# Thread: Black Hole at near-light speed?

1. If mass increases as speed accelerates, what would happen to a Black Hole that was travelling at almost light speed?

Would the event horizon expand?

2.

3. This is equivalent to asking what a blackhole looks like from a moving frame of reference.

4. Originally Posted by KJW
This is equivalent to asking what a blackhole looks like from a moving frame of reference.
I'm afraid my knowledge of physics is somewhat lacking. I'm not sure I fully understand your analogy.

5. This is related to your question: If you go too fast, do you become a black hole?

Basically, a black hole's event horizon depends on its rest mass. (And the idea of mass increasing with velocity is not generally used nowadays as it leads to confusion.)

Also, note that velocity is relative so everything (black holes, you, etc.) is travelling at near light speed relative to something such as a high energy cosmic ray.

6. Originally Posted by Strange
This is related to your question: If you go too fast, do you become a black hole?
Is it truly the object's length that decreases (thus increasing the object's mass:volume ratio) or is it just a case of the 3rd dimension shortening relative to the other two planes when observed from an outside perspective? Would the object notice itself growing shorter?

If it's the dimension itself that shrinks rather than the physical substance of the object growing denser, then there'd be no reason to assume an object would become a black hole at light speed, would there?

7. Originally Posted by Daecon
Would the object notice itself growing shorter?
No. An object is at rest in its own frame of reference, and therefore has the same properties in that rest frame as it would in any other rest frame. This is what is meant by the laws of physics being the same in all inertial frames of reference. The consequences of this are far-reaching. Basically, it tells us everything about a moving object from the properties of the same object at rest.

8. Originally Posted by Daecon
Is it truly the object's length that decreases (thus increasing the object's mass:volume ratio) or is it just a case of the 3rd dimension shortening relative to the other two planes when observed from an outside perspective?
Length contraction is a relation between two inertial frames of reference in relative motion; it is a result of the geometry of Minkowski space-time.

Would the object notice itself growing shorter?
No. The object in its own frame of reference notices nothing special.

If it's the dimension itself that shrinks rather than the physical substance of the object growing denser, then there'd be no reason to assume an object would become a black hole at light speed, would there?
Right, an object doesn't turn into a black hole just because it is going fast relative to some other observer. It either is a black hole for all observers, or it is not.

9. Originally Posted by KJW
This is what is meant by the laws of physics being the same in all inertial frames of reference. The consequences of this are far-reaching. Basically, it tells us everything about a moving object from the properties of the same object at rest.
Given the elegant simplicity of this, it is amazing how many anti-relativity cranks there are out there.

10. Originally Posted by KJW
Given the elegant simplicity of this, it is amazing how many anti-relativity cranks there are out there.
Indeed. That is something that will never change, and is to be expected with anything that isn't intuitive from everyday life, such as SR and GR. The really sad part is that many of those cranks just spout the same nonsense for years and years, despite all evidence to the contrary; it's like they are walking around with blindfolds on. That bit is just incomprehensible to me - ignorance of certain aspects of physics is fine, but remaining ignorant is a choice, and unacceptable. Our member Farsight is a prime example - consider this :

At Rest with our Hubble view

This thread stands at 1329 posts, and just goes round and round in circles. Very sad indeed. And he has been doing this for many years now.

11. I think one of the "difficulties" of relativity is in explaining how the laws of physics have no absolute frame of reference while the physical universe does (e.g. the cosmic microwave background). While I see no inconsistency, I can understand why some people do as it is quite difficult to explain why there is no inconsistency.

12. Originally Posted by KJW
I think one of the "difficulties" of relativity is in explaining how the laws of physics have no absolute frame of reference while the physical universe does (e.g. the cosmic microwave background). While I see no inconsistency, I can understand why some people do as it is quite difficult to explain why there is no inconsistency.
Yes, that is the incompatability with everyday life I was referring to, because in our normal experience there is always some form of absolute frame, and be it just the earth's surface. It's very ingrained, and hard to let go of.

