# Questions after reading The Beginning of Infinity

1. Sep 25, 2015

### TheTuringTester

I listened to the audiobook version of The Beginning of Infinity by David Deutsch on a recent vacation. I'll always associate the drive from Seattle to Gold Beach, Oregon with Physics! I've read some of Stephen Hawking's and Brian Greene's books, but the way David Deutsch worded some of his explanations, it really made me question my assumptions. I made a list of very fundamental questions that I realized I didn't know the answer to. I hope this forum can help.

1. I've often read about spaghettification as you get closer to the center of a black hole. How does space latch onto your atoms and differentially stretch them, overcoming their binding force? I don't think there is friction or any grip between spacetime and subatomic particles, is there?

2. When virtual particles to steal energy from the Universe, there is a time limit to balance the books proportional to the amount of energy. Does this give us any clue as to the nature of time itself? By what mechanism does time enforce the energy limit?

3. Could particle superposition be due to our perception of time rather than the particle really being in to separate spatial positions at the same time?

4. I believe objects that are traveling below the speed of light can't be sped up to go faster than the speed of light and hypothetical tachyons that are going faster than the speed of light can't be slowed down to below the speed of light. What if two tachyons collided? Would a collision produce regular quarks traveling below the speed of light? Would a glancing collision possibly slow a tachyon down below light speed without exploding it into new particles? If so, what kind of particle might it be?

Thanks for the insight and answers!
Paul

2. Sep 25, 2015

### Orodruin

Staff Emeritus
1. You do not need GR or black holes to understand this, it is a tidal force which results in different parts of an extended object being accelerated at different rates. The same type of effect is responsible for the tides on Earth.

4. Tachyons are connected with significant problems regarding causality and other things. If they do exist, finding out how they would scatter to normal particles might be the least of our problems when it comes to making a consistent theory.

3. Sep 25, 2015

### TheTuringTester

I think my misunderstanding is even more fundamental than I thought. I still don't understand how spacetime can pull on subatomic particles. I assume my concept of friction or grip is flawed, but I don't know what to replace it with.

4. Sep 25, 2015

### Staff: Mentor

Consider two rocks that, at some instant of time, are at slightly different high altitudes above the Earth, one directly above the other, and both at rest relative to the Earth (and each other). As time passes, the rocks will start to accelerate downward, and the one that is lower will do so a little more than the higher one. So the rocks will gradually separate. There is no force pushing or pulling them apart--remember that, in GR, gravity is not a force; the rocks are in free fall, feeling no force. How then can they separate? Spacetime curvature.

In other words, every object is constantly "moving through spacetime", and because of the presence of matter and energy, spacetime is curved, and that curvature affects how objects move through spacetime.

Now suppose we start with two rocks as above, but this time they are connected by a spring. Now what will happen? The rocks will start accelerating downward at slightly different rates; but as they start to separate, the spring will pull on them, resisting the effects of spacetime curvature. So they won't separate as much; depending on how stiff the spring is, they might oscillate back and forth relative to each other as they fall, or they might just stay at almost the same separation (with a really stiff spring), so that they behave like a single object instead of two objects. It all depends on the relationship between the stiffness of the spring and the curvature of spacetime. The interactions between subatomic particles are more or less equivalent to the spring in this example; they act between the particles to resist the effects of spacetime curvature as they move. (Note that spacetime curvature can cause objects to move closer to each other, as well as further away from each other--consider, for example, two rocks that start out at rest at the same high altitude above the Earth, but separated horizontally by a small distance. As they fall, they come closer together.)

What happens near the singularity of a black hole is that the curvature of spacetime is so large that, no matter how strong the "springs" (interactions) between subatomic particles are, they aren't strong enough to resist the effects of the curvature. So objects get stretched and squeezed in different directions by the curvature.

The key thing to understand is that objects don't need to be "gripped" by spacetime in order to move through it. Objects can't help moving through spacetime; it's the natural thing for them to do. But because spacetime is curved, even though the objects are all doing their natural thing, they don't all move the same way.

5. Sep 25, 2015

### TheTuringTester

I just remembered my fifth question...I will try not to cram any more questions in a single thread, but in a way, they all are similar in the sense that they ask about very fundamental properties of the Universe and this way anyone not interested can just skip the thread instead of 4 or 5 threads. Thanks again for any replies.

