I’ve never been able to wrap my head around the idea of string theory and someone talking about membrane theory may as well be talking to me in Chinese.

my question is this.
what does string theory say about black holes? string theory sais that there are 11 dimensions. is that correct? my understanding is that black holes only effect the 4 known dimensions of space-time. does string theory make any predictions for the effects of a black hole on the other 7 dimensions? As a thought experiment, if we could travel to another dimension could we travel to the inside of a black hole?

utunga
Member
utunga
February 15, 2010 9:45 AM

this is a fascinating question.. what confused me is your reference to the ‘proper rest’ frame.. my recollection of relativity was that all inertial frames, are, well equal..

I googled ‘proper rest’ and came up with this Quote:
http://www.physicsforums.com/archive/index.php/t-74178.html
“Before I write a long response, are you looking for someone to explain your misconceptions about relativity or not?

There is no such thing as a “proper rest frame” in relativity – it does not matter which reference frame one adopts as far as observations go”

So now I’m really confused.

Surely the concept of relativity is that if it looks like a duck and quacks like a duck then it ‘is’ a duck – at least in that frame of reference.. no?

PIKA
Member
PIKA
February 15, 2010 11:14 AM

utunga,
A proper rest frame is the inertial (traveling at a constant velocity) reference frame from which the object is at rest. If you’re in another reference frame, the length and mass of an object can change.

I think what that person was referring to is that there are never any real inertial frames because there is always some force acting on an object so that any reference frame where that object is at rest is accelerating and thus non-inertial. However, we sometimes assume (when they aren’t accelerating “too” much) that they are inertial so we can calculate what they are doing.

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
February 15, 2010 11:17 AM
a spacecraft which is approaching the speed of light, is indeed both gaining mass and shrinking in volume. Well, seeing an ever accelerating rocket go like that would make a fun picture. The real fun thing is that you can’t observe a contraction. Well, not as a contraction as such, as explained by theoretical physicist John Baez: “Oddly enough, though Einstein published his famous relativity paper in 1905, and Fitzgerald proposed his contraction several years earlier, no one seems to have asked this question until the late ’50s. Then Roger Penrose and James Terrell independently discovered that the object will not appear flattened [1,2]. People sometimes say that the object appears rotated, so this effect is called the… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
February 15, 2010 11:38 AM
@ dimtick: Let’s hope some string theorist will answer your question. Meanwhile, it is my understanding that: “what does string theory say about black holes?” You can make it predict the same effects as the semiclassical theory, i.e. entropy goes as area et cetera. “string theory sais [sic] that there are 11 dimensions. is that correct?” The full blown theory, M theory, predicts that all versions of string theories are the same modulo dualisms. So the 10 dimensional string theory hides an 11th one, which is the maximal number of dimensions in super-symmetry M-theory (else you have 26, IIRC): “The original string theories from the 1980s describe special cases of M-theory where the eleventh dimension is a very… Read more »
Lawrence B. Crowell
Member
Lawrence B. Crowell
February 15, 2010 4:48 PM
The relationship between string theory and black holes is deep, and you have to wrap your mind around quantum holography. Susskind and Lindesay have written a moderately technical book on the subject http://books.google.com/books?id=cxJCBRUNmVYC&pg=PP3&lpg=PP1#v=onepage&q=&f=false For anyone with a reasonable background in physics should read this book. It is a book which requires some familiarity with some advanced concepts in physics at the BS level or first year graduate school. The upshot is that if you watch a string fall towards a black hole the transverse modes of the string are observed by a stationary observer to slow down. In effect very high energy or UV modes are then transformed into low energy or IR modes. Further, since the black… Read more »
davesmith_au
Member
February 16, 2010 2:55 AM

Sounds to me more like a bunch of back-slapping, convoluted, ego-stroking gobbledegook to me. When are physicists going to start talking about the physical again?

Lawrence B. Crowell
Member
Lawrence B. Crowell
February 16, 2010 4:40 AM

Physics has taken us further away from a classical notion of things. As we have pushed deeper into foundations the implications of things run counter to our ordinary expectations of things.

LC

Dark Gnat
Member
Dark Gnat
February 16, 2010 6:12 AM

There’s nothing wrong with a good thought experiment.

So basically, this means that there can be a percieved black hole, which makes sense, as you are traveling as fast or faster than the light from an object behind you. If that light never catches up to you.

Light from an object ahead of you would be very blue shifted, enough that it might even “feel” like gamma rays, which would suck. Is this accurate?

Dark Gnat
Member
Dark Gnat
February 16, 2010 6:14 AM

Forgot to finish a sentence:

If that light never catches up to you the object that’s emmitting the light should appear to go black. smile

Lawrence B. Crowell
Member
Lawrence B. Crowell
February 16, 2010 9:01 AM
Dark Gnat hits on an important thing here. A much closer flat space analogy to a black hole is an accelerated frame. An observer suspended by some means which provides an outwards acceleration close to the event horizon of a black hole observes physics in a way nearly identical to what an accelerated observer would see. An accelerated observer, with a very significant acceleration I might add, observes a type of event horizon behind them. This horizon splits the spacetime into a Rindler wedge which is the domain of observation accessible to this accelerated observer. So an observer accelerating in a certain direction can drop a mass and watch it move away as it approaches this event horizon… Read more »
Uncle Fred
Member
Uncle Fred
February 16, 2010 3:38 PM
LC I think that the average person is still going to be somewhat confused by your method of explanation. Obviously, you are well versed in physics, but I feel at times your manner of addressing these issues narrows your audience to just those with a similar background. I see this frustration in many of those who respond to your comments. This is not in any means a personal criticism, just an idea to help others (such as myself) receive the full benefit of your insight. Here is what I feel would help me and others: 1. Don’t use prerequisite terminology Ex. “Nyquist” “transverse modes” 2. Avoid explaining with math 3. Use visual viewpoints to explain. What does the… Read more »
Lawrence B. Crowell
Member
Lawrence B. Crowell
February 16, 2010 4:06 PM

Sorry if this was a bit abstract. Some questions were raised about the relationship between strings and black holes. This is an interesting area of research, for I do think that black hole physics determines the structure of elementary particles.

LC

Lawrence B. Crowell
Member
Lawrence B. Crowell
February 17, 2010 4:01 AM

Right, the speed of light is a universal invariant. If you are travelling at a relativistic velocity away from a source of light those photons reach you at the speed of light, but their wavelength is stetched out or Doppler shifted.

If you head directly towards a source of light it is blue shifted. Further, if you accelerate at a constant g = 10m/sec^2 then within about a year’s time the cosmic background radiation in microwave band will appear as X-ray radiation, and eventually as gamma radiation. This radiation will then destroy your spacecraft — and you along with it.

LC

Uncle Fred
Member
Uncle Fred
February 17, 2010 6:34 PM

It’s understandable.

Regards,

UF

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