13. Originally Posted by Markus Hanke
Originally Posted by KJW
I think one of the "difficulties" of relativity is in explaining how the laws of physics have no absolute frame of reference while the physical universe does (e.g. the cosmic microwave background). While I see no inconsistency, I can understand why some people do as it is quite difficult to explain why there is no inconsistency.
Yes, that is the incompatability with everyday life I was referring to, because in our normal experience there is always some form of absolute frame, and be it just the earth's surface. It's very ingrained, and hard to let go of.
But it's not just the seeming incompatibility with everyday experience, the CMB is a cosmic frame of reference that people with some interest in astronomy (noting that the cranks aren't just ordinary members of the public who have no interest in science) can point to and say: "See, here we have an absolute universal frame of reference (that contradicts what relativity says)."

14. I'm sorry if I came across as a crank, I'm really just unfamiliar with this area of physics.

15. Originally Posted by KJW
But it's not just the seeming incompatibility with everyday experience, the CMB is a cosmic frame of reference that people with some interest in astronomy (noting that the cranks aren't just ordinary members of the public who have no interest in science) can point to and say: "See, here we have an absolute universal frame of reference (that contradicts what relativity says)."
That makes no sense to me, since the CMB is just photons; photons aren't inertial frames at all, you can't tell your relative velocity "with respect to a photon". Thus the CMB is most certainly not an absolute frame; in fact it never really occured to me that someone might even think that. Am I missing something here ??

16. Originally Posted by Daecon
I'm sorry if I came across as a crank, I'm really just unfamiliar with this area of physics.
Don't worry, you are not at all coming across as a crank - we understand that your questions are genuine. It is just that recently we have had a large number of "anti-relativity" threads, which is what got us talking. Nothing to do with you

17. Ah, that's a relief. I am genuinely interested in science even though I have no formal scientific education other than what I learned at school up until the age of 16, so I guess a lot of the questions I'd have probably seem quite amateurish.

18. Originally Posted by Daecon
Ah, that's a relief. I am genuinely interested in science even though I have no formal scientific education other than what I learned at school up until the age of 16, so I guess a lot of the questions I'd have probably seem quite amateurish.
That's quite ok
The difference between a crank and a genuine enquirer is that the latter will listen to what is being explained, whereas cranks are forever stuck in their own misconceptions. So in a way the difference is easy enough to tell by the reaction you get when explaining something.

19. Originally Posted by Markus Hanke
Originally Posted by KJW
But it's not just the seeming incompatibility with everyday experience, the CMB is a cosmic frame of reference that people with some interest in astronomy (noting that the cranks aren't just ordinary members of the public who have no interest in science) can point to and say: "See, here we have an absolute universal frame of reference (that contradicts what relativity says)."
That makes no sense to me, since the CMB is just photons; photons aren't inertial frames at all, you can't tell your relative velocity "with respect to a photon". Thus the CMB is most certainly not an absolute frame; in fact it never really occured to me that someone might even think that. Am I missing something here ??
It does make sense to "them" (the cranks). The idea comes t them from the bastardization of the correct term: "the frame in which CMBR (not CMB) is isotropic". The cranks drop the correct definition, "forget" that the frame in discussion is just another frame and declare it "privileged". This is the genesis of the crackpottery "CMB (sic!) is a preferred frame", best I could reconstruct it.

20. Originally Posted by xyzt
It does make sense to "them" (the cranks). The idea comes t them from the bastardization of the correct term: "the frame in which CMBR (not CMB) is isotropic". The cranks drop the correct definition, "forget" that the frame in discussion is just another frame and declare it "privileged". This is the genesis of the crackpottery "CMB (sic!) is a preferred frame", best I could reconstruct it.
To me, it is not just not preferred, but it isn't even a valid inertial frame at all. The whole concept of this somehow violating SR is just ludicrous.

21. The "preferred" frame of reference isn't the frame of reference of the photons, it is the frame of reference in which the CMBR from one half of the sky isn't redshifted relative to the CMBR from the opposite half of the sky. Our local group of galaxies is moving at about 370 km/s relative to this frame of reference.

