A photon’s point of view

Correction. The astronaut can determine if he is accelerating, even though he can only assume (having a constant velocity) that he rather than the rest of the universe is moving.

Hey, great explanation. I have never thought of it quite like that, and it does seem to clear up GR a bit.

I am not so sure about the discussion on evolution.

This was on the level of some Ladies' Home Journal. If I were to read this again, spaghettification would result.

Correction. The astronaut can determine if he is accelerating, even though he can only assume (having a constant velocity) that he rather than the rest of the universe is moving.

But only by looking out, no windows no chance.

Correction. The astronaut can determine if he is accelerating, even though he can only assume (having a constant velocity) that he rather than the rest of the universe is moving.

But only by looking out, no windows no chance.

Tidal forces. Other than that, can't measure it. And they wouldn't really measure "acceleration" as that is also a relative idea when it comes to acceleration due to gravity. But, they could determine that tidal forces are increasing and presume they are approaching a gravitational body (or a gravitational body is approaching THEM! :)

So what's stopping the astronaut from simply stopping his spacecraft with a quick dab of thrusters and hanging there in the blue sky, if there is no force pulling?

I don't like these geometry analogies, because they provide no reason why the objects should follow the geometry of space-time. If the universe is a "sheet" full of dips where objects fall, then you still haven't explained what draws them there.

For example, if the astronaut seems to be accelerating towards the planet while simply being at rest, why can't he go the other way just the same?

The same astronaut would be standing on the planet's surface and suddenly pick up an enormous velocity upwards and dissapear into space, while experiencing no acceleration in his subjective point of view.

What makes it work one way, but not the other?

For example, if the astronaut seems to be accelerating towards the planet while simply being at rest, why can't he go the other way just the same?

The same astronaut would be standing on the planet's surface and suddenly pick up an enormous velocity upwards and dissapear into space, while experiencing no acceleration in his subjective point of view.

What makes it work one way, but not the other?

Relativity and gravity.

and this artilce was rated 4.8/5... huh!?

@Eikka, Because it requires a force NOT to move along a geodesic. In free-fall, the observer shuts off the space-ship engines,.. and takes the natural path, a geodesic,.... any other path would require a force from the space-ship.

A geodesic is the "shortest" possible path in space-time, by depending on the distribution of mass-energy, may not be straight.

If two black holes 'merge' or (collide head on ), is space-time itself an object of 'spaghettification'?.
Not familiar with black hole 'merger' mechanics.

If two black holes 'merge' or (collide head on ), is space-time itself an object of 'spaghettification'?.
Not familiar with black hole 'merger' mechanics.

As I understand it, Yes...

The event horizon remaining after the merger of two black holes is always larger than the sum of the original volumes.

If two black holes 'merge' or (collide head on ), is space-time itself an object of 'spaghettification'?.
Not familiar with black hole 'merger' mechanics.

Look up "Cosmic Quandaries with Dr. Neil deGrasse Tyson" on Youtube. In the Q&A session towards the end a kid asks what happens when black holes collide and Tyson talks a bit about it.

We can travel 'faster than light' now?

Why is this article here? It just doesn't seem like news.

"But only by looking out, no windows no chance." - whomever

When released in an accelerating free fall, the ball will move with the astronaut. If released when the frame is being accelerated forward then when released the ball will move toward the ship.

"We can travel 'faster than light' now?" - hmmm

Only Randite/Libertarian/Republican lies spread faster than the speed of light.

Why is this article here? It just doesn't seem like news.

Nerlich has a pretty good record of bringing cutting edge research down to the layman's level and explaining some of the implications of said research. His weekly contribution to Universe Today is almost always a great read.

This week is just alright and lacking the news content. Still a succinct read for the layman (me). If you don't get GR and spacetime curvature I think it is a great perspective. Not news though.

The article tends to confuse perception with reality. If you are falling towards a black hole you may not experience acceleration but you are indeed acceleratng at a very high rate and you would observe the surface moving towards you so from any point of view you would know you were accelerating.

I've been torn a new one for proposing this little gedanken, so seeing it here, I feel a little less butt-hurty. Thank you Mr. Nerlich .