5. If heat is simply the average speed (motion) of particles and all motion is relative, is heat and all its consequences (phase transitions, pressure, radiant light, etc.) relative?

6. Sep 25, 2015

### Staff: Mentor

Spacetime isn't pulling on or gripping anything. If I'm falling into a black hole, I'm in free fall, no forces at all acting on me (search this forum for a video made by member @A.T. to see how that works and why gravity doesn't count as a force in general relativity). However, because of the curvature of spacetime, the natural free-fall trajectory of my head diverges from the natural free fall trajectory of my feet. Thus, my feet and my head will drift apart - or at least they would if they weren't connected by my spine, which is pulling them both off their natural free fall trajectories. If the curvature is extreme enough, then the divergence between the two trajectories will be very high, meaning that my spine must exert a very great force my head and feet to keep them near one another. If the required force exceeds the strength of my spine.... Something breaks.

Last edited: Sep 25, 2015
7. Sep 25, 2015

### Staff: Mentor

You'll find these two questions discussed in many threads over in the Quantum Mechanics forum. The quick answer is that virtual particles don't steal energy from the universe and superposition does not mean that the particle is in two places at the same time. There's a lot of misleading stuff about QM in the popular press, mainly because it's very hard to talk about it without using a fair amount of math.

8. Sep 25, 2015

### TheTuringTester

I think I'm getting closer to an understanding and Nugatory and Peterdonis have made me look at it a different way. I believe that energy and matter are equivalent as far as their effect on spacetime distortion, so I'll stick to just asking about matter. If space is distorted, curved, by matter, what leverage does matter have to bend space? It's really the same question, but in reverse. If space without matter is geometrically flat without the presence of matter and it bends proportionally to the amount of matter, what is it about matter that can apply a force or leverage to bend space? If there is no friction or grip how does it bend?

9. Sep 26, 2015

### Staff: Mentor

No one knows. You'll notice, however, that this isn't a new problem. For example, Newton's $F=Gm_1m_2/r^2$ from classical physics predicts the force between two objects but doesn't tell us why there's a force, or by what mechanism one object exerts an influence over the other.

The natural sciences are about describing the rules by which the universe works. They don't always explain why we have those rules and not some others, and there are some questions for which the best known answer is "because that's how the universe we live in works".

10. Sep 26, 2015

### TheTuringTester

Nugatory,

Thank you. I was not expecting "No one knows." It was never explained in any of the popular science books I read and now I know why. Thanks for taking the time to answer some of my questions and clarify some new ones to explore. In a lot of the books, they start with classical physics and then give the quote about physics being solved in the late 1800s and only needing to fill out the extra decimal places except for the two issues that led to QM and GR. I am definitely not of that mindset to think we're almost done and I'm certainly not on board with the "shut up and calculate" school of physics. I want to know the answers to the biggies...

These are rhetorical...I know there are no answers to be had at this point to the following.
What is time?
What is energy?
What is motion?
What is space?
and two more, thanks to this thread...
How does matter bend space?
How does matter distort time?

11. Sep 26, 2015

### Staff: Mentor

The short answer is "no". The reason why it's "no" is that heat is the average speed of particles in the center of mass frame of the system. In other words, from a relativistic point of view, "heat" is really part of the rest mass of the system, and rest mass is an invariant; it's not relative.

12. Sep 26, 2015

### Staff: Mentor

Neither am I.

I don't think the viewpoint Nugatory was describing is "shut up and calculate". It's more like "we don't have all the answers right now, but that doesn't invalidate the answers we do have". We don't really have an answer to "how does matter bend spacetime"; but that doesn't invalidate the theory, General Relativity, that says that matter bends spacetime. No matter how much we learn, there will always be some questions to which our best current answer is "No one knows". But that doesn't mean we know nothing, and it doesn't mean we can't get on with physics using the best theories we have.

13. Sep 27, 2015

### TheTuringTester

Sorry about that. I didn't mean to come across as so negative. My vocabulary has fallen victim to all the books I've read referring to the Copenhagen Interpretation as the "Shut Up and Calculate" Interpretation. What I meant to express is that I hope we don't ever give up on trying to understand the how's and why's of science, being satisfied with the practical and pragmatic what's of science.