22. Originally Posted by KJW
The "preferred" frame of reference isn't the frame of reference of the photons, it is the frame of reference in which the CMBR from one half of the sky isn't redshifted relative to the CMBR from the opposite half of the sky. Our local group of galaxies is moving at about 370 km/s relative to this frame of reference.
Ok, that's true, but such a frame is by no means preferred or universal.

23. Originally Posted by Markus Hanke
Originally Posted by KJW
The "preferred" frame of reference isn't the frame of reference of the photons, it is the frame of reference in which the CMBR from one half of the sky isn't redshifted relative to the CMBR from the opposite half of the sky. Our local group of galaxies is moving at about 370 km/s relative to this frame of reference.
Ok, that's true, but such a frame is by no means preferred or universal.
But it is universal because the CMBR is everywhere and there is only one frame of reference in which there is no dipolar anisotropy at any location. Everyone can measure their velocity relative to this frame of reference so it is in a sense preferred. The challenge is to explain why this does not violate relativity.

24. In my mind, I do understand why this (or any other seemingly preferred frame of reference) doesn't violate relativity. But, I do find it difficult to explain it to other people. The symmetry of relativity is about the laws of physics, whereas physical fields break this symmetry. The problem then becomes explaining what the laws of physics are, and in particular how they are distinct from the physical fields. The problem is that in some sense, the laws of physics do depend on the physical environment, whereas in the sense required by relativity, they don't.

25. Originally Posted by Markus Hanke
Originally Posted by xyzt
It does make sense to "them" (the cranks). The idea comes t them from the bastardization of the correct term: "the frame in which CMBR (not CMB) is isotropic". The cranks drop the correct definition, "forget" that the frame in discussion is just another frame and declare it "privileged". This is the genesis of the crackpottery "CMB (sic!) is a preferred frame", best I could reconstruct it.
To me, it is not just not preferred, but it isn't even a valid inertial frame at all. The whole concept of this somehow violating SR is just ludicrous.
The frame in which CMBR is isotropic is used routinely in literature, specifically in the Mansouri-Sexl test theory of SR.

26. Only an outside observer would detect an increase of mass concerning an object of any mass approaching the speed of light.

But the interesting thing about the question of the OP is that it does warrant a paradoxical situation; it is in fact extremely similar to the time-radiation paradox of the black hole itself.

Basically, if someone (in their frame of reference) watches something else fall towards the horizon - the dilation effects are so strong that it would seem like that the objects never actually passes it. But if it never falls in, why then can anything fall into a black hole?

Obviously the answer to this question is on par of the OP question. The answer is that obviously the object falls into the black hole, in the same line, obviously the black hole will eventually evaporate. The black hole approaching a speed of light however, could be anything like a very dense small black hole that has an extended life-time because the time frame of the object will be stretched.

But someone outside won't see this object - this object will emit practically no radiation as it passes you by - so a black hole moving at nearly lightspeed would be pretty much undetectable - we would still be able to detect the gravitational force. I'd reckon.

27. But if it never falls in, why then can anything fall into a black hole?
Because the in-falling object, in its own frame, experiences nothing special. Space-time looks regular and normal at the event horizon, and the object will hit the singularity in a finite proper time as seen from its own FOR. The in-fall time from event horizon to singularity can be anything between fractions of a second to several days. I did this calculation for a supermassive Schwarzschild black hole ( approx 15 billion solar masses ) as an exercise once, and the result was just over 72 hours.

A far-away outside observer will argue that the object never reaches the event horizon; of course he is also correct, but only in his own frame of reference.

28. Originally Posted by Geometrogenesis
Basically, if someone (in their frame of reference) watches something else fall towards the horizon - the dilation effects are so strong that it would seem like that the objects never actually passes it. But if it never falls in, why then can anything fall into a black hole?
It is a common belief that an external observer will see something falling in "frozen" at the event horizon for ever. There are several reasons why this is not the case.