"But only by looking out, no windows no chance." - {OJorf}

When released in an accelerating free fall, the ball will move with the astronaut. If released when the frame is being accelerated forward then when released the ball will move toward the ship.

Because in the former case the astronaut is following a geodesic while in the latter he is not. The article speaks of free fall, in which case your first post was false, except maybe for the irrelevent exception as correctly pointed out by Ojorf.

I'm not sure that I can believe that centripetal force cannot be felt. I imagine an instance where I am slingshot around a planet. It would seem that the difference in direction would prevail as a potential difference and thus be experienced by the traveler(Newton on a rocket). In this event, the traveler must experience the gain in momentum by the rocket from the planet. equal and opposite reactions... In the case of the rocket landing on the planet, any difference in velocity between the objects will be felt. The only instance is going into orbit which must return the difference to zero in order to land. the article is right to one extent; a craft that intersects an orbital trajectory will follow a straight line around the planet with no detection due to the same "difference". an orbit is a straight line in four dimensions.

Gravity is not real, its just our dumbed-down interpretation of space-time curvature.

Hmmm, i think gravity is as unreal as 'space-time curvature' as neither are 'real' in the sense of being a discoverable physical substance existing independently of their application in relating things. Space and time are fundamentally relations between things and nothing more. For example, the time between two events is Defined as the number of cycles of a particular atom that are congruencent with the two events; simply comparing the number of occurences of one event to another.

The failure of simultaneity shows that space-time is not a absolute existing entity of itself.

The presence of mass-energy somehow effects the ceasium atom, so the arena of events, ...the geometry of space-time has to be modified. In general relativity, the metric tensor describes the distribution of mass-energy, and is something that has to be observed and put into the equations.

,... if one wants to talk evolution,.. given that out minds evolved with a means of ordering experience for consciousness,.. our a-priori intuitions of space and time are not quit right; we can not conform realty within these conceptual structures with consistency at all levels of reality. In GR they have to be slightly modified by observational input, and in QM break down completely. This shows that space-time is the subjective part of phenomenal reality.

The space-time model only describes paths. Something must still drive you around those paths. Assume a massive body hanging out there in space all alone, and somehow you managed to get a stone somewhere near that body and left it there motionless relative to the body. The stone will actually initiate acceleration towards that body. Why can't it just remain where it is forever?

I'm not sure that I can believe that centripetal force cannot be felt. I imagine an instance where I am slingshot around a planet. It would seem that the difference in direction would prevail as a potential difference and thus be experienced by the traveler

Only because you assume that the space-ship should align with the geodesic as compared with the back of the space-ship, and thus have to steer the ship yourself thus inducing the centripetal force you speak of, otherwise if you have no such preference (as a comet) following such a sling shot path will not cause a force.

Im not sure if or how a centripetal force would be compatible with GR in general though (?).

The space-time model only describes paths. Something must still drive you around those paths. Assume a massive body hanging out there in space all alone, and somehow you managed to get a stone somewhere near that body and left it there motionless relative to the body. The stone will actually initiate acceleration towards that body. Why can't it just remain where it is forever?

No, GR describes motion as well, not just static paths!!! An object remaining at the same distance from a massive body requires a force to NOT accelerate toward the massive body, therefore the 'natural state' of the stone, with no stone-propulsion mechanism, is to accelerate toward the massive body. Now, why is such a natural path a (curved) geodesic, nobody knows.

No, GR describes motion as well, not just static paths!!! An object remaining at the same distance from a massive body requires a force to NOT accelerate toward the massive body, therefore the 'natural state' of the stone, with no stone-propulsion mechanism, is to accelerate toward the massive body. Now, why is such a natural path a (curved) geodesic, nobody knows.

Ok, the question is, why does it need a force to hold it in place where the natural state is to accelerate into its geodesic? Why doesn't the opposite happen?

As I said, why does mass cause space-time to 'curve', I don't know. What is certain is that treating gravity as a Newtonian force plus special relativity does not work as accurately as general relativity which treats space-time as curved.

Maybe someone else can answer.

Perhaps gravitational waves are more akin to the forces that cause a meniscus in fluids ?

