Spacetime Definition and 1000 Threads

I was chatting to someone recently about the motion of objects and whether they propagate through spacetime or not. They were/are convinced that motion through spacetime is simply not possible arguing something along the lines of the following: "Objects move through space. If you depict an...

I'm having a difficult time researching the answer to my question about the speed of light. Now obviously it is a speed not only reserved for light but also all other massless particles/waves. It's obviously a constant property of our Spacetime since we can manipulate th speeds of different...

I think I may have finally had my eureka moment in understanding how curved spacetime could cause an object at rest to be attracted to another very massive object due to gravity. Could someone please confirm whether what I've written below is correct please? The unit vectors associated with a...

Putting aside gravity, if there were some way to stretch and twist spacetime, what effect would that have on the matter inside the volume? Say we were talking about a part of spacetime that enclosed the earth, would the Earth itself be stretched and twisted? Also, wouldn't a ruler I used be...

I recently had someone ask me why we use 4-vectors in special relativity and what is the motivation for introducing them in the first place. This is the response I gave: From Einstein's postulates( i.e. 1. the principle of relativity - the laws of physics are identical (invariant) in all...

I have just read an article http://phys.org/news/2016-05-space-three-dimensional.html about the following paper: http://arxiv.org/pdf/1502.01843v2.pdf and wonder what specialist think about it, resp. says their intuition.

I came across this issue a while ago, when spacetime expands, then energy doesn' seem to be conserved? But does not that violate the law of conservation of energy? I don't get it, how can spacetime expansion happen without energy issues? Thanks in advance

Do space-time have such behavior that if we give force on it and eventually it reacts according to the third law of Newton?

May be this experiment will be able tell us soon https://www.sciencedaily.com/releases/2016/04/160422115329.htm One of the problems to be solved in this respect is that if space-time is granular beyond a certain scale it means that there is a "basic scale," a fundamental unit that cannot be...

Would general relativity (axiom: spacetime is continuous) still work if we changed the axiom to: spacetime is discrete (but each individual chunk of it is so incredibly small* relative to a quark or gluon that it makes no difference)? *e.g. 3D's of Planck length, 1D of Planck time

According to current theory, high concentrations of matter warp space-time and create gravity. The Einstein field equations EFE describe the fundamental interaction of gravitation as a result of spacetime being curved by matter and energy. Would not a reinterpretation of the EFE lead to the...

In this http://web.stanford.edu/~oas/SI/SRGR/notes/SRGRLect6_2007.pdf, it is stated: Likewise, objects in spacetime all move at constant speed c in spacetime but if you change its direction, say by moving at speed v in the x direction, then spatial speed will change and so will the speed along...

Homework Statement Show that the isotropy and homogeneity of space-time and equivalence of different inertial frames (first postulate of relativity) require that the most general transformation between the space-time coordinates (x, y, z, t) and (x', y', z', t') is the linear transformation...

Does length contraction means the contraction of space time?

What exactly are the theoretical motivations for considering space and time as a four dimensional continuum? Is it a natural consequence of requiring that the speed of light is independent of the frame of reference that it is measured in, since this implies that time and time are not absolute...

I was looking at the Static Weak Field Metric, which Hartle gives as: ##ds^2 = (1- \frac{2\Phi(x^i)}{c^2})(dx^2 + dy^2 + dz^2)## For a fixed time. Where, for example, ##\Phi(r) = \frac{-GM}{r}## I was trying to figure out how the coordinates (x, y, z) could be defined. Clearly, they can't...

In a gravitational field you can put a particle and it is immediately subject to a force that tries to accelerate it. But in a curved spacetime notion if you put a particle it has no reason to move. Right?

[Moderator's note: this post has been spun off into its own thread.] I'm a retired engineer trying to get my head around GR, its effects in our everyday non-relativistic world, and its reduction to Newtonian gravity. I hope this is not too much of a digression from the current string. As I...

I watched this video : https://www.youtube.com/watch?v=sOiifkFYck4 Here, the lecturer said that if someone wants a spacetime which contains spin structure (physically equal to the existence of fermions, CMIIW) should topologically ℝ×Σ, where Σ is the Cauchy surface. Is that true? If so, then...

Hello, I am reading through some GR lecture notes and have come across the following: "A spacetime is static if there exists a coordinate chart where: ∂0gμν = 0 g0i = 0 This spacetime admits a Timelike Killing vector X that satisfies: X[α∇βXγ] = 0 " How do I go about proving that this...

Would, assuming a quantum theory of gravity, space time be able to be torn. Or is this only possible withe the assumption of string theory? If such a tear were possible what implications would it have on the universe?

I don't really understand energy-momentum... Regarding the fundamental fields in the standard model: Could the gluons in an atom (and the atom itself), ever be stationary to spacetime? I'm assuming not because that would seem like the atom didn't have gravity or ...whatever? (I can't really word...

From what I read attempts to measure the curvature of space have not succeeded. It would seem there may not be a curvature of space time. If this is true then what may be implied is that space goes on forever. If this is true how could the big bang theory, if it could, give a reasonable answer...

Homework Statement This is Exercise 15.2 in MTW - See attachment Homework Equations See attachment The Attempt at a Solution [/B] My attempt at a solution is also in the attachment. Are my initial assumptions OK? If not can someone nudge me in the right direction. If my initial...

I've been trying to learn about cosmological expansion (some weeks ago), I think I understand as much as any lay-person could, regarding why everything is moving away from our galaxy. However I still don't understand what spacetime is. The fact that space can deform indicates to me that...

I wanted to post the conjecture from this new paper which has recently been published within this past month - here's a direct link to the paper: http://arxiv.org/pdf/1602.01439v1.pdf Could an experimental apparatus consisting of oscillating masses be used to generate miniscule gravity waves...

Physicists refer to "spacetime", lumping together the dimensions of X, Y, Z, and T as if they're all common and same. This reductionism is the product of mathematical rigor. But in our daily lives, we don't experience T in the same way we experience X, Y, and Z. I can arbitrarily set the...

so i am somewhat new to the theory of general relativity but in none of the papers i have read does anyone seem to explain what causes matter to attract... for example, the moon and the Earth are attracted to each other because each of them warps spacetime around themselves. these warped pockets...

Are spacetime and the gravitational quantum field (still hypothetical) separate entities? Would the gravitational field be more fundamental, one of the various entities from which spacetime as a whole is composed? Gravitons, which are believed to transmit the force of gravity, would surely be...

In QFT, an ultraviolet cutoff is imposed to avoid singularities. One physical reason for why this works may be that there actually is an ultraviolet cutoff from spacetime being quantized. Since in GR spacetime is responsible for the force of gravity, and gravitons are the hypothesized quanta of...

If general relativity, space and time are postulated as a 4D non euclidean space that exist absolutely. All past, present, and future events are static moments in this model. In this model, the present moment can access and has knowledge of past events, but cannot access or have knowledge of...

Any papers about manifolds beyond spacetime? It's because the quantum objects may not be embedded in spacetime at all (which is where our current quantum gravity programmes work).. but other kind of manifolds where Einsteinian spacetime and quantum objects are emergent from them? What are the...

Hello. I'm not sure if this is an A level thread however I'll just post it here. I have a question that regards dark energy in relation to space time. I've learned that dark energy only affects space and not time. However I've also learned that space and time are intrinsically intertwined as...

Does a large amount of mass in a 'small' area cause spacetime to bend inwards to the centre of mass? If so: Since the path of light changes because it is traveling with the bent spacetime, why wouldn't other moving particles also bend their path while moving in a bent spacetime? So if it is...

Lorentz contraction problem: By Bertrand Boucquillon Components of the problem: - Bob (observer) - 2 identical rods that both measure 1 meter. Let's call them rod X and rod Y - Point A - Point B Scenario (step by step): 1) Bob is at point A, and is at rest with both rods in his hands 2) Bob...

I was messing around with the relativistic energy equation and stumbled upon something that looks like the spacetime interval equation. So, I'm wondering if there is some deeper connection there, or if it's just an interesting coincidence. I'll just go through it really quickly. E2 = m2c4 +...

I understand that large objects that have an extreme amount of mass results in the warping of space-time, however my question is more fundamental than that. I'm wondering why the presence of mass causes the warped space-time. Is there an intrinsic property within mass that causes this warping...

So I saw that claims are being made that LIGO may have detected gravitational waves. http://www.nature.com/news/has-giant-ligo-experiment-seen-gravitational-waves-1.18449 My question is, if the universe were in fact multidimensional as string theory predicts, would gravitational waves propagate...

While learning about SR, I encountered the famous Minkowski spacetime. At first I thought it was just a useful mathematical trick, with no particular physical meaning. But reading more about it, it seems that this notion of spacetime is often (always?) regarded as the actual physical reality...

Is the inertial frame of reference the same thing as space time?

We now know two things distort space time: relative speed [we call it time dilation and length contraction] and gravity. Gravity IS the 'curvature' of space and time. "Mass tells spacetime how to curve; spacetime tells mass how to move." [John Wheeler, I think.]

Is QFT on curved spacetime BtSM

Hello, I am re-reading a book about quantum physics and general relativity. To introduce representation of the lorentz group, they explain the definition of lorentz group as the group of transformation that let x² + y² ... -t² unchanged. But in cuved space the distance is not the same as in...

I'm trying to understand why timelike geodesics in Anti de-Sitter space are plotted as sinusoidal waves on a Penrose diagram (a nice example of the Penrose diagram for AdS is given in Figure 2.3 of this thesis: http://www.nbi.dk/~obers/MSc_PhD_files/MortenHolm_Christensen_MSc.pdf). Bearing in...

The Schwarzschild spacetime is defined by the following line element \begin{equation*} ds^2 = - \left( 1 - \frac{2m}{r} \right)dt^2 + \frac{1}{1-\frac{2m}{r}}dr^2 + r^2 d\theta^2 + r^2\sin \theta^2 d\phi^2. \end{equation*} We can use the isotropic coordinates, obtained from the Schwarzschild...

As I understand it, the standard spacetime model is a 4-dimensional geometric object, and that my personal history is just a particular curve in the model. Reality as I experience it though is that I move through time, and thus I have the concept of the present in which "now" corresponds to the...

Hello I searched a lot but I am not sure if I understood correctly the change in the shape of light cone while speeding up. I am aware that the x and ct axis are getting closer to each other like scissors while you speed up as the graph below shows, both symmetricaly approaching the ct=x or v=c...

Can you tie this together for me? My understanding: If something does not have mass, it does not interact with time. If something has mass it interacts with spacetime. When mass interacts with spacetime, and creates gravity, it may travel through n+1 dimensions. My questions: A.Time is...

Would it be misleading (to myself) to define mass as that which (insert a group of words like interacts with/experiences/ travels through/ is affected by/...your preference) time? That seems more useful than "that which warps spacetime."

What do you think of Stephen Wolfram's blog post about spacetime? http://blog.stephenwolfram.com/2015/12/what-is-spacetime-really