As I understand it, the "infinite time dilation" at the event horizon would only be true for an observer at infinity. (Edit: apparently, I don't )

The falling object will emit a finite number of photons before it crosses the event horizon. These will leave the black hole in finite time. (I have seen a calculation that shows this time would be pretty short ... I don't remember now, but of the order of milliseconds maybe?)

And, of course, the final photons will be massively red-shifted.

Also, as the mass of the black hole increases, the event horizon radius will increase to include the falling object.

This also says that the Schwarzschild solution is only an approximate description for this situation as it strictly applies only to an unchanging black hole in an empty universe.

So, there appears to be no problem with things falling into a black hole.

29. Originally Posted by Strange
As I understand it, the "infinite time dilation" at the event horizon would only be true for an observer at infinity.
Incorrect. If you are stationary relative to the blackhole, no matter how close you are to the event horizon (but remaining definitely outside), the time dilation will be "infinite" at the event horizon.

30. Originally Posted by KJW
Originally Posted by Strange
As I understand it, the "infinite time dilation" at the event horizon would only be true for an observer at infinity.
Incorrect. If you are stationary relative to the blackhole, no matter how close you are to the event horizon (but remaining definitely outside), the time dilation will be "infinite" at the event horizon.
Thanks!

31. I believe (but have not proven) that an object falling into a blackhole will appear (if one ignores the effect of redshift) to be falling at increasing speed into an infinitely deep hole, with the event horizon at the "bottom" (at infinity). That is, I don't think the object will appear to be frozen onto a hypothetical spherical surface. In other words, although the event horizon is at a finite proper distance, I believe that stereoscopically, the light will appear to come from an infinite distance.

32. Originally Posted by KJW
I believe (but have not proven) that an object falling into a blackhole will appear (if one ignores the effect of redshift) to be falling at increasing speed into an infinitely deep hole, with the event horizon at the "bottom" (at infinity). That is, I don't think the object will appear to be frozen onto a hypothetical spherical surface. In other words, although the event horizon is at a finite proper distance, I believe that stereoscopically, the light will appear to come from an infinite distance.
...in other words, it will simply become dimmer and dimmer, and eventually fade away as it approaches the event horizon.

33. Originally Posted by Markus Hanke
Originally Posted by KJW
I believe (but have not proven) that an object falling into a blackhole will appear (if one ignores the effect of redshift) to be falling at increasing speed into an infinitely deep hole, with the event horizon at the "bottom" (at infinity). That is, I don't think the object will appear to be frozen onto a hypothetical spherical surface. In other words, although the event horizon is at a finite proper distance, I believe that stereoscopically, the light will appear to come from an infinite distance.
...in other words, it will simply become dimmer and dimmer, and eventually fade away as it approaches the event horizon.
Sorry, I was also excluding the dimming effect when I excluded the effect of redshift (by decreasing the frequency, one is also decreasing the rate of emission). My point was about where the light-rays appear to come from, the idea being that a blackhole looks like a hole in space, and also that event horizons (of all types) appear to be infinitely far away even if they are not. The general idea is that regions beyond an event horizon do not create voids in our perceived space, that our perceived space is complete even if it does not include all of the physical space.

34. [QUOTE=Strange;453713]
Originally Posted by Geometrogenesis
So, there appears to be no problem with things falling into a black hole.

I'm sorry, I perhaps wasn't clear. The question was rhetoric. The question was asked to highlight an important relationship to the OP and the paradox of infalling bodies and their relation also to the thermodynamics of the black hole.

Basically, yes, bodies still fall into the horizon, just as a black hole will still radiate away even though when moving at near lightspeed even though there is a time dilation factor to take into account (which may extend the lifetime of a black hole) according to our measurements.

 Bookmarks
##### Bookmarks
 Posting Permissions
 You may not post new threads You may not post replies You may not post attachments You may not edit your posts   BB code is On Smilies are On [IMG] code is On [VIDEO] code is On HTML code is Off Trackbacks are Off Pingbacks are Off Refbacks are On Terms of Use Agreement