In general relativity, the metric tensor describes the distribution of mass-energy, and is something that has to be observed and put into the equations.

Before I get busted on this; I meant the stress-energy tensor above.

@Kontrast
Thks for the link.
The second Grader, Neil Tyson, and I share a common denominator. We are not able to follow the math of merger mechanics for holes.

Really, biological evolution is driven more by chance chemical reactions (resulting the formation and mutation of lifeforms) and inter-organismal relationships than by geographical (including climate) changes. Just because blooms permitted the growth of more complex life does not make such things the driving force.

"We can travel 'faster than light' now?" - hmmm

Only Randite/Libertarian/Republican lies spread faster than the speed of light.

As the left continues its collective nihilistic plunge into oblivion; doing everything in their power to take the rest of us with them.

therefore the 'natural state' of the stone, with no stone-propulsion mechanism, is to accelerate toward the massive body.

Can someone please clarify. Why would the stone that is traveling at a constant velocity not accelerate while a stationary stone would? I guess the difference is the initial conditions of the two cases?

I guess what is confusing is that in the article it says

Put a planet in their line of trajectory.

Does that mean, put a planet instantly where a there was not planet before, or consider the case where there is now a planet in their path. Or is this the same thing?

Wrong: "Time and distance are just two apparently different aspects of the same thing.
Reason: Einstein established "3-D space plus 4th D time" not the theory "distance is the same thing a time" theory.

Wrong:" photon not experience the passage of time, it does not experience the passage of distance either. Since you cant move a mass less consciousness at the speed of light in a vacuum
Reason: So you are basing your entire argument on the hunch that a photon has a human like consciousness ? You are mixing up human think with quantum think.

Wrong: "doomed astronauts will not themselves experience any such change to their velocity. After all, if they were accelerating, surely they would be pushed back into their seat as a consequence."
Reason: They would be pushed back in their seats. Einstein established the "equivalence principle", the mass in front of the rocket ship would exert and acceleration pull indistinguishable from the pull of gravity.

We are assuming that time is a euclidean dimension like our other 3-dimensions for this article. It is my understanding that this is a flawed view. In all euclidean dimensions you could from your location being equal to 0, define a positive vector and a negative vector. The time vector is a velocity vector in a sense and you can not have a negative velocity vector because the negative component would be included in the spatial vector instead. This does not remove the idea of time compression by GR.

I just wanted to remind everybody that symmetry of time is more likely an illusion created by the human memory and that there is no such thing as a place in time as time is merely the speed at which change occurs.

i'm not sure that my explanation of my thoughts makes sense, i'm merely a sophomore in college and have not yet perfected the art of articulating my thoughts.

As for evolution there are fossils, vestigial organs and mitochondrial DNA. Get real.

Good man.

Only Randite/Libertarian/Republican lies spread faster than the speed of light.

And monarchy (upon the death of a soverign the sovereignity(?) passes instantaneously to the next in line).

This is why Douglas Adams propsed faster than light communication by the very careful torturing of a small king.

I hope to see this discussion continue and not fade into the archives of physorg, it is very interesting.

Can someone please clarify. Why would the stone that is traveling at a constant velocity not accelerate while a stationary stone would?

The stone traveling at a constant velocity near a massive object indeed would accelerate. Acceleration is described as a vector, so if there is just a change of direction it counts as an acceleration, even though from the stones perspective it's going at the same speed; i.e.. the observer watching the stone would describe it using a vector.

We are assuming that time is a euclidean dimension like our other 3-dimensions for this article{...} In all euclidean dimensions you could from your location being equal to 0, define a positive vector and a negative vector. The time vector is a velocity vector in a sense and you can not have a negative velocity vector because the negative component would be included in the spatial vector instead.

In special relativity, the 'distance' between events are described by just one vector, the spacetime interval,... not a time vector and a spatial vector. The spacetime interval is a 'mixture' of time and space components with a Lorentzian metric signature,...i.e. time is given the opposite sign of the spatial components. say (-, , , ).

... This editor does't allow plus symbols for some reason. I mean (minus (t), plus (X), plus (y), plus (Z)) or the